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Old 8th August 2005, 05:02 PM   #691
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ctong,

Your understanding is correct IF the AC current gain is set at 50%. the Aleph 30 must have more than 50% AC current gain.
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Old 8th August 2005, 05:26 PM   #692
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Bob,

I still do not understand. Can you be more specific? Thanks.
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Old 8th August 2005, 05:57 PM   #693
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Take some time to review the Aleph current source patent.

As used here, 50% AC current gain means that at maximum output half the current comes from the Aleph Curretn source's AC current gain, half is the static bias. 67% gain would mean that at max output, 67% of the current comes from the AC gain, so a bias of 1 amp could produce an ouput current of 3 amps (1 amp bias, 2 from AC gain)

As shown in the Aleph 30 service manual, the Ratios of R114 and R115 and the parallel combinations of R120,121 and R124-7 set the current gain. Since the output resistors add up to half the value of the emitter resistors in parallel, the AC gain would be 50% if R114=R115. Reducing R114 increases the AC gain. Since the Aleph 30 has R114 at 750R and R115 at 1K, the gain is greater than 50%

Hope this helps
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Old 8th August 2005, 06:02 PM   #694
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Here is my explincation. It is probably releted to the nonlinearity of the transconductance curve: upswings have a larger ac current than down swings for both the output transisters and the current source, therefore for both positive and negative signals the max output current can be somewhat larger than twice the bias current. This, however, might result in high levels of distortion.
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Old 8th August 2005, 06:25 PM   #695
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Bob,

Thanks for the information. I now understand the ac gain issue. But it seems that what you said is true before Q106-7 shut off during a down swing. After that point the output current cannot increase unless the current through Q108-9 keeps increasing.
The nonlinear transconductance curve provides a way for that to happen.
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Old 8th August 2005, 07:04 PM   #696
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Ctong,

when setting the amp up for more than 50% ac-current-gain the upper half (current source) will shutt off during negative current peaks causing the amp to get in some sort of AB mode. I donīt think this has anything to do with the nonlinear transconductance curve. You can set the ac-current gain for 4 times the bias or more if you want although it wonīt sound very nice. An ac-current-gain of around 60-65% is acceptable though and always better than a clipping amp.

Like Bob said, try to read the patent or the articles on the Zen variations (here the active current source is explained in part 3 or 4 I think)

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Old 8th August 2005, 07:45 PM   #697
ctong is offline ctong  United States
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William,

I see what you mean. Thanks.
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Old 9th August 2005, 10:57 PM   #698
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Quote:
Originally posted by wuffwaff
when setting the amp up for more than 50% ac-current-gain the upper half (current source) will shutt off during negative current peaks causing the amp to get in some sort of AB mode.
It's worth noting that if you bias the amplifier at the expected
peak output current, you won't get this effect, and you can set
the current gain arbitrarily.

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Old 10th August 2005, 03:27 AM   #699
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Let me ask a dump question.
How to set the current gain?
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Old 10th August 2005, 09:28 AM   #700
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Hi,

itīs not a dumb question but often asked. Just search for "ac-current-gain" and "setting".

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