On Monday, December 11, 2023 at 6:49:30 PM UTC, Glen Walpert wrote:> On Mon, 11 Dec 2023 15:01:25 +0000, Clive Arthur wrote: > > > On 11/12/2023 14:05, legg wrote: > >> On Wed, 06 Dec 2023 23:22:26 GMT, Glen Walpert <nos...@null.void> > >> wrote: > > > > <snip> > > > >>> Some years ago Jim Thompson posted an audio amplifier design which > >>> used current mirrors to provide bias to the output transistors for the > >>> express purpose of keeping crossover distortion low over a large > >>> temperature range. He claimed it was the bees knees, but a quick > >>> search failed to turn it up. Perhaps someone else saved it or > >>> remembers the thread? > >>> > >>> Glen > >> > >> From old LTSpice trash here; > >> > >> http://ve3ute.ca/query/Half_Bridge_for_77_280Z_thompson.pdf > >> > >> RL > > > > Thanks! > > > > Looks like the top output Darlington is AC coupled and when the > > comparator detects a quiescent current through the output resistors > > transitioning to less than some value, it pumps the upper Darlington > > base voltage up a bit, otherwise, the upper Darlington base voltage > > drifts down. > > > > Is that about right? > There was a lot of discussion of this circuit when it was posted, and Jim > posted some models and simulations possibly still available on Phil's > archive. I don't have time to actually think about it right now, but here > are some post snips with comments and model links, sorry about the length: > > ------------ > Here's half of the full H-bridge amplifiers that I built for my 1977 > 280Z... Image scanned in quarters and pieced together for easier > understanding... > > <http://www.analog-innovations.com/SED/Half_Bridge_for_77_280Z.pdf> > -- > >If both output transistors are briefly off or very nearly off while the > >output is increasing through crossover (zero or near zero current through > >both emitter resistors), then the LM311 goes high due to the lag through > >the RC on the negative input, delivering additional current to the > >current mirror with illegible designations through D1, pulling current > >from the 20uF 10V capacitor, increasing it's voltage thus increasing the > >bias offset provided by Q5 and Q6 until there is enough bias voltage > >difference to insure some small overlap in the on time of the output > >transistors. Q1 and Q2 appear to keep the bias voltages centered between > >the rails, and possibly Q8 pulls the negative input of the comparator > >down enough to prevent noise from turning it on with no input?. (Not at > >all sure about Q8, it might do more than that). > > > >Am I close? Hints on Q8? > > > >Regards, > >Glen > > You are virtually on the money! > > Q8 is just a current mirror operating on R13 to establish the bias current > at the zero crossing (your observation that corrections only occur while > passing thru the zero crossing are dead-on... except that the Q8 current > prevents both off). > > ...Jim Thompson > > The Q5/Q6 Darlington is simply to knock down the base current so that a > long R/C time constant dominates. > > ...Jim Thompson > > >What's the SPICE quiescent bias? Back of the envelope, I get 75 or 80mA. > > > >How do you pick R15/R16/C4? Looks like it's to bootstrap the bias above > >the 13.3V rail with a time constant longer than the roll-off of the > >amplifier. > > > > > >Best regards, > >Spehro Pefhany > > In a later life I might have used a diode. We improve our skill-set over > the years... at least some of us do... some just bloviate >:-} > > ...Jim Thompson > > See... > > As requested, entered into PSpice and simulated.... > > <http://www.analog-innovations.com/SED/My_1977_Z_Amp.pdf> > > for the simulation (and a readable schematic). > > Betwixt the "honey-do", I ran intermod distortion, comparing class-B to my > class-A-B method, zip file now updated... > > <http://www.analog-innovations.com/SED/JimThompsons_A-B- > Bias_Amplifier.zip> > > ...Jim Thompson > > > To go along with that schematic, here is the subcircuit that should work > in all modern flavors of Spice... > > <http://www.analog-innovations.com/SED/My_1977_Z_Amp.zip> > > To simulate my circuit in LTspice, open a text editor and