Aleph5 "degrade" down to Aleph3/30?

gogo · 27th February 2003, 06:29 AM

Hello all!

I found a heatsinks for my Aleph5 (see my previous post), but after calculations, these heatsinks will be not suitable for Aleph5 (300W total disipation), but are enough for Aleph3/30 (200W of heat). But because I have ready-to-use Aleph5 boards (thank you again Kristijan), I will use these boards, lowering the supply voltage from +/-34V to +/-25V. But after that, I must recalculate some values for the resistors (this is my opinion). And because I am not so good with these calculations, I just think to use values from Aleph30 (Aleph3 has some different configuration). So (according to original PASS schematic for Aleph5) here are the swap: R13 (from 10k to 4.75k), R5 and R4(from 10k to 68.1k), R17 (from 4.75k to 1.5k), R19 (from 221k to 47.5k), C7 (must be connected B-C - from 0.047uF to 0.001uF), R21 (from 453 to 820Ohm), C6 (removed), R16 (from 75 to 392Ohm), R40-42, R64-66 (from 0.1 to 0.47Ohm). A new values are taken directly form originnal PASS Aleph30 schematic (as I say, I am not good in calculations). So here is the question (maybe Mr. Pass can give the best answer): are these swaps enough (or are too much) for that lowering the voltage? I will use 6 output transistors.
Thank you inn advance!
Regards,
Gogo

MikeW · 27th February 2003, 03:49 PM

I did pretty much the same changes and the amp sounds great. I left C6 at 1000pf. and C7 at 1000pf where it was. You can always increase R19 to get more bias.

Nelson Pass · 27th February 2003, 03:57 PM

I don't have the schematics in front of me, but in general the only changes you might consider when altering the supply rails down are probably the bias feed resistors in two spots:

1) the value of the resistor feeding the zener diode on the positive current source to the diff pair can be lowered proportionately to the supply voltage (not a big difference). This resistor attaches to the Zener at one end and ground on the other.

2) the resistor going from the + supply and feeding the drive for the positive output current source can also be reduced in the same way. I think the default is 4.7K, but you can take it down to 3.3K without any complaints.

In both cases, we are simply compensating so that the drive circuits get the same amount of bias current, just a few mA without any close tolerance.

27th February 2003, 06:29 AM	#1
gogo diyAudio Member Join Date: Jan 2003 Location: Bulgaria, Sofia	Aleph5 "degrade" down to Aleph3/30? Hello all! I found a heatsinks for my Aleph5 (see my previous post), but after calculations, these heatsinks will be not suitable for Aleph5 (300W total disipation), but are enough for Aleph3/30 (200W of heat). But because I have ready-to-use Aleph5 boards (thank you again Kristijan), I will use these boards, lowering the supply voltage from +/-34V to +/-25V. But after that, I must recalculate some values for the resistors (this is my opinion). And because I am not so good with these calculations, I just think to use values from Aleph30 (Aleph3 has some different configuration). So (according to original PASS schematic for Aleph5) here are the swap: R13 (from 10k to 4.75k), R5 and R4(from 10k to 68.1k), R17 (from 4.75k to 1.5k), R19 (from 221k to 47.5k), C7 (must be connected B-C - from 0.047uF to 0.001uF), R21 (from 453 to 820Ohm), C6 (removed), R16 (from 75 to 392Ohm), R40-42, R64-66 (from 0.1 to 0.47Ohm). A new values are taken directly form originnal PASS Aleph30 schematic (as I say, I am not good in calculations). So here is the question (maybe Mr. Pass can give the best answer): are these swaps enough (or are too much) for that lowering the voltage? I will use 6 output transistors. Thank you inn advance! Regards, Gogo

27th February 2003, 03:49 PM	#2
MikeW diyAudio Member Join Date: Sep 2002 Location: North of Boston	I did pretty much the same changes and the amp sounds great. I left C6 at 1000pf. and C7 at 1000pf where it was. You can always increase R19 to get more bias. __________________ MikeW

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27th February 2003, 03:57 PM	#3
Nelson Pass The one and only Join Date: Mar 2001	I don't have the schematics in front of me, but in general the only changes you might consider when altering the supply rails down are probably the bias feed resistors in two spots: 1) the value of the resistor feeding the zener diode on the positive current source to the diff pair can be lowered proportionately to the supply voltage (not a big difference). This resistor attaches to the Zener at one end and ground on the other. 2) the resistor going from the + supply and feeding the drive for the positive output current source can also be reduced in the same way. I think the default is 4.7K, but you can take it down to 3.3K without any complaints. In both cases, we are simply compensating so that the drive circuits get the same amount of bias current, just a few mA without any close tolerance.

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