On Fri, 8 Dec 2023 19:41:08 -0800 (PST), Fred Bloggs wrote:> On Wednesday, December 6, 2023 at 6:22:34 PM UTC-5, Glen Walpert wrote: >> On Wed, 6 Dec 2023 22:49:40 +0000, Clive Arthur wrote: >> >> > On 06/12/2023 21:19, Phil Hobbs wrote: >> >> On 2023-12-06 10:26, Clive Arthur wrote: >> >>> I'm not an analog design expert, but needs must. >> >>> >> >>> I recently adapted a Doug Self audio amplifier design for use on a >> >>> 60V single supply, and to go up to 100kHz with a working ambient >> >>> temperature of -20'C to 180'C. >> >>> >> >>> I can't show the circuit, but it was based on Fig 1a here... >> >>> http://www.douglas-self.com/ampins/dipa/dipa.htm >> >>> >> >>> Cdom needs to come down for this higher frequency application, but >> >>> too far and you have oscillation. Also, problems occur with slew >> >>> rate limiting due to Cdom, the TR5 constant current, and the >> >>> increase in hFE of TR4 with temperature. >> >>> >> >>> So I added an emitter degeneration resistor to TR4 to tame the hFE >> >>> variation, removed Cdom and put a smaller C across Rf1 instead. >> >>> Seems stable and not slew rate limiting. Took a lot longer to do >> >>> than that sentence might imply. >> >>> >> >>> However. >> >>> >> >>> The Vbias is a single diode, can't risk 2 diodes as TR6 & TR8 >> >>> probably get hotter and their Vbe would drop by more than the >> >>> diodes, so it's class B. As it runs at a high temperature, I >> >>> obviously want to reduce dissipation, and if I'd used say a rubber >> >>> diode to get some quiescent current, I think it would be very >> >>> difficult to control Iq well enough over the temperature range. >> >>> >> >>> So I have a circuit which works well enough, but could be better >> >>> with regard to crossover distortion (though it's lower than I would >> >>> have thought). In this application, the better the signal quality, >> >>> the higher the data rate. >> >>> >> >>> Any ideas for improving crossover distortion, bearing in mind the >> >>> temperature range? The signal is OFDM, so pretty much a load of >> >>> 'random' steps, some of which may be small and at a crossover >> >>> point. >> >>> >> >> Some local feedback around the output stage would get my vote. The >> >> Sziklai pairs have their own local feedback, but that doesn't fix >> >> the crossover problem. >> >> >> >> Another approach would be to turn TR4 into a diff pair. TR3's >> >> collector swing is going to waste, and that would let you keep the >> >> open-loop gain the same, while stabilizing the tail current. >> >> >> >> Cheers >> >> >> >> Phil Hobbs >> > >> > Thanks, that second one particularly sounds like a good idea. I'll >> > see what the sim says. Local feedback on the output stage sounds >> > trickier, >> > I'll have to think. >> > >> > One thing I thought of was to use multiple smaller output pairs in >> > parallel, and have a DC offset for each one. Imagine replacing the >> > Vbias diode with a string of a few series diodes, and connecting the >> > bases of one output pair across D1, the next pair across D2 etc. >> > Would need resistors from each pair of emitters to the output. That >> > should give lower crossover in more places. Maybe. >> 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? > > This circuit looks like what you're talking about, MC34071 : > > https://www.javanelec.com/CustomAjax/GetAppDocument/9feb1d77-b648-447f-a4ec-f89b60d40589?type=1&inlineName=True> > But it's a typical OA input and intermediate gain stage so the gain is > very large. The discrete Self circuit doesn't come close. Without gain > with bw near 10MHz, suppression of large signal output distortion is > going to be kinda weak. >That may use the same bias method, but I was thinking of a discrete transistor design, posted on his web site and possibly still available somewhere: From: Jim Thompson <To-Email-Use-The-Envelope-Icon@On-My-Web-Site.com> Newsgroups: sci.electronics.design Subject: Unusual Bias Method Date: Sun, 24 Nov 2013 08:53:30 -0700 Message-ID: <sl749997np4gq2giqk9d8k77orheh6qt3d@4ax 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> No one has commented on the unusual bias scheme in this amplifier since I originally posted it. No actual circuit designers in our midst ?:-} ...Jim Thompson
Power Amplifier for 100kHz.
