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Old 26th April 2012, 05:04 PM   #1171
AndrewT is offline AndrewT  Scotland
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Since I can't measure output distortion, I rely on the Vre method for optimum ClassAB for output bias setting, unless the designer tells me otherwise.

You have to decide how close the amp output stage is to similar designs where the "professional designers" tell us of their method in comparison to Carlos' DX amp.
You have to decide who to put your faith in.
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Old 27th April 2012, 12:32 AM   #1172
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Hmmmm...

I did some testing increasing the quiescent current (bias) measuring the Vq that crosses the two emitter resistors (to prevent getting skewed by any offset).

When I gently increase the bias current, it gradually goes up to 43mV, then suddenly jumps to 140mV. Then I turn the other way around and it gently descent down to 110mV, then suddenly jumps to 30mV. I have to go back many turns to get back below the 110mV mark.

Looks like past 43mV, something is triggered that makes the quiescent current jump all in a sudden.

That was for one card, for the other, when I reach 35mV it jumps to 165mV, then when I go back it jumps from 80mV to 6mV...

Anybody have a clue for me?

Thanks,
Martin.

BTW, I checked for some "consumer" amps, and for example the Adcom GFA-555, the measured voltage drop across on resistor should be set to 16mV cold, resistor is 0.82r
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Old 27th April 2012, 09:36 AM   #1173
AndrewT is offline AndrewT  Scotland
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The parameters of the output stage are changing.
The drivers and/or the outputs are going into oscillation.
The DX range of amplifiers are not designed in the conventional manner. They were put together with guesswork based on listening tests. Stability is not guaranteed.
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Old 27th April 2012, 11:41 AM   #1174
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Well, compared to the base design of Self's amps, the Hx is not totally different. This abrupt jump of emitter bias voltage is so unexpected that I'm wondering of the problem resides onto the design itself or onto components that reaches a breakdown point.

Il the simulator, I cannot reproduce that behavior, I can increment from a few uV to about 350mV without any jump.

I cannot agree with your comment "put together with guesswork". Dx did apply Self's "best practices" and he fine-tuned some values by ear. Look at the schematics of the Hx and compare it to Self's base amp... I don't speak about the Es amp Dx has put out, it was ... well, not good.

Still if someone else increases its bias voltage, please post results.
Martin.
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Old 27th April 2012, 11:52 AM   #1175
PMA is offline PMA  Europe
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Oscillations, 100%.


Quote:
Originally Posted by canonnica View Post
Hmmmm...

I did some testing increasing the quiescent current (bias) measuring the Vq that crosses the two emitter resistors (to prevent getting skewed by any offset).

When I gently increase the bias current, it gradually goes up to 43mV, then suddenly jumps to 140mV. Then I turn the other way around and it gently descent down to 110mV, then suddenly jumps to 30mV. I have to go back many turns to get back below the 110mV mark.

Looks like past 43mV, something is triggered that makes the quiescent current jump all in a sudden.

That was for one card, for the other, when I reach 35mV it jumps to 165mV, then when I go back it jumps from 80mV to 6mV...

Anybody have a clue for me?

Thanks,
Martin.

BTW, I checked for some "consumer" amps, and for example the Adcom GFA-555, the measured voltage drop across on resistor should be set to 16mV cold, resistor is 0.82r
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Old 27th April 2012, 03:02 PM   #1176
a.wayne is offline a.wayne  United States
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Quote:
Originally Posted by canonnica View Post
Well, compared to the base design of Self's amps, the Hx is not totally different. This abrupt jump of emitter bias voltage is so unexpected that I'm wondering of the problem resides onto the design itself or onto components that reaches a breakdown point.

Il the simulator, I cannot reproduce that behavior, I can increment from a few uV to about 350mV without any jump.

I cannot agree with your comment "put together with guesswork". Dx did apply Self's "best practices" and he fine-tuned some values by ear. Look at the schematics of the Hx and compare it to Self's base amp... I don't speak about the Es amp Dx has put out, it was ... well, not good.

Still if someone else increases its bias voltage, please post results.
Martin.
Did you use matched outputs/drivers .....?
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Old 27th April 2012, 04:10 PM   #1177
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Nope. But this would have given some DC offset, not that bias voltage jump. Since I don't have a scope, I cannot confirm oscillations, but everything points toward that.

M.
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Old 27th April 2012, 04:19 PM   #1178
a.wayne is offline a.wayne  United States
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Yes, but you may have more than one wayward tranny making it easier to push the amp into oscillation . If so you will not be able to bias up the amplifier ..
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Old 27th April 2012, 04:25 PM   #1179
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It is quite usual that oscillations appear at some value of the idle current. This seems to be your case, when they appear the meter reads the step in the emitter voltage.
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Old 27th April 2012, 05:04 PM   #1180
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I guess the amp hasn't been built to have such a high bias voltage, I'm gonna stick to the suggested value until I have a better understanding of it.
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