type the > following... > > * Jim Thompson's 1977 Z Amplifier * > ** Analysis setup ** > .tran 0 10m 0 100n .OPTIONS ITL1=1500 .OPTIONS ITL2=2000 .OPTIONS > ITL4=1000 .OPTIONS STEPGMIN .OP X1 IN OUT VCC 0 My_1977_Z_Amp VCC VCC 0 > 13.3V VIN IN 0 SIN 0 4 1K 0 0 0 .INC > "C:\InsertYourPathToCopyOf\My_1977_Z_Amp.sub" > * > .END > > Save as whatever name rings your chime, say... > > "JimThompson'sMarvelousAmplifier.cir" >:-} > > Then open LTspice. On the Tools/Control Panel/Save Defaults section check > both Save Subcircuits... check-boxes. > > Then Open "JimThompson'sMarvelousAmplifier.cir" > > Then Run > > View whatever node voltage or device current you like. > > Irrespective of Larkin's stone throwing, it doesn't fail for several > reasons... one specifically because it was 1977. Can anyone guess what > that was? > > Interestingly it takes LTspice _much_longer_ to run this circuit than it > does PSpice, particularly the bias point calculation is butt slow. > > Note that you _do_not_ need to draw a schematic in LTspice (or any other > Spice, for that matter) to simulate someone else's circuit. Many of my > clients only have LTspice, so I just pass them a PDF schematic and a > netlist, and they can verify my work just fine. > > ...Jim Thompson > > > >Latest version.... > > > ><http://www.analog-innovations.com/SED/JimThompsons_A-B- > Bias_Amplifier.zip> > > Turns out that my A-B bias is STUNNINGLY better than the conventional > diode-biased class-B... almost 30dB better on intermod distortion! > > Intermod is what gives you those nasty atonal ear-piercing sounds when you > play a Mozart wood-wind ensemble with French horn accompaniment. > > After 36 years, revisiting my scheme, and fixing the bias droop, it's time > for me to go back and roll my own sound system from scratch... like I did > up until my late 30's... then I got "too busy" ;-) > > I'll toss the TL091 and put some discrete's in there... maybe even use my > TL431 diff-pair >:-} > > ...Jim ThompsonHere is a link to Mr Thompson's file (JimThompsons_A-B- Bias_Amplifier.zip) should anyone want to simulate the circuit. https://1drv.ms/u/s!AkjNCaVyTfIag2w_3ym3b15HkMyP?e=PWxkbm
Power Amplifier for 100kHz.
Started by ●December 6, 2023
Reply by ●December 11, 20232023-12-11
Reply by ●December 11, 20232023-12-11
On 2023-12-11 12:09, Fred Bloggs wrote:> On Monday, December 11, 2023 at 10:50:04 AM UTC-5, legg wrote: >> On Mon, 11 Dec 2023 15:01:25 +0000, Clive Arthur >> <cl...@nowaytoday.co.uk> wrote: >> >>> On 11/12/2023 14:05, legg wrote: >>>> On Wed, 06 Dec 2023 23:22:26 GMT, Glen Walpert >>>> <nos...@null.void> wrote: >>> >>> <snip> >>> >>>>> Some years ago Jim Thompson posted an audio amplifier design >>>>> which used current mirrors to provide bias to the output >>>>> transistors for the express purpose of keeping crossover >>>>> distortion low over a large temperature range. He claimed it >>>>> was the bees knees, but a quick search failed to turn it up. >>>>> Perhaps someone else saved it or remembers the thread? >>>>> >>>>> Glen >>>> >>>> From old LTSpice trash here; >>>> >>>> http://ve3ute.ca/query/Half_Bridge_for_77_280Z_thompson.pdf >>>> >>>> RL >>> >>> Thanks! >>> >>> Looks like the top output Darlington is AC coupled and when the >>> comparator detects a quiescent current through the output >>> resistors transitioning to less than some value, it pumps the >>> upper Darlington base voltage up a bit, otherwise, the upper >>> Darlington base voltage drifts down. >>> >>> Is that about right? >>> >>> Not sure it would work in my application as my signal isn't >>> continuous - it spends some proportion of the time idling at half >>> supply. Still, I could probably arrange a clock to force a >>> comparator sample somehow. >>> >>> Or maybe make the adjustment non-volatile (digipot?) and clock it >>> both up and down. The signal comes from a DAC, so I do have >>> access to timing signals. >> A lot of the bumph is dedicated only to biasing and it would take >> some doing to get it to work over temperature given those >> polarized cap sizes. Integrated darlingtons are also best avoided. >> By 'wide range', the author was talking standard industrial >> temperatures. >> >> You'd also have to do some thin'in around the gain-setting regime. >> Doubt this was a consideration in this drawing ( . . . 