More A/B/Y Switcher questions

[Greg Moss]

I just finished the active A/B/Y switcher from the GEO site.  It seems to work great.  I used the power supply circuit (the "turn on 2 batteries with one switching jack") from the transformer splitter diagram.  I was able to get it working using a 3PDT in place of one of the DPDT's and having that extra pole graound the input of the buffered output. So that's fine.

My new problem is this:  I am experiencing fairly loud pops when switching output channels.  I have a friend who experiences the same problem, when using his bass setup, but not using his guitar setup. I am used to the necessity of pulldown resistors when using bypass switching in order to bleed off charge from coupling caps at the input and output of the effect, butthis circiut has no coupling caps.  Maybe I need to poke around the "history of bypasses," to find my answer, but does anyone have any hints offhand....

Greg

[R.G.]

Does one of the amps have DC coming out the input? I've seen that before.

You shouldn't have DC out of either output of the AB/Y.

I did find some earlier notes. The ground connection is a typo, it should be deleted, and that should give you humless A/B/Y from the circuit. Did you have trouble with that incarnation?

[Peter Snowberg]

If you have DC coming out of one of your amps you should be able to find that quickly with a meter. You could always try adding a large value coupling cap to the offending input.

-Peter

[Greg Moss]

R.G.:

I did find some earlier notes. The ground connection is a typo, it should be deleted, and that should give you humless A/B/Y from the circuit. Did you have trouble with that incarnation?

The circuit seemed to work  pretty well provided you DO ground the input to the "B" output buffer when running only out of the "A" (unbuffered) output.  Thus, a 3PDT was required to get it to work buzzlessly.  Basicly the A/B/Y as in the schematic, but with the extra pole of the "A" or "B" footswitch making the connection to the grounded pole of the "Both" switch (the one you mentioned as being a typo).

In the pedal I built, at least, when just output "A" was selected, and "B" wasn't grounded, I got a substancial buzz.  That was where I thought I might have a problem with the output buffer.

Peter Snowberg:

If you have DC coming out of one of your amps you should be able to find that quickly with a meter. You could always try adding a large value coupling cap to the offending input.

Is that common?  I have some pretty oddball amps around (which is kind of good for testing worse case scenario situations).  I would be having problems with a gorilla 15W combo, and my friend would have been having problems switching into a GK bass head.

Speaking of DC, this is sort of off topic:  My band used to play a song where I would hold a photoflash up to my bass, and I would get numerous weird sounds out of it.  (Short delay seemed to intensify the effect) Low ocillating tones, high stuff too.  I could never get anyone to explain why that would happen.  My favorite explaination was that you were able to hear some large capacitors charging and discharging, but that was never voiced with much confidance.  Anyway in hopes of protecting my speakers, a friend wired a pair of large caps in series with the mains output of my bass head in order to block strange, theorized ultra-low frequency information which might have been costing me some trips to New England Speaker.  We never figured out what kept blowing speakers, or whether the caps helped.  At this point, we don't play the song any more, and I've stopped blowing 15" speakers (so regularly anyway).

I should have proofread, I know,  thanks for reading anyway,

Greg[/b]

[Greg Moss]

Well I should have asked: How do you check for DC?  just turn on the amp and measure tip to sleeve on the input jack?

[Peter Snowberg]

Yep.

Ideally the ins are always at 0 volts with the AC signal on top of that nothingness. In tube circuits that's the natural tendancy since the cathode is slighly elevated and the grid is connected to ground with a 1 meg resistor.

In the real world things are not ideal and you can get some current flowing out the grid. In that case the grid resistor makes the 2nd part of a resistive divider and you get a tiny bit of DC in the in.

In transistor circuits you more commonly see a coupling cap going to a base which is held at a constant bias voltage. If that input cap leaks, you can get DC in the ins as well.

-Peter

[Greg Moss]

One more question and I'm through, I swear.

The buffered output is picking up a lot of radio.  do i tie a small cap from the input of the output buffer to ground?  Would you think that the culprit is crossed input and output wiring?  The unbuffered output (which does follow the input buffer stage) isn't experiencing the RF problem so maybe the hookup wire leading to the output buffer and the one coming out of the output buffer are too close together?  would shielded hookup wire be the solution?

This is the first "non-high gain" (I'm assuming stages with a gain of 1 don't count as high gain) pedal I've tried, and the first run in with the unwanted RF....

Greg

[Peter Snowberg]

Uh oh, now you're in voodoo land.

You could try adding a 22pF cap across the input and ground and/or a series resistor (1-10K) in the signal path in front of the cap.

Those are just twiddling guesses.

A ferrite filter chokes in the input may be the best cure here. Pick up a ferrite doughnut of the right frequency ferrite. Is the radio AM? if so get #73, #75, #77, or Type J ferrite. If the radio is FM, get #43 ferrite.

Wrap 10-20 turns of wire around it and put it in line with your input. Add little caps after the coil to fine tune.

It is truely amazing (to me at least) how smiple a radio can be.



AM:


Look how closely that resembles the path from a guitar to an amp if you look at real cable and component characteristics and omit the detector diode.

Historic note: The first use of electrolytic capacitors was actually for rectification. (Thank you Radio Electronics trivia)

http://www.mikroelektronika.co.yu/english/product/books/rrbook/rrbook.htm

-Peter