PSU voltage difference?

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matta

Well-known member
Joined
Jan 11, 2005
Messages
1,640
Location
Cape Town, South Africa
Hi Guys,

I'm working on a 15V power supply for another Green Pre. I am done but the voltages I get are +14.9V, 0V, -14.7V and 47.8V (unloaded).

It was an existing PSU that was outputing + and - 10V so I swopped out some of the resistors to up it to 15V, but I am now left with a .2V difference between the negative and positive rails. Should this be a concern?

My gut says yes, but I am still a Newbie and need to know. I assume if it is that I need to swop out a resistor or 2 to try and balance them or could this be a cap related difference?

I've fired it up and recorded a couple tracks and can't hear anything different, not sure if I should, or should I?

Thanks

Mattt
 
[quote author="matta"]I'm working on a 15V power supply for another Green Pre. I am done but the voltages I get are +14.9V, 0V, -14.7V and 47.8V (unloaded).

It was an existing PSU that was outputing + and - 10V so I swopped out some of the resistors to up it to 15V, but I am now left with a .2V difference between the negative and positive rails. Shoudl this be a concern?[/quote]

It sounds like this is an unregulated supply with a dropping resistor for each leg. The difference between + and - is not important and is not likely to have any effect on the circuits connected to it. However, you should check the voltages under load rather than unloaded, and adjust resistors to give something close to 15V on each amplifier supply rail. Better still, modify the circuit by installing regulators. Your amplifiers will thank you for it.

Peace,
Paul
 
Hey Pstamler,

Thanks for the input. It is indeed regulated, an LM317 on the + rail and LM3327 on the negative, plus a LM317 on the Phantom.

My only thoughts are that it could be the resistors and maybe that some of the point values are different causing the difference, but I don't want to take them out and replace if I don't have to. The board is home etched and it looks like it has been used a couple times, I am worried the track might lift if I do any more desoldering and resoldering.

Thanks for the advise on checking it loaded versus unloaded.

My thoughts are if the .2V is harmfull (which it may not be from what you shared) I could mount a trimpot offboard and adjust it to match the V.

Cheers

Matt
 
Sounds like the differences are entirely normal for the range of internal reference voltages and resistor tolerances. Unless your circuit is ultra-sensitive to supply variations I wouldn't be concerned.
 
Bcarso to the rescue ;-)

Thanks man. I get the feeling I'm okay. I have out the pre through it's paces and really can't hear anything 'wrong' and I guess that is the main thing. It is just that my last PSU (which I built from scratch) was dead on, whereas this one differed by .2V. Thanks for all the input and feedback, I'll rest easy now :)

Cheers

Matt
 
Hi Peter,

Those are unloaded voltages, I power up a the pre as mentioned and did some track all seems well to the ear. Will try and check the loaded values and post details.

Thanks

Matt
 

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