How to set AC current gain in Aleph-X ?

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yoke

yoke

Member
Joined 2004
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
 
yoke

yoke

Member
Joined 2004
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 ???
 
L

ljozsef

Member
Joined 2003
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
 
Nelson Pass

Nelson Pass

The one and only
Joined 2001
Paid Member
Re: Yes, Yoke,

ljozsef said:
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.
Click to expand...

Any wave you like is fine with me.

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

:cool:
 
yoke

yoke

Member
Joined 2004
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…:bawling: :bawling: :bawling:

So, what else I can do ???

:smash: :smash: :smash:
 
yoke

yoke

Member
Joined 2004
I have replace R2,R3 with 0.44 Ohm resistor ( 2 x 0.22 in series )...
now I can set it to 50% by triming R12...
Whell I try to set current gain by method describe by Nellson but that didn't give me result...whell I mesure voltage on R6 ( don't know was this corect , maybe I should mesure on R5 ) I can could lower it from 20mV to 15mV...maybe I understand it wrong...
but when I mesure voltage over R5 and R2,R3 and put it in formula describe on Wiki I get this...

0,0195 / ( 0,44 / 2 ) = 0,0886
0,0111 / ( 0,5 / 2 ) = 0,0444
( 1 – ( 0,0444 / 0,0886 )) = 49,9%

Now, did I did it right ? :confused:

once agian I have 8 mosfets per chanel, source resistors are 0.5 Ohm and I have 2 x R2,R3 per chanel ( R2,R3=0,44 Ohm )

:smash:
 
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