Reply by Phil Hobbs June 4, 20192019-06-04
On 6/3/19 2:17 PM, John Larkin wrote:
> On Sun, 2 Jun 2019 19:15:52 -0400, Phil Hobbs > <pcdhSpamMeSenseless@electrooptical.net> wrote: > >> On 6/1/19 11:44 AM, John Larkin wrote: >>> On Sat, 01 Jun 2019 10:59:58 +0300, upsidedown@downunder.com wrote: >>> >>>> On Fri, 31 May 2019 15:36:39 -0700, John Larkin >>>> <jjlarkin@highland_snip_technology.com> wrote: >>>> >>>>> I want to make a class-D audio amp, 150 watts or so, using a TI >>>>> TPA3255 maybe. It's good for 600 watts mono! >>>>> >>>>> I'll use it full-bridge to drive a step-up transformer, probably a >>>>> custom toroid. But toroids are especially unhappy with any DC drive, >>>>> and the class D part will surely have some DC offset. The TI spec is >>>>> 60 mV max output offset, which could be a problem into a good >>>>> transformer. Speakers don't mind a little DC, but transformers do. DC >>>>> can cause stairstepped increase in circulating current, the Devil's >>>>> Staircase, until they saturate. >>>> >>>> Is it necessary to use a toroid, why not ordinary EI transformer with >>>> possibly air gap ? >>> >>> A toroid would be about half the footprint and half the weight of a >>> regular transformer, and would have lower output impedance. I know a >>> couple of guys who do nice toroids. One did this for us, lower power. >>> >>> https://www.dropbox.com/s/0bl6jdkw4ljx4e2/400_Hz_Toroid.JPG?dl=0 >>> >>> >>> >>>> >>>> Or how about a high inductance but low resistance external solenoid >>>> across toroid primary ? Apparently the frequency is in the 400 Hz >>>> ballpark, so even less solenoid inductance should suffice, compared to >>>> 50/60 Hz. >>> >>> I'm guessing that the TI amp would fight to maintain its output offset >>> voltage. A series cap would be a lot easier. I could quit bitching and >>> use some big caps. Probably don't need the diodes. >> >> Voltages are relative, so it might fight to maintain its output offset >> compared to what? The ground pins, I expect. If you used only one >> output per package, you could maybe servo the grounds to eliminate the >> offset. >> >> Cheers >> >> Phil Hobbs > > The output offset is what a speaker (or my transformer) would see, at > the full-bridge output. There's probably a feedback loop from the > outputs to the input diffamp; the numbers look about right. > >
Right, I knew that. Moving right along.... ;) Cheers Phil Hobbs -- Dr Philip C D Hobbs Principal Consultant ElectroOptical Innovations LLC Optics, Electro-optics, Photonics, Analog Electronics 160 North State Road #203 Briarcliff Manor NY 10510 hobbs at electrooptical dot net http://electrooptical.net
Reply by Chris Jones June 3, 20192019-06-03
On 03/06/2019 10:04, Lasse Langwadt Christensen wrote:
> mandag den 3. juni 2019 kl. 01.36.05 UTC+2 skrev John Larkin: >> On Sun, 2 Jun 2019 16:05:10 -0700 (PDT), Lasse Langwadt Christensen >> <langwadt@fonz.dk> wrote: >> >>> mandag den 3. juni 2019 kl. 00.23.03 UTC+2 skrev John Larkin: >>>> On Sun, 2 Jun 2019 18:07:37 -0400, bitrex <user@example.net> wrote: >>>> >>>>> On 6/2/19 6:04 PM, bitrex wrote: >>>>>> On 6/1/19 11:23 AM, John Larkin wrote: >>>>>>> On Sat, 1 Jun 2019 00:42:55 -0400, bitrex <user@example.net> wrote: >>>>>>> >>>>>>>> On 6/1/19 12:23 AM, bitrex wrote: >>>>>>>>> On 5/31/19 10:55 PM, John Larkin wrote: >>>>>>>>>> On Fri, 31 May 2019 22:22:37 -0400, krw@notreal.com wrote: >>>>>>>>>> >>>>>>>>>>> On Fri, 31 May 2019 15:36:39 -0700, John Larkin >>>>>>>>>>> <jjlarkin@highland_snip_technology.com> wrote: >>>>>>>>>>> >>>>>>>>>>>> >>>>>>>>>>>> >>>>>>>>>>>> I want to make a class-D audio amp, 150 watts or so, using a TI >>>>>>>>>>>> TPA3255 maybe. It's good for 600 watts mono! >>>>>>>>>>>> >>>>>>>>>>>> I'll use it full-bridge to drive a step-up transformer, probably a >>>>>>>>>>>> custom toroid. But toroids are especially unhappy with any DC drive, >>>>>>>>>>>> and the class D part will surely have some DC offset. The TI spec is >>>>>>>>>>>> 60 mV max output offset, which could be a problem into a good >>>>>>>>>>>> transformer. Speakers don't mind a little DC, but transformers >>>>>>>>>>>> do. DC >>>>>>>>>>>> can cause stairstepped increase in circulating current, the Devil's >>>>>>>>>>>> Staircase, until they saturate. >>>>>>>>>>>> >>>>>>>>>>>> So I'm thinking I'll add a series blocking cap so I can ignore >>>>>>>>>>>> any DC >>>>>>>>>>>> problems. It will have to be big, 10s of millifarads at least. >>>>>>>>>>>> Biggest >>>>>>>>>>>> thing on the board. Maybe use a low voltage electrolytic with >>>>>>>>>>>> antiparallel power diodes, or a shorted bridge, to protect it from >>>>>>>>>>>> accidental forward or backwards over-voltage. >>>>>>>>>>>> >>>>>>>>>>>> Lytics will be big, and supercaps don't seem to like ripple >>>>>>>>>>>> current. I >>>>>>>>>>>> think. >>>>>>>>>>>> >>>>>>>>>>>> Any other ideas about driving a transformer from an audio amp? >>>>>>>>>>> >>>>>>>>>>> Feed back a (heavily) filtered signal to the input?&nbsp; The problem is >>>>>>>>>>> that they don't really tell you what the input of the 3255 looks >>>>>>>>>>> like, >>>>>>>>>>> IIRC (none do).