Forums

Class D commercial audio amp

Started by N_Cook November 29, 2016
Using PWM input to  a Si8244BB predriver soic. After the inductor HF cut 
filter, on the 8 ohm speaker load there is about 1.5V pk-pk 200Kz 
constant level of switcher waveform, is that normal? what sort of 
background level would be normal ?
On Tue, 29 Nov 2016 16:03:40 +0000, N_Cook <diverse@tcp.co.uk> wrote:

> >Using PWM input to a Si8244BB predriver soic. After the inductor HF cut >filter, on the 8 ohm speaker load there is about 1.5V pk-pk 200Kz >constant level of switcher waveform, is that normal? what sort of >background level would be normal ?
"Class-D" isn't "audio"... the "D" stands for "distortion" >:-} ...Jim Thompson -- | James E.Thompson | mens | | Analog Innovations | et | | Analog/Mixed-Signal ASIC's and Discrete Systems | manus | | STV, Queen Creek, AZ 85142 Skype: skypeanalog | | | Voice:(480)460-2350 Fax: Available upon request | Brass Rat | | E-mail Icon at http://www.analog-innovations.com | 1962 | I'm looking for work... see my website.
On 11/29/2016 11:03 AM, N_Cook wrote:
> > Using PWM input to a Si8244BB predriver soic. After the inductor HF cut > filter, on the 8 ohm speaker load there is about 1.5V pk-pk 200Kz > constant level of switcher waveform, is that normal? what sort of > background level would be normal ?
You think you've got it bad. ;) "Inductorless" Class D amps use the speaker inductance and rolloff to smooth out the waveform. No EMI problems, no sir, never, of course not. Sounds like your filter is just knocking the sharp edges off the switching waveform in order to pass EMI, rather than actually getting rid of the fundamental. 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
On 11/29/2016 11:37 AM, Phil Hobbs wrote:
> On 11/29/2016 11:03 AM, N_Cook wrote: >> >> Using PWM input to a Si8244BB predriver soic. After the inductor HF cut >> filter, on the 8 ohm speaker load there is about 1.5V pk-pk 200Kz >> constant level of switcher waveform, is that normal? what sort of >> background level would be normal ? > > You think you've got it bad. ;) "Inductorless" Class D amps use the > speaker inductance and rolloff to smooth out the waveform. No EMI > problems, no sir, never, of course not. > > Sounds like your filter is just knocking the sharp edges off the > switching waveform in order to pass EMI, rather than actually getting > rid of the fundamental. > > Cheers > > Phil Hobbs >
If the OP were to give us the approximate reactances of the L and C at 200kHz and the supply voltages, we could probably actually tell him if 1.5V pk-pk was a reasonable residual amount of fundamental for a likely two-pole Butterworth having that break frequency...;)
Jim Thompson <To-Email-Use-The-Envelope-Icon@On-My-Web-Site.com> wrote:
> On Tue, 29 Nov 2016 16:03:40 +0000, N_Cook <diverse@tcp.co.uk> wrote: > >> >>Using PWM input to a Si8244BB predriver soic. After the inductor HF cut >>filter, on the 8 ohm speaker load there is about 1.5V pk-pk 200Kz >>constant level of switcher waveform, is that normal? what sort of >>background level would be normal ? > > "Class-D" isn't "audio"... the "D" stands for "distortion" >:-}
Why? Of course a Class-D amplifier could have better linearity than a Class-A or Class-AB can achieve.
On 29 Nov 2016 17:02:55 GMT, Rob <nomail@example.com> wrote:

