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Old 12th October 2009, 12:18 AM   #101
Bigun is offline Bigun  Canada
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Try it the other way around. Buffer first, VAS 2nd. Then you will find that you've unloaded the LTP from driving Cdom. It might be counter-intuitive to put the buffer first but I think it provides some interesting benefits.
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Old 12th October 2009, 10:05 PM   #102
GEirin is offline GEirin  Argentina
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OK Bigun. Buffer + Vas.
You had tried this schema: Buffer + Lender Vas?
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Old 13th October 2009, 01:53 AM   #103
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nope - haven't tried it. It's just theory (simulation) but it looked interesting

What you will find is that the compensation cap goes on the VAS (2nd device), whereas you need only a very small compensation cap on the first device to ensure stability of the LTP-VAS.

So, if you put the comp. cap on the 2nd device it's now actively driven from both ends. The emitter of the 1st device and the collector of the 2nd device. Slew rate is now more symmetrical and no longer limited by the current output from the LTP.

Will I build it -- well it's in the queue along with a few other things
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Old 15th October 2009, 10:20 PM   #104
GEirin is offline GEirin  Argentina
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Hi Bigun.
Other compensation. No Miller cap. RC filter in base 1st. device.
What is your opinion?
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Old 15th October 2009, 10:28 PM   #105
Bigun is offline Bigun  Canada
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I like to use the free LTspice tool to simulate circuits and understand at least in principle how they might behave. Have you tried this ?

The RC input filter - not sure if you need it, what are you trying to filter ? - do you expect to operate in a noisy RF environment ?

The Miller cap is almost always needed to make an amplifier of this topology stable. I would doubt you can eliminate it without some other equivalent compensation method. The miller cap is controversial, some hate it, some don't. My own listening tests tell me that it has an optimum value.
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Old 15th October 2009, 11:12 PM   #106
GEirin is offline GEirin  Argentina
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Hi. Thanks for you reply.
I'm diy audio, not an expert in amplifier design. I cant simulate circuit.
Old amplifier, Phase Linear .........actual Yamaha use RC compensation, no miller cap.
Im interested to know the "best" Vas and compensation.
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Old 15th October 2009, 11:29 PM   #107
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Actually this amp is highly mature and uses several stabilising techniques, most simply preventing excessive slew and thus inhibiting oscillation. It also has a very small output inductor, with split feedback before and after presumably to promote stability into capacitive loads.

1. C508 across the bases of the long tailed pair.
2. C517 across the feedback resistor, R536, reducing gain with frequency.
3. C519 in series with R540 across collector load of Q507.
4. C529 and C526 shunting VAS output to AC ground.

Certainly there is no formal lag compensation across the VAS collector/base, but the foregoing demonstrate that stability has been very carefully configured regardless.

Hugh
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Old 15th October 2009, 11:34 PM   #108
GEirin is offline GEirin  Argentina
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Default My amplifier

My idea is to improve a old amplifier.
My thread is "my update lin topology"
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Old 16th October 2009, 12:02 AM   #109
Bigun is offline Bigun  Canada
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Your circuit looks familiar... 25W Class-A Power Amplifier


Well, two newbies isn't the best approach here (meaning you and I) - it perhaps deserves a new thread where you can attract some help from people like Hugh. I am happy to chime in with my thoughts but the TGM amplifier is my only project so far
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Old 16th October 2009, 12:18 AM   #110
GEirin is offline GEirin  Argentina
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Bigun
Sorry...
I understood you explanation
GEirin
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