Quick question about a microphone pre-amp

[Howard]

Originally posted by: mindless1
Maybe overkill, but the same power rail noise is present on USB as on your integrated audio, it too being designed to run from that power.

But how do we know that the noise is being caused by the +5V rail?

 

[bobsmith1492]

Originally posted by: Howard

Originally posted by: Howard

Originally posted by: bobsmith1492
I wouldn't worry about noise specs on voltage regulators. Modern regulators all have extremely low noise (i.e. don't use an LM317 because they're ancient...). Plus, if you're simply powering op-amps, the op-amp will have tens of dBs of power supply rejection.

If you're worried about a -118dB noise floor, you're slightly crazy. You'll never see that little noise which means the regulator's noise is inconsequential.

I ran the preamp (in a case) without a mic hooked up and I could definitely hear some noise after turning it up. Not sure if it's the 84.77uV from the feedback resistors or whatever noise from my own sound card or some crap picked up by the dollar-store interconnect, but it's there.

I recorded without the preamp hooked up (no interconnect connected, even) and there's still quite a bit of noise. God damned onboard audio.

What is your setup - what is the input coming from and I'm assuming the output from your preamp goes into line-in on your computer?

 

[Howard]

Yep. WM-61a -> preamp -> line-in - KRISTAL

 

[mindless1]

Originally posted by: Howard

Originally posted by: mindless1
Maybe overkill, but the same power rail noise is present on USB as on your integrated audio, it too being designed to run from that power.

But how do we know that the noise is being caused by the +5V rail?

The ground noise effects all rails, and even if it's a different rail it's still one with HF digital parts sitting on it, plus powered by a switching PSU. 5V is common and typical, but it could have a linear regulator sitting on the 12V rail and yet that often has fan noise even if separate from 12V VRM CPU supply subcircuits.

My point is, you have to decide just how ideal you want to make it, I could've told you originally that none of the changes made will matter as much as noisey integrated audio but the refinement process is one of making each stage as good as can be (reasonably, within design parameters like cost and time), considering that if your pre is good it's more versatile than only being connected to a computer's integrated audio input.

Putting the input on a USB device, if it is reasonably designed, is a step forward. The remaining question is how many steps to take. You notice noise, but remember this is SNR, if one opamp alone, on a stripboard, can't give you the resolution you need, by using a 2nd opamp for more gain you raise the SNR with the constant of integrated audio noise pickup, until you reach the max input level it accepts.

This is a quick, instead of lengthly, question about a mic pre-amp.

 

[bobsmith1492]

Originally posted by: Howard
Yep. WM-61a -> preamp -> line-in - KRISTAL

This WM-61a?

If you're running a microphone directly, how are you connecting it to the preamp? You need a differential input, proper mic biasing, and good cap filtering on the mic for a quiet input...

I'm working on a remote audio link at work and the mic input is nice and quiet... see the microphone configuration on page 16.

 

[Howard]

Originally posted by: bobsmith1492

Originally posted by: Howard
Yep. WM-61a -> preamp -> line-in - KRISTAL

This WM-61a?

If you're running a microphone directly, how are you connecting it to the preamp? You need a differential input, proper mic biasing, and good cap filtering on the mic for a quiet input...

I'm working on a remote audio link at work and the mic input is nice and quiet... see the microphone configuration on page 16.

The very same.

http://www.linkwitzlab.com/sys_test.htm#Mic