>Jim Thompson <To-Email-Use-The-Envelope-Icon@On-My-Web-Site.com> wrote: >> On Tue, 29 Nov 2016 16:03:40 +0000, N_Cook <diverse@tcp.co.uk> wrote: >> >>> >>>Using PWM input to a Si8244BB predriver soic. After the inductor HF cut >>>filter, on the 8 ohm speaker load there is about 1.5V pk-pk 200Kz >>>constant level of switcher waveform, is that normal? what sort of >>>background level would be normal ? >> >> "Class-D" isn't "audio"... the "D" stands for "distortion" >:-} > >Why? Of course a Class-D amplifier could have better linearity than >a Class-A or Class-AB can achieve.
(1) The damping factor sucks the most sour of lemons ;-) (2) And your "better linearity" claim has the same engineering fact level as would be put forth by a used car salesman. (3) Class-D pros: low heat, small size, CHEAP, thus manufacturers of consumer grade (aka junk) audio products love it. ...Jim Thompson -- | James E.Thompson | mens | | Analog Innovations | et | | Analog/Mixed-Signal ASIC's and Discrete Systems | manus | | STV, Queen Creek, AZ 85142 Skype: skypeanalog | | | Voice:(480)460-2350 Fax: Available upon request | Brass Rat | | E-mail Icon at http://www.analog-innovations.com | 1962 | I'm looking for work... see my website.
On 29/11/2016 16:45, bitrex wrote:
> On 11/29/2016 11:37 AM, Phil Hobbs wrote: >> On 11/29/2016 11:03 AM, N_Cook wrote: >>> >>> Using PWM input to a Si8244BB predriver soic. After the inductor HF cut >>> filter, on the 8 ohm speaker load there is about 1.5V pk-pk 200Kz >>> constant level of switcher waveform, is that normal? what sort of >>> background level would be normal ? >> >> You think you've got it bad. ;) "Inductorless" Class D amps use the >> speaker inductance and rolloff to smooth out the waveform. No EMI >> problems, no sir, never, of course not. >> >> Sounds like your filter is just knocking the sharp edges off the >> switching waveform in order to pass EMI, rather than actually getting >> rid of the fundamental. >> >> Cheers >> >> Phil Hobbs >> > > If the OP were to give us the approximate reactances of the L and C at > 200kHz and the supply voltages, we could probably actually tell him if > 1.5V pk-pk was a reasonable residual amount of fundamental for a likely > two-pole Butterworth having that break frequency...;)
Certainly, the rail voltages for a pair of powerFETs is + and -80V, C is .68uF 400V, inductor is about 30mm diameter toroid with about 45 turns of about 1.5mm wire
On 11/29/2016 12:12 PM, Jim Thompson wrote:
> On 29 Nov 2016 17:02:55 GMT, Rob <nomail@example.com> wrote: > >> Jim Thompson <To-Email-Use-The-Envelope-Icon@On-My-Web-Site.com> wrote: >>> On Tue, 29 Nov 2016 16:03:40 +0000, N_Cook <diverse@tcp.co.uk> wrote: >>> >>>> >>>> Using PWM input to a Si8244BB predriver soic. After the inductor HF cut >>>> filter, on the 8 ohm speaker load there is about 1.5V pk-pk 200Kz >>>> constant level of switcher waveform, is that normal? what sort of >>>> background level would be normal ? >>> >>> "Class-D" isn't "audio"... the "D" stands for "distortion" >:-} >> >> Why? Of course a Class-D amplifier could have better linearity than >> a Class-A or Class-AB can achieve. > > (1) The damping factor sucks the most sour of lemons ;-) > > (2) And your "better linearity" claim has the same engineering fact > level as would be put forth by a used car salesman. > > (3) Class-D pros: low heat, small size, CHEAP, thus manufacturers of > consumer grade (aka junk) audio products love it. > > ...Jim Thompson >
They're good for controlling thermoelectrics, but you have to filter the daylights out of it to avoid switching crap getting into whatever sensitive item is on the cold plate. I've used capacitance multipliers with parallelled NPN and PNP pass transistors for that. Works fine as long as you don't mind a volt or so of crossover distortion. ;) (You also have to avoid forward-biasing the BC junction of the nominally-off transistor.) It's basically limited by the Early voltage of the transistors. 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
On Tue, 29 Nov 2016 13:10:09 -0500, Phil Hobbs
<pcdhSpamMeSenseless@electrooptical.net> wrote:

