Gurus: Need Help With Opamp Hum/Buzz Problem

[Paul Marossy]

I hope someone here can help me with this very baffling problem I've never run across before.  

I'm messing around with a circuit that uses a cascaded dual opamp, in an inverted configuration. The positive inputs are connected to Vref (1/2 of power supply, 9V). First stage has a gain of 31 (15K input resistor/470K in loop) and the second stage has a maximum gain of 67 (15K input resistor/1M loop). There is a 50K interstage volume control which I am calling a "Master Volume". Circuit input cap is 0.022uF. Got the 1M resistor to ground on the input too. 50K volume pot on the output.

I measured voltages this morning and found that I had 4.5V on Pins 1, 2 & 3. Does that sound right? Pins 5 & 6 also measured 4.5V. IIRC, Pin 7 had 0V.

So my problem is that when I have the gain up at 100%, there is an objectionable buzzing/hum sound (sounds like it's both to my ears) that I can not get rid of to save my life. It is VERY apparent with single coil pickups. It's still there with humbuckers and is still way too apparent to be acceptable to me. I have tried all sorts of things... pushing around wires, shortening them, shielded input cable, looked for ground loops, checked for continuity between all grounds, all to no effect. I've tried various opamps, OP275, JRC4558, JRC4580, LT013, still the same problem. I've messed with the values of the Vref resistors, tried everything from 10K to 100K, no effect. I've got a 100uF cap on the 9V power supply and a 22uF on the Vref supply. Different guitars, still the same problem. I am completely mystified as this circuit isn't anything that different from things I have built before, but yet I have this problem which I have never encountered in the many things I have built that were built much more sloppily in terms of lead dress, etc. A circuit that is very similar to the one I'm messing around with is very quiet, which makes this even more baffling to me.

All that to ask, does anyone have any ideas what the problem may be? Is it a problem with my circuit? I don't see how when I compare it to things I've done in the past...  

[Kesh]

that's 66db of gain, you might expect hiss and buzz from a guitar as input.

pin 7 shouldn't have 0V on 9V/0V rails

[Paul Marossy]

that's 66db of gain, you might expect hiss and buzz from a guitar as input.

Yeah, but very similar circuit with exactly the same amount of gain does not have this problem. That's why I don't get it. This is like MAJOR noise in comparison.

pin 7 shouldn't have 0V on 9V/0V rails

That's what I thought. I'll have to measure it again when I get home from work. Probably was just that the DMM wasn't making a good contact cause of solder flux or something.

[Paul Marossy]

Posting this just for the record in case one day someone does a search and this topic comes up.

The problem can be traced to my own dumb mistake! I inadvertently wired a 1.2K resistor in parallel with the 1M feedback resistor in the second stage. So I guess I had something approaching open loop gain and it was magnifying the heck out of the tiny amount of hum present in the signal from the previous stage - even with humbuckers there can a very slight hum which you can hear if you amplifiy it enough. Anyway, glad I figured it out. It really was driving me crazy for a little while.