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Yet another Class AB autobias

Inspired by the late Jim Thompson's autobias design that we saw in the thread "Power Amplifier for 100kHz" I thought his use of a bang-bang comparator in setting bias was non-optimal but I liked his idea of running the pa in pure class B for large signals and setting a minimum quiescent class A current for small signals.

My idea uses JT's way of sensing output device current falling below threshold but instead of pumping up a "rubber Vbe diode" I pull the bias point apart with resistors and a pair of current mirrors. As an IC designer JT was liberal with current mirrors so I hope he would approve.

I expect over the decades many clever ways have been devised at eliminating crossover distortion and autobiasing class A/B so I have no shame in adding another. Does this look interesting or has it been done already?

Here is a pdf schematic:

formatting link

Here is the LT Spice wirelist:

Version 4 SHEET 1 2520 916 WIRE 720 -768 720 -816 WIRE 528 -752 528 -816 WIRE 1088 -752 1088 -800 WIRE -512 -704 -512 -800 WIRE 528 -640 528 -672 WIRE 720 -640 720 -688 WIRE 1088 -640 1088 -672 WIRE 640 -592 592 -592 WIRE 896 -592 784 -592 WIRE 1024 -592 896 -592 WIRE 1888 -576 1888 -624 WIRE 1888 -576 1680 -576 WIRE -512 -560 -512 -624 WIRE -464 -560 -512 -560 WIRE 1680 -528 1680 -576 WIRE -464 -512 -464 -560 WIRE 1616 -480 1536 -480 WIRE 640 -464 640 -592 WIRE 896 -464 896 -592 WIRE 896 -464 640 -464 WIRE 1088 -464 1088 -544 WIRE 1088 -464 896 -464 WIRE -512 -432 -512 -560 WIRE 1888 -432 1888 -576 WIRE 1088 -416 1088 -464 WIRE 1264 -416 1264 -544 WIRE 1680 -384 1680 -432 WIRE 1824 -384 1680 -384 WIRE 1088 -320 1088 -336 WIRE 1264 -320 1264 -336 WIRE -512 -272 -512 -352 WIRE 1024 -272 928 -272 WIRE 1888 -272 1888 -336 WIRE 1888 -272 1328 -272 WIRE 1888 -240 1888 -272 WIRE 1168 -224 1088 -224 WIRE 1264 -224 1168 -224 WIRE 928 -192 928 -272 WIRE 1168 -176 1168 -224 WIRE 1680 -112 1680 -384 WIRE 1888 -112 1888 -160 WIRE 2080 -112 1888 -112 WIRE 2208 -112 2080 -112 WIRE 1888 -64 1888 -112 WIRE 1168 -32 1168 -96 WIRE 928 64 928 -112 WIRE 1888 64 1888 16 WIRE 1888 64 928 64 WIRE 720 128 720 -544 WIRE 864 128 720 128 WIRE 1536 128 1536 -480 WIRE 1536 128 864 128 WIRE 720 160 720 128 WIRE -112 240 -112 176 WIRE -496 256 -624 256 WIRE -304 256 -416 256 WIRE -144 256 -304 256 WIRE 864 256 864 128 WIRE 224 272 -80 272 WIRE 720 272 720 240 WIRE 720 272 224 272 WIRE 2208 272 2208 -112 WIRE -144 288 -224 288 WIRE 720 304 720 272 WIRE 1888 304 1888 64 WIRE 1680 352 1680 -32 WIRE 1824 352 1680 352 WIRE -224 368 -224 288 WIRE -112 384 -112 304 WIRE 1680 384 1680 352 WIRE -624 432 -624 256 WIRE 528 432 528 -544 WIRE 624 432 528 432 WIRE 720 432 720 384 WIRE 864 432 864 320 WIRE 864 432 720 432 WIRE 1616 432 864 432 WIRE 2208 464 2208 352 WIRE 528 480 528 432 WIRE 720 480 720 432 WIRE 1680 512 1680 480 WIRE 1888 512 1888 400 WIRE 1888 512 1680 512 WIRE 624 528 624 432 WIRE 624 528 592 528 WIRE 656 528 624 528 WIRE -304 560 -304 256 WIRE -48 560 -304 560 WIRE 224 560 224 272 WIRE 224 560 16 560 WIRE 528 608 528 576 WIRE 720 608 720 576 WIRE -624 656 -624 512 WIRE 1888 