&nbsp; It's intended to be AC coupled. >>>>>>>>>> >>>>>>>>>> TI does spec 60 mV max DC offset at the output, which is pretty good, >>>>>>>>>> but even that could push a lot of DC into my transformer. >>>>>>>>>> >>>>>>>>>> There is probably some way to tweak the input with a little DC. >>>>>>>>>> Feedback loop or even a trimpot. >>>>>>>>>> >>>>>>>>>> >>>>>>>>> >>>>>>>>> You can drive the transformer bridged but using current-sense feedback >>>>>>>>> instead of voltage feedback; put a small sense resistor in line with >>>>>>>>> each amp output going to each end of the transformer and take off >>>>>>>>> the DC >>>>>>>>> feedback to the opposite amp from the junction. That way the amps >>>>>>>>> should >>>>>>>>> act as their own servo to keep DC out of the transformer. >>>>>>>>> >>>>>>>>> Lower power example like this for driving audio isolation transformer >>>>>>>>> for XLR cable: >>>>>>>>> >>>>>>>>> <https://www.dropbox.com/s/8d1flmr8lko2nf1/Screenshot_2019-06-01_00-12-49.png?dl=0> >>>>>>>>> >>>>>>>>> >>>>>>>>> >>>>>>>> >>>>>>>> In the case of the TPA3255 I think you would put a low-offset op amp in >>>>>>>> front of the inputs and AC couple into that and do something similar but >>>>>>>> the 3255 just acts as a power buffer, I don't immediately see anything >>>>>>>> in the datasheet that says you can't intentionally apply a small DC >>>>>>>> feedback generated offset to its single input per channel as an error >>>>>>>> signal. >>>>>>> >>>>>>> It's not stated how the normally AC coupled inputs affect the DC >>>>>>> offset, but we have the eval board and I could have a scut bunny set >>>>>>> it up and try it. >>>>>>> >>>>>>> One trimpot and 30 seconds of tech time, to turn it, is sure >>>>>>> appealing. If it turns out we don't need it, we can leave it off the >>>>>>> board. >>>>>>> >>>>>>> The input resistance of the TPA3255 is 10K, and they want 10 uF input >>>>>>> coupling caps. Tau is 100 ms, which is 1.6 Hz corner. That suggests to >>>>>>> me that the input caps are also used as lowpass filters for the DC >>>>>>> feedback loop, which then suggests we could push the input pins gently >>>>>>> to change the output offset. >>>>>>> >>>>>>> Datasheets tend to hide the good stuff. >>>>>>> >>>>>>> >>>>>>> >>>>>> >>>>>> Does LTC make any class D amp modules included with LTSpice? I'm curious >>>>>> now about how using a DC servo loop into a first stage opamp driving the >>>>>> D-amp as a power buffer, instead of a cap would work out in practice. >>>>>> >>>>>> The problem with that XLR transformer driver circuit as drawn in a >>>>>> modification is that while there are two feedback loops, one local to >>>>>> the op-amps and one around the opposite side current sense resistor, >>>>>> it's assumed that the phase of the AC signals on both inputs of the op >>>>>> amps will be similar. >>>>>> >>>>>> but the phase shift produced by the class D output filter complicates >>>>>> things, where to put the sense resistor. If after the output filter need >>>>>> to compensate the phase shift somehow and if before need to filter the >>>>>> switching frequency down to DC. It may not be nearly as easily workable >>>>>> in the class D bridged power-buffer topology as with linear amplifiers >>>>>> driving the transformer, bridged. >>>>> >>>>> Putting it after the LC also means have to take the (possibly not well- >>>>> defined) R of the L into account as a component of the current sense >>>>> impedance. >>>> >>>> >>>> We plan to put the current sensor just before the customer output, >>>> which is after the filter inductors and the isolation transformer. >>>> Probably a resistive shunt, isolated by a small audio transformer. >>>> >>>> It will be an interesting control loop, synthesizing the output >>>> impedance. >>>> >>> >>> use a h-bridge instead and do everything in software/FPGA ? >>> >> >> The TI switcher is a class-D h-bridge. I wouldn't want to try to go >> linear. >> >> We do plan to do all the processing in an FPGA. > > I wasn't thinking linear but pwm directly from the FPGA to a powerstage, just like numerous variable frequency drives and brushless DC motor drives > > >
The open-loop all-digital approach tends to produce less accurate output waveforms than DAC + class D amplifier. If the power supply rails are not perfectly smooth, or if the propagation delays or rise and fall times or on-resistance of the output devices have some dependence on the output current, then you will get distortion, that can best be corrected with feedback. The all-digital approach might be ok for a motor drive but you won't reach the kind of distortion performance that can be achieved by a class D amplifier that takes analogue feedback from the output terminals, e.g. these Hypex ones: https://www.hypex.nl/oem-audio-amplifiers/
Reply by Chris Jones June 3, 20192019-06-03
On 03/06/2019 13:31, John Larkin wrote:
> On Mon, 3 Jun 2019 11:53:23 +1000, Chris Jones > <lugnut808@spam.yahoo.com> wrote: > >> On 03/06/2019 08:22, John Larkin wrote: >>> On Sun, 2 Jun 2019 18:07:37 -0400, bitrex <user@example.net> wrote: >>> >>>> On 6/2/19 6:04 PM, bitrex wrote: >>>>> On 6/1/19 11:23 AM, John Larkin wrote: >>>>>> On Sat, 1 Jun 2019 00:42:55 -0400, bitrex <user@example.net> wrote: >>>>>> >>>>>>> On 6/1/19 12:23 AM, bitrex wrote: >>>>>>>> On 5/31/19 10:55 PM, John Larkin wrote: >>>>>>>>> On Fri, 31 May 2019 22:22:37 -0400, krw@notreal.com wrote: >>>>>>>>> >>>>>>>>>> On Fri, 31 May 2019 15:36:39 -0700, John Larkin >>>>>>>>>> <jjlarkin@highland_snip_technology.com> wrote: >>>>>>>>>> >>>>>>>>>>> >>>>>>>>>>> >>>>>>>>>>> I want to make a class-D audio amp, 150 watts or so, using a TI >>>>>>>>>>> TPA3255 maybe. It's good for 600 watts mono! >>>>>>>>>>> >>>>>>>>>>> I'll use it full-bridge to drive a step-up transformer, probably a >>>>>>>>>>> custom toroid. But toroids are especially unhappy with any DC drive, >>>>>>>>>>> and the class D part will surely have some DC offset. The TI spec is >>>>>>>>>>> 60 mV max output offset, which could be a problem into a good >>>>>>>>>>> transformer. Speakers don't mind a little DC, but transformers >>>>>>>>>>> do. DC >>>>>>>>>>> can cause stairstepped increase in circulating current, the Devil's >>>>>>>>>>> Staircase, until they saturate. >>>>>>>>>>> >>>>>>>>>>> So I'm thinking I'll add a series blocking cap so I can ignore >>>>>>>>>>> any DC >>>>>>>>>>> problems. It will have to be big, 10s of millifarads at least. >>>>>>>>>>> Biggest >>>>>>>>>>> thing on the board. Maybe use a low voltage electrolytic with >>>>>>>>>>> antiparallel power diodes, or a shorted bridge, to protect it from >>>>>>>>>>> accidental forward or backwards over-voltage. >>>>>>>>>>> >>>>>>>>>>> Lytics will be big, and supercaps don't seem to like ripple >>>>>>>>>>> current. I >>>>>>>>>>> think. >>>>>>>>>>> >>>>>>>>>>> Any other ideas about driving a transformer from an audio amp? >>>>>>>>>> >>>>>>>>>> Feed back a (heavily) filtered signal to the input?