Hum in battery-powered preamp -- very frustrating

[David]

I got sent out of town for several weeks, so I haven't been doing any reading, posting or circuit-building.    AHHH!

Anyway, I have this minimalist acoustic guitar preamp built on pad-per-hole and almost ready to go.  I say almost because it's humming so bad the noise practically swamps out the audio signal.  VERY FRUSTRATING!  The circuit's based largely on Orman's "Basic Buffers" article.  I'm using a voltage divider of 2 1M resistors to provide bias.  The input goes to the noninverting input of the op-amp.  Gain is fixed at about 50 using a 100K and a 2.2K resistor in the feedback loop.  The 2.2K then connects to ground through a 100uF electrolytic.  I'm going to have a passive tone control and a gain pot on the output of the circuit.  The tone control's out right now while I debug the circuit.

I'm stumped.

I've already done some detective work.  Here's what I know:

The circuit worked fine on breadboard.  It hummed, but everything I breadboard hums somewhat.  It's "working" now because I hear guitar.  It's just that the hum drowns it out.  This is on a battery, by the way.

The noise is not coming from the pickup, headphone amplifier or shielded patch cable that connected the preamp to the headphone amp.  They all test out fine without the preamp.

The circuit is in a metal box.  The input jack ground is wired to the circuit ground.  The output jack ground is unconnected.  I have continuity between the negative terminal of the battery wire and the input ground.  Also to the ground bus on the circuit board and the ground pin of the op-amp socket.

I subbed a 5534 for the TL071 I had been using.  The hum didn't go away.

I resoldered my input and output connections.

I am not using shielded wire.  I'm beginning to think that maybe I should shield the input because of the high gain factor.  Or, perhaps I should pull out the 2.2K resistor and replace it with a 4.7K or even a 10K.

Anyone got any ideas or advice?

[niftydog]

what are the last few components before the output?

Do you have a series capacitor of the appropriate value to block DC going out the output?

[David]

Output pin (I think it's 6) has the feedback loop connector and also connects to the positive side of a 1uF electrolytic cap.  The cap then feeds the volume and gain control combo.  Before you ask, there's a cap on the input too.  The buffer called for a 0.1.  I found a cap that made my acoustic sound really sparkly and used that instead.  I've had trouble deciphering the logo because the cap is so small.  It could be a 0.22.  It could also be a 0.022uf.

[puretube]

do the in- & outputjackgrounds have contact with each other and the chassis?

[Ge_Whiz]

Yeah, the output jack needs to be earthed.

[David]

Unit is in a metal box.  Jacks are not insulated;  they would be interconnected by virtue of the box, I would think.  Input jack is grounded.  Actually, the output jack WAS grounded, but I removed it because I thought I was getting ground looping.

[niftydog]

The cap then feeds the volume and gain control combo.

what is the volume and gain control combo exactly?

I'm just trying to get at the possibilty of DC on your output jack.

Don't suppose you have access to an oscilloscope?!

[niftydog]

what am I talking about!?  You don't need an o-scope...


to be sure of zero volts in, ground the input.  Your DMM should read zero on the output.  Check it on the most sensitive DC range.

[spongebob]

Sounds much like a grounding problem...

Is the 2K2 resistor (from the feedback loop to ground) located at the right pin? I have a habit of sometimes connecting it to the output pin rather than the inverting input. And the output jack should be connected to ground, don't rely on the metal case to carry the ground connection.

Can you post the voltages measured at each opamp pin?

[David]

Sponge:

I agree that it's a grounding problem, but it's got to be a very sneaky one.  Actually, I did some work on the board last night.  I shielded the lead from the input jack hot to the input blocking capacitor.  The shield is connnected to the ground lead on the input jack and doesn't connect to the board.  Unfortunately, this didn't get rid of the noise.

I started moving wires around.  The connections for the feedback resistors were squirrely, so I fixed those.  Still noise.  I started wiggling wires.  No change in noise until I wiggled my newly-shielded input lead or the input capacitor itself.  Then I got BIG noise!  I've already remelted and retouched these connections more than once.  Is it possible the cap itself is bad?

I'm beginning to think I need to trash this board and start all over.  Does anyone have any ideas that will give it a last-minute stay of execution?

[Paul Perry (Frostwave)]

Have you checked the ground from the far end of the guitar lead to the far end of the lead to the amp?
And, caps can be bad, for sure.

[David]

I tried replacing the input cap last night.  No change.  The offending preamp was executed.  Guess I'll have to start all over.  So much for the comment "I can build one in my sleep".  Apparently I did!

You know, I can see why people say "I quit!" and run screaming from this stuff.  I did -- twice!

So how come I don't feel like running this time?

[gez]

So how come I don't feel like running this time?

Forget heroin and crack, they've got nothing on Stompbox building!  :twisted:

[David]

Blimey, Gez, I guess you're right!  I'm already thinking of my next breadboard "fix"!   :twisted:

[niftydog]

I tried replacing the input cap last night. No change.

Dude, the trial and error method is for the propellorheads, let them aimlessly poke and prod - we know better!  Come at it from one direction and work it systematically.

If I went prodding through a professional TV monitor circuit hoping to find a faulty cap, I'd be there for months working on one monitor... and I'd probably never find the fault.

If I were doing this, I'd first try to work out what is the hum.  Three things leap to mind;

1 - DC on the output

2 - unwanted audio oscillations

3 - external noise not being shielded out

DC on the output is a quick one to check.  Short the input, and you should be expecting zero volts DC on the output.  If you have a sine wave generator, plug it in.  You should still have zero VDC on the output.

Since "hum" is normally 50Hz hum from the mains supply, we can almost discount this thanks to the battery factor.  But it's still possible that something is oscillating at low frequencies.  It's also possible that it's leaking in.  Double check your shielding, move to another part of the house etc.

If it's DC, look at the blocking caps, consider changing to tantalums, try different values.

If it's oscillating, you'll need to look at the design and maybe adjust some resistors.