'or' . . >> .), nor was 100KHz ( hence zobel network ). >> >> I don't see quiescent conditions being an issue, but start-up and >> shutdown could be surprising. Not sure that was Thompson's strong >> point. >> >> RL > > That 100u base-to-base bias cap is probably needed to dominant poll > stabilize the LM311 amp more than anything else. >I'm not sure that we're looking at the same schematic. In the one I have, "Half_Bridge_for_77_280Z_thompson.pdf", the LM311 is a weird sort of switching bias supervisor, not a linear amp. It's running as a normal open-collector comparator, with its output wire-ORed with the shutdown transistor Q7. When it fires, or the shutdown line is high, it steals Q3's base bias. That causes Darlington Q5/Q6 to turn on more, with a TC of about 2 seconds. That reduces the quiescent bias. In small-signal conditions, that'll just oscillate irregularly and keep the class-A bias current of very roughly 60 mA. (*) In large-signal conditions, the comparator will be pulling low most of the time, which reduces the quiescent bias progressively. (If the gain of the bias loop is high enough, it may not drift that far, but I'd probably need to use SPICE to find that out.) When the shutdown pin is active, the class-A bias will gradually go to 0, turning the output totem pole into a really bad class B. (One gathers that the shutdown turns off the audio input as well.) One aspect that I don't understand well is Q3. With a 3k/100 ohm voltage divider in the Q4 leg, ISTM that the base voltage of Q3 will be nearly the same as that of Q2, which is driven from a 620 ohm / 22 ohm divider (plus various V_BEs). That makes the current through the 220 ohm hard to estimate by eyeball. (The average current is obviously going to be small, on account of that 100k resistor.) (I sort of gather that it's pretty tweaky, due to that scribbled-in 10k to ground from the bases of Q3 and Q4.) Cheers Phil Hobbs (*) That 60 mA number is based on the ~20 mA emitter current of Q1. That'll drop about 450 mV across the 22 ohms, which translates to about 1.5 mA collector current in Q8. That puts roughly 40 mV across the 27 ohms, which divided by 0.66 ohms gets you roughly 60 mA of Class-A bias current. -- Dr Philip C D Hobbs Principal Consultant ElectroOptical Innovations LLC / Hobbs ElectroOptics Optics, Electro-optics, Photonics, Analog Electronics Briarcliff Manor NY 10510 http://electrooptical.net http://hobbs-eo.com
Reply by ●December 12, 20232023-12-12
On Mon, 11 Dec 2023 18:05:46 -0500, Phil Hobbs <pcdhSpamMeSenseless@electrooptical.net> wrote:>On 2023-12-11 12:09, Fred Bloggs wrote: >> On Monday, December 11, 2023 at 10:50:04?AM UTC-5, legg wrote: >>> On Mon, 11 Dec 2023 15:01:25 +0000, Clive Arthur >>> <cl...@nowaytoday.co.uk> wrote: >>> >>>> On 11/12/2023 14:05, legg wrote: >>>>> On Wed, 06 Dec 2023 23:22:26 GMT, Glen Walpert >>>>> <nos...@null.void> wrote: >>>> >>>> <snip> >>>> >>>>>> Some years ago Jim Thompson posted an audio amplifier design >>>>>> which used current mirrors to provide bias to the output >>>>>> transistors for the express purpose of keeping crossover >>>>>> distortion low over a large temperature range. He claimed it >>>>>> was the bees knees, but a quick search failed to turn it up. >>>>>> Perhaps someone else saved it or remembers the thread? >>>>>> >>>>>> Glen >>>>> >>>>> From old LTSpice trash here; >>>>> >>>>> http://ve3ute.ca/query/Half_Bridge_for_77_280Z_thompson.pdf >>>>> >>>>> RL >>>> >>>> Thanks! >>>> >>>> Looks like the top output Darlington is AC coupled and when the >>>> comparator detects a quiescent current through the output >>>> resistors transitioning to less than some value, it pumps the >>>> upper Darlington base voltage up a bit, otherwise, the upper >>>> Darlington base voltage drifts down. >>>> >>>> Is that about right? >>>> >>>> Not sure it would work in my application as my signal isn't >>>> continuous - it spends some proportion of the time idling at half >>>> supply. Still, I