&nbsp; The problem is >>>>>>>>>> that they don't really tell you what the input of the 3255 looks >>>>>>>>>> like, >>>>>>>>>> IIRC (none do).&nbsp; It's intended to be AC coupled. >>>>>>>>> >>>>>>>>> TI does spec 60 mV max DC offset at the output, which is pretty good, >>>>>>>>> but even that could push a lot of DC into my transformer. >>>>>>>>> >>>>>>>>> There is probably some way to tweak the input with a little DC. >>>>>>>>> Feedback loop or even a trimpot. >>>>>>>>> >>>>>>>>> >>>>>>>> >>>>>>>> You can drive the transformer bridged but using current-sense feedback >>>>>>>> instead of voltage feedback; put a small sense resistor in line with >>>>>>>> each amp output going to each end of the transformer and take off >>>>>>>> the DC >>>>>>>> feedback to the opposite amp from the junction. That way the amps >>>>>>>> should >>>>>>>> act as their own servo to keep DC out of the transformer. >>>>>>>> >>>>>>>> Lower power example like this for driving audio isolation transformer >>>>>>>> for XLR cable: >>>>>>>> >>>>>>>> <https://www.dropbox.com/s/8d1flmr8lko2nf1/Screenshot_2019-06-01_00-12-49.png?dl=0> >>>>>>>> >>>>>>>> >>>>>>>> >>>>>>> >>>>>>> In the case of the TPA3255 I think you would put a low-offset op amp in >>>>>>> front of the inputs and AC couple into that and do something similar but >>>>>>> the 3255 just acts as a power buffer, I don't immediately see anything >>>>>>> in the datasheet that says you can't intentionally apply a small DC >>>>>>> feedback generated offset to its single input per channel as an error >>>>>>> signal. >>>>>> >>>>>> It's not stated how the normally AC coupled inputs affect the DC >>>>>> offset, but we have the eval board and I could have a scut bunny set >>>>>> it up and try it. >>>>>> >>>>>> One trimpot and 30 seconds of tech time, to turn it, is sure >>>>>> appealing. If it turns out we don't need it, we can leave it off the >>>>>> board. >>>>>> >>>>>> The input resistance of the TPA3255 is 10K, and they want 10 uF input >>>>>> coupling caps. Tau is 100 ms, which is 1.6 Hz corner. That suggests to >>>>>> me that the input caps are also used as lowpass filters for the DC >>>>>> feedback loop, which then suggests we could push the input pins gently >>>>>> to change the output offset. >>>>>> >>>>>> Datasheets tend to hide the good stuff. >>>>>> >>>>>> >>>>>> >>>>> >>>>> Does LTC make any class D amp modules included with LTSpice? I'm curious >>>>> now about how using a DC servo loop into a first stage opamp driving the >>>>> D-amp as a power buffer, instead of a cap would work out in practice. >>>>> >>>>> The problem with that XLR transformer driver circuit as drawn in a >>>>> modification is that while there are two feedback loops, one local to >>>>> the op-amps and one around the opposite side current sense resistor, >>>>> it's assumed that the phase of the AC signals on both inputs of the op >>>>> amps will be similar. >>>>> >>>>> but the phase shift produced by the class D output filter complicates >>>>> things, where to put the sense resistor. If after the output filter need >>>>> to compensate the phase shift somehow and if before need to filter the >>>>> switching frequency down to DC. It may not be nearly as easily workable >>>>> in the class D bridged power-buffer topology as with linear amplifiers >>>>> driving the transformer, bridged. >>>> >>>> Putting it after the LC also means have to take the (possibly not well- >>>> defined) R of the L into account as a component of the current sense >>>> impedance. >>> >>> >>> We plan to put the current sensor just before the customer output, >>> which is after the filter inductors and the isolation transformer. >>> Probably a resistive shunt, isolated by a small audio transformer. >> >> If you're going to use transformer isolation for the shunt, you might as >> well use a current transformer instead.
>>That way you can have less >> voltage per turn of the core for a given customer-visible output >> current, so a smaller core and/or better accuracy.
I have now realised that this might not be true. There could be arbitrarily many turns on the isolation transformer after a shunt, though I still don't think it is a better solution.
>> Probably better >> frequency response too. You'd also face less modifications if you ever >> needed to switch to a hall effect sort of current sensor, if you wanted >> to extend the frequency range downwards. >> >> >> > > A shunt+signal transformer is a lot smaller than a CT, and generally > has better frequency response. I could use a tiny surface-mount shunt > and a small step-up transformer, standard parts. 50 or 100 mV drop in > the shunt would be fine.
Although I am no longer convinced of the inferiority of your solution, I would be surprised if there are not some small CTs available these days. As I replied to myself above, I no longer think the CT necessarily offers better linearity. A possible advantage of the CT is that the manufacturer will expect it to be used like this, whereas the audio transformer manufacturer will not, so that the applications support and characterisation data available might be better for the CT.
