SPST preamp fx switch?

[freebird1127]

Hey all,

Boy have I not been around for a while, hope some of you remember me! 

Well I know it's not technically stompbox related, but I'm redesigning the preamp for my 250W amplifier.  I'd like to keep it how it is, a basic clean preamp with 3 band EQ, digital VU meter, and a switchable TS9.  Now, i get horrible transients when I switch between the clean channel and distortion, and I think this is a good opportunity to address the problem.  I would like to accomplish a clean switch between the two channels with a single external SPST footswitch.  Anyone have any thoughts on the best way to do this, with popless fluidity?  I can use any internal electronics suggested, so go crazy.

Thanks all!
Evan Haklar

P.S. -> Aron, I haven't been around since the old site.  Looks great!

[freebird1127]

I guess I should also mention that the preamp should be set to clean channel when the SPST footswitch is not plugged in.

Thanks for any ideas!
Ev

[RaceDriver205]

Could you explain the setup a bit more? Is the TS9 the "distortion channel"?
By horrible transients, do you mean a big pop?

[freebird1127]

Yeah... BIG pop.  BIG BAD pop.  Especially driving 250 Watts.
Well as far as the setup goes, I can set it up however really since this is a redesign.  But basically what I had in mind was the preamp has the "clean" (undistorted) signal path, and the TS9 circuitry is switchable with the SPST footswitch.  Whether the channel selected is clean or TS9, the output of the preamp goes to the input of the power amp.  If the SPST footswitch is not plugged in, the preamp is defaulted to clean channel.

Again, I'm open to any ideas as I'm scratch rebuilding this thing and can make it however it is best suited to be built.
Thanks!

[RaceDriver205]

Well the TS9 should have input and output resistors to ground, does yours have these?

[freebird1127]

Well sorta.  Input and output signal level are controlled through potentiometers, but should I have included other resistors in parallel as well?  I'm sure the pop is coming (largely) from my switching system.

The old switching system I had thrown together was sort of a last minute thing, it was the SPST footswitch driving an SPDT relay on or off.  The Pole of the relay was tied to the output of the clean stage.  One of the Throws was tied directly to the input of the power amp.  The other throw was tied to the input of the TS9 stage, whose output was tied directly into the power amp.  Is that confusing?  I'd show you a diagram but I have nowhere to put it online...

Anyways, I know a relay is a BAD way of doing these things (now), so what is a better option?  Any thoughts?

[RaceDriver205]

Well a relay should work fine. Id ask from which schematic/layout you built your TS9, or did you buy it?
The pop is generally caused by the input/ouput capacitors remaining charged, and then discharging suddenly when you reconnect them (i.e. switch the effect on). This discharge causes a pop. So if you put a large (say 1Meg) resistor from the input to ground, and another from the output to ground, this problem should to some extent disappear. If I understood your explanation correctly, perhaps just the ouput resistor would suffice.

Of course, unless I can see a diagram I really can't be sure. You can grab a geocities site real fast, and its simple to upload images.

[Somicide]

^^^^
photobucket.com for image hosting.  Not sure about the pop, but it's less of a hassle, and allows direct linking.

[freebird1127]

http://i101.photobucket.com/albums/m70/freebird1127/footswitch.jpg

This is the quickest little diagram I could draw to illustrate how my switching arrangement works.  Is this an OK method of doing things?  RaceDriver, could you maybe doodle in where your recommended changes are?

Thanks!

[Seljer]

I think you'd want the switching part of the relay selecting either the output of the preamp or the output of the tubescreamer. Not selecting between running the singal into the poweramp or into the tubescreamer.
Or get a DPDT relay and wire the tubescreamer for true bypass.

[freebird1127]

Excellent Idea, DPDT relay.  I think I'll do that, thanks for the suggestion. 
To go with the DPDT, what are ways to ensure... poplessness?

What do you think about perhaps using a chip like one of Analog Devices analog switches...
perhaps the ADG888 (I'm not worried about the small footprint BTW)
http://www.analog.com/en/prod/0,,768_833_ADG888%2C00.html
and triggering it using the SPST footswitch, along with maybe a small debounce circuit?  Or has anyone tried using this type of device with not-so-satisfactory results?

Evan

[RaceDriver205]

Aye, use a DPDT relay. Having the clean signal connected to the ouput of the TS causes problems. One of the ways to ensure poplessness with a DPDT relay is to have one of the throws connect the input of the TS to ground. This keeps it discharged.
See in this example:

Note also the 4M7 resistors on the far right and left. More pop protection.

[freebird1127]

Thereby assuring the input/output caps are discharged to the common voltage level.  Well that makes sense :-)
Thanks all.