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Old 22nd January 2011, 10:31 PM   #3391
jcx is offline jcx  United States
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High loop Gain Composite Op Amp Circuits

shows a low distortion diode clipper inside the feedback loop - you may not like the step recovery on clipping but I would usually try to size line level or headphone amplifier circuits made with these op amp techniques with considerable headroom to avoid clipping (structure system gain for >120 dB SPL output)

I believe low single digit uS clipping recovery details to unlikely to be audible especially if the clipping itself is really infrequent - few dyanmic tranducers will have as much as an octave more than 20 KHz 2nd order Fc

it is possible that better clipping recovery could be managed as promised by BJ Lurie's "nonlinear dynamic compensator" examples but I don't know of a systematic way to design such circuits - the greater access to internal stages in discrete circuits helps - as has been shown in a couple of amps here at diyAudio


phono preamps should show distortion, accuracy improvements with 2 properly chosen op amps in the feedback RIAA correction loop - you do have to shape the loop gain with local feedback around the amps to give linear stability with the RIAA feedback reducing to unity high frequency gain

typically a fast output op amp can have flat enough gain around the slower input op amp dominant pole loop gain intercept to avoid problems - fast unity gain buffers in the loop have long been popular but I see advantages to added loop gain at lower frequencies

a high current, fast, low distortion CFA output op amp allows driving very low RIAA feedback network Z at high frequency (~10-100 Ohms feedback R could be appropriate for MC) for low noise while eliminating the heavy loading thermal, ps pin common impedance errors of the input op amp

loop gain also has powerful linearizing effect on diff input linearity - 10x smaller input diff V from +20 dB audio frequency gain from a 2nd op amp in the loop gives ~ 100x less distortion from the 3rd order gm nonlinearity term that dominates in well balanced diff pair at the composit input

the same added 10x audio frequency gain means the input op amp swings 10x less V at its output too, as well as just having to drive the output op amp high Z positive input - almost certainly staying in "deep Class A" even with only 100-200 uA bias current

Last edited by jcx; 22nd January 2011 at 10:46 PM.
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Old 23rd January 2011, 11:07 PM   #3392
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High loop Gain Composite Op Amp Circuits

shows a low distortion diode clipper inside the feedback loop - you may not like the step recovery on clipping but I would usually try to size line level or headphone amplifier circuits made with these op amp techniques with considerable headroom to avoid clipping (structure system gain for >120 dB SPL output)

I believe low single digit uS clipping recovery details to unlikely to be audible especially if the clipping itself is really infrequent - few dyanmic tranducers will have as much as an octave more than 20 KHz 2nd order Fc

it is possible that better clipping recovery could be managed as promised by BJ Lurie's "nonlinear dynamic compensator" examples but I don't know of a systematic way to design such circuits - the greater access to internal stages in discrete circuits helps - as has been shown in a couple of amps here at diyAudio


phono preamps should show distortion, accuracy improvements with 2 properly chosen op amps in the feedback RIAA correction loop - you do have to shape the loop gain with local feedback around the amps to give linear stability with the RIAA feedback reducing to unity high frequency gain

typically a fast output op amp can have flat enough gain around the slower input op amp dominant pole loop gain intercept to avoid problems - fast unity gain buffers in the loop have long been popular but I see advantages to added loop gain at lower frequencies

a high current, fast, low distortion CFA output op amp allows driving very low RIAA feedback network Z at high frequency (~10-100 Ohms feedback R could be appropriate for MC) for low noise while eliminating the heavy loading thermal, ps pin common impedance errors of the input op amp

loop gain also has powerful linearizing effect on diff input linearity - 10x smaller input diff V from +20 dB audio frequency gain from a 2nd op amp in the loop gives ~ 100x less distortion from the 3rd order gm nonlinearity term that dominates in well balanced diff pair at the composit input

the same added 10x audio frequency gain means the input op amp swings 10x less V at its output too, as well as just having to drive the output op amp high Z positive input - almost certainly staying in "deep Class A" even with only 100-200 uA bias current
Hi jcx,

Thanks for describing this. It is quite a creative approach that appears to yield very good performance. I have to admit that I have not generally built circuits with two op amps in the loop in this fashion. I must admit that I also shy away from circuits where the phase lag goes beyond 180 degrees before the loop gain crossover. For things like RIAA preamps, I'm less creative, dividing the required gain across multiple closed loops.

Cheers,
Bob
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Old 2nd October 2012, 06:47 PM   #3393
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It's finally happened, I've come across the impenetrable local oscillation of the outputs at... 50MHz.

