Diode switching

Keppy · June 07, 2011, 11:13:22 PM

I have a Rat on the breadboard based on Beavis' FKR. Working on the switching diodes, I've encountered something I don't understand: I thought I could leave one side of the diodes in the feedback loop connected at all times and just switch the other side, similar to selecting different resistors. I've found on the breadboard that this is not the case at all times. For example, when using 1N914 diodes in the feedback loop, if I connect one side of the MOSFET clippers the sound changes.

How are the diodes conducting if the two sides are not connected across a voltage difference?

Sanguinicus · June 07, 2011, 11:49:45 PM

I'm currently working on the FKR also. I don't understand what you mean. Are you altering the Beavis project? If you follow the schematic in the PDF (hard to find on the website) it should work. You can select between two feedback clipping options or none at all using an on-off-on SPDT.

blooze_man · June 08, 2011, 12:05:43 AM

That's not really what he was asking. He wants to know why the sound changes when only one side of a set of diodes is in the circuit. I don't know, but I'm interested.

Sanguinicus · June 08, 2011, 12:11:15 AM

Is it possible to provide a schematic of what you are doing?

Keppy · June 08, 2011, 12:38:49 AM

I hadn't found Beavis' schem before, so thanks for alerting me to it, and for attempting to help my sorry butt.

As for my confusion and yours: turns out the MOSFETs I was using were shorted on the breadboard to the diodes next to them, causing the issues I was having. My understanding was correct after all, meaning my question was nonsense, hence your confusion.

And now my stupid nonsense question will live on the forum forever...

Sanguinicus · June 08, 2011, 12:50:57 AM

No worries mate, here's the PDF for ya: http://www.beavisaudio.com/projects/FKR/BAR_FKR_Mark_2_Build_Documentation.pdf

I wonder if Dano (that's his name right? The Beavis guy) will chime in, BAR hasn't been updated in a while. Wanna know what other cool shit is he's been up to...

Keppy · June 08, 2011, 12:59:16 AM

Thanks for the link!

As for the schematic you asked me for, check out the one you linked to. See how the MOSFET/Ge clippers are always connected to the circuit on one side, then switched with a SPDT? I'm essentially attempting that with 4 sets of diodes. I understand the latest FKR revision does this as well, but the schematic is an earlier version.

Sanguinicus · June 08, 2011, 02:13:58 AM

Yep, that's how to do it. I did a similar thing on a Tube Screamer I made a year ago. 4 clipping options controlled with a rotary switch.

I know what you're talking about now! I was looking at this over the weekend and decided it was an error. it MUST be an error. All the clippers should be on one side of the drive pot then switch goes between clippers and the other side of the pot. Like this (Excuse the awful crudeness):

----mos----------------|
----Diodes-------------|
--------SW1---- ----something else----|
| |
| |
------------Drive Pot----------------------

Keppy · June 08, 2011, 03:52:49 AM

Quote from: Sanguinicus on June 08, 2011, 02:13:58 AM
I know what you're talking about now! I was looking at this over the weekend and decided it was an error. it MUST be an error. All the clippers should be on one side of the drive pot then switch goes between clippers and the other side of the pot. Like this (Excuse the awful crudeness):

----mos----------------|
----Diodes-------------|
--------SW1---- ----something else----|
| |
| |
------------Drive Pot----------------------

Do you mean an error in Beavis' schematic? Looking at his layout, the overdrive switch is indeed wired as you describe, and not as he presents it in the schematic.

Guess we caught him out!

And it only took 4 years... and my dumb mistake...

Sanguinicus · June 08, 2011, 06:00:18 AM

I hope he sees this haha. With the one I described, depending on your switch you can switch between any clippers or disconnect them so it returns to stock. In the next few weeks I'll chime back in and tell you how I went. I'm waiting on parts (read: money) at the moment

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Diode switching