could probably arrange a clock to force a >>>> comparator sample somehow. >>>> >>>> Or maybe make the adjustment non-volatile (digipot?) and clock it >>>> both up and down. The signal comes from a DAC, so I do have >>>> access to timing signals. >>> A lot of the bumph is dedicated only to biasing and it would take >>> some doing to get it to work over temperature given those >>> polarized cap sizes. Integrated darlingtons are also best avoided. >>> By 'wide range', the author was talking standard industrial >>> temperatures. >>> >>> You'd also have to do some thin'in around the gain-setting regime. >>> Doubt this was a consideration in this drawing ( . . . 'or' . . >>> .), nor was 100KHz ( hence zobel network ). >>> >>> I don't see quiescent conditions being an issue, but start-up and >>> shutdown could be surprising. Not sure that was Thompson's strong >>> point. >>> >>> RL >> >> That 100u base-to-base bias cap is probably needed to dominant poll >> stabilize the LM311 amp more than anything else. >> > >I'm not sure that we're looking at the same schematic. In the one I >have, "Half_Bridge_for_77_280Z_thompson.pdf", the LM311 is a weird sort >of switching bias supervisor, not a linear amp. It's running as a >normal open-collector comparator, with its output wire-ORed with the >shutdown transistor Q7. > >When it fires, or the shutdown line is high, it steals Q3's base bias. >That causes Darlington Q5/Q6 to turn on more, with a TC of about 2 >seconds. That reduces the quiescent bias. > >In small-signal conditions, that'll just oscillate irregularly and keep >the class-A bias current of very roughly 60 mA. (*) In large-signal >conditions, the comparator will be pulling low most of the time, which >reduces the quiescent bias progressively. (If the gain of the bias loop >is high enough, it may not drift that far, but I'd probably need to use >SPICE to find that out.) > >When the shutdown pin is active, the class-A bias will gradually go to >0, turning the output totem pole into a really bad class B. (One >gathers that the shutdown turns off the audio input as well.) > >One aspect that I don't understand well is Q3. With a 3k/100 ohm >voltage divider in the Q4 leg, ISTM that the base voltage of Q3 will be >nearly the same as that of Q2, which is driven from a 620 ohm / 22 ohm >divider (plus various V_BEs). That makes the current through the 220 >ohm hard to estimate by eyeball. (The average current is obviously >going to be small, on account of that 100k resistor.) > >(I sort of gather that it's pretty tweaky, due to that scribbled-in 10k >to ground from the bases of Q3 and Q4.) > >Cheers > >Phil Hobbs > >(*) That 60 mA number is based on the ~20 mA emitter current of Q1. >That'll drop about 450 mV across the 22 ohms, which translates to about >1.5 mA collector current in Q8. That puts roughly 40 mV across the 27 >ohms, which divided by 0.66 ohms gets you roughly 60 mA of Class-A bias >current.Phil, The 'bumph' controlling the biasing is a sub-audible current switch into fairly large capacitors, so simulation would have to take this into account. In my internet/bugs/ampjt folder there are a number of simulations posted by Jim around 2013. They used the .op spice directive to establish DC operating point values only. I couldn't get it to operate as an amplifier (starts with static latched-off biasing) using his TL081/071 subcircuit. Using the basic LTSpice single or doublepole OA would allow it to demonstrate a signal path in a .tran simulation. His sims left out the electrolytic cap on the base of Q2 and the gain was set to simple unity (inverting) using 10K resistors and a cap-coupled source. Crossover distortion was easily visible. As I couldn't see any slow-moving voltages or currents in the biasing section to correct this, while a signal was being processed, or any reason why they should change (with the LM311 inputs overloaded by normal operating current), I set the thing aside. RL
Reply by ●December 12, 20232023-12-12
On Mon, 11 Dec 2023 18:05:46 -0500, Phil Hobbs <pcdhSpamMeSenseless@electrooptical.net> wrote: In practise, of the LM311 sees zero current, it allows the bias to increase. RL
