Hello all,

I'm making an audio mixer based on the following schematic, good news is it works fine.
http://www.all-electric.com/schematic/simp_mix.gif

I'm powering it with a 9v unregulated DC power supply, and the following virtual ground schematic:
http://www.hqew.net/files/Images/Article/Circuit_Diagram/9-V-battery-powered.jpg

The trouble I'm having is that there is a constant and audible hum that I can't get rid of.

I've tried adding a lowpass filter to the DC in, this had no effect.

I've tried adding a voltage regulator to the input in the hope that this would sort it out, again it had no effect.

Two relevant things I've noticed:

1. If I use another power supply on the same socket as the mixer, it adds to the humming sound.

2. If I touch any part of the circuit the humming sound is reduced. Maybe this means it is a grounding issue?


So basically I've run out of ideas, I don't have enough experience to know if this is a PSU issue, a grounding issue, or what?

Any help would be greatly appreciated,
thanks Ewan

 

One thing I'd try immediately is using a 9V battery, or a 12V battery with a regulator to take it down to 9V. Your problem sounds like power supply hum, but you can confirm that easily before proceeding.

 

A lot of mixers have high gain, and when repairing them it was common for them to hum pretty badly with the metal covers removed. But when the covers were replaced they were fine.

Have you considered putting the project in a metal box? That might be required.

 

I tried a 9v battery, the hum waveform is exactly the same.

I think you're right THE_RB, highly amplified external signal noise from power supplies or something. Will put the final project in a decent metal box.

Anything else I can do to minimise the problem?

Thanks a lot people!

 

Sounds like ground loop issues. http://www.jensen-transformers.com/an/generic seminar.pdf Lots of good info in this document that pertains to interconnecting several different electronic devices like what a mixer is used for.

 

I don't think so, because I have run the entire setup using battery powered devices and it persists.

My understanding is that ground loop happens when devises have a mutual earth ground?

 

This circuit doesn't have high gain, but it has 6 inputs, and that equals 6 antennas. I agree with finishing the metal enclosure.

It is possible to create a ground problem in a single circuit like this, but you description about how it acts seems to say that's not the problem.

 

You need to have a single ground earthing (ground) point.
Everything needs to be screened with metal connected to earth.
The power supply needs to be batteries or a well-regulated mains supply.

Most mixers have all the gain at the input. Before the fader. This gain stage (one for each channel) is very carefully designed and screened. Use low-impedance balanced inputs for microphones. Even better, use balanced input transformers.

After that the rest of the mixer should operate with a fairly high signal level (0dBm. 0.775VRMS) that reduces noise and hum problems. Keep internal wiring short.

The best way to learn audio electronics design is to build something, then discover and correct the problems!

 

I tried a 9v battery, the hum waveform is exactly the same.

Well done. You see now how working with DC filters and regulators would never have solved the problem. Diagnosis is key.

 

another fix for hum is to use audio transformers to isolate each input from any ground loops that may occur.