supply rail voltage on 3875 output ? - diyAudio
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Old 5th November 2006, 11:35 AM   #1
Buhl is offline Buhl  Denmark
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Default supply rail voltage on 3875 output ?

Hello
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.....
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Old 5th November 2006, 12:08 PM   #2
Leolabs is offline Leolabs  Malaysia
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Single supply or dual supply???
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Old 5th November 2006, 07:49 PM   #3
Nuuk is offline Nuuk  United Kingdom
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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!
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Old 5th November 2006, 07:50 PM   #4
Buhl is offline Buhl  Denmark
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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 !
Buhl
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Old 5th November 2006, 07:51 PM   #5
Nuuk is offline Nuuk  United Kingdom
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So you are getting the full voltage on the output and I am getting 25 volts from 37 volt rails! Even more mysterious!
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Old 5th November 2006, 08:08 PM   #6
Buhl is offline Buhl  Denmark
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exactly - 25 volts in and 25 volts out - nice equation.....
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Old 5th November 2006, 08:46 PM   #7
Nordic is offline Nordic  South Africa
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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...
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Old 5th November 2006, 10:06 PM   #8
Shigeko is offline Shigeko  Germany
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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...
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Old 7th November 2006, 10:39 AM   #9
Buhl is offline Buhl  Denmark
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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 ground....works 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 !
Hans
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Old 7th November 2006, 04:14 PM   #10
CarlosT is offline CarlosT  United States
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When everyone talk about DC rail voltage, is that measured between the V+ and PGN+ and not V+ and V-?
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