DC offset with phones plugged in...

[Pars]

I ran across a weird problem the other night when using my Dynalo (headamp V2 board). I have an amb e22 DC offset protection circuit on the output, and when I have a set of headphones plugged in (T50rp fostex) and the volume turned all the way down, the protect circuit activates (cycles). Once I turn the volume up a little it stops doing that and works normally. Before I dig into it to try to figure out why, I just thought I'd ask here to see if this might just be normal?

 

EDIT: answered that. My work dynalo does not do this, and it also has an e22 on it. Same phones (not the same pair) as well.

[kevin gilmore]

put 100 ohms on the output of the pot to the amplifier.

[Pars]

In series? Or pot output to ground?

 

EDIT: series it is

Edited October 31, 2013 by Pars

[kevin gilmore]

may need to be as high as 200 ohms.

what is happening is that its oscillating with a zero impedance

input.  Same thing a buf634 does.

[RudeWolf]

Isn't this what the R65/66 prevents on the Rev. C boards? So the input does't get completely lifted.

[Pars]

No, those are in parallel with the wiper and ground and are there in case the wiper loses contact, and are normally a very large resistance (500k-1M). Kevin is talking about putting in a series resistance so that the pot minimum stays 100-200 ohms above ground basically. No spot for it on the board.

 

This is on my Dynalo built on a Headamp V2 board (didn't I sell you the other one I had? Or was that a Rev. B or Rev. C board?).

 

My work Dynalo does not exhibit this problem.

 

EDIT: I haven't gotten around to implementing this yet; maybe tomorrow. Wifey has plans for me today

Edited November 2, 2013 by Pars

[Pars]

Finally got around to looking at this today. Of course Mr. Scope confirmed Kevin's diagnosis (and cure); it was definitely oscillating. 200 ohms in series with the pot wiper -> amp took care of it.

 

Funny, it didn't oscillate when I built it and worked on checking dc offset (with board input shorted). Must be the additional inductance/capacitance of the input and pot wiring. I would guess that had I board mounted the pot (this is one of Justin's V2 boards), it would not have done this.

 

Thanks for the quick help Kevin!

[spritzer]

I had something similar happen with one of my Dynalo's.  It oscillated a bit depending on where the input cable was positioned underneath the pcb.  This is well away from any AC source and the chassis is grounded so very odd indeed.  The TKD pot might have been the culprit though given their odd construction.