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Old 29th November 2006, 11:18 AM   #11
h_a is offline h_a  Europe
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Hey thanks a lot to all for your replies!

First, I'm not sure what voltage I have after the diodes. It's stupid, but I don't have access to a multimeter at the moment.

Actually it's a 12V AC wall wart (12V at 200mA that is) that has higher idling voltage at low current (about 30mA (max) I guess) - that's the reason that it is possible to use +-18V regulators (that need +1.7V themselves plus 0.7V for the diode yields at least 20.7V idle voltage).

pcb is a project phonobox (see for a discussion here Schematic for Pro-ject phono box).

@peranders: well since the phonobox uses cheap components I replaced 90% of its parts with better ones (better in terms of closer tolerances and better specs) and of course I wanted to improve filtering (from original 470muF to 2200uF). The PSU powers 6 opa134 in total.

@AndrewT: well I thought about dropouts, but this hum is already present without any signal, so the current is lowest and therefore idle voltage highest. It should get far worse with music playing but it isn't!

@lineup: phono+psu schematic is right here:
http://www.angelfire.com/az3/dimitri...s/phonobox.pdf

However I found a far better solution (IMHO), if I'm lucky I can try it today in the evening.

I'm not sure that my improved solution has not this problem, but I will report.

With kind regards, Hannes
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Old 29th November 2006, 11:35 AM   #12
AndrewT is online now AndrewT  Scotland
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how can anyone make any progress with this hobby/addiction without some tools?
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regards Andrew T.
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Old 29th November 2006, 12:29 PM   #13
h_a is offline h_a  Europe
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Tools locked. Guy with key away. Hannes
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