supply rail voltage on 3875 output ?

Buhl · 5th November 2006, 11:35 AM

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.....

Leolabs · 5th November 2006, 12:08 PM

Single supply or dual supply???

Nuuk · 5th November 2006, 07:49 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!

Buhl · 5th November 2006, 07:50 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 !
Buhl

Nuuk · 5th November 2006, 07:51 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, 08:08 PM

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

Nordic · 5th November 2006, 08:46 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, 10:06 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, 10:39 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 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

CarlosT · 7th November 2006, 04:14 PM

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

5th November 2006, 11:35 AM	#1
Buhl diyAudio Member Join Date: May 2002 Location: Aalborg, Denmark	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..... __________________ "Calculations? We don't need no stinkin calculations! We just make them really big!" Quote: Nelson Pass

5th November 2006, 07:49 PM	#3
Nuuk diyAudio Member Join Date: Feb 2003 Location: Somerset, SW England	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! __________________ The truth need not be veiled, for it veils itself from the eyes of the ignorant.

5th November 2006, 07:50 PM	#4
Buhl diyAudio Member Join Date: May 2002 Location: Aalborg, Denmark	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 __________________ "Calculations? We don't need no stinkin calculations! We just make them really big!" Quote: Nelson Pass

5th November 2006, 07:51 PM	#5
Nuuk diyAudio Member Join Date: Feb 2003 Location: Somerset, SW England	So you are getting the full voltage on the output and I am getting 25 volts from 37 volt rails! Even more mysterious! __________________ The truth need not be veiled, for it veils itself from the eyes of the ignorant.

5th November 2006, 08:08 PM	#6
Buhl diyAudio Member Join Date: May 2002 Location: Aalborg, Denmark	exactly - 25 volts in and 25 volts out - nice equation..... __________________ "Calculations? We don't need no stinkin calculations! We just make them really big!" Quote: Nelson Pass


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5th November 2006, 12:08 PM	#2
Leolabs diyAudio Member Join Date: Dec 2004 Location: Bukit Mertajam	Single supply or dual supply???

5th November 2006, 08:46 PM	#7
Nordic diyAudio Member Join Date: Sep 2005	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...

5th November 2006, 10:06 PM	#8
Shigeko diyAudio Member Join Date: Sep 2005	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...

7th November 2006, 10:39 AM	#9
Buhl diyAudio Member Join Date: May 2002 Location: Aalborg, Denmark	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 __________________ "Calculations? We don't need no stinkin calculations! We just make them really big!" Quote: Nelson Pass

7th November 2006, 04:14 PM	#10
CarlosT diyAudio Member Join Date: Oct 2006	When everyone talk about DC rail voltage, is that measured between the V+ and PGN+ and not V+ and V-?

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