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 8th August 2005, 05:02 PM #691 BobEllis   diyAudio Member     Join Date: Sep 2003 Location: Upstate NY 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.
 8th August 2005, 05:26 PM #692 ctong   diyAudio Member   Join Date: Jun 2005 Location: Clemson, South Carolina Bob, I still do not understand. Can you be more specific? Thanks. __________________ CT
 8th August 2005, 05:57 PM #693 BobEllis   diyAudio Member     Join Date: Sep 2003 Location: Upstate NY 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
 8th August 2005, 06:02 PM #694 ctong   diyAudio Member   Join Date: Jun 2005 Location: Clemson, South Carolina 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. __________________ CT
 8th August 2005, 06:25 PM #695 ctong   diyAudio Member   Join Date: Jun 2005 Location: Clemson, South Carolina 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. __________________ CT
 8th August 2005, 07:04 PM #696 wuffwaff   diyAudio Member     Join Date: Aug 2001 Location: Ingolstadt Germany 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) William __________________ een ooievaar is geen konijn want zijn oren zijn te klein!
 8th August 2005, 07:45 PM #697 ctong   diyAudio Member   Join Date: Jun 2005 Location: Clemson, South Carolina William, I see what you mean. Thanks. __________________ CT
Nelson Pass
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Join Date: Mar 2001
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.

 10th August 2005, 03:27 AM #699 santi   diyAudio Member   Join Date: Nov 2004 Location: Bangkok Let me ask a dump question. How to set the current gain?
 10th August 2005, 09:28 AM #700 wuffwaff   diyAudio Member     Join Date: Aug 2001 Location: Ingolstadt Germany Hi, itīs not a dumb question but often asked. Just search for "ac-current-gain" and "setting". William __________________ een ooievaar is geen konijn want zijn oren zijn te klein!

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