I got this amp from China years ago and it has held up and sounded pretty good. Lately one channel has cutting in and out, loss of power, distorted sound. I found with the inputs shorted the right channel has a 50kHz oscillation. I do see an inductor that might be for an SMPS on the side with problematic channel.
Is the first stage and SRPP? If it is why? I thought they were good for driving heavy loads.....looks like it's driving 470k? I do remember reading that with the correct load they can cancel distortion like any push pull stage.
Is the first stage and SRPP? If it is why? I thought they were good for driving heavy loads.....looks like it's driving 470k? I do remember reading that with the correct load they can cancel distortion like any push pull stage.
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Yes.famousmockingbird said:
Good question. Two possible answers:
1. Chinese audio designers like SRPP, even when it is unnecessary.
2. Chinese audio designers believe their customers like SRPP, even when it is unnecessary.
I can't think of any more plausible reason for using an SRPP than the two given by DF96.
Your circuit seems incomplete.
The advanatges of a SRPP in audio, where there is a separate output stage are :-
a) ability to drive a somewhat lower impedance load - not relavent here;
-or-
b) ability to produce a higher output voltage at a given distortion level - not relavent here as there is an intermediate amplifying stage;
-or-
c) The ability to (for a given load) produce low distortion due to an even order cancelation mechanism simlar to other push-pull topologies.
Advantage (c) is not releavent here as the SE output stage distortion will completely dominate. Only if the SRPP stage is used without voltgae gain betwen it and the output stage, and the output stage push pull, would there be any benefit. In any case, simple "back-of-the-envelope" calculation shows that the SRPP stage in your circuit is far from balance and so distortion cancellation will be ineffective.
Triodes produce predominately 2nd harmonic distortion, with a tiny bit of 3rd. The amount of 2nd (as a perecentage) falls directly with a fall in signal level, and the third fall of even more rapidly. Hence distortion in input stages in 3-gain-stage amps is never an issue, even with push-pull UL output. There have even been amplifiers designed that use local positive feedback around the input stage - this worsens input stage distortion, but as the output stage dominates, and gain is increased, negative feedback is more effective and overall distortion is reduced.
Your circuit seems incomplete.
The advanatges of a SRPP in audio, where there is a separate output stage are :-
a) ability to drive a somewhat lower impedance load - not relavent here;
-or-
b) ability to produce a higher output voltage at a given distortion level - not relavent here as there is an intermediate amplifying stage;
-or-
c) The ability to (for a given load) produce low distortion due to an even order cancelation mechanism simlar to other push-pull topologies.
Advantage (c) is not releavent here as the SE output stage distortion will completely dominate. Only if the SRPP stage is used without voltgae gain betwen it and the output stage, and the output stage push pull, would there be any benefit. In any case, simple "back-of-the-envelope" calculation shows that the SRPP stage in your circuit is far from balance and so distortion cancellation will be ineffective.
Triodes produce predominately 2nd harmonic distortion, with a tiny bit of 3rd. The amount of 2nd (as a perecentage) falls directly with a fall in signal level, and the third fall of even more rapidly. Hence distortion in input stages in 3-gain-stage amps is never an issue, even with push-pull UL output. There have even been amplifiers designed that use local positive feedback around the input stage - this worsens input stage distortion, but as the output stage dominates, and gain is increased, negative feedback is more effective and overall distortion is reduced.
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I don't see any high-frequency compensation for the negative feedback. With most transformers there will be a gain peak in the ultrasonic range, and a small cap (in the 10s to 100S of pF) across the feedback resistor helps cure it.
It was working before though. Therefore he needs to look for a faulty part, not a part missing through design error.
It looks as though he drew the circuit himself, and it is not a complete cuircuit of the amplifier anyway.
Thanks guys. Yes I drew the schematic myself because I couldn't find any online. I will go back and look if there is a compensation capacitor in the feedback loop. Like stated it was working fine for years, and usually those small value capacitors either being film or ceramic disc usually never fail, but anything is possible.
I just thought it was strange that every time I get it to oscillate it is the same exact frequency 50kHz. I have some more time later to look at the amp and figure out if that is an SMPS. I don't have much experience with SMPS, I only have a vague understanding of the theory of operation.
So the first stage is an SRPP. And Like I thought it was driving a pretty easy load. I forgot that SRPP's are fashionable over seas and is probably the only reason why it was used.
