Using a SPDT ON-ON toggle to add in an optional circuit before a main effect?

[Andon]

Howdy! I tried searching for this, but I couldn't really find examples that were entirely applicable to what I'm looking to do. Is it possible to use a SPDT ON-ON toggle switch to switch between a base circuit and that same circuit with an additional "block" before it? For example:



You'll have to excuse the quick MSPaint mock-up, but it should get the point across. The top path is the original circuit (say a distortion, fuzz, etc.), while the bottom path has the added block (maybe a boost, extra gain stage, etc.) before the main circuit.

I feel like this should work, but I don't know if this would cause any issues. I know it works with DPDT toggle switch because that's what I'm currently using, but I'd like all of this to fit into a 1590A-sized enclosure, and between the board-mounted components (Neutrik and DC jacks, LED, etc.) I can't seem to make everything fit without switching to a SPDT if it's a viable option. Thanks!

[idy]

The additional block will probably make noise when not engaged, the open input being an antenna. Also its output section will change the input impedance of the second circuit. (you would be "feeding" both the main circuit in and the optional circuit output section in parallel.)

So usually we use a dpdt, and are able to disconnect the first stage completely, even grounding its input.

[r080]

You also put the switch on the other side, and start the whole chain with a buffer (buffered bypass). If you were getting a board manufactured with SMD parts, this would probably be the better option. However, in your case, given the amount of space the buffer takes up and the cost of a JFET, you should probably just use a DPDT switch unless your circuit was already planned to start with a buffer.

[anotherjim]

If you must, place the switch After the optional input and make sure both circuits have a high input impedance - say 1M - and always connect the input to the optional circuit input. The SPDT will either connect the main circuit to the pedal input or the optional circuit output.  When not used, the optional circuit will be working for nothing, but so what?
Well, as warned above, a high gain circuit can have noise/squeal side effects as it's so sensitive to stray pickup, but if you lay it out carefully, it should be ok.

[Andon]

Ah yep, that makes sense. I assumed the pulldown resistor was a given, which was why I didn't include it (same with the input buffer, which the optional circuit also has), but as it stands there's a 1M to ground before the toggle so no matter which path is selected it will have that impedance (unless you think I should have a 1M pulldown on each circuit, or a higher impedance).