704 1888 512 WIRE 528 736 528 688 WIRE 720 736 720 688 WIRE -304 800 -304 560 WIRE -64 800 -304 800 WIRE 2080 800 2080 -112 WIRE 2080 800 16 800 FLAG -512 -800 P15 FLAG -112 176 P15 FLAG 1888 -624 P15 FLAG 1088 -800 P15 FLAG -512 -272 N15 FLAG -112 384 N15 FLAG 1888 704 N15 FLAG 1168 -32 N15 FLAG 2208 464 0 FLAG -624 656 0 FLAG -224 368 0 FLAG -464 -512 0 FLAG 720 736 N15 FLAG 528 736 N15 FLAG 720 -816 P15 FLAG 528 -816 P15 FLAG 1264 -544 P15 SYMBOL voltage -512 -720 R0 WINDOW 123 0 0 Left 0 WINDOW 39 0 0 Left 0 SYMATTR InstName V1 SYMATTR Value 15 SYMBOL voltage -512 -448 R0 WINDOW 123 0 0 Left 0 WINDOW 39 0 0 Left 0 SYMATTR InstName V2 SYMATTR Value 15 SYMBOL voltage -624 416 R0 WINDOW 123 0 0 Left 0 WINDOW 39 0 0 Left 0 SYMATTR InstName V3 SYMATTR Value SINE(0 1 100k) SYMBOL voltage 928 -208 R0 WINDOW 123 0 0 Left 0 WINDOW 39 0 0 Left 0 SYMATTR InstName V4 SYMATTR Value 0.05 SYMBOL res 2192 256 R0 SYMATTR InstName R1 SYMATTR Value 8 SYMBOL pnp 1824 400 M180 SYMATTR InstName Q1 SYMATTR Value 2SAR533P SYMBOL pnp 1616 480 M180 SYMATTR InstName Q2 SYMATTR Value 2N2907 SYMBOL pnp 784 -544 R180 SYMATTR InstName Q10 SYMATTR Value 2N3906 SYMBOL npn 1824 -432 R0 SYMATTR InstName Q3 SYMATTR Value 2SCR533P SYMBOL npn 1616 -528 R0 SYMATTR InstName Q4 SYMATTR Value 2N2222 SYMBOL npn 1024 -320 R0 SYMATTR InstName Q5 SYMATTR Value 2N2222 SYMBOL npn 1328 -320 M0 SYMATTR InstName Q6 SYMATTR Value 2N2222 SYMBOL OpAmps\\UniversalOpAmp2 -112 272 R0 SYMATTR InstName U1 SYMBOL res 1872 -256 R0 SYMATTR InstName R2 SYMATTR Value 0.22 SYMBOL res 1872 -80 R0 SYMATTR InstName R3 SYMATTR Value 0.22 SYMBOL res 1696 -16 R180 WINDOW 0 36 76 Left 2 WINDOW 3 36 40 Left 2 SYMATTR InstName R4 SYMATTR Value 220 SYMBOL current 1168 -176 R0 WINDOW 123 0 0 Left 0 WINDOW 39 0 0 Left 0 SYMATTR InstName I1 SYMATTR Value 10m SYMBOL res 32 784 R90 WINDOW 0 0 56 VBottom 2 WINDOW 3 32 56 VTop 2 SYMATTR InstName R6 SYMATTR Value 10k SYMBOL res -400 240 R90 WINDOW 0 0 56 VBottom 2 WINDOW 3 32 56 VTop 2 SYMATTR InstName R5 SYMATTR Value 1k SYMBOL cap -48 544 M90 WINDOW 0 0 32 VBottom 2 WINDOW 3 32 32 VTop 2 SYMATTR InstName C1 SYMATTR Value 5p SYMBOL npn 656 480 R0 SYMATTR InstName Q13 SYMATTR Value 2N3904 SYMBOL npn 592 480 M0 SYMATTR InstName Q12 SYMATTR Value 2N3904 SYMBOL pnp 1024 -544 M180 SYMATTR InstName Q9 SYMATTR Value 2N3906 SYMBOL pnp 592 -544 R180 SYMATTR InstName Q11 SYMATTR Value 2N3906 SYMBOL res 1072 -768 R0 SYMATTR InstName R9 SYMATTR Value 22 SYMBOL res 704 -784 R0 SYMATTR InstName R10 SYMATTR Value 22 SYMBOL res 512 -768 R0 SYMATTR InstName R11 SYMATTR Value 22 SYMBOL res 704 592 R0 SYMATTR InstName R13 SYMATTR Value 22 SYMBOL res 512 592 R0 SYMATTR InstName R12 SYMATTR Value 22 SYMBOL res 704 144 R0 SYMATTR InstName R7 SYMATTR Value 220 SYMBOL res 704 288 R0 SYMATTR InstName R8 SYMATTR Value 220 SYMBOL res 1072 -432 R0 SYMATTR InstName R14 SYMATTR Value 1m SYMBOL res 1248 -432 R0 SYMATTR InstName R15 SYMATTR Value 1m SYMBOL cap 848 256 R0 SYMATTR InstName C2 SYMATTR Value 100p TEXT -656 848 Left 2 !.tran 1m TEXT 2128 -728 Left 2 ;EPW SED DEC 2023 TEXT 2128 -672 Left 2 ;CLASS AB AMPLIFIER TEXT 2128 -616 Left 2 ;CURRENT MIRROR ELASTIC BIAS

