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Old 8th September 2005, 06:17 AM   #21
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I have to agree that transistors with a higher gain sound better in circuit.

Also that Low gain HV transistors aren't that great for amps.

I tried to make push-pull and other amps with HV 400V transistors from computer powersupplies. If I remember correctly, HFE = 15 VCE= 400V Ic = 8A

It was harder to correctly bias these and get rid of distortion.

Using output transistors with a higher beta IMO works better, and sounds better. Having high beta drivers also, I would highly recommend, especially if you do use low beta outputs.
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Old 8th September 2005, 06:45 AM   #22
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Default You can use them in the GEM amplifier, being the extra NPN, the one that provides

class A current all time long.

In that position, the gain will be not problem.... dissipation and current limit will be important...it is producing some crossover correction, conducting always.

I tested and worked fine....of course, not needing gain and linearity will avoid problems.

Having them at hand, why not to try for evaluation purposes?

regards,

Carlos
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Old 8th September 2005, 02:05 PM   #23
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A really good input stage can eliminate the non-linearity problems caused by output transistors.

That's really my point.
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Old 8th September 2005, 03:12 PM   #24
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I suppose you would want to run the input transistors to where the current-gain bandwidth product is the highest possible(on graph) for that device to cancel any non-linearities of outputs? or does it really matter?
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Old 9th September 2005, 02:21 AM   #25
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I think that's a good measure, but importantly, the feedback and current drive of previous stages is useful. (Linearity of the feedback loop itself is essential of course.
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Old 9th September 2005, 03:00 AM   #26
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Are you saying that the driver stage must be able to handle the outputs after they become non-linear and the Hfe drops? The VAS would still have to produce nonlinear current in order to correct, IOW, create harmonics that cancel those produce by the output's nonlinearity. Possibly higer frequency harmonics? (mathmatically speaking)
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Old 9th September 2005, 03:08 AM   #27
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Yes, of course the VAS still has to amplify an inversely distorted signal in order to cancel the non-linearities. The feedback loop of course causes this.

I certainly don't purport to this being the ultimate amplifier design; however, I'm of solid belief that a good amplifier -can- be made with the horizontal output transistors as long as the design of the circuit takes care of this.
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Old 9th September 2005, 03:22 AM   #28
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Now I have heard that to choose a VAS transistor that is very fast is not as good as higher gain because it is more likely to occilate. Only a rumer to me, but I suppose it would require less miller fixing to make it stable. Since the correction signal is generated by the diff by way of neg. feedback, would miller feedback or miller compensation on the VAS transistor cut the amount of correction fed to the outputs(and drivers)?

I personally think that both (Ft & Hfe) are good for VAS, if you can keep it stable without dampening the HF correction harmonics.
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Old 9th September 2005, 04:40 AM   #29
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Well, what the miller stabilization tends to do is limit the high frequency response of the amplifier as a whole. Then again, if there are high harmonics in the output, the miller cap will change how they are corrected for somewhat.

Also, I have had the oscillation problem occur with faster transistors in the VAS, but if you can stabilize them without too much capacitance, then they seem a bit nicer.

There are lots of ways to get around things though.
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Old 9th September 2005, 07:06 AM   #30
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Umm....cant you just add a small transistor (also found in the TV, monitor etc.) and create a darlington ? Gain problem solved.

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