Bob Cordell Interview: BJT vs. MOSFET - Page 62 - diyAudio
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Old 16th January 2007, 08:29 PM   #611
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Default KSA50:

Used 2 pairs of 15003/4, with (IIRC) ~1.9A total bias.

The amp was only biased for 50w classA into 8ohms, so if the slowness of the outputs was an issue it would/should manifest itself while the amp was driving 2ohm reactive loads, for which it was considered one of the best amps around...not that I've looked at the spectrum of the output while driving 2ohms, so I have no clue what the real change from classA to classB does to the distortion.

Are crossover non-linearities mitigated in their effect or character by their occurence at higher output levels in a heavily biased amp?

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Old 16th January 2007, 09:41 PM   #612
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Yes.
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Old 16th January 2007, 10:22 PM   #613
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Default Re: KSA50:

Quote:
Originally posted by Stuart Easson
Used 2 pairs of 15003/4, with (IIRC) ~1.9A total bias.

The amp was only biased for 50w classA into 8ohms, so if the slowness of the outputs was an issue it would/should manifest itself while the amp was driving 2ohm reactive loads, for which it was considered one of the best amps around...not that I've looked at the spectrum of the output while driving 2ohms, so I have no clue what the real change from classA to classB does to the distortion.

Are crossover non-linearities mitigated in their effect or character by their occurence at higher output levels in a heavily biased amp?

Stuart

Stu, thanks for pointing this out. Good point about the 2 ohm load. I agree with John that XO distortion will tend to be less audible if the transition is at high levels.

Bob
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Old 17th January 2007, 08:15 AM   #614
AndrewT is offline AndrewT  Scotland
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Quote:
I agree with John that XO distortion will tend to be less audible if the transition is at high levels
if this high bias makes the distortion less audible , but measure worse, when looking at the KSA50 specifically,
then can the corollary apply to any high bias BJT amplifier?
i.e. move the bias up from optimum to high bias and the distortion increases but becomes less audible.
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Old 17th January 2007, 09:44 PM   #615
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Quote:
Originally posted by AndrewT
if this high bias makes the distortion less audible , but measure worse, when looking at the KSA50 specifically,
then can the corollary apply to any high bias BJT amplifier?
i.e. move the bias up from optimum to high bias and the distortion increases but becomes less audible.

This is hard to answer in the general case, but do keep in mind that the percentage of time that the amplifier is spending at higher power levels is quite small. This is because of the high crest factor of music. So if that is the only time it distorts significantly, it might be perceived as not so bad. Taken to an extreme, realize that it is likely that tube amplifiers, with their limited power, are probably clipping a lot more than solid state amplifiers (but of course they often are clipping in a softer way, and that compressive type of distortion is different than crossover distortion). I apologize for having to be so speculative here; there may be others who have a better answer to this question.

If I had to err on one side or the other of bias, I would always err on the side of an amplifier that was over-biased than under-biased, as long as no thermal stability or reliability problems arose.

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Old 18th January 2007, 08:15 AM   #616
AndrewT is offline AndrewT  Scotland
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I would always err on the side of an amplifier that was over-biased than under-biased, as long as no thermal stability or reliability problems arose.
absolutely.
I just rebiased my Leach from 20mV to 25mV on the basis of this thread. But it seems to hold bias value quite closely IF the mains voltage does not vary too much.
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Old 18th January 2007, 10:01 AM   #617
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Quote:
Originally posted by Bob Cordell



Thanks, Glen, this seems to make sense. Sounds like you are building a version of the amplifier Pioneer did a long time ago in the 70's and was written up in the JAES. It was a really neat concept. The REAL amplifier was a low-voltage Class-A amplifier whose rails were boostrapped by a big Class-AB amplifier driving the "ground" of the floating LV power supply of the Class-A amp. I've always been tempted to build one. So, if I understood, your op amp supplies are bootstrapped in an essentially "feedforward" way by the output of the Class AB amp. Moreover, I guess you could just be using the LV rails that drive the Class A amp for this. This approach means no feedback through the bootstrap and alleviates that stability concern. Is that about right? Cool.

This is not an inexpensive amplifier, right? Each of the Class A amplifiers needs an independent floating LV supply, right? If you are going fully balanced bridging, that means two of these extra floating supplies per channel, right?

Bob

Never seen that JAES paper, but it probably pretty much the same thing. You are corrrect about the opamp rails, although I have now changed my mind and will derive the opamp rails from the input signal. That will avoid the ripple on the ground driven power supplies.
No, this isn't a cheap design. It's an esoteric amplifier for my loungeroom. Rated at 512W rms per channel into 4ohms in class A. 350W total idle dissipation in each channel, 8A bias for each of the four class A stages.
Each channel heatsink measures 1500X240X90mm. The final unit will also have built in oscilloscopes as signal monitors to give it that 50's RF transmitter look
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Old 18th January 2007, 11:03 AM   #618
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Quote:
Originally posted by G.Kleinschmidt



Never seen that JAES paper, but it probably pretty much the same thing. You are corrrect about the opamp rails, although I have now changed my mind and will derive the opamp rails from the input signal. That will avoid the ripple on the ground driven power supplies.
No, this isn't a cheap design. It's an esoteric amplifier for my loungeroom. Rated at 512W rms per channel into 4ohms in class A. 350W total idle dissipation in each channel, 8A bias for each of the four class A stages.
Each channel heatsink measures 1500X240X90mm. The final unit will also have built in oscilloscopes as signal monitors to give it that 50's RF transmitter look

Wow!! That should keep you warm on those cold winter nights! Keep us posted on your progress.

Cheers,
Bob
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Old 18th January 2007, 12:26 PM   #619
AndrewT is offline AndrewT  Scotland
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Quote:
350W total idle dissipation in each channel, 8A bias for each of the four class A stages.
I may have asked this before in another thread but please forgive me if I repeat myself.
How does 350W dissipation tally with 8A bias in 4 stages?
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Old 18th January 2007, 12:57 PM   #620
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Quote:
Originally posted by AndrewT
I may have asked this before in another thread but please forgive me if I repeat myself.
How does 350W dissipation tally with 8A bias in 4 stages?

The Class A stages are low voltage, each with a dedicated +/-10V power supply which is ground driven by a Class AB stage to follow the output signal.
+/-10V @ 8A = 160W. The amplifier is a bridged design so there are two class A and two class AB stages in each channel. There is a total of 320W idle dissipation in the class A stages of one channel plus another 30W idle dissipation in the class AB stages.
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