piglet

Reply to
piglet
Loading thread data ...

JT's amp actually holds its bias very nicely in large signal operation. There's enough gain in the loop that the LM311 only needs to fire occasionally near a zero-crossing for it to work fine.

(Don't have time to look at your amp very closely just now, Erich--maybe over the weekend.)

Cheers

Phil Hobbs

Reply to
Phil Hobbs

Where those diodes could also be emitter followers for more output current... Nice!

Arie

Reply to
Arie de Muijnck

Spoil sport! You have deprived me a nice intellectual challenge :>

piglet

Reply to
piglet

We like the THS4361, which is a 33-V FET-input amp with 210 MHz GBW,

1000 V/us slewing, and a 100-mA current limit.

Cheers

Phil Hobbs

Reply to
Phil Hobbs

THS4631,

Reply to
Phil Hobbs

On 14/12/2023 10:40 pm, piglet wrote:

The addition of a second current sense diff amp completely transforms the idea. The first design went into class A only around zero crossings or at very low levels and was in class B on large signals. As Mike Monett pointed out it created new distortion. The below adaptation senses both output devices currents and selects the lowest of the two to create the bias feedback - this keeps the output slightly out of pure class B.

Seems better and works like JT's but without the irregular comparator action.

PDF Schematic here:

formatting link

LT Spice wirelist here:

