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Old 9th April 2006, 10:44 PM   #1
Bobken is offline Bobken  United Kingdom
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Join Date: Dec 2002
Location: UK
Default Aleph 30 queries.


I'm constructing an Aleph 30 using Peter Daniel's excellent boards, but since I got these PCBs Nelson was forced to remove the full A30 schematic from his site because of IP problems, and I didn't save it previously, unfortunately.
When I had the boards last year I made a list of components I needed to obtain, and I do have Kristian's schematic of the Aleph 3 which is similar in layout, but I am now a bit in the dark as there were some component changes made between them.

When using single-ended inputs, R3 (10k) then being grounded would be in parallel with R6 (68k1) to give 8k72 in combination, which is also the 'lower limb' in the feedback path to ground. With a main feedback resistor of 100k (R7), if my calculations are correct, this gives a total system gain of approx 12.5 X (or 22dB).
Except for the now missing schematic, the A30 manual is still on line and this clearly states a single-ended gain of 26dB, or approx. 20X, which is significantly different from my calculation.
Can someone please explain this apparent difference, or did I write one of these resistor values down wrongly, perhaps?

Also, looking at the PCB, R4 (100k) appears to be in parallel with R5 (68k1) which gives a combined resistance of 40k5 here to ground, and this, together with R2 (10k) seems to be mainly for establishing the amp's input impedance. Is there any known reason why Nelson didn't use a single 40k2 resistor here (would still be within 1%), or was it simply the availability of existing stock resistor values, maybe, which gave rise to this choice?

I have searched the Forum thoroughly, but cannot find any comments which cover either of these two particular points. Any help or clarification here will be much appreciated.

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Old 10th April 2006, 09:09 PM   #2
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As a practical matter, I always set balanced input networks up
with identical resistors, which insures the match and keeps
the calculation simple. Usually I get .1% matching for about
-60 dB CMRR.
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Old 10th April 2006, 10:08 PM   #3
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Location: Netherlands
If you send me your email address, I could send some more info about the A30.

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Old 11th April 2006, 05:14 PM   #4
Bobken is offline Bobken  United Kingdom
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Join Date: Dec 2002
Location: UK
Thank you very much for these responses.

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