Reply by Lasse Langwadt Christensen June 3, 20192019-06-03
mandag den 3. juni 2019 kl. 05.24.59 UTC+2 skrev John Larkin:
> On Sun, 2 Jun 2019 17:04:11 -0700 (PDT), Lasse Langwadt Christensen > <langwadt@fonz.dk> wrote: > > >mandag den 3. juni 2019 kl. 01.36.05 UTC+2 skrev John Larkin: > >> On Sun, 2 Jun 2019 16:05:10 -0700 (PDT), Lasse Langwadt Christensen > >> <langwadt@fonz.dk> wrote: > >> > >> >mandag den 3. juni 2019 kl. 00.23.03 UTC+2 skrev John Larkin: > >> >> On Sun, 2 Jun 2019 18:07:37 -0400, bitrex <user@example.net> wrote: > >> >> > >> >> >On 6/2/19 6:04 PM, bitrex wrote: > >> >> >> On 6/1/19 11:23 AM, John Larkin wrote: > >> >> >>> On Sat, 1 Jun 2019 00:42:55 -0400, bitrex <user@example.net> wrote: > >> >> >>> > >> >> >>>> On 6/1/19 12:23 AM, bitrex wrote: > >> >> >>>>> On 5/31/19 10:55 PM, John Larkin wrote: > >> >> >>>>>> On Fri, 31 May 2019 22:22:37 -0400, krw@notreal.com wrote: > >> >> >>>>>> > >> >> >>>>>>> On Fri, 31 May 2019 15:36:39 -0700, John Larkin > >> >> >>>>>>> <jjlarkin@highland_snip_technology.com> wrote: > >> >> >>>>>>> > >> >> >>>>>>>> > >> >> >>>>>>>> > >> >> >>>>>>>> I want to make a class-D audio amp, 150 watts or so, using a TI > >> >> >>>>>>>> TPA3255 maybe. It's good for 600 watts mono! > >> >> >>>>>>>> > >> >> >>>>>>>> I'll use it full-bridge to drive a step-up transformer, probably a > >> >> >>>>>>>> custom toroid. But toroids are especially unhappy with any DC drive, > >> >> >>>>>>>> and the class D part will surely have some DC offset. The TI spec is > >> >> >>>>>>>> 60 mV max output offset, which could be a problem into a good > >> >> >>>>>>>> transformer. Speakers don't mind a little DC, but transformers > >> >> >>>>>>>> do. DC > >> >> >>>>>>>> can cause stairstepped increase in circulating current, the Devil's > >> >> >>>>>>>> Staircase, until they saturate. > >> >> >>>>>>>> > >> >> >>>>>>>> So I'm thinking I'll add a series blocking cap so I can ignore > >> >> >>>>>>>> any DC > >> >> >>>>>>>> problems. It will have to be big, 10s of millifarads at least. > >> >> >>>>>>>> Biggest > >> >> >>>>>>>> thing on the board. Maybe use a low voltage electrolytic with > >> >> >>>>>>>> antiparallel power diodes, or a shorted bridge, to protect it from > >> >> >>>>>>>> accidental forward or backwards over-voltage. > >> >> >>>>>>>> > >> >> >>>>>>>> Lytics will be big, and supercaps don't seem to like ripple > >> >> >>>>>>>> current. I > >> >> >>>>>>>> think. > >> >> >>>>>>>> > >> >> >>>>>>>> Any other ideas about driving a transformer from an audio amp? > >> >> >>>>>>> > >> >> >>>>>>> Feed back a (heavily) filtered signal to the input?&nbsp; The problem is > >> >> >>>>>>> that they don't really tell you what the input of the 3255 looks > >> >> >>>>>>> like, > >> >> >>>>>>> IIRC (none do).&nbsp; It's intended to be AC coupled. > >> >> >>>>>> > >> >> >>>>>> TI does spec 60 mV max DC offset at the output, which is pretty good, > >> >> >>>>>> but even that could push a lot of DC into my transformer. > >> >> >>>>>> > >> >> >>>>>> There is probably some way to tweak the input with a little DC. > >> >> >>>>>> Feedback loop or even a trimpot. > >> >> >>>>>> > >> >> >>>>>> > >> >> >>>>> > >> >> >>>>> You can drive the transformer bridged but using current-sense feedback > >> >> >>>>> instead of voltage feedback; put a small sense resistor in line with > >> >> >>>>> each amp output going to each end of the transformer and take off > >> >> >>>>> the DC > >> >> >>>>> feedback to the opposite amp from the junction. That way the amps > >> >> >>>>> should > >> >> >>>>> act as their own servo to keep DC out of the transformer. > >> >> >>>>> > >> >> >>>>> Lower power example like this for driving audio isolation transformer > >> >> >>>>> for XLR cable: > >> >> >>>>> > >> >> >>>>> <https://www.dropbox.com/s/8d1flmr8lko2nf1/Screenshot_2019-06-01_00-12-49.png?dl=0> > >> >> >>>>> > >> >> >>>>> > >> >> >>>>> > >> >> >>>> > >> >> >>>> In the case of the TPA3255 I think you would put a low-offset op amp in > >> >> >>>> front of the inputs and AC couple into that and do something similar but > >> >> >>>> the 3255 just acts as a power buffer, I don't immediately see anything > >> >> >>>> in the datasheet that says you can't intentionally apply a small DC > >> >> >>>> feedback generated offset to its single input per channel as an error > >> >> >>>> signal. > >> >> >>> > >> >> >>> It's not stated how the normally AC coupled inputs affect the DC > >> >> >>> offset, but we have the eval board and I could have a scut bunny set > >> >> >>> it up and try it. > >> >> >>> > >> >> >>> One trimpot and 30 seconds of tech time, to turn it, is sure > >> >> >>> appealing. If it turns out we don't need it, we can leave it off the > >> >> >>> board. > >> >> >>> > >> >> >>> The input resistance of the TPA3255 is 10K, and they want 10 uF input > >> >> >>> coupling caps. Tau is 100 ms, which is 1.6 Hz corner. That suggests to > >> >> >>> me that the input caps are also used as lowpass filters for the DC > >> >> >>> feedback loop, which then suggests we could push the input pins gently > >> >> >>> to change the output offset. > >> >> >>> > >> >> >>> Datasheets tend to hide the good stuff. > >> >> >>> > >> >> >>> > >> >> >>> > >> >> >> > >> >> >> Does LTC make any class D amp modules included with LTSpice? I'm curious > >> >> >> now about how using a DC servo loop into a first stage opamp driving the > >> >> >> D-amp as a power buffer, instead of a cap would work out in practice. > >> >> >> > >> >> >> The problem with that XLR transformer driver circuit as drawn in a > >> >> >> modification is that while there are two feedback loops, one local to > >> >> >> the op-amps and one around the opposite side current sense resistor, > >> >> >> it's assumed that the phase of the AC signals on both inputs of the op > >> >> >> amps will be similar. > >> >> >> > >> >> >> but the phase shift produced by the class D output filter complicates > >> >> >> things, where to put the sense resistor. If after the output filter need > >> >> >> to compensate the phase shift somehow and if before need to filter the > >> >> >> switching frequency down to DC. It may not be nearly as easily workable > >> >> >> in the class D bridged power-buffer topology as with linear amplifiers > >> >> >> driving the transformer, bridged. > >> >> > > >> >> >Putting it after the LC also means have to take the (possibly not well- > >> >> >defined) R of the L into account as a component of the current sense > >> >> >impedance. > >> >> > >> >> > >> >> We plan to put the current sensor just before the customer output, > >> >> which is after the filter inductors and the isolation transformer. > >> >> Probably a resistive shunt, isolated by a small audio transformer. > >> >> > >> >> It will be an interesting control loop, synthesizing the output > >> >> impedance. > >> >> > >> > > >> >use a h-bridge instead and do everything in software/FPGA ? > >> > > >> > >> The TI switcher is a class-D h-bridge. I wouldn't want to try to go > >> linear. > >> > >> We do plan to do all the processing in an FPGA. > > > >I wasn't thinking linear but pwm directly from the FPGA to a powerstage, just like numerous variable frequency drives and brushless DC motor drives > > > > > > We considered that, using GaN fets. The amp would be tiny. But the TI > thing has current limiting, thermal limiting, all sorts of > protections. We'd have to add that to a home-made H-bridge. > > Are there any integrated h-bridges with all the suitable protections? > My FPGA guys wouldn't mind outputting PWM (or delta-sigma?). I should > look into that. >
this is a bit slow https://www.infineon.com/cms/en/product/power/motor-control-ics/intelligent-motor-control-ics/single-half-bridge-driver/btn8982ta/ maybe? http://www.ti.com/product/TAS5631B
Reply by June 3, 20192019-06-03
On Saturday, June 1, 2019 at 11:44:56 AM UTC-4, John Larkin wrote:
> On Sat, 01 Jun 2019 10:59:58 +0300, upsidedown@downunder.com wrote: > > >On Fri, 31 May 2019 15:36:39 -0700, John Larkin > ><jjlarkin@highland_snip_technology.com> wrote: > > > >>I want to make a class-D audio amp, 150 watts or so, using a TI > >>TPA3255 maybe. It's good for 600 watts mono! > >> > >>I'll use it full-bridge to drive a step-up transformer, probably a > >>custom toroid. But toroids are especially unhappy with any DC drive, > >>and the class D part will surely have some DC offset. The TI spec is > >>60 mV max output offset, which could be a problem into a good > >>transformer. Speakers don't mind a little DC, but transformers do. DC > >>can cause stairstepped increase in circulating current, the Devil's > >>Staircase, until they saturate. > > > >Is it necessary to use a toroid, why not ordinary EI transformer with > >possibly air gap ? > > A toroid would be about half the footprint and half the weight of a > regular transformer, and would have lower output impedance. I know a > couple of guys who do nice toroids. One did this for us, lower power. > > https://www.dropbox.com/s/0bl6jdkw4ljx4e2/400_Hz_Toroid.JPG?dl=0 > > > > > > >Or how about a high inductance but low resistance external solenoid > >across toroid primary ? Apparently the frequency is in the 400 Hz > >ballpark, so even less solenoid inductance should suffice, compared to > >50/60 Hz. > > I'm guessing that the TI amp would fight to maintain its output offset > voltage. A series cap would be a lot easier. I could quit bitching and > use some big caps. Probably don't need the diodes.