I just thought it was strange that every time I get it to oscillate it is the same exact frequency 50kHz. I have some more time later to look at the amp and figure out if that is an SMPS. I don't have much experience with SMPS, I only have a vague understanding of the theory of operation.
So the first stage is an SRPP. And Like I thought it was driving a pretty easy load. I forgot that SRPP's are fashionable over seas and is probably the only reason why it was used.
Yes, it is. The SRPP is a "fad" design. The original SRPP was designed as an active pull-up/active pull-down circuit. It operates as a balanced circuit under these conditions:
1) Dead short (makes both halves grounded cathode amps -- not a practical proposition.)
2) Open circuit (balance assured if the only current path is through both tubes)
3) The one impedance where the source current equals the sink current. Stray from this load condition and distortion rises rapidly.
The last case applies when using it as a driver operating into the Z0 of a T-line. In that case, it stays in balance and nulls even order harmonics. You could also run it into a very light load, as is done here in this design. However, you'd do better with an active plate load: more voltage gain, and lower distortion since triodes like the lightest possible plate loading. The closer you come to a horizontal loadline, the less distortion you have.
There's a lot of folk "wisdom" out there about the supposed magical properties of the SRPP, but that's all it is: audiophoolery.
I think DF96 meant Chinese manufacturers, not DIYer's, still it was "mildly racist"... like "Asians are the worst drivers..."
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No racism. I was referring to culturally Chinese, nationally Chinese, commercially Chinese - not ethnically Chinese. There is a wide range of ethnicities in China - but it appears that almost all of them put SRPP front-ends in their ebay offerings.
No worries, just having some fun with culture stereotypes, e.g., what would be the reason that makes the Chinese culturelly pre-disposed to SRPP?😀
Balancing ying and yang? 😎
Or just smart marketing to people who have heard of SRPP but know nothing about it?
Or just smart marketing to people who have heard of SRPP but know nothing about it?
With a 470k load, that stage is actually not an SRPP, just stacked triodes. The topology is the same as SRPP, but the distortion performance is worse since it does't actually push and pull. This was my own misconception as well until I read Merlin Blencowe's excellent papers with analysis and measurements of the circuit.
Interesting philosophical question: can you change a circuit (or at least its name) just by changing the external load applied to the circuit?
I'm not sure the distortion performance would be worse than an SRPP. Light loading is one condition for good performance. Optimum loading is another (but rarely seen in audio circuits).
I think I would prefer to say that it is an SRPP, but with such a light load it functions just like an active load circuit. A poorly designed active load circuit: because balance is not needed the upper triode could provide a much higher load and so get lower distortion.
I'm not sure the distortion performance would be worse than an SRPP. Light loading is one condition for good performance. Optimum loading is another (but rarely seen in audio circuits).
I think I would prefer to say that it is an SRPP, but with such a light load it functions just like an active load circuit. A poorly designed active load circuit: because balance is not needed the upper triode could provide a much higher load and so get lower distortion.
That's why I asked SY for a clarification, why would the name of the topology change depedning on the loading condition?
SRPP does in fact depend on operating condition, not just topology. I hate analogies, but here's one- suppose you had a conventional push pull output stage and one tube was biased well past cutoff. Would the circuit still be operating as push-pull, despite the topology?
See, for example, http://valvewizard2.webs.com/SRPP_Blencowe.pdf.
See, for example, http://valvewizard2.webs.com/SRPP_Blencowe.pdf.
Merlin Blencoe said:
Poor analogy. Unbalanced bias is a change of the circuit, not something external.
Note that a lightly loaded SRPP still has the lower output impedance of an SRPP, not the Ra/2 of an active load circuit. This may have no consequences, but it is still true.
What we should be stamping out is people who draw an active load and then call it an SRPP, despite taking output from the lower anode instead of the upper cathode.
Note that a lightly loaded SRPP still has the lower output impedance of an SRPP, not the Ra/2 of an active load circuit. This may have no consequences, but it is still true.
What we should be stamping out is people who draw an active load and then call it an SRPP, despite taking output from the lower anode instead of the upper cathode.
The lower output impedance is a function of the CF operation of the active load. It is not because of any push-pull action. When loaded to attain push-pull, the distortion nulls quite nicely.
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