Version 4 SHEET 1 2520 916 WIRE 512 -768 512 -816 WIRE 320 -752 320 -816 WIRE 1184 -752 1184 -800 WIRE 1776 -704 1776 -752 WIRE 1776 -704 1568 -704 WIRE 1568 -656 1568 -704 WIRE -304 -640 -304 -736 WIRE 320 -640 320 -672 WIRE 512 -640 512 -688 WIRE 1184 -640 1184 -672 WIRE 1504 -608 1488 -608 WIRE 432 -592 384 -592 WIRE 1056 -592 576 -592 WIRE 1120 -592 1056 -592 WIRE 1776 -560 1776 -704 WIRE 432 -512 432 -592 WIRE 736 -512 432 -512 WIRE 1056 -512 1056 -592 WIRE 1056 -512 736 -512 WIRE 1184 -512 1184 -544 WIRE 1184 -512 1056 -512 WIRE 1568 -512 1568 -560 WIRE 1712 -512 1568 -512 WIRE -304 -496 -304 -560 WIRE -256 -496 -304 -496 WIRE 1184 -480 1184 -512 WIRE -256 -448 -256 -496 WIRE 736 -416 736 -512 WIRE 1184 -384 1184 -400 WIRE 1376 -384 1376 -544 WIRE -304 -368 -304 -496 WIRE 1120 -336 1072 -336 WIRE 1776 -336 1776 -464 WIRE 1776 -336 1440 -336 WIRE 736 -320 736 -336 WIRE 736 -320 720 -320 WIRE 912 -320 912 -432 WIRE 1296 -288 1184 -288 WIRE 1376 -288 1296 -288 WIRE 656 -272 624 -272 WIRE 1008 -272 976 -272 WIRE 1776 -272 1776 -336 WIRE 1072 -256 1072 -336 WIRE 1296 -256 1296 -288 WIRE 816 -224 720 -224 WIRE 912 -224 816 -224 WIRE -304 -208 -304 -288 WIRE 624 -176 624 -272 WIRE 816 -176 816 -224 WIRE 1296 -144 1296 -176 WIRE 1776 -144 1776 -192 WIRE 1968 -144 1776 -144 WIRE 2032 -144 1968 -144 WIRE 2096 -144 2032 -144 WIRE 1008 -80 1008 -272 WIRE 1072 -80 1072 -176 WIRE 1072 -80 1008 -80 WIRE 1776 -80 1776 -144 WIRE 1776 -80 1072 -80 WIRE 816 -64 816 -96 WIRE 1776 -48 1776 -80 WIRE 624 64 624 -96 WIRE 1776 64 1776 32 WIRE 1776 64 624 64 WIRE 512 128 512 -544 WIRE 1120 128 512 128 WIRE 1488 128 1488 -608 WIRE 1488 128 1120 128 WIRE 512 160 512 128 WIRE 1568 208 1568 -512 WIRE 0 240 0 176 WIRE -384 256 -512 256 WIRE -192 256 -304 256 WIRE -32 256 -192 256 WIRE 176 272 32 272 WIRE 512 272 512 240 WIRE 512 272 176 272 WIRE 2096 272 2096 -144 WIRE -32 288 -112 288 WIRE 512 304 512 272 WIRE 1776 304 1776 64 WIRE 1568 352 1568 288 WIRE 1712 352 1568 352 WIRE -112 368 -112 288 WIRE 0 384 0 304 WIRE 1568 384 1568 352 WIRE -512 432 -512 256 WIRE 320 432 320 -544 WIRE 416 432 320 432 WIRE 512 432 512 384 WIRE 1136 432 512 432 WIRE 1504 432 1136 432 WIRE 2096 464 2096 352 WIRE 320 480 320 432 WIRE 512 480 512 432 WIRE 1568 512 1568 480 WIRE 1776 512 1776 400 WIRE 1776 512 1568 512 WIRE 416 528 416 432 WIRE 416 528 384 528 WIRE 448 528 416 528 WIRE -192 560 -192 256 WIRE -16 560 -192 560 WIRE 176 560 176 272 WIRE 176 560 48 560 WIRE 320 608 320 576 WIRE 512 608 512 576 WIRE -512 656 -512 512 WIRE 1776 704 1776 512 WIRE 320 736 320 688 WIRE 512 736 512 688 WIRE -192 800 -192 560 WIRE 48 800 -192 800 WIRE 1968 800 1968 -144 WIRE 1968 800 128 800 FLAG -304 -736 P15 FLAG 0 176 P15 FLAG 1776 -752 P15 FLAG 1184 -800 P15 FLAG -304 -208 N15 FLAG 0 384 N15 FLAG 1776 704 N15 FLAG 2096 464 0 FLAG -512 656 0 FLAG -112 368 0 FLAG -256 -448 0 FLAG 512 736 N15 FLAG 320 736 N15 FLAG 512 -816 P15 FLAG 320 -816 P15 FLAG 1376 -544 P15 FLAG 1296 -144 N15 FLAG 816 -64 N15 FLAG 912 -432 P15 FLAG 2032 -144 vout FLAG 1120 128 pdrive FLAG 1136 432 ndrive