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RKC 21st December 2002 04:41 PM

New aleph 5 up and running, have a few ??'s about my measurements.
Hello all, I've read/learned quite a lot over the last year on this site while building up my aleph 5 clone. I finally got a chance to turn on one channel yesterday and except for a small wiring oversight everything worked wonderfully. My DC offset was 13mv, and powersupply hum is about 75 mv with only 1/2 of my capacitor bank hooked up. The sound was fanatastic, as good as can be expected with one channel anyway. I made some measurements at all of the voltage test points in the aleph 5 schematics and most of them are fine. But a few are off a and I was going to see what you folks thoughts on these measurements are. I think most of my differences are due to running a higher rail voltage +/- 35.6V. The main differences I have from the sch voltages are the bias I'm seeing about .66v across the source resistors. I've read some other posts on this and I think I know what I need to do to correct this. But I'm seeing 5.2 V across r11 to Q3 and I'm seeing 3.55V on the source's for Q1/Q2 which according to the sch. should be 4V. Any thoughts on this, should I change the R11 value to change the voltage drop? Any input would be appreciated. Thx...

Jmertz 21st December 2002 05:36 PM

Your voltages all look fine. The only thing you should do is change the value of R19. Making it smaller will decrease the bias across the source resistors. You could solder a large (>500K) resistor in parallel with the 221K.
Or just replace it with something smaller around 200K is a good start. Keep decreasing it until you get the bias you want. Changing R11 will do nothing to change the voltage across it, you would only change the tail current which biases the differential input pair.
Hope this helps

Good Luck

Mad_K 22nd December 2002 11:32 AM

-Don't change anything!

Your voltages are right on spec.! :)

The voltage points in the scematic are not that exact. For example, there SOULD BE 0,66V across the source resistors, and not 0,5V as stated. (Total bias now: 0,66*3=1,98A = Perfect !)

RKC 23rd December 2002 04:36 AM

Thx for the replies, After sitting down and figuring it up. It looks like I'm running about 4amps total across all 6 of the IRF240's for the one channel. Which figuring in my rail voltage puts me at about 140w of idle power. Which is pretty close to what the book and others have said is the normal power draw for one channel(150w).

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