How can i use a noisy PSU ?

Garbz · 2nd January 2004, 03:18 AM

Hi all.

I recently built an OpAmp based headphone amplifier using OPA2134 chips from BurrBrown. The amplifier itself works perfectly and sounds pretty good considering the price and the cheap parts.

The problem is the amp is powered by a computer PSU and while it sounds great from a 9v battery, it sounds rather noisy when running in the computer. If i turn my fans on the amp sounds like a lawnmower!

Basically the amp doesn't draw power through a rail splitter because of the nature of a computer's sound design, and there's nothing fancy at all.

Currently the +12v rail of the PSU is connected 200ohm resistor (the original attempt to reduce noise). From there +12v and -12v connect in parrallel to a 1000uf capacitor, and then are connected straight to the power pins on the opamp.

The circuit and amp itself generate no noise at all, it all comes in on the +/-12v rails of the power supply.
How can i reduce noise in this situation? Currently the amp is utterly useless!

dhaen · 2nd January 2004, 11:04 AM

Much of the noise will be common-mode.
IMO Fixing it will be more trouble that building a new (linear) power supply.

crown300 · 2nd January 2004, 12:16 PM

Is the noise high frequency or 50/60 Hz ?

My 2cents is buy a wall wort PS and run it off that.

Garbz · 2nd January 2004, 01:57 PM

i was afraid something like that would have happened.

Like i said it sounds like a quiet lawnmower. I didn't think it would be an easy fix. It's just transformers cost the best part of $30AUD here.

tpenguin · 2nd January 2004, 04:01 PM

I'm certainly no expert, but I'll give you my thoughts anyway. When your computer is running, the fans are going to generate some fairly large, oscillating magnetic fields. Any inductive part prior to an opamp (or even inside one) is going to pick up a tiny bit of AC from these oscilations, and this includes the PSU's wiring. I would suggest putting the amp in a separate enclosure and powering it from a small Wall-mountable power supply with a couple of extra filtering caps to smooth out the ripple. This should make it sound better. If you have to have it inside your computer, I'd still recommend you use an external wall-mount PSU, and just run shielded power cables into your case. If that doesn't clear up the noise, try a shielded sub-enclosure inside your computer. I don't know if that will help or not, but that's what I would try.

Garbz · 3rd January 2004, 12:15 AM

thanks everyone but this is getting too hard.

The amp itself doesn't pick up the noise from the fans when i'm using a 9v battery. So i'm just gonna put the amp unshielded into the side of the case and simply run an additional powersupply into the computer.

Personally i'm not gonna worry about the ripple either since the sound is comming from a computer and is therefore quite substandard anyway!

Thanks again.

2nd January 2004, 03:18 AM	#1
Garbz diyAudio Member Join Date: Dec 2003 Location: Brisbane	How can i use a noisy PSU ? Hi all. I recently built an OpAmp based headphone amplifier using OPA2134 chips from BurrBrown. The amplifier itself works perfectly and sounds pretty good considering the price and the cheap parts. The problem is the amp is powered by a computer PSU and while it sounds great from a 9v battery, it sounds rather noisy when running in the computer. If i turn my fans on the amp sounds like a lawnmower! Basically the amp doesn't draw power through a rail splitter because of the nature of a computer's sound design, and there's nothing fancy at all. Currently the +12v rail of the PSU is connected 200ohm resistor (the original attempt to reduce noise). From there +12v and -12v connect in parrallel to a 1000uf capacitor, and then are connected straight to the power pins on the opamp. The circuit and amp itself generate no noise at all, it all comes in on the +/-12v rails of the power supply. How can i reduce noise in this situation? Currently the amp is utterly useless!

2nd January 2004, 11:04 AM	#2
dhaen diyAudio Moderator Emeritus Join Date: Jun 2002 Location: U.K.	One setp back, then two forward.. Much of the noise will be common-mode. IMO Fixing it will be more trouble that building a new (linear) power supply.

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2nd January 2004, 12:16 PM	#3
crown300 diyAudio Member Join Date: Oct 2002 Location: pittsboro, NC	Is the noise high frequency or 50/60 Hz ? My 2cents is buy a wall wort PS and run it off that.

2nd January 2004, 01:57 PM	#4
Garbz diyAudio Member Join Date: Dec 2003 Location: Brisbane	i was afraid something like that would have happened. Like i said it sounds like a quiet lawnmower. I didn't think it would be an easy fix. It's just transformers cost the best part of $30AUD here.

2nd January 2004, 04:01 PM	#5
tpenguin diyAudio Member Join Date: Dec 2003 Location: Atlanta, GA, US	I'm certainly no expert, but I'll give you my thoughts anyway. When your computer is running, the fans are going to generate some fairly large, oscillating magnetic fields. Any inductive part prior to an opamp (or even inside one) is going to pick up a tiny bit of AC from these oscilations, and this includes the PSU's wiring. I would suggest putting the amp in a separate enclosure and powering it from a small Wall-mountable power supply with a couple of extra filtering caps to smooth out the ripple. This should make it sound better. If you have to have it inside your computer, I'd still recommend you use an external wall-mount PSU, and just run shielded power cables into your case. If that doesn't clear up the noise, try a shielded sub-enclosure inside your computer. I don't know if that will help or not, but that's what I would try.

3rd January 2004, 12:15 AM	#6
Garbz diyAudio Member Join Date: Dec 2003 Location: Brisbane	thanks everyone but this is getting too hard. The amp itself doesn't pick up the noise from the fans when i'm using a 9v battery. So i'm just gonna put the amp unshielded into the side of the case and simply run an additional powersupply into the computer. Personally i'm not gonna worry about the ripple either since the sound is comming from a computer and is therefore quite substandard anyway! Thanks again.

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