Upgraded opamp in preamp and now there is a hum/buzz

[theAnonymous1]

Originally Posted by sardonx

To explain it in my electronically uneducated manner, when i hooked up my volt meter to pins 4 and 8 the readout was 29.5 volts. It's a dual opamp, so it only has one rail. I hope i'm explaining this correctly and not miscommunicating.

Communication seems fine; definitely explaining it wrong.

The circuit has two rails (+/- supplies), you're just measuring incorrectly. You need to measure from GROUND to each supply pin, not from the negative supply to the positive supply.

Anyway, I guess the newer Bursons don't have the ground wire. I wasn't just making that up though....

Burson Audio opamp modules and buffer review - [English]
help:ground pin for opamp?

[rabbitz]

All the older Bursons I've used have a ground wire. I don't know about the new ones.

[sardonx]

Ok, thanks for telling me the proper way to measure the voltage. I measured from ground to the rails and it comes up as 13.4 volts. So I should be good there. What I have done though is shorten the wires which i used to connect the burson. It has made things better. Now once the volume gets past the fourth click the noise goes away 100%. Dead quiet. I just need to improve it even further so there is no noise at any level. Why would it matter how long the wires are? I mean we're talking inches here..

[Mooly]

Someone mention wires... wires to opamps are not good. Inches... even a few millimeters or less can cause issues.

I would ditch these Burson things (not for the first time problems have been reported with these on here) and try a different quality opamp.

As we don't know what is fitted I would suggest something sensible like an OPA2134 and see how you get on.

[godfrey]

Originally Posted by Mooly

As we don't know what is fitted I would suggest something sensible like an OPA2134 and see how you get on.

Assuming the unmarked IC he pulled out was actually a dual op-amp to start with. According to the Blue Circle website:

The BC101 is a dual mono solid state linestage...

If that was actually true, then they wouldn't be using a dual opamp for stereo. They also claim "minimal circuit design", so I wouldn't expect more than one opamp per channel unless it caters for balanced inputs or outputs.

[Mooly]

Originally Posted by godfrey

Assuming the unmarked IC he pulled out was actually a dual op-amp to start with. According to the Blue Circle website:

I was going off the diagram in post #10 and the fact it does still work with the Burson. The supplies seem to be where they should be too on pin 8 and 4.

[sardonx]

I wish I had a better camera but the pictures from the cell phone are horrific and won't help. I guess since it has two stepped attenuators they figure they can call it a dual mono. It is a minimal circuit. All the case has is a 1 small toroid, 4 diodes, 8 filter caps, 1 opamp, 1 selector switch, and two attenuators with a low/high gain switch on each of them. That's it. I don't see what asides from a dual opamp it could be. If it was anything else where the pinout was different it wouldn't work at all. And it does work perfectly.. past the forth click :/

[rabbitz]

If it works past the 4th click, could be a grounding issue as the lower clicks shunt most of the signal to ground. You could also try a normal pot to check.

[Mooly]

Managed to find a description on Google for it. Bing didn't turn anything up.

It says it's DC coupled from input to output. That limits you to a FET opamp to minimise DC offset.

Not sure where the "the same high speed polypropylene caps as the BC103 and BC3000MKII come into it.

They can't be in the signal path or else it's not DC coupled. PSU maybe ?

It would be really interesting to see pics. Can you tell if there are coupling caps at input and/or output ?

I would still go with the OPA2134 or maybe an OPA2604 is you want to try something more "coloured" (but coloured in a good way... particularly on less than perfect digital sources).

Voltage gain of 13.5db or around "5" in real money.

[sardonx]

Hi Mooly, thanks for looking that up. I'm trying to find whether the burson is a fet opamp or not. It's too bad if i can't make it work cuz it actually sounds real nice. I'll borrow a good camera and take a pic for you to see. Either way, i'm not sure if i can find any opa2604s laying around but i do have an adaptor with two opa627bp's in it..From the good ol days when they sent out free samples!