Reply by ●December 12, 20232023-12-12
On 2023-12-12 09:17, legg wrote:> On Mon, 11 Dec 2023 18:05:46 -0500, Phil Hobbs > <pcdhSpamMeSenseless@electrooptical.net> wrote: > >> On 2023-12-11 12:09, Fred Bloggs wrote: >>> On Monday, December 11, 2023 at 10:50:04?AM UTC-5, legg wrote: >>>> On Mon, 11 Dec 2023 15:01:25 +0000, Clive Arthur >>>> <cl...@nowaytoday.co.uk> wrote: >>>> >>>>> On 11/12/2023 14:05, legg wrote: >>>>>> On Wed, 06 Dec 2023 23:22:26 GMT, Glen Walpert >>>>>> <nos...@null.void> wrote: >>>>> >>>>> <snip> >>>>> >>>>>>> Some years ago Jim Thompson posted an audio amplifier design >>>>>>> which used current mirrors to provide bias to the output >>>>>>> transistors for the express purpose of keeping crossover >>>>>>> distortion low over a large temperature range. He claimed it >>>>>>> was the bees knees, but a quick search failed to turn it up. >>>>>>> Perhaps someone else saved it or remembers the thread? >>>>>>> >>>>>>> Glen >>>>>> >>>>>> From old LTSpice trash here; >>>>>> >>>>>> http://ve3ute.ca/query/Half_Bridge_for_77_280Z_thompson.pdf >>>>>> >>>>>> RL >>>>> >>>>> Thanks! >>>>> >>>>> Looks like the top output Darlington is AC coupled and when the >>>>> comparator detects a quiescent current through the output >>>>> resistors transitioning to less than some value, it pumps the >>>>> upper Darlington base voltage up a bit, otherwise, the upper >>>>> Darlington base voltage drifts down. >>>>> >>>>> Is that about right? >>>>> >>>>> Not sure it would work in my application as my signal isn't >>>>> continuous - it spends some proportion of the time idling at half >>>>> supply. Still, I could probably arrange a clock to force a >>>>> comparator sample somehow. >>>>> >>>>> Or maybe make the adjustment non-volatile (digipot?) and clock it >>>>> both up and down. The signal comes from a DAC, so I do have >>>>> access to timing signals. >>>> A lot of the bumph is dedicated only to biasing and it would take >>>> some doing to get it to work over temperature given those >>>> polarized cap sizes. Integrated darlingtons are also best avoided. >>>> By 'wide range', the author was talking standard industrial >>>> temperatures. >>>> >>>> You'd also have to do some thin'in around the gain-setting regime. >>>> Doubt this was a consideration in this drawing ( . . . 'or' . . >>>> .), nor was 100KHz ( hence zobel network ). >>>> >>>> I don't see quiescent conditions being an issue, but start-up and >>>> shutdown could be surprising. Not sure that was Thompson's strong >>>> point. >>>> >>>> RL >>> >>> That 100u base-to-base bias cap is probably needed to dominant poll >>> stabilize the LM311 amp more than anything else. >>> >> >> I'm not sure that we're looking at the same schematic. In the one I >> have, "Half_Bridge_for_77_280Z_thompson.pdf", the LM311 is a weird sort >> of switching bias supervisor, not a linear amp. It's running as a >> normal open-collector comparator, with its output wire-ORed with the >> shutdown transistor Q7. >> >> When it fires, or the shutdown line is high, it steals Q3's base bias. >> That causes Darlington Q5/Q6 to turn on more, with a TC of about 2 >> seconds. That reduces the quiescent bias. >> >> In small-signal conditions, that'll just oscillate irregularly and keep >> the class-A bias current of very roughly 60 mA. (*) In large-signal >> conditions, the comparator will be pulling low most of the time, which >> reduces the quiescent bias progressively. (If the gain of the bias loop >> is high enough, it may not drift that far, but I'd probably need to use >> SPICE to find that out.) >> >> When the shutdown pin is active, the class-A bias will gradually go to >> 0, turning the output totem pole into a really bad class B. (One >> gathers that the shutdown turns off the audio input as well.) >> >> One aspect that I don't understand well is Q3. With a 3k/100 ohm >> voltage divider in the Q4 leg, ISTM that the base voltage of Q3 will be >> nearly the same as that of Q2, which is driven from a 620 ohm / 22 ohm >> divider (plus various V_BEs). That makes the current through the 220 >> ohm hard to estimate by eyeball. (The average