As just a half-baked idea, could you use two transformers? Keep your toroid for the power, plus a second light-weight transformer in parallel that's exposed to the toroid's same volt-seconds, to sense any imbalance? Then the sensing transformer could be anything you like (E-core, pot-core, gapped, toroid, saturable cored, etc.), and you could sense saturation, hysteresis, use a flux gate, Hall sensor, or whatever crazy scheme you wanted to monitor any imbalance. Just a notion for the notion bin... Cheers, James
Reply by John Larkin June 3, 20192019-06-03
On Sun, 2 Jun 2019 19:15:52 -0400, Phil Hobbs
<pcdhSpamMeSenseless@electrooptical.net> wrote:

>On 6/1/19 11:44 AM, John Larkin wrote: >> On Sat, 01 Jun 2019 10:59:58 +0300, upsidedown@downunder.com wrote: >> >>> On Fri, 31 May 2019 15:36:39 -0700, John Larkin >>> <jjlarkin@highland_snip_technology.com> wrote: >>> >>>> I want to make a class-D audio amp, 150 watts or so, using a TI >>>> TPA3255 maybe. It's good for 600 watts mono! >>>> >>>> I'll use it full-bridge to drive a step-up transformer, probably a >>>> custom toroid. But toroids are especially unhappy with any DC drive, >>>> and the class D part will surely have some DC offset. The TI spec is >>>> 60 mV max output offset, which could be a problem into a good >>>> transformer. Speakers don't mind a little DC, but transformers do. DC >>>> can cause stairstepped increase in circulating current, the Devil's >>>> Staircase, until they saturate. >>> >>> Is it necessary to use a toroid, why not ordinary EI transformer with >>> possibly air gap ? >> >> A toroid would be about half the footprint and half the weight of a >> regular transformer, and would have lower output impedance. I know a >> couple of guys who do nice toroids. One did this for us, lower power. >> >> https://www.dropbox.com/s/0bl6jdkw4ljx4e2/400_Hz_Toroid.JPG?dl=0 >> >> >> >>> >>> Or how about a high inductance but low resistance external solenoid >>> across toroid primary ? Apparently the frequency is in the 400 Hz >>> ballpark, so even less solenoid inductance should suffice, compared to >>> 50/60 Hz. >> >> I'm guessing that the TI amp would fight to maintain its output offset >> voltage. A series cap would be a lot easier. I could quit bitching and >> use some big caps. Probably don't need the diodes. > >Voltages are relative, so it might fight to maintain its output offset >compared to what? The ground pins, I expect. If you used only one >output per package, you could maybe servo the grounds to eliminate the >offset. > >Cheers > >Phil Hobbs
The output offset is what a speaker (or my transformer) would see, at the full-bridge output. There's probably a feedback loop from the outputs to the input diffamp; the numbers look about right. -- John Larkin Highland Technology, Inc picosecond timing precision measurement jlarkin att highlandtechnology dott com http://www.highlandtechnology.com
Reply by Bill Sloman June 3, 20192019-06-03
On Saturday, June 1, 2019 at 12:37:06 AM UTC+2, John Larkin wrote:
> I want to make a class-D audio amp, 150 watts or so, using a TI > TPA3255 maybe. It's good for 600 watts mono! > > I'll use it full-bridge to drive a step-up transformer, probably a > custom toroid. But toroids are especially unhappy with any DC drive, > and the class D part will surely have some DC offset. The TI spec is > 60 mV max output offset, which could be a problem into a good > transformer. Speakers don't mind a little DC, but transformers do. DC > can cause stairstepped increase in circulating current, the Devil's > Staircase, until they saturate. > > So I'm thinking I'll add a series blocking cap so I can ignore any DC > problems. It will have to be big, 10s of millifarads at least. Biggest > thing on the board. Maybe use a low voltage electrolytic with > antiparallel power diodes, or a shorted bridge, to protect it from > accidental forward or backwards over-voltage. > > Lytics will be big, and supercaps don't seem to like ripple current. I > think. > > Any other ideas about driving a transformer from an audio amp?
