Line out troubles

macumbista · Posted: Mon Jan 23, 2012 11:55 am Post subject: Line out troubles

This should be really simple, but it's a problem I never had when building my modular, so I'll put it to you folks.

I've been building small wooden speaker-box instruments, basically a contact mic run through a Germanium distortion (FzzzFctrry clone) and an LM-386 amp to a speaker. Schemo and info for the soundboxes are here: http://macumbista.net/?page_id=1897

I've been asked to add a line out to this arrangement. When I do, suddenly I get radio stations.

A normal circuit bender's line out usually disconnects the speaker, and when I do this I get no radio. This would be totally normal with a circuit-bent toy (you don't need to hear the crappy built-in speaker when you play over the PA), however since you play this instrument by making feedback with the speaker, that doesn't really help me much.

I first tried adding a 1K between the speaker + and line out, then added a 100nF between line out and ground, swapped that for a 20dB pad (10K from speaker to line out, 1K to ground) and even tried running the line out through a DI box but nothing got rid of it. This is probably because of the induction of the speaker coil combined with the high gain of the distortion circuit--plus the fact that it's in a wooden rather than metal enclosure... and I'm not such an Electronics Engineering wizard as to be able to fix that easily right now.

The other thing I tried was tapping the line out before it goes into the LM-386, but this ended up cutting out the signal to the amp unless I loaded the line out with so much resistance that I barely got any signal out of it.

Does anyone have any suggestions to solve this problem?

Thx in advance...

umschmitt · Posted: Tue Jan 24, 2012 9:33 am Post subject:

I'd be curious to hear a specialist's explanation about this. From the result of your last experiment, I'd humbly suggest that you try buffering the unamplified line output, at the cost of one transistor and two resistors. Or maybe use a two conductors cable with shielding to ground at one end… Just my two cents.
*************
[off topic : Do you plan more workshops in Berlin soon ?]

macumbista · Posted: Tue Jan 24, 2012 12:34 pm Post subject:

Thanks for your thoughts. I also thought a buffer of some kind (although I was thinking an op-amp). I might give this a shot.

As for Berlin workshops, no plans yet but that doesn't mean it won't happen. Possibly in the spring. My blog has an RSS feed if you are so inclined and everything I do gets announced there: macumbista.net

Best!
D.

mosc · Posted: Wed Jan 25, 2012 10:22 am Post subject:

This is a really interesting problem; I love these kinds of problems.

Are you putting the output in series with the speaker or are you drawing it off in parallel with the speaker? If you're doing it in parallel, the speaker becomes a coil and the amplifier on the out is probably adding some sort of rectification that is in a allowing signal to get picked up. This is a crystal radio, of sorts; maybe.

Do you hear the radio from the speaker, or just from the line out?

I agree that a buffer will be the most logical solution to try.

You may have discovered something to use in future instruments.

macumbista · Posted: Wed Jan 25, 2012 10:29 am Post subject:

oooo... didn't think of trying it in series cyclops

mosc · Posted: Wed Jan 25, 2012 12:08 pm Post subject:

macumbista · Posted: Wed Jan 25, 2012 12:12 pm Post subject:

Probably why I didn't try it myself. Thx!

macumbista · Posted: Tue Feb 28, 2012 7:11 pm Post subject:

I discovered that the radio problem I was getting came from the volume pot. Moving my hand near it (not even touching it, also through a plastic knob) changed the capacitance of the circuit and introduced the interference. Grounding the case of the pot fixed the problem.

mosc · Posted: Fri Mar 02, 2012 11:27 pm Post subject:

Great