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Buhl 5th November 2006 12:35 PM

supply rail voltage on 3875 output ?
Having assembled my god knows what number gainclone, I find it to behave strange - that is I have 25 volts on the output og leg no.3 - I have double, triple and quadruple checked everything.
Both channels behave this way.
Any suggestions ?- blown chips ? anything ?

Cheers !
Buhl - who's getting more grey hair than I should.....

Leolabs 5th November 2006 01:08 PM

Single supply or dual supply???

Nuuk 5th November 2006 08:49 PM

Buhl, I have also got quite a few successfully completed GC's under my belt but I started making a paralelled LM3875 and found that I had 25 volts on the output (37 volt rails). I separated both sections and found that they also both have 25 votls on the output.

I have been over this amp I don't know how many times and cannot find anything wrong so I too would welcome any suggestions! :xeye:

Buhl 5th November 2006 08:50 PM

Dual supply, 18-0-18 vac supply, 25 volts rectrified - one rec. bridge / channel - 25volt caps (mabye cutting it close, but they are good so i would like to use them.

Cheers !

Nuuk 5th November 2006 08:51 PM

So you are getting the full voltage on the output and I am getting 25 volts from 37 volt rails! Even more mysterious! ;)

Buhl 5th November 2006 09:08 PM

exactly - 25 volts in and 25 volts out - nice equation.....

Nordic 5th November 2006 09:46 PM

Which at least in theory = a dead short... oh well glad to see I'm not the only one in chipamp hell this weekend. At the rate I'm loosing hair graying is not going to be a big problem...

Shigeko 5th November 2006 11:06 PM

I've heard you can have that problem if you dont have a 47k Resistor to ground behind your dc filter Capacitor at the input. And I had the problem because the negative Input was connected to the wrong side of the 20k negative feedback Resistor...

Buhl 7th November 2006 11:39 AM

Yehaa !
Thanx for helping me turn the magnifying glass in the rigth direction - I had looked only at the amp board again and again, and found no errors, turns out I had forgot to attact signal ground to star ground with a lead from the volume pot, hence no resistor from signal input to like a charm now !

By the way, the Elna Cerafine caps i use in the PSU(1000uF/25V) is a lot better than the "standard" panasonic FC caps to my ears - with the pana caps the highs got a little sharp og edgy, now they are detailed and not straining to listen to - well worth their relativly low price I think.

Thank you!

Cheers !

CarlosT 7th November 2006 05:14 PM

When everyone talk about DC rail voltage, is that measured between the V+ and PGN+ and not V+ and V-?

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