Why not just monitor the current flowing through the transformer with a low value resistor, low pass filter the voltage and look at it with a low-offset op amp? -- Bill Sloman, Sydney
Reply by John Larkin June 3, 20192019-06-03
On Mon, 3 Jun 2019 11:53:23 +1000, Chris Jones
<lugnut808@spam.yahoo.com> wrote:

>On 03/06/2019 08:22, John Larkin wrote: >> On Sun, 2 Jun 2019 18:07:37 -0400, bitrex <user@example.net> wrote: >> >>> On 6/2/19 6:04 PM, bitrex wrote: >>>> On 6/1/19 11:23 AM, John Larkin wrote: >>>>> On Sat, 1 Jun 2019 00:42:55 -0400, bitrex <user@example.net> wrote: >>>>> >>>>>> On 6/1/19 12:23 AM, bitrex wrote: >>>>>>> On 5/31/19 10:55 PM, John Larkin wrote: >>>>>>>> On Fri, 31 May 2019 22:22:37 -0400, krw@notreal.com wrote: >>>>>>>> >>>>>>>>> On Fri, 31 May 2019 15:36:39 -0700, John Larkin >>>>>>>>> <jjlarkin@highland_snip_technology.com> wrote: >>>>>>>>> >>>>>>>>>> >>>>>>>>>> >>>>>>>>>> I want to make a class-D audio amp, 150 watts or so, using a TI >>>>>>>>>> TPA3255 maybe. It's good for 600 watts mono! >>>>>>>>>> >>>>>>>>>> I'll use it full-bridge to drive a step-up transformer, probably a >>>>>>>>>> custom toroid. But toroids are especially unhappy with any DC drive, >>>>>>>>>> and the class D part will surely have some DC offset. The TI spec is >>>>>>>>>> 60 mV max output offset, which could be a problem into a good >>>>>>>>>> transformer. Speakers don't mind a little DC, but transformers >>>>>>>>>> do. DC >>>>>>>>>> can cause stairstepped increase in circulating current, the Devil's >>>>>>>>>> Staircase, until they saturate. >>>>>>>>>> >>>>>>>>>> So I'm thinking I'll add a series blocking cap so I can ignore >>>>>>>>>> any DC >>>>>>>>>> problems. It will have to be big, 10s of millifarads at least. >>>>>>>>>> Biggest >>>>>>>>>> thing on the board. Maybe use a low voltage electrolytic with >>>>>>>>>> antiparallel power diodes, or a shorted bridge, to protect it from >>>>>>>>>> accidental forward or backwards over-voltage. >>>>>>>>>> >>>>>>>>>> Lytics will be big, and supercaps don't seem to like ripple >>>>>>>>>> current. I >>>>>>>>>> think. >>>>>>>>>> >>>>>>>>>> Any other ideas about driving a transformer from an audio amp? >>>>>>>>> >>>>>>>>> Feed back a (heavily) filtered signal to the input?&#2013266080; The problem is >>>>>>>>> that they don't really tell you what the input of the 3255 looks >>>>>>>>> like, >>>>>>>>> IIRC (none do).&#2013266080; It's intended to be AC coupled. >>>>>>>> >>>>>>>> TI does spec 60 mV max DC offset at the output, which is pretty good, >>>>>>>> but even that could push a lot of DC into my transformer. >>>>>>>> >>>>>>>> There is probably some way to tweak the input with a little DC. >>>>>>>> Feedback loop or even a trimpot. >>>>>>>> >>>>>>>> >>>>>>> >>>>>>> You can drive the transformer bridged but using current-sense feedback >>>>>>> instead of voltage feedback; put a small sense resistor in line with >>>>>>> each amp output going to each end of the transformer and take off >>>>>>> the DC >>>>>>> feedback to the opposite amp from the junction. That way the amps >>>>>>> should >>>>>>> act as their own servo to keep DC out of the transformer. >>>>>>> >>>>>>> Lower power example like this for driving audio isolation transformer >>>>>>> for XLR cable: >>>>>>> >>>>>>> <https://www.dropbox.com/s/8d1flmr8lko2nf1/Screenshot_2019-06-01_00-12-49.png?dl=0> >>>>>>> >>>>>>> >>>>>>> >>>>>> >>>>>> In the case of the TPA3255 I think you would put a low-offset op amp in >>>>>> front of the inputs and AC couple into that and do something similar but >>>>>> the 3255 just acts as a power buffer, I don't immediately see anything >>>>>> in the datasheet that says you can't intentionally apply a small DC >>>>>> feedback generated offset to its single input per channel as an error >>>>>> signal. >>>>> >>>>> It's not stated how the normally AC coupled inputs affect the DC >>>>> offset, but we have the eval board and I could have a scut bunny set >>>>> it up and try it. >>>>> >>>>> One trimpot and 30 seconds of tech time, to turn it, is sure >>>>> appealing. If it turns out we don't need it, we can leave it off the >>>>> board. >>>>> >>>>> The input resistance of the TPA3255 is 10K, and they want 10 uF input >>>>> coupling caps. Tau is 100 ms, which is 1.6 Hz corner. That suggests to >>>>> me that the input caps are also used as lowpass filters for the DC >>>>> feedback loop, which then suggests we could push the input pins gently >>>>> to change the output offset. >>>>> >>>>> Datasheets tend to hide the good stuff. >>>>> >>>>> >>>>> >>>> >>>> Does LTC make any class D amp modules included with LTSpice? I'm curious >>>> now about how using a DC servo loop into a first stage opamp driving the >>>> D-amp as a power buffer, instead of a cap would work out in practice. >>>> >>>> The problem with that XLR transformer driver circuit as drawn in a >>>> modification is that while there are two feedback loops, one local to >>>> the op-amps and one around the opposite side current sense resistor, >>>> it's assumed that the phase of the AC signals on both inputs of the op >>>> amps will be similar. >>>> >>>> but the phase shift produced by the class D output filter complicates >>>> things, where to put the sense resistor. If after the output filter need >>>> to compensate the phase shift somehow and if before need to filter the >>>> switching frequency down to DC. It may not be nearly as easily workable >>>> in the class D bridged power-buffer topology as with linear amplifiers >>>> driving the transformer, bridged. >>> >>> Putting it after the LC also means have to take the (possibly not well- >>> defined) R of the L into account as a component of the current sense >>> impedance. >> >> >> We plan to put the current sensor just before the customer output, >> which is after the filter inductors and the isolation transformer. >> Probably a resistive shunt, isolated by a small audio transformer. > >If you're going to use transformer isolation for the shunt, you might as >well use a current transformer instead. That way you can have less >voltage per turn of the core for a given customer-visible output >current, so a smaller core and/or better accuracy. Probably better >frequency response too. You'd also face less modifications if you ever >needed to switch to a hall effect sort of current sensor, if you wanted >to extend the frequency range downwards. > > >
A shunt+signal transformer is a lot smaller than a CT, and generally has better frequency response. I could use a tiny surface-mount shunt and a small step-up transformer, standard parts. 50 or 100 mV drop in the shunt would be fine. -- John Larkin Highland Technology, Inc lunatic fringe electronics