current is obviously >> going to be small, on account of that 100k resistor.) >> >> (I sort of gather that it's pretty tweaky, due to that scribbled-in 10k >> to ground from the bases of Q3 and Q4.) >> >> Cheers >> >> Phil Hobbs >> >> (*) That 60 mA number is based on the ~20 mA emitter current of Q1. >> That'll drop about 450 mV across the 22 ohms, which translates to about >> 1.5 mA collector current in Q8. That puts roughly 40 mV across the 27 >> ohms, which divided by 0.66 ohms gets you roughly 60 mA of Class-A bias >> current. > > Phil, > > The 'bumph' controlling the biasing is a sub-audible current switch > into fairly large capacitors, so simulation would have to take this > into account. > > In my internet/bugs/ampjt folder there are a number of simulations > posted by Jim around 2013. They used the .op spice directive to > establish DC operating point values only. > > I couldn't get it to operate as an amplifier (starts with static > latched-off biasing) using his TL081/071 subcircuit. Using the > basic LTSpice single or doublepole OA would allow it to demonstrate > a signal path in a .tran simulation. > > His sims left out the electrolytic cap on the base of Q2 and the > gain was set to simple unity (inverting) using 10K resistors > and a cap-coupled source. > > Crossover distortion was easily visible. As I couldn't see any > slow-moving voltages or currents in the biasing section to > correct this, while a signal was being processed, or any reason > why they should change (with the LM311 inputs overloaded by > normal operating current), I set the thing aside. > > RL >In 1977 it wasn't that easy to do a fully-differential measurement of a 40-mV signal sitting on 12 Vpp of audio. Re-framing the problem as preventing the measured voltage from falling much below 40 mV, and letting it gradually decrease otherwise, is an interesting approach. JT was a smart guy, even if he was a tiny bit too aware of that. ;) May God hold him in memory eternal. Cheers Phil Hobbs -- Dr Philip C D Hobbs Principal Consultant ElectroOptical Innovations LLC / Hobbs ElectroOptics Optics, Electro-optics, Photonics, Analog Electronics Briarcliff Manor NY 10510 http://electrooptical.net http://hobbs-eo.com
Reply by ●December 12, 20232023-12-12
On Wednesday, December 13, 2023 at 10:44:03 AM UTC+11, Phil Hobbs wrote:> On 2023-12-12 09:17, legg wrote: > > On Mon, 11 Dec 2023 18:05:46 -0500, Phil Hobbs <pcdhSpamM...@electrooptical.net> wrote: > >> On 2023-12-11 12:09, Fred Bloggs wrote: > >>> On Monday, December 11, 2023 at 10:50:04?AM UTC-5, legg wrote: > >>>> On Mon, 11 Dec 2023 15:01:25 +0000, Clive Arthur <cl...@nowaytoday.co.uk> wrote: > >>>>> On 11/12/2023 14:05, legg wrote: > >>>>>> On Wed, 06 Dec 2023 23:22:26 GMT, Glen Walpert <nos...@null.void> wrote:<snip>> JT was a smart guy, even if he was a tiny bit too aware of that. ;)Technically adept, but not in the Barry Gilbert or Bob Widlar class. He claimed to have reported me to the FBI as dangerously anti-American, which showed a surprising lack of trust in Amercian military intelligence, when I'd had a high enough security rating to get into US Army ECOM back when it was at fort Monmouth, New Jersey.> May God hold him in memory eternal.And keep quiet about his less desirable aspects. -- Bill Sloman, Sydney
Reply by ●December 13, 20232023-12-13
On 12/12/2023 23:43, Phil Hobbs wrote: <snip>> > In 1977 it wasn't that easy to do a fully-differential measurement of a > 40-mV signal sitting on 12 Vpp of audio. > > Re-framing the problem as preventing the measured voltage from falling > much below 40 mV, and letting it gradually decrease otherwise, is an > interesting approach.Yes, it's a good idea. I'd go for a current monitor - eg INA169 - to get things down to the 5V domain (I have a 60V supply) and take it from there. -- Cheers Clive
Reply by ●December 14, 20232023-12-14