It's only on the positive signals. This is a unique output stage in that the outputs are driven from a capacitive impedance, from the collectors of the drivers. Just an EF driven by collectors. I remember there as an RC netork across the B-C junction that orked most times? I thought it as in Cordell's book but I just can't find it. It is basically a snubber across the B-C I thought.

The drivers are 2SC4793/A1837, and the outputs are MJL0302/3281 or an unmatched mix, like suggested by Ostripper at one time. The oscillation begins hen I go over 20V rails. I already have 2.2R carbon comp base stoppers.

I kno there are a number of ays people have solved this problem in triple output stages but I liked the snubber option the best.
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Old 2nd October 2012, 06:57 PM   #3394
ingrast is offline ingrast  Uruguay
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.....

The drivers are 2SC4793/A1837, and the outputs are MJL0302/3281 or ....
Schematics available ??

Rodolfo
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Old 2nd October 2012, 07:06 PM   #3395
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I don't expect anyone to calculate anything, I just ant to kno hat I'm looking for. My simulations appear to sho the 50MHz oscillation and indeed a 150p+47R snub across the B-C seems to be the ay to go.

The circuit is a good one of my design after all, I don't ant to reveal it ust yet...
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Old 2nd October 2012, 07:26 PM   #3396
davada is offline davada  Canada
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Originally Posted by keantoken View Post
I don't expect anyone to calculate anything, I just ant to kno hat I'm looking for. My simulations appear to sho the 50MHz oscillation and indeed a 150p+47R snub across the B-C seems to be the ay to go.

The circuit is a good one of my design after all, I don't ant to reveal it ust yet...
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Last edited by davada; 2nd October 2012 at 07:30 PM.
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Old 2nd October 2012, 07:34 PM   #3397
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I am getting one, thanks to a very good friend, it should be here soon. Hang in there. Yes I am a bit challenged, as alays.
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Old 2nd October 2012, 07:50 PM   #3398
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Originally Posted by keantoken View Post
It's finally happened, I've come across the impenetrable local oscillation of the outputs at... 50MHz.

It's only on the positive signals. This is a unique output stage in that the outputs are driven from a capacitive impedance, from the collectors of the drivers. Just an EF driven by collectors. I remember there as an RC netork across the B-C junction that orked most times? I thought it as in Cordell's book but I just can't find it. It is basically a snubber across the B-C I thought.

The drivers are 2SC4793/A1837, and the outputs are MJL0302/3281 or an unmatched mix, like suggested by Ostripper at one time. The oscillation begins hen I go over 20V rails. I already have 2.2R carbon comp base stoppers.

I kno there are a number of ays people have solved this problem in triple output stages but I liked the snubber option the best.
Hi keantoken,

I came up with the snubber idea in my MOSFET power amplifier with error correction. I don't recall having used it in BJT output stages, although it might be useful there as well. I refer to them as gate zobel networks on page 226 of my book in the MOSFET chapter, but unfortunately do not show a schematic there with them. I cite as a reference my MOSFET amplifier paper. That paper is available on my website at CordellAudio.com - Home.

The basic idea is to damp out resonances that can be part of forming an oscillator topology, such as a Hartley or Colpitts. The technique may very well work with BJT arrangements as well. Don't rule out the possibility it is the drivers oscillating.

It was a little unclear whether the problem you saw was in simulation or prototype, but it sounds like the latter. In that case, Triples of any type can be susceptible to power supply wiring inductances and HF feedback through the power rails. When using ordinary thriple EFs, I often recommend some small-resistance R-C filtering in the rail as it travels back to the driver and thence the pre-driver (i.e., two LPF stages, often with resistances on the order of 1 ohm and 10 ohms, respectively). If the shunt capacitors are of a moderate value, the R-C combination not only acts as an LPF, but also acts as a Zobel on the rails.


Cheers,
Bob
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Old 2nd October 2012, 08:07 PM   #3399
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Yes, this is a real prototype. I simulated it and then built it. I have a 40MHz signal generator so I have some indication hen simulations and reality don't converge.

Thanks for the info. I onder if I coul find the source of the oscillation by probing the air around the traces. I suppose this ould be a job better suited for an FET probe, unfortunately I don't have one.
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Old 2nd October 2012, 11:58 PM   #3400
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Okay, I hooked up the 150p+47R B-C snubbers. I am totally amazed. The sound is so much more real no. I couldn't spot any oscillation on the scope. The output impedance still has the 50MHz spike, but no more signs of actual oscillation. That last spike seems really hard to get rid of.

Just letting you guys kno it orked.
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