How to set AC current gain in Aleph-X ? - diyAudio
 How to set AC current gain in Aleph-X ?
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 22nd August 2004, 10:00 AM #1 diyAudio Member     Join Date: Jan 2004 Location: Split How to set AC current gain in Aleph-X ? Nead help how to set AC current gain in Aleph-X ? I have put together one channel of Aleph-X…but have some question…I try to find answer by searching, but no luck…to many posts… Any way… I have connect PC in input and play some sine wave of 100Hz and this is what I measure R5=0,5 Ohm V=0,0064 R2,R3=0,22 Ohm V=0,008 So this gives me by this formula ; 0.05V / ( 0.22 / 2 ) = 0.45A 0.05V / 0.22 = 0.227A (1 - (0.227 / 0.45)) * 100 = 50% 0,008 / ( 0,22 / 2 ) = 0,0727 0,0064 / ( 0,5 / 2 ) = 0,0256 ( 1 – ( 0,0256 / 0,0727 )) = 64.8% Now, I have solder already R12=1,2 Kohm...But If I replace it with 1,5K pot , will it give me enough room to trim this to 50% ? Or is there some thing else I can do ??? What if I add some other resistors, other value, in parallel with R2,R3, how do you calculate it then ??? Thanks Josip
 22nd August 2004, 08:33 PM #2 The one and only     Join Date: Mar 2001 There's a ton of postings on this issue, many by me. It's the same as an Aleph. Rather than me trying to recapitulate them, try a search.
 23rd August 2004, 07:09 AM #3 diyAudio Member     Join Date: Nov 2003 Location: bolton AC Current gain Hi Yoke spotted your question..............please could you post any usefull links on this thread to help others.................. Cheers
 23rd August 2004, 07:21 AM #4 diyAudio Member     Join Date: Jan 2004 Location: Split Thanks Mr. Pass for the answer, last night I spend some time reading and trying to find something, at the and I get sleepy and give up... So I have tried again today O.K. I have found this but it's different method then describe on Wiki Aleph-X tread... Wrote by Mr. Nelson Pass You will note in the Aleph schematics that the Base of the NPN transistor which controls the current source attaches to the output of the amplifier through an RC network. The value for C is set arbitrarily high so that it does not interfere with audio frequencies, and it is the value of R which is the most convenient spot to adjust the gain of the current source. For the purpose of this discussion, I'll call it R0. If you set the amplifier driving a sine wave into a load (let's say 16 Vrms into 8 ohms at 100 Hz), you can measure the current variation of the gain N channel Mosfets (whose Sources attach through power resistors to the - supply rail) with a cheap AC voltmeter placed across one of these Source resistors. With R0 taken out of the circuit, you will get one AC value across the Source resistor (say 470 mV, for example). As you put a value for R0 in the circuit, this will decline, and when it measures 1/2 the value without R0, you have reached 50%. If it measures 1/4 the value, the current gain of the Aleph source is 75%, and this figure is too high for a standard Aleph. Most listeners like the Alephs at 50% or lower, so I recommend between 50% and 100% of the AC voltage value compared with no R0. So in Aleph-X revision V1 ; R0 would be R12...so I pull out ( disolder ) R12 and measure voltage on R5 ( source resistor )...then place pot in a place of R12 and trim it until I get 50% of the value, I get without pot in place of R12, again on R5 ( lets say voltage on R5 was 0.5V I trim R12 ( R0 ) until I get 0.25V on R5 ) ...replace pot with resistor... Is this correct ??? Will I damage amplifier if I don’t have R12 in place ???
 23rd August 2004, 11:25 AM #5 diyAudio Member   Join Date: Apr 2003 Location: Budapest Yes, Yoke, Basically, for 50% it were correctly described. However, be careful: Nelson's explanation was I think for measuring the R6 voltage. At 50% the values are identical on the R5, too, only for different tuning have care. But another question: Nelson mentions sine waves - would it be with square waves possible, too (scope calibrating signal), supposing that measurements are made with a true-RMS meter? I did it in that manner, and the A-X sounds marvellous (despite of the failing NFB 5-10 pFcapacitors and grounding output resistors). I don't know if there were paras. oscillations, but the absolut offset w/o resistors remains under 0.5 V, hot 0.15 V. Diff. offset 8 mV. May I ignore them? Have success, Yoke! Laci
The one and only

Join Date: Mar 2001
Re: Yes, Yoke,

Quote:
 Originally posted by ljozsef Nelson mentions sine waves - would it be with square waves possible, too (scope calibrating signal), supposing that measurements are made with a true-RMS meter? I don't know if there were paras. oscillations, but the absolut offset w/o resistors remains under 0.5 V, hot 0.15 V. Diff. offset 8 mV.
Any wave you like is fine with me.

Your DC offset figures are just fine, better than average.

 25th August 2004, 11:24 AM #7 diyAudio Member     Join Date: Jan 2004 Location: Split Here I’ am again... First I would like to say thanks to everyone who help me with their answers, not only here but on AX builders tread to... Now…I have replace R12 with 2,5Kohm trim pot. But I was not able to set current gain to 50%... I have disolder R12 and play 100Hz sine wave…voltage on R6 was 20.9mV…I try to trim R12 to get 10.5mV but only could get to something like 16.4mV…Resistance on trim pot was around 2K…If I rise or drop resistance, voltage on R6 starts to rise… So, what else I can do ???
 25th August 2004, 11:36 AM #8 diyAudio Member     Join Date: Nov 2003 Location: bolton Sine Wave Hi Yoke............what...are you using to generate sine wave????
 25th August 2004, 11:42 AM #9 diyAudio Member     Join Date: Jan 2004 Location: Split Now when you mention it...I am wondering am I doing it right ?!?...but I think this is correct... I am using signal generator from WinISD...I play it on PC and connect it to preamp then to amp...
 25th August 2004, 12:19 PM #10 diyAudio Member     Join Date: Jan 2004 Location: Split Can I replace R2,3 which are 0,22Ohm with 0,44 ( 0,22+0,22 )...would that help ???

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