>On 11/29/2016 12:12 PM, Jim Thompson wrote: >> On 29 Nov 2016 17:02:55 GMT, Rob <nomail@example.com> wrote: >> >>> Jim Thompson <To-Email-Use-The-Envelope-Icon@On-My-Web-Site.com> wrote: >>>> On Tue, 29 Nov 2016 16:03:40 +0000, N_Cook <diverse@tcp.co.uk> wrote: >>>> >>>>> >>>>> Using PWM input to a Si8244BB predriver soic. After the inductor HF cut >>>>> filter, on the 8 ohm speaker load there is about 1.5V pk-pk 200Kz >>>>> constant level of switcher waveform, is that normal? what sort of >>>>> background level would be normal ? >>>> >>>> "Class-D" isn't "audio"... the "D" stands for "distortion" >:-} >>> >>> Why? Of course a Class-D amplifier could have better linearity than >>> a Class-A or Class-AB can achieve. >> >> (1) The damping factor sucks the most sour of lemons ;-) >> >> (2) And your "better linearity" claim has the same engineering fact >> level as would be put forth by a used car salesman. >> >> (3) Class-D pros: low heat, small size, CHEAP, thus manufacturers of >> consumer grade (aka junk) audio products love it. >> >> ...Jim Thompson >> >They're good for controlling thermoelectrics, but you have to filter the >daylights out of it to avoid switching crap getting into whatever >sensitive item is on the cold plate. > >I've used capacitance multipliers with parallelled NPN and PNP pass >transistors for that. Works fine as long as you don't mind a volt or so >of crossover distortion. ;) (You also have to avoid forward-biasing the >BC junction of the nominally-off transistor.) It's basically limited by >the Early voltage of the transistors. > >Cheers > >Phil Hobbs
Class-D audio systems, such as common TV sets, play hob (pun intended) with hearing aids which also use class-D :-( ...Jim Thompson -- | James E.Thompson | mens | | Analog Innovations | et | | Analog/Mixed-Signal ASIC's and Discrete Systems | manus | | STV, Queen Creek, AZ 85142 Skype: skypeanalog | | | Voice:(480)460-2350 Fax: Available upon request | Brass Rat | | E-mail Icon at http://www.analog-innovations.com | 1962 | I'm looking for work... see my website.
On 11/29/2016 12:39 PM, N_Cook wrote:
> On 29/11/2016 16:45, bitrex wrote: >> On 11/29/2016 11:37 AM, Phil Hobbs wrote: >>> On 11/29/2016 11:03 AM, N_Cook wrote: >>>> >>>> Using PWM input to a Si8244BB predriver soic. After the inductor HF >>>> cut >>>> filter, on the 8 ohm speaker load there is about 1.5V pk-pk 200Kz >>>> constant level of switcher waveform, is that normal? what sort of >>>> background level would be normal ? >>> >>> You think you've got it bad. ;) "Inductorless" Class D amps use the >>> speaker inductance and rolloff to smooth out the waveform. No EMI >>> problems, no sir, never, of course not. >>> >>> Sounds like your filter is just knocking the sharp edges off the >>> switching waveform in order to pass EMI, rather than actually getting >>> rid of the fundamental. >>> >>> Cheers >>> >>> Phil Hobbs >>> >> >> If the OP were to give us the approximate reactances of the L and C at >> 200kHz and the supply voltages, we could probably actually tell him if >> 1.5V pk-pk was a reasonable residual amount of fundamental for a likely >> two-pole Butterworth having that break frequency...;) > > Certainly, the rail voltages for a pair of powerFETs is + and -80V, C is > .68uF 400V, inductor is about 30mm diameter toroid with about 45 turns > of about 1.5mm wire
Sounds like something on the order of 100 uH, depending on the core material. In that case the LC rolloff starts at f_3dB = 1/(2 pi sqrt(LC)) ~ 19 kHz. It rolls off asymptotically like (f_3dB/f)**2, which at 200 kHz is 40 dB down (1/100 times the amplitude). So 160V p-p ought to roll off to, yeah, about a volt and a half. 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