[awagner]

Hello-

I recently built two MINT amps. One is encased, other is just wired together. However, I'm having a weird problem where some sort of noise/distortion is being added to the signal at very specific volume levels.

The noise, but doesn't sound periodic, and is certainly not a specific tone. Its volume relative to the music is such that a non-audio geek might not notice it, but it is not acceptable for me.

This only occurs within a certain range of the pot, about 15% to 25%.

I'm using the (crap) audio output of my Mac G5 as my audio source, so I'm apt to blame a lot on that. However, this noise is not present at the same output setting on the mac using the same headphones plugged directly into the mac. This, and the fact that the noise is correlated to a specific pot setting leads me to blame the amp.

Any ideas? There are a lot of obvious debugging measures for me to take (try different sources, drive different headphones, double check all connections, replace pot, listen more to the other amp), but I thought I'd ask if anyone has experienced this before.

My mint uses all components recommended by Tangent right down to a penguin mint tin; I'll post my parts list.

The configuration in the case is somewhat non-standard. I have the pot sticking out the left side, with the switch and the jacks on the right side. The board is split and two fresh 9V batteries are wired in the standard way (series). I hope to post a picture of this some time later in the week.

Things are isolated from the case and from each other by electrical tape.

Thanks!

 

[jerb]

this could be a number of things: you could be getting interfierence (sp?) and thats causing the noise. there is a ressistor in the cmoy(it might be R5) that you add to reduce noise( i think its like 50 ohm). also an unfiltered, unregulated powersupply can cause this.

 

[awagner]

Quote:

Originally Posted by jerb this could be a number of things: you could be getting interfierence (sp?) and thats causing the noise. there is a ressistor in the cmoy(it might be R5) that you add to reduce noise( i think its like 50 ohm). also an unfiltered, unregulated powersupply can cause this.

Thanks for your reply.

I believe the MINT amp has a corresponding resistor, which I installed. (The only things I did not populate were the diodes to bias the op-amp)

I doubt my two 9V batteries are causing power supply noise. Furthermore, the computer's power supply doublessly adds some small amount of noise, but what I'm describing is worse than what (expected) noise is coming from the PC.

(FWIW, the G5's audio is better than most. I can hear both the graphics card and the DVD drive, but I have to have no music playing and volume turned up. In any case it is below the noise floor of most recordings I listen to)

 

[Pars]

Could be oscilating. You have a DMM? If so, set it to measure current (mA), put it in series with the batteries, and take a look at the current draw as you take the pot from min volume up. If it spikes up in current draw at about the point of pot movement that you notice the noise, it could be oscillating. Also feel the opamp and buffers to see if they get warm or hot. Or anything else hot for that matter.

 

[tangent]

A brute force way to attack this is to lower the gain, so you never use that section of the pot at all. You'll get better tracking as a bonus.

 

[jerb]

Quote:

I doubt my two 9V batteries are causing power supply noise.

you never know if you were also using an external PSU so i assumed the worst (un filtered, unregulated wallwart)

i dont understand gain much but i know that optimizing it for your particular phones will help. im also pretty sure that lower gain is for more efficient phones (please correct me if im wrong) so if you have efficient headphones lower your gain like tangent said.

also how long is your interconnect and is it shielded?