On Tue, 12 Dec 2023 18:43:44 -0500, Phil Hobbs <pcdhSpamMeSenseless@electrooptical.net> wrote:>On 2023-12-12 09:17, legg wrote: >> On Mon, 11 Dec 2023 18:05:46 -0500, Phil Hobbs >> <pcdhSpamMeSenseless@electrooptical.net> wrote: >> >>> On 2023-12-11 12:09, Fred Bloggs wrote:<snip>>>> One aspect that I don't understand well is Q3. With a 3k/100 ohm >>> voltage divider in the Q4 leg, ISTM that the base voltage of Q3 will be >>> nearly the same as that of Q2, which is driven from a 620 ohm / 22 ohm >>> divider (plus various V_BEs). That makes the current through the 220 >>> ohm hard to estimate by eyeball. (The average current is obviously >>> going to be small, on account of that 100k resistor.) >>> >>> (I sort of gather that it's pretty tweaky, due to that scribbled-in 10k >>> to ground from the bases of Q3 and Q4.) >>> >>> Cheers >>> >>> Phil Hobbs >>> >>> (*) That 60 mA number is based on the ~20 mA emitter current of Q1. >>> That'll drop about 450 mV across the 22 ohms, which translates to about >>> 1.5 mA collector current in Q8. That puts roughly 40 mV across the 27 >>> ohms, which divided by 0.66 ohms gets you roughly 60 mA of Class-A bias >>> current. >> >> Phil, >> >> The 'bumph' controlling the biasing is a sub-audible current switch >> into fairly large capacitors, so simulation would have to take this >> into account. >> >> In my internet/bugs/ampjt folder there are a number of simulations >> posted by Jim around 2013. They used the .op spice directive to >> establish DC operating point values only. >> >> I couldn't get it to operate as an amplifier (starts with static >> latched-off biasing) using his TL081/071 subcircuit. Using the >> basic LTSpice single or doublepole OA would allow it to demonstrate >> a signal path in a .tran simulation. >> >> His sims left out the electrolytic cap on the base of Q2 and the >> gain was set to simple unity (inverting) using 10K resistors >> and a cap-coupled source. >> >> Crossover distortion was easily visible. As I couldn't see any >> slow-moving voltages or currents in the biasing section to >> correct this, while a signal was being processed, or any reason >> why they should change (with the LM311 inputs overloaded by >> normal operating current), I set the thing aside. >> >> RL >> > >In 1977 it wasn't that easy to do a fully-differential measurement of a >40-mV signal sitting on 12 Vpp of audio. > >Re-framing the problem as preventing the measured voltage from falling >much below 40 mV, and letting it gradually decrease otherwise, is an >interesting approach. >The concept is sound, but not actually demonstrated here. In the presence of signal, the biasing reverts to bad classB, as you've suggested. The op amp is stressed to supply the ~ 20mA in Q2, while slewing a full darlington Veb at crossover, even for 40mV signal amplitudes. All subckts in the sim are questionable in function, even the simple darlingtons (from Modpex 2004?). There are no thermal spice parameters included. High temperature 100KHz is probably out of the question. RL RL
Reply by ●December 14, 20232023-12-14
On Wed, 13 Dec 2023 16:03:21 +0000, Clive Arthur <clive@nowaytoday.co.uk> wrote:>On 12/12/2023 23:43, Phil Hobbs wrote: > ><snip> >> >> In 1977 it wasn't that easy to do a fully-differential measurement of a >> 40-mV signal sitting on 12 Vpp of audio. >> >> Re-framing the problem as preventing the measured voltage from falling >> much below 40 mV, and letting it gradually decrease otherwise, is an >> interesting approach. > >Yes, it's a good idea. I'd go for a current monitor - eg INA169 - to >get things down to the 5V domain (I have a 60V supply) and take it from >there.Something like an LM10 will operate at low power within a few Vebs of local supply voltage, in a bootstrapped circuit. Not sure if it was there in 1977; definitely 1979. Possible that wafer size will prevent miniturization in SMD, but hermetic packages may make more sense anyways, at high temperature. A range of integrated circuits will intentionally shut down above a certain temperature (~ overload protection circuitry), so 'simplicity' or low junction/device count is probably better. RL
Reply by ●December 14, 20232023-12-14