Reply by John Larkin June 3, 20192019-06-03
On Sun, 2 Jun 2019 17:04:11 -0700 (PDT), Lasse Langwadt Christensen
<langwadt@fonz.dk> wrote:

>mandag den 3. juni 2019 kl. 01.36.05 UTC+2 skrev John Larkin: >> On Sun, 2 Jun 2019 16:05:10 -0700 (PDT), Lasse Langwadt Christensen >> <langwadt@fonz.dk> wrote: >> >> >mandag den 3. juni 2019 kl. 00.23.03 UTC+2 skrev John Larkin: >> >> On Sun, 2 Jun 2019 18:07:37 -0400, bitrex <user@example.net> wrote: >> >> >> >> >On 6/2/19 6:04 PM, bitrex wrote: >> >> >> On 6/1/19 11:23 AM, John Larkin wrote: >> >> >>> On Sat, 1 Jun 2019 00:42:55 -0400, bitrex <user@example.net> wrote: >> >> >>> >> >> >>>> On 6/1/19 12:23 AM, bitrex wrote: >> >> >>>>> On 5/31/19 10:55 PM, John Larkin wrote: >> >> >>>>>> On Fri, 31 May 2019 22:22:37 -0400, krw@notreal.com wrote: >> >> >>>>>> >> >> >>>>>>> On Fri, 31 May 2019 15:36:39 -0700, John Larkin >> >> >>>>>>> <jjlarkin@highland_snip_technology.com> wrote: >> >> >>>>>>> >> >> >>>>>>>> >> >> >>>>>>>> >> >> >>>>>>>> I want to make a class-D audio amp, 150 watts or so, using a TI >> >> >>>>>>>> TPA3255 maybe. It's good for 600 watts mono! >> >> >>>>>>>> >> >> >>>>>>>> I'll use it full-bridge to drive a step-up transformer, probably a >> >> >>>>>>>> custom toroid. But toroids are especially unhappy with any DC drive, >> >> >>>>>>>> and the class D part will surely have some DC offset. The TI spec is >> >> >>>>>>>> 60 mV max output offset, which could be a problem into a good >> >> >>>>>>>> transformer. Speakers don't mind a little DC, but transformers >> >> >>>>>>>> do. DC >> >> >>>>>>>> can cause stairstepped increase in circulating current, the Devil's >> >> >>>>>>>> Staircase, until they saturate. >> >> >>>>>>>> >> >> >>>>>>>> So I'm thinking I'll add a series blocking cap so I can ignore >> >> >>>>>>>> any DC >> >> >>>>>>>> problems. It will have to be big, 10s of millifarads at least. >> >> >>>>>>>> Biggest >> >> >>>>>>>> thing on the board. Maybe use a low voltage electrolytic with >> >> >>>>>>>> antiparallel power diodes, or a shorted bridge, to protect it from >> >> >>>>>>>> accidental forward or backwards over-voltage. >> >> >>>>>>>> >> >> >>>>>>>> Lytics will be big, and supercaps don't seem to like ripple >> >> >>>>>>>> current. I >> >> >>>>>>>> think. >> >> >>>>>>>> >> >> >>>>>>>> Any other ideas about driving a transformer from an audio amp? >> >> >>>>>>> >> >> >>>>>>> Feed back a (heavily) filtered signal to the input?&#2013266080; The problem is >> >> >>>>>>> that they don't really tell you what the input of the 3255 looks >> >> >>>>>>> like, >> >> >>>>>>> IIRC (none do).&#2013266080; It's intended to be AC coupled. >> >> >>>>>> >> >> >>>>>> TI does spec 60 mV max DC offset at the output, which is pretty good, >> >> >>>>>> but even that could push a lot of DC into my transformer. >> >> >>>>>> >> >> >>>>>> There is probably some way to tweak the input with a little DC. >> >> >>>>>> Feedback loop or even a trimpot. >> >> >>>>>> >> >> >>>>>> >> >> >>>>> >> >> >>>>> You can drive the transformer bridged but using current-sense feedback >> >> >>>>> instead of voltage feedback; put a small sense resistor in line with >> >> >>>>> each amp output going to each end of the transformer and take off >> >> >>>>> the DC >> >> >>>>> feedback to the opposite amp from the junction. That way the amps >> >> >>>>> should >> >> >>>>> act as their own servo to keep DC out of the transformer. >> >> >>>>> >> >> >>>>> Lower power example like this for driving audio isolation transformer >> >> >>>>> for XLR cable: >> >> >>>>> >> >> >>>>> <https://www.dropbox.com/s/8d1flmr8lko2nf1/Screenshot_2019-06-01_00-12-49.png?dl=0> >> >> >>>>> >> >> >>>>> >> >> >>>>> >> >> >>>> >> >> >>>> In the case of the TPA3255 I think you would put a low-offset op amp in >> >> >>>> front of the inputs and AC couple into that and do something similar but >> >> >>>> the 3255 just acts as a power buffer, I don't immediately see anything >> >> >>>> in the datasheet that says you can't intentionally apply a small DC >> >> >>>> feedback generated offset to its single input per channel as an error >> >> >>>> signal. >> >> >>> >> >> >>> It's not stated how the normally AC coupled inputs affect the DC >> >> >>> offset, but we have the eval board and I could have a scut bunny set >> >> >>> it up and try it. >> >> >>> >> >> >>> One trimpot and 30 seconds of tech time, to turn it, is sure >> >> >>> appealing. If it turns out we don't need it, we can leave it off the >> >> >>> board. >> >> >>> >> >> >>> The input resistance of the TPA3255 is 10K, and they want 10 uF input >> >> >>> coupling caps. Tau is 100 ms, which is 1.6 Hz corner. That suggests to >> >> >>> me that the input caps are also used as lowpass filters for the DC >> >> >>> feedback loop, which then suggests we could push the input pins gently >> >> >>> to change the output offset. >> >> >>> >> >> >>> Datasheets tend to hide the good stuff. >> >> >>> >> >> >>> >> >> >>> >> >> >> >> >> >> Does LTC make any class D amp modules included with LTSpice? I'm curious >> >> >> now about how using a DC servo loop into a first stage opamp driving the >> >> >> D-amp as a power buffer, instead of a cap would work out in practice. >> >> >> >> >> >> The problem with that XLR transformer driver circuit as drawn in a >> >> >> modification is that while there are two feedback loops, one local to >> >> >> the op-amps and one around the opposite side current sense resistor, >> >> >> it's assumed that the phase of the AC signals on both inputs of the op >> >> >> amps will be similar. >> >> >> >> >> >> but the phase shift produced by the class D output filter complicates >> >> >> things, where to put the sense resistor. If after the output filter need >> >> >> to compensate the phase shift somehow and if before need to filter the >> >> >> switching frequency down to DC. It may not be nearly as easily workable >> >> >> in the class D bridged power-buffer topology as with linear amplifiers >> >> >> driving the transformer, bridged. >> >> > >> >> >Putting it after the LC also means have to take the (possibly not well- >> >> >defined) R of the L into account as a component of the current sense >> >> >impedance. >> >> >> >> >> >> We plan to put the current sensor just before the customer output, >> >> which is after the filter inductors and the isolation transformer. >> >> Probably a resistive shunt, isolated by a small audio transformer. >> >> >> >> It will be an interesting control loop, synthesizing the output >> >> impedance. >> >> >> > >> >use a h-bridge instead and do everything in software/FPGA ? >> > >> >> The TI switcher is a class-D h-bridge. I wouldn't want to try to go >> linear. >> >> We do plan to do all the processing in an FPGA. > >I wasn't thinking linear but pwm directly from the FPGA to a powerstage, just like numerous variable frequency drives and brushless DC motor drives > >
