Proper Current Source Adjustment

[highbias]

As it turns out I've apparently set the current gain improperly on my 2's although they still do sound excellent. I've searched high and low through these threads for anything on this adjustment but keep comming up with all sorts of other threads except current gain adjustment.

Would someone out there please clear this one up for me and either point me to the correct thread or take a min. and post the formula and or proceedure. My time here has been very limited lately but I want to be sure that my 2's are properly set.

Thanks in advance!!

Mark

[rtirion]

Attached an old reply from Mr. Pass in one of the many threads
on this issue.

Enjoy

[Nelson Pass]

I'm not sure that the quote addresses the question. I
interpret it as wanting to know how to set the current
gain of the Aleph current source, not the DC bias.

For the former, you generally want to set the gain so that
the AC variation of the current source is about 1/2 the output
current. It can be more or less, but 50% gives the optimal
energy efficiency figure.

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.

[highbias]

Thanks Nelson for making that very easy to understand. I really appreciate that. When I get back home from this road trip I will measure it and see exactly how far off I am. Judging by how good they sound I can't be too far off the 50% mark. They continue to astound vistors that come over to hear them.

Mark

[Stones]

Originally posted by 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.

It seams that setting "R0" to get 25% of the AC value compared to not having "R0" gives 50% share.

Could someone please tell me if I am correct or not before my head explodes?

[wuffwaff]

Hi,

not correct. If the current through the lower source resistors halfes you´ve got 50%. Half of the current is then delivered by the current source. So 500mV without R0 = 0%, then 250mV with R0 is 50%.

william

[Stones]

Thanks wuffwaff,

With the standard A5 supply voltage +-34vdc, bias at 1.9a, 7.5v into 8ohm load, 50hz I got 99.3mv ac with "R0" (R21 in the A5) removed. I found that putting a value of 453r (as in the original A5) reduced the figure down to 24.9mv ac, and fitting a value of 430r took it down to around 23mv ac.
Is the standard A5 set to 75%? or is there something else i missed?

[wuffwaff]

Hi,

the standard A5 has to be set to 60-70% to meet the specs (90watts 4 ohms). Although this depends a bit on the bias.

I measured ac-current-gain at a few resistor values for my Aleph5 with BC550C:

390R 66%
430R 61%
470R 55%
510R 52%
560R 47%

I tried this at different bias values (from 1,9 to 2,4A) and loads (4 and 8 Ohms) and frequencies (1k / 10k) and the gain faktor stayed more or less the same. I somehow always thought that ac-current-gain would change with different bias values but this doesn´t seem to be the case......
Between channels there was a difference of around 1%

William

[Netlist]

If you have a regulated sine wave generator, you can it so the AC voltage over the source resistor is say 100mV which would then represent 100%.
Putting in R0 shows immediately what percentage you get when looking at the voltmeter. If the voltage drops to say 62mV you got 62%. Lowering the resistor value will lower the percentage, as seen in Williams post.

/Hugo

[Stabist]

Originally posted by wuffwaff
Hi,

the standard A5 has to be set to 60-70% to meet the specs (90watts 4 ohms). Although this depends a bit on the bias.

I measured ac-current-gain at a few resistor values for my Aleph5 with BC550C:

390R 66%
430R 61%
470R 55%
510R 52%
560R 47%

Hmmm - what are values of your source resistors and output sense resistors??

Because if calculating AC c.g. based on original numbers in A5 service manual - with combination of 0R47 at the output, 1R as source resistors and 1k for R20 and 453R for R21 - the ratio is 78%!!!