[DigiPete]

Folks,

I'm having trouble with noise when I rotate the 4-gang pot in my Dynamite using my balanced DT880s. When I tie the 2 left and 2 right amp board groundplanes together, the pot wiper noise goes away.

When I hook up my balanced K701s I get worse wiper noise, butwhen tying the groundpances together, I get lots of loud hiss.

If I run the amp in single-ended mode I get no pot noise while I rotate the knob for any configuration of headphones or grounding scheme.

What gives? Need some advice

Pete

 

[Gontran]

Maybe the added complexity of running balanced in short run home setups ain't all it's cracked up to be?

(running and ducking)

 

[Francis_Vaughan]

A common cause of noisy pots is DC on the pots. This sounds suspiciously possible given the symptoms you describe. I would check the ground potentials and PS layout. Something sounds very odd here, not just the pot noise. There is a hint of somthing amiss in the wiring or layout that is leading to these syptoms. Hiss can come from a very low level oscillation in an amp, and this too may be related to the same root cause.

 

[Francis_Vaughan]

Bah! Double post....

 

[DigiPete]

QUOTE]

Thanks, I'll start checking DC offsets and see what I can dig up.

Pete

 

[Garbz]

Also check the grounding outside the amp. Like over the ground lug of the mains, and out via pin 1 on the balanced cables. http://www.rane.com/note151.html Has everything you ever wanted to know about grounding. and more.

Quote:

Originally Posted by Gontran Maybe the added complexity of running balanced in short run home setups ain't all it's cracked up to be? (running and ducking)

Spoken like someone who's never tried it

 

[DigiPete]

Thanks Garbz,

Time to hit the books, and the multimeter...

Quote:

Originally Posted by Garbz Also check the grounding outside the amp. Like over the ground lug of the mains, and out via pin 1 on the balanced cables. http://www.rane.com/note151.html Has everything you ever wanted to know about grounding. and more. Spoken like someone who's never tried it

 

[DigiPete]

When the K701s are plugged in and I am getting the clicking and noise, I found 30-50mv dc offset. Plug in the DT880s and offsets are back down to <1mv dc offset and all noise is gone. This was the case with and without opamps in the servo loops.

The gain resistors on my amps boards are dropped from the 1.2k to 500R to lower the gain

Any ideas?

Pete

 

[Garbz]

Sounds like the amp isn't stable at that low gain. Not all amps can be run low. Either put a small capacitor in parallel to the resistor in the feedbackloop to compensate at higher frequencies like 1-5nF which cuts outside the audible range, or take the gain up to stock again.

Remember when you run an amp in a balanced bridged config then each channel of the amp sees only half of the load impednace they are driving. In this case 30ohms.

/Edit: You can also try a 120ohm resistor in series with the output, or rather 60ohm / side.

 

[DigiPete]

Quote:

Originally Posted by Garbz Sounds like the amp isn't stable at that low gain. Not all amps can be run low. Either put a small capacitor in parallel to the resistor in the feedbackloop to compensate at higher frequencies like 1-5nF which cuts outside the audible range, or take the gain up to stock again. Remember when you run an amp in a balanced bridged config then each channel of the amp sees only half of the load impednace they are driving. In this case 30ohms. /Edit: You can also try a 120ohm resistor in series with the output, or rather 60ohm / side.

Thanks Garbz,

I'll try the series resistance first and see how that goes, if not, then I'll try bumping the gain back up.

Pete