On Tue, 12 Dec 2023 18:43:44 -0500, Phil Hobbs <pcdhSpamMeSenseless@electrooptical.net> wrote:>On 2023-12-12 09:17, legg wrote: >> On Mon, 11 Dec 2023 18:05:46 -0500, Phil Hobbs >> <pcdhSpamMeSenseless@electrooptical.net> wrote: >> >>> On 2023-12-11 12:09, Fred Bloggs wrote:<snip>>>> >>> I'm not sure that we're looking at the same schematic. In the one I >>> have, "Half_Bridge_for_77_280Z_thompson.pdf", the LM311 is a weird sort >>> of switching bias supervisor, not a linear amp. It's running as a >>> normal open-collector comparator, with its output wire-ORed with the >>> shutdown transistor Q7. >>> >>> When it fires, or the shutdown line is high, it steals Q3's base bias. >>> That causes Darlington Q5/Q6 to turn on more, with a TC of about 2 >>> seconds. That reduces the quiescent bias. >>> >>> In small-signal conditions, that'll just oscillate irregularly and keep >>> the class-A bias current of very roughly 60 mA. (*) In large-signal >>> conditions, the comparator will be pulling low most of the time, which >>> reduces the quiescent bias progressively. (If the gain of the bias loop >>> is high enough, it may not drift that far, but I'd probably need to use >>> SPICE to find that out.) >>> >>> When the shutdown pin is active, the class-A bias will gradually go to >>> 0, turning the output totem pole into a really bad class B. (One >>> gathers that the shutdown turns off the audio input as well.) >>> >>> One aspect that I don't understand well is Q3. With a 3k/100 ohm >>> voltage divider in the Q4 leg, ISTM that the base voltage of Q3 will be >>> nearly the same as that of Q2, which is driven from a 620 ohm / 22 ohm >>> divider (plus various V_BEs). That makes the current through the 220 >>> ohm hard to estimate by eyeball. (The average current is obviously >>> going to be small, on account of that 100k resistor.) >>> >>> (I sort of gather that it's pretty tweaky, due to that scribbled-in 10k >>> to ground from the bases of Q3 and Q4.) >>> >>> Cheers >>> >>> Phil Hobbs >>> >>> (*) That 60 mA number is based on the ~20 mA emitter current of Q1. >>> That'll drop about 450 mV across the 22 ohms, which translates to about >>> 1.5 mA collector current in Q8. That puts roughly 40 mV across the 27 >>> ohms, which divided by 0.66 ohms gets you roughly 60 mA of Class-A bias >>> current. >> >> Phil, >> >> The 'bumph' controlling the biasing is a sub-audible current switch >> into fairly large capacitors, so simulation would have to take this >> into account. >> >> In my internet/bugs/ampjt folder there are a number of simulations >> posted by Jim around 2013. They used the .op spice directive to >> establish DC operating point values only. >> >> I couldn't get it to operate as an amplifier (starts with static >> latched-off biasing) using his TL081/071 subcircuit. Using the >> basic LTSpice single or doublepole OA would allow it to demonstrate >> a signal path in a .tran simulation. >> >> His sims left out the electrolytic cap on the base of Q2 and the >> gain was set to simple unity (inverting) using 10K resistors >> and a cap-coupled source. >> >> Crossover distortion was easily visible. As I couldn't see any >> slow-moving voltages or currents in the biasing section to >> correct this, while a signal was being processed, or any reason >> why they should change (with the LM311 inputs overloaded by >> normal operating current), I set the thing aside. >> >> RL >> > >In 1977 it wasn't that easy to do a fully-differential measurement of a >40-mV signal sitting on 12 Vpp of audio. > >Re-framing the problem as preventing the measured voltage from falling >much below 40 mV, and letting it gradually decrease otherwise, is an >interesting approach. > >JT was a smart guy, even if he was a tiny bit too aware of that. ;) > >May God hold him in memory eternal. > >Cheers > >Phil HobbsInstead of just bitching about the sims, I replaced both the TL071 subckt and the LM311 subckt with a default OA and an LT1011 from the standard LTspice library so that the thing runs. ( Be sure to remove or terminate the old subcircuits to ensure easy sim startup.) This ran with low crossover distortion at 100KHz and about 50mA bias current at xover. The bias current could be raised or lowered proportionally through comparator input adjustment. No big OA slewing requirements. So the concept could be demonstrated ~accurately in LTSpice. RL