Started by ●December 6, 2023
Reply by ●December 10, 20232023-12-10
Reply by ●December 10, 20232023-12-10
Glen Walpert <nospam@null.void> wrote:> That may use the same bias method, but I was thinking of a discrete > transistor design, posted on his web site and possibly still available > somewhere: > > From: Jim Thompson <To-Email-Use-The-Envelope-Icon@On-My-Web-Site.com> > Newsgroups: sci.electronics.design > Subject: Unusual Bias Method > Date: Sun, 24 Nov 2013 08:53:30 -0700 > Message-ID: <sl749997np4gq2giqk9d8k77orheh6qt3d@4ax > > 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> > > No one has commented on the unusual bias scheme in this amplifier since I > originally posted it. > > No actual circuit designers in our midst ?:-} > > ...Jim ThompsonLink times out. Doesn't exist. I seem to recall Phil Hobbs copied Jim's web site before it was taken down. -- MRM
Reply by ●December 11, 20232023-12-11
Mike Monett VE3BTI <spamme@not.com> wrote:> Glen Walpert <nospam@null.void> wrote: > >> That may use the same bias method, but I was thinking of a discrete >> transistor design, posted on his web site and possibly still available >> somewhere: >> >> From: Jim Thompson <To-Email-Use-The-Envelope-Icon@On-My-Web-Site.com> >> Newsgroups: sci.electronics.design >> Subject: Unusual Bias Method >> Date: Sun, 24 Nov 2013 08:53:30 -0700 >> Message-ID: <sl749997np4gq2giqk9d8k77orheh6qt3d@4ax >> >> 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> >> >> No one has commented on the unusual bias scheme in this amplifier since >> I originally posted it. >> >> No actual circuit designers in our midst ?:-} >> >> ...Jim Thompson > > Link times out. Doesn't exist. I seem to recall Phil Hobbs copied Jim's > web site before it was taken down.Found it. Thanks Phil! Home Page https://electrooptical.net/static/oldsite/www.analog- innovations.com/logo.html Links https://electrooptical.net/static/oldsite/www.analog- innovations.com/analog-innovations.html SED Postings (Unusual Bias Method not found) https://electrooptical.net/static/oldsite/www.analog- innovations.com/SED.html Secret Sauce - collection of Spice amplifiers https://electrooptical.net/static/oldsite/www.analog- innovations.com/SecretSauce_001.zip -- MRM
Reply by ●December 11, 20232023-12-11
On Wed, 06 Dec 2023 23:22:26 GMT, Glen Walpert <nospam@null.void> wrote:>On Wed, 6 Dec 2023 22:49:40 +0000, Clive Arthur wrote: > >> On 06/12/2023 21:19, Phil Hobbs wrote: >>> On 2023-12-06 10:26, Clive Arthur wrote: >>>> I'm not an analog design expert, but needs must. >>>> >>>> I recently adapted a Doug Self audio amplifier design for use on a 60V >>>> single supply, and to go up to 100kHz with a working ambient >>>> temperature of -20'C to 180'C. >>>> >>>> I can't show the circuit, but it was based on Fig 1a here... >>>> http://www.douglas-self.com/ampins/dipa/dipa.htm >>>> >>>> Cdom needs to come down for this higher frequency application, but too >>>> far and you have oscillation.� Also, problems occur with slew rate >>>> limiting due to Cdom, the TR5 constant current, and the increase in >>>> hFE of TR4 with temperature. >>>> >>>> So I added an emitter degeneration resistor to TR4 to tame the hFE >>>> variation, removed Cdom and put a smaller C across Rf1 instead.� Seems >>>> stable and not slew rate limiting.� Took a lot longer to do than that >>>> sentence might imply. >>>> >>>> However. >>>> >>>> The Vbias is a single diode, can't risk 2 diodes as TR6 & TR8 probably >>>> get hotter and their Vbe would drop by more than the diodes, so it's >>>> class B.� As it runs at a high temperature, I obviously want to reduce >>>> dissipation, and if I'd used say a rubber diode to get some quiescent >>>> current, I think it would be very difficult to control Iq well enough >>>> over the temperature range. >>>> >>>> So I have a circuit which works well enough, but could be better with >>>> regard to crossover distortion (though it's lower than I would have >>>> thought).� In this application, the better the signal quality, the >>>> higher the data rate. >>>> >>>> Any ideas for improving crossover distortion, bearing in mind the >>>> temperature range?� The signal is OFDM, so pretty much a load of >>>> 'random' steps, some of which may be small and at a crossover point. >>>> >>> Some local feedback around the output stage would get my vote.� The >>> Sziklai pairs have their own local feedback, but that doesn't fix the >>> crossover problem. >>> >>> Another approach would be to turn TR4 into a diff pair.� TR3's >>> collector swing is going to waste, and that would let you keep the >>> open-loop gain the same, while stabilizing the tail current. >>> >>> Cheers >>> >>> Phil Hobbs >> >> Thanks, that second one particularly sounds like a good idea. I'll see >> what the sim says. Local feedback on the output stage sounds trickier, >> I'll have to think. >> >> One thing I thought of was to use multiple smaller output pairs in >> parallel, and have a DC offset for each one. Imagine replacing the >> Vbias diode with a string of a few series diodes, and connecting the >> bases of one output pair across D1, the next pair across D2 etc. Would >> need resistors from each pair of emitters to the output. That should >> give lower crossover in more places. Maybe. > >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? > >GlenFrom old LTSpice trash here; http://ve3ute.ca/query/Half_Bridge_for_77_280Z_thompson.pdf RL
