Mutron III clone - 9V input PNP?

[ChrisKelly]

Hi

Quick question about the Mutron III clone on the GGG site (the 'Neutron'):

What is Q1 for? On the schematic it's a 2N3906 directly where the 9V power input jack is.

http://www.generalguitargadgets.com/effects-projects/filters-envelope/neutron/

Cheers
Chris

[PRR]

> What is Q1 for?

It's drawn a little roundabout to keep the drawing tight. I've extracted and drawn "current flows down" for my mental clarity.

Q1 Base connects to input jack Ring. No plug in jack, Q1 Base connects to nothing. No base current, thus no collector current. No power to pedal-guts.

Stick a TS plug in the TRS jack. This shorts jack R to S, which connects R20 to zero V. Now Base current flows. Now collector current flows. Now pedal-guts get power.

My sim is a little awkward about "zero!" current. (So is the real world). With switch U6 tuned to as "open" as the sim's tiny mind can stand, it figures 24 NANO Volts to "power" the pedal-guts, which is "no" power. OTOH when switch U8 is set "short" (tenths-Ohm), a couple mA of base current flows, transistor is turned-ON, pedal-guts gets pulled-up to practically 9V (0.030V drop).

The time-curve assumes you plug-in at 100 milliSeconds. Power to guts is "zero" before that, rapidly rises to "full" after that.

[Rob Strand]

What is Q1 for? On the schematic it's a 2N3906 directly where the 9V power input jack is.

For most effects the input jack carries the switches the 0V.  The current on the 0V rail therefore passes through the input socket.  That's OK for clean DC.  When you use charge pumps (like ICL7660) and switch-mode chips there can be large high-frequency current pulses going down the input jack and this can easily get in the audio.  So that's the problem.
(FYI: Marcos-Munky had this problem recently with a switchmode.)

The solution is to hard-wire the grounds.

However, now you have a secondary problem of how to switch out the power.  So that's where Q1 comes in.

So the reason why it's there is more devious than how it works.

[ChrisKelly]

Wow thanks for the responses! Makes total sense now.

I'm always amazed by how helpful people are on forums like this - v much appreciated.   

Chris