SYMBOL voltage -304 -656 R0 WINDOW 123 0 0 Left 0 WINDOW 39 0 0 Left 0 SYMATTR InstName V1 SYMATTR Value 15 SYMBOL voltage -304 -384 R0 WINDOW 123 0 0 Left 0 WINDOW 39 0 0 Left 0 SYMATTR InstName V2 SYMATTR Value 15 SYMBOL voltage -512 416 R0 WINDOW 123 0 0 Left 0 WINDOW 39 0 0 Left 0 SYMATTR InstName V3 SYMATTR Value SINE(0 1 100k) SYMBOL voltage 1072 -272 R0 WINDOW 123 0 0 Left 0 WINDOW 39 0 0 Left 0 SYMATTR InstName V4 SYMATTR Value 0.02 SYMBOL res 2080 256 R0 SYMATTR InstName R1 SYMATTR Value 8 SYMBOL pnp 1712 400 M180 SYMATTR InstName Q1 SYMATTR Value 2SAR533P SYMBOL pnp 1504 480 M180 SYMATTR InstName Q2 SYMATTR Value 2N2907 SYMBOL pnp 576 -544 R180 SYMATTR InstName Q9 SYMATTR Value 2N3906 SYMBOL npn 1712 -560 R0 SYMATTR InstName Q4 SYMATTR Value 2SCR533P SYMBOL npn 1504 -656 R0 SYMATTR InstName Q3 SYMATTR Value 2N2222 SYMBOL npn 1120 -384 R0 SYMATTR InstName Q6 SYMATTR Value 2N2222 SYMBOL npn 1440 -384 M0 SYMATTR InstName Q7 SYMATTR Value 2N2222 SYMBOL OpAmps\\UniversalOpAmp2 0 272 R0 SYMATTR InstName U1 SYMBOL res 1760 -288 R0 SYMATTR InstName R2 SYMATTR Value 0.33 SYMBOL res 1760 -64 R0 SYMATTR InstName R3 SYMATTR Value 0.33 SYMBOL res 1584 304 R180 WINDOW 0 36 76 Left 2 WINDOW 3 36 40 Left 2 SYMATTR InstName R4 SYMATTR Value 220 SYMBOL res 144 784 R90 WINDOW 0 0 56 VBottom 2 WINDOW 3 32 56 VTop 2 SYMATTR InstName R7 SYMATTR Value 10k SYMBOL res -288 240 R90 WINDOW 0 0 56 VBottom 2 WINDOW 3 32 56 VTop 2 SYMATTR InstName R8 SYMATTR Value 1k SYMBOL cap -16 544 M90 WINDOW 0 0 32 VBottom 2 WINDOW 3 32 32 VTop 2 SYMATTR InstName C1 SYMATTR Value 5p SYMBOL npn 448 480 R0 SYMATTR InstName Q11 SYMATTR Value 2N3904 SYMBOL npn 384 480 M0 SYMATTR InstName Q12 SYMATTR Value 2N3904 SYMBOL pnp 1120 -544 M180 SYMATTR InstName Q5 SYMATTR Value 2N3906 SYMBOL pnp 384 -544 R180 SYMATTR InstName Q10 SYMATTR Value 2N3906 SYMBOL res 1168 -768 R0 SYMATTR InstName R5 SYMATTR Value 22 SYMBOL res 496 -784 R0 SYMATTR InstName R9 SYMATTR Value 22 SYMBOL res 304 -768 R0 SYMATTR InstName R10 SYMATTR Value 22 SYMBOL res 496 592 R0 SYMATTR InstName R11 SYMATTR Value 22 SYMBOL res 304 592 R0 SYMATTR InstName R12 SYMATTR Value 22 SYMBOL res 496 144 R0 SYMATTR InstName R13 SYMATTR Value 220 SYMBOL res 496 288 R0 SYMATTR InstName R14 SYMATTR Value 220 SYMBOL res 1168 -496 R0 SYMATTR InstName Pleast SYMATTR Value 1m SYMBOL current 1296 -256 R0 WINDOW 123 0 0 Left 0 WINDOW 39 0 0 Left 0 SYMATTR InstName I1 SYMATTR Value 10m SYMBOL voltage 624 -192 R0 WINDOW 123 0 0 Left 0 WINDOW 39 0 0 Left 0 SYMATTR InstName V5 SYMATTR Value 0.02 SYMBOL npn 656 -320 R0 SYMATTR InstName Q13 SYMATTR Value 2N2222 SYMBOL npn 976 -320 M0 SYMATTR InstName Q14 SYMATTR Value 2N2222 SYMBOL res 720 -432 R0 SYMATTR InstName Nleast SYMATTR Value 1m SYMBOL current 816 -176 R0 WINDOW 123 0 0 Left 0 WINDOW 39 0 0 Left 0 SYMATTR InstName I2 SYMATTR Value 10m TEXT -544 848 Left 2 !.tran 1m TEXT 1856 -712 Left 2 ;EPW SED DEC 2023 TEXT 1856 -656 Left 2 ;CLASS AB AMPLIFIER TEXT 1856 -600 Left 2 ;CURRENT MIRROR ELASTIC BIAS TEXT 1856 -544 Left 2 ;REGULATE TO LEAST OF EACH PHASE