Reply by ●December 11, 20232023-12-11
On Monday, December 11, 2023 at 9:05:07 AM UTC-5, legg wrote:> On Wed, 06 Dec 2023 23:22:26 GMT, Glen Walpert <nos...@null.void> > wrote: > > >On Wed, 6 Dec 2023 22:49:40 +0000, Clive Arthur wrote: > > > >> On 06/12/2023 21:19, Phil Hobbs wrote: > >>> On 2023-12-06 10:26, Clive Arthur wrote: > >>>> I'm not an analog design expert, but needs must. > >>>> > >>>> I recently adapted a Doug Self audio amplifier design for use on a 60V > >>>> single supply, and to go up to 100kHz with a working ambient > >>>> temperature of -20'C to 180'C. > >>>> > >>>> I can't show the circuit, but it was based on Fig 1a here... > >>>> http://www.douglas-self.com/ampins/dipa/dipa.htm > >>>> > >>>> Cdom needs to come down for this higher frequency application, but too > >>>> far and you have oscillation. Also, problems occur with slew rate > >>>> limiting due to Cdom, the TR5 constant current, and the increase in > >>>> hFE of TR4 with temperature. > >>>> > >>>> So I added an emitter degeneration resistor to TR4 to tame the hFE > >>>> variation, removed Cdom and put a smaller C across Rf1 instead. Seems > >>>> stable and not slew rate limiting. Took a lot longer to do than that > >>>> sentence might imply. > >>>> > >>>> However. > >>>> > >>>> The Vbias is a single diode, can't risk 2 diodes as TR6 & TR8 probably > >>>> get hotter and their Vbe would drop by more than the diodes, so it's > >>>> class B. As it runs at a high temperature, I obviously want to reduce > >>>> dissipation, and if I'd used say a rubber diode to get some quiescent > >>>> current, I think it would be very difficult to control Iq well enough > >>>> over the temperature range. > >>>> > >>>> So I have a circuit which works well enough, but could be better with > >>>> regard to crossover distortion (though it's lower than I would have > >>>> thought). In this application, the better the signal quality, the > >>>> higher the data rate. > >>>> > >>>> Any ideas for improving crossover distortion, bearing in mind the > >>>> temperature range? The signal is OFDM, so pretty much a load of > >>>> 'random' steps, some of which may be small and at a crossover point. > >>>> > >>> Some local feedback around the output stage would get my vote. The > >>> Sziklai pairs have their own local feedback, but that doesn't fix the > >>> crossover problem. > >>> > >>> Another approach would be to turn TR4 into a diff pair. TR3's > >>> collector swing is going to waste, and that would let you keep the > >>> open-loop gain the same, while stabilizing the tail current. > >>> > >>> Cheers > >>> > >>> Phil Hobbs > >> > >> Thanks, that second one particularly sounds like a good idea. I'll see > >> what the sim says. Local feedback on the output stage sounds trickier, > >> I'll have to think. > >> > >> One thing I thought of was to use multiple smaller output pairs in > >> parallel, and have a DC offset for each one. Imagine replacing the > >> Vbias diode with a string of a few series diodes, and connecting the > >> bases of one output pair across D1, the next pair across D2 etc. Would > >> need resistors from each pair of emitters to the output. That should > >> give lower crossover in more places. Maybe. > > > >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.pdfWould be better to discretize the ideas used in the MC34071, which is still available for $0.23 in quantity.> > RL
Reply by ●December 11, 20232023-12-11
On 11/12/2023 14:05, legg wrote:> On Wed, 06 Dec 2023 23:22:26 GMT, Glen Walpert <nospam@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 > > RLThanks! 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. -- Cheers Clive
Reply by ●December 11, 20232023-12-11
On Mon, 11 Dec 2023 15:01:25 +0000, Clive Arthur <clive@nowaytoday.co.uk> wrote:>On 11/12/2023 14:05, legg wrote: >> On Wed, 06 Dec 2023 23:22:26 GMT, Glen Walpert <nospam@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
Reply by ●December 11, 20232023-12-11
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.He's bootstrapping the output back into his base drive with a humongous cap. Seems there are a bunch of big caps in there, like 100u to stabilize base to base on the output. The LM311 is being used as a linear high gain amplifier, not a comparator, negative feedback around it is a clue. RC phase compensations are all over the place as well as inductors. He had a time stabilizing it. All the current source biasing is good, but there's so much of it to make it a chore to implement. Looks like 'shutdown' kills the power drive base-base bias and shunts the input signal to ground, so that's good.> > RL
Reply by ●December 11, 20232023-12-11
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. > > RLThat 100u base-to-base bias cap is probably needed to dominant poll stabilize the LM311 amp more than anything else.
Reply by ●December 11, 20232023-12-11
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 <nospam@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, >GlenYou 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 PefhanyIn 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 Thompson