We considered that, using GaN fets. The amp would be tiny. But the TI thing has current limiting, thermal limiting, all sorts of protections. We'd have to add that to a home-made H-bridge. Are there any integrated h-bridges with all the suitable protections? My FPGA guys wouldn't mind outputting PWM (or delta-sigma?). I should look into that. -- John Larkin Highland Technology, Inc lunatic fringe electronics
Reply by Chris Jones June 2, 20192019-06-02
On 03/06/2019 08:22, John Larkin wrote:
> On Sun, 2 Jun 2019 18:07:37 -0400, bitrex <user@example.net> wrote: > >> On 6/2/19 6:04 PM, bitrex wrote: >>> On 6/1/19 11:23 AM, John Larkin wrote: >>>> On Sat, 1 Jun 2019 00:42:55 -0400, bitrex <user@example.net> wrote: >>>> >>>>> On 6/1/19 12:23 AM, bitrex wrote: >>>>>> On 5/31/19 10:55 PM, John Larkin wrote: >>>>>>> On Fri, 31 May 2019 22:22:37 -0400, krw@notreal.com wrote: >>>>>>> >>>>>>>> On Fri, 31 May 2019 15:36:39 -0700, John Larkin >>>>>>>> <jjlarkin@highland_snip_technology.com> wrote: >>>>>>>> >>>>>>>>> >>>>>>>>> >>>>>>>>> I want to make a class-D audio amp, 150 watts or so, using a TI >>>>>>>>> TPA3255 maybe. It's good for 600 watts mono! >>>>>>>>> >>>>>>>>> I'll use it full-bridge to drive a step-up transformer, probably a >>>>>>>>> custom toroid. But toroids are especially unhappy with any DC drive, >>>>>>>>> and the class D part will surely have some DC offset. The TI spec is >>>>>>>>> 60 mV max output offset, which could be a problem into a good >>>>>>>>> transformer. Speakers don't mind a little DC, but transformers >>>>>>>>> do. DC >>>>>>>>> can cause stairstepped increase in circulating current, the Devil's >>>>>>>>> Staircase, until they saturate. >>>>>>>>> >>>>>>>>> So I'm thinking I'll add a series blocking cap so I can ignore >>>>>>>>> any DC >>>>>>>>> problems. It will have to be big, 10s of millifarads at least. >>>>>>>>> Biggest >>>>>>>>> thing on the board. Maybe use a low voltage electrolytic with >>>>>>>>> antiparallel power diodes, or a shorted bridge, to protect it from >>>>>>>>> accidental forward or backwards over-voltage. >>>>>>>>> >>>>>>>>> Lytics will be big, and supercaps don't seem to like ripple >>>>>>>>> current. I >>>>>>>>> think. >>>>>>>>> >>>>>>>>> Any other ideas about driving a transformer from an audio amp? >>>>>>>> >>>>>>>> Feed back a (heavily) filtered signal to the input?&nbsp; The problem is >>>>>>>> that they don't really tell you what the input of the 3255 looks >>>>>>>> like, >>>>>>>> IIRC (none do).&nbsp; It's intended to be AC coupled. >>>>>>> >>>>>>> TI does spec 60 mV max DC offset at the output, which is pretty good, >>>>>>> but even that could push a lot of DC into my transformer. >>>>>>> >>>>>>> There is probably some way to tweak the input with a little DC. >>>>>>> Feedback loop or even a trimpot. >>>>>>> >>>>>>> >>>>>> >>>>>> You can drive the transformer bridged but using current-sense feedback >>>>>> instead of voltage feedback; put a small sense resistor in line with >>>>>> each amp output going to each end of the transformer and take off >>>>>> the DC >>>>>> feedback to the opposite amp from the junction. That way the amps >>>>>> should >>>>>> act as their own servo to keep DC out of the transformer. >>>>>> >>>>>> Lower power example like this for driving audio isolation transformer >>>>>> for XLR cable: >>>>>> >>>>>> <https://www.dropbox.com/s/8d1flmr8lko2nf1/Screenshot_2019-06-01_00-12-49.png?dl=0> >>>>>> >>>>>> >>>>>> >>>>> >>>>> In the case of the TPA3255 I think you would put a low-offset op amp in >>>>> front of the inputs and AC couple into that and do something similar but >>>>> the 3255 just acts as a power buffer, I don't immediately see anything >>>>> in the datasheet that says you can't intentionally apply a small DC >>>>> feedback generated offset to its single input per channel as an error >>>>> signal. >>>> >>>> It's not stated how the normally AC coupled inputs affect the DC >>>> offset, but we have the eval board and I could have a scut bunny set >>>> it up and try it. >>>> >>>> One trimpot and 30 seconds of tech time, to turn it, is sure >>>> appealing. If it turns out we don't need it, we can leave it off the >>>> board. >>>> >>>> The input resistance of the TPA3255 is 10K, and they want 10 uF input >>>> coupling caps. Tau is 100 ms, which is 1.6 Hz corner. That suggests to >>>> me that the input caps are also used as lowpass filters for the DC >>>> feedback loop, which then suggests we could push the input pins gently >>>> to change the output offset. >>>> >>>> Datasheets tend to hide the good stuff. >>>> >>>> >>>> >>> >>> Does LTC make any class D amp modules included with LTSpice? I'm curious >>> now about how using a DC servo loop into a first stage opamp driving the >>> D-amp as a power buffer, instead of a cap would work out in practice. >>> >>> The problem with that XLR transformer driver circuit as drawn in a >>> modification is that while there are two feedback loops, one local to >>> the op-amps and one around the opposite side current sense resistor, >>> it's assumed that the phase of the AC signals on both inputs of the op >>> amps will be similar. >>> >>> but the phase shift produced by the class D output filter complicates >>> things, where to put the sense resistor. If after the output filter need >>> to compensate the phase shift somehow and if before need to filter the >>> switching frequency down to DC. It may not be nearly as easily workable >>> in the class D bridged power-buffer topology as with linear amplifiers >>> driving the transformer, bridged. >> >> Putting it after the LC also means have to take the (possibly not well- >> defined) R of the L into account as a component of the current sense >> impedance. > > > We plan to put the current sensor just before the customer output, > which is after the filter inductors and the isolation transformer. > Probably a resistive shunt, isolated by a small audio transformer.
If you're going to use transformer isolation for the shunt, you might as well use a current transformer instead. That way you can have less voltage per turn of the core for a given customer-visible output current, so a smaller core and/or better accuracy. Probably better frequency response too. You'd also face less modifications if you ever needed to switch to a hall effect sort of current sensor, if you wanted to extend the frequency range downwards.