Hissing noise goes away when pad is touched

[almondcity]

I built a circuit which works, but has a pretty large amount of white noise in the background. I've found that touching the circled pad with anything makes it go away.

What could this be a symptom of, or what could be a remedy? I don't really understand how me holding a resistor with tweezers and touching this could make the noise go away but it does

 

[Robert]

Would really need to see a schematic to understand what is happening at that point.

 

[almondcity]

@Robert It's the anode of D14 in this schematic. I've clipped both of these diodes out with no change to the behavior

 

[xefned]

Is it a mic preamp? Or something to run a microphone into stompboxes? I'd be interested in such a circuit.

I have no idea what's causing the hiss, but I wonder if you could solder a dangling resistor to that pad to make the hiss go away permanently.

 

[almondcity]

Is it a mic preamp? Or something to run a microphone into stompboxes? I'd be interested in such a circuit.

I have no idea what's causing the hiss, but I wonder if you could solder a dangling resistor to that pad to make the hiss go away permanently.

It's partly a mic preamp, yes. I'm going to solder a wire to this and mess around with it today because I have no other ideas

 

[JTEX]

Does it do the same thing with the input shorted or with a mic plugged in?

 

[JTEX]

Also, you have a DC-DC switching supply extremely close to the most sensitive audio path in the whole circuit: the gain loop. That really won't help with noise. Try disabling it and using an external +/-15v supply for troubleshooting.

 

[almondcity]

It still makes the noise with no mic plugged in. It seems like if I have phantom power on, touching the pad actually produces more noise.

I also get some noise if I touch the top of the bipolar electrolytic caps which I'm not sure if that's normal behavior.


Like, clearly I have no idea what I'm doing, but I have made a box very similar to this one and it works perfectly so that's why I'm confused. I feel like somehow the hot and cold paths are not balanced or whatever

 

[JTEX]

Any mic pre will make more noise (hiss) when there's nothing plugged into the input. Mic pres need to have a low impedance source (on the order of 200 ohms or less) to achieve their lowest noise. Short the input XLR pins 2 and 3 together. Still have noise? I'd try troubleshooting with a clean, off-board power supply at that point. Your DC-DC converter is way too close to the mic pre chip.

 

[almondcity]

Any mic pre will make more noise (hiss) when there's nothing plugged into the input. Mic pres need to have a low impedance source (on the order of 200 ohms or less) to achieve their lowest noise. Short the input XLR pins 2 and 3 together. Still have noise? I'd try troubleshooting with a clean, off-board power supply at that point. Your DC-DC converter is way too close to the mic pre chip.

It still makes the noise shorting those pins

 

[benny_profane]

Is that network of 100n caps intended to be power decoupling for the opamps/ICs? This may be unrelated, but good design practice would have them placed as close as possible to the ICs' power pins.

 

[darwin999]

I'm not sure I understand the circuit (part of it is surely that it's very hard for my aged eyes to read)...

So the resistance Rg between RG1 and RG2 on the THAT1510 sets the voltage gain, right? Av = 1 + (10kohm/Rg). So if Av is high and there is no input signal, then presumably you get lots of noise at the output.

Hmm, so there are two paths in parallel for determining Rg, (1) the path through diodes D13 and D14 (that resistance is quite high when input voltage is low, thus on its own it would set Av ~1), and (2) the path through the Gain 1 pot (which looks to have impedance Z<<10kohm at audio frequencies, thus driving Av high).

 

[almondcity]

I'm not sure I understand the circuit (part of it is surely that it's very hard for my aged eyes to read)...

So the resistance Rg between RG1 and RG2 on the THAT1510 sets the voltage gain, right? Av = 1 + (10kohm/Rg). So if Av is high and there is no input signal, then presumably you get lots of noise at the output.

Hmm, so there are two paths in parallel for determining Rg, (1) the path through diodes D13 and D14 (that resistance is quite high when input voltage is low, thus on its own it would set Av ~1), and (2) the path through the Gain 1 pot (which looks to have impedance Z<<10kohm at audio frequencies, thus driving Av high).

That part is pretty much straight out of the datasheet for this preamp chip

 

[JTEX]

The circuit diagram itself is fine, I have built it in the past and so did quite a few other people around me. It makes a great mic pre, on par with just about anything out there in the pro audio world.