piglet

Reply to
piglet

I spent a bit more time on JT's design, just for interest's sake. I replaced the op amp with a UniversalOpAmp2 that mimics a THS4631 (210 MHz GBW, 1000 V/us), and used the RH111 (an old rad-hard AD version of the LM311).

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The bias doesn't become Class B for large signals--it's solid as a rock, weirdly. You just get a 300-ns-ish bias current pulse near the output zero crossings. With a nearly full-scale output at 20 kHz, its distortion is only 0.04%, not bad for having no local feedback.

The LTspice 17.1 library has the D45H11 low-sat PNP, but weirdly not its NPN complement, the D44H11. I put both libraries in the zipfile.

It works exactly the same with BCV61C duals for the mirrors, which would save a couple of packages.

Cheers

Phil Hobbs

Reply to
Phil Hobbs

Bit of a brain fart. The 0.04% is actually the peak-to-peak summing junction error divided by the input voltage, which is fine as far as it goes, but I omitted to subtract off the fundamental. Once that's done, the p-p error at the SJ is less than 4 uV, with a 2 V p-p input.

That's actually 0.0002% THD at 20 kHz with an 8 V p-p output, not bad at all.

Cheers

Phil Hobbs

Reply to
Phil Hobbs

Why not try JT's updated design posted by myself and legg?

Reply to
sunaeconospam

I can see JT’s works but it seemed ugly to me , I wanted to see if there was a wholly linear way.

My second version (bias is set by lesser of the two currents) looks very promising and now turns out has indeed been done before, I just found a Bang & Olufsen patent from 2014 using similar approach.

Dang - fame and fortune elude me again :)

piglet

Reply to
piglet

Phil,

What's the point of sticking an ideal fixed offset voltage in a circuit that is attenpting to demonstrate practical circuitry? It's what mirrors are for . . .

With all later revs of LTspiceXVII, you have to be pretty careful when pulling parts from cmp\standard.xxx. This is particularly true if demonstrating temperature effects. I'm not sure what an ako:2n2904 (or even a stray 2n2906) looks like on your machine.

The second sim in your zip produces an error in a not-recently- updated LTSpiceXVII installation ;

Missing closing '}' in "{cb)"

. . . and won't start.

I see no obvious editable brackets to correct this.

RL

Reply to
legg
<snip>

You can get some idea of the flavors of different bipolar models there are floating around in the LTSpice arena, often with the same ident, from a spreadsheet thrown together last year:

formatting link
RL

Reply to
legg

Sorry about that. Just replace the closing paren with a right brace. I obviously didn’t save the file between fixing that and zipping it.

Re The bias voltage source: it makes zero difference to the performance of the circuit. It’s the comparator loop that interested me, and I thought it might interest others too.

I really like things that work very well using cheap components in non-obvious ways. I wouldn’t have expected 0.0004% THD for an 8V p-p output into 4 ohms.

Cheers

Phil Hobbs

Reply to
Phil Hobbs

Thanks, I’ll check it out.

Cheers

Phil Hobbs

Reply to
Phil Hobbs

I updated the zipfile with the correction and an auxiliary source to cancel the fundamental part of the SJ error voltage. Use the plot settings file to look at the THD waveform. (The aux source is calibrated by eye.)

I put in an annotated screen shot that shows an honest 0.0005% (5 ppm amplitude) p-p harmonic distortion. The actual THD is even better than that, because the p-p calculation is a bit pessimistic.)

I think the novel part of JT's design is that it controls what you actually care about, namely the minimum quiescent bias under transient conditions. Linear methods only get you the average, which doesn't guarantee what happens with a large output signal. Mike's method has a large transient error at the zero crossing, which gets worse at smaller inputs.

Cheers

Phil Hobbs

Reply to
Phil Hobbs

No sure about that: JT's design has two big capacitors (100uF and 22uF I recall) in the bias control so it will always be sluggish on transients. My second version has no capacitors in the bias control so should be very adaptive?

piglet

Reply to
piglet

U were designed before the invention of the opamp

Reply to
bitrex

Thank goodness the thread seems to have designed a satisfactory audio amp using a 250 MHz 900 V/usec op amp.

Reply to
bitrex

It’s not as good, though. JT’s gizmo samples the quiescent bias only at the zero crossings, and accumulates the results on the big cap on the base of the rubber diode.

That takes account of large signal effects, capacitance, and so on. Jim’s

100 uF is too big—something around 1-5 uF is better. You want just enough to avoid distortion in the low bass.

The current version of the zip file I posted has a plot file and screen shot that shows an honest 0.0005 % THD while putting 8 V pp of 20 kHz into

4 ohms.

Swapping out the $7 THS4631 for a 50-cent OPA172 still does 0.01% THD, which is much better than your average car speaker.

Cheers

Phil Hobbs

Reply to
Phil Hobbs

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