"Series" vs. "Shunt" switching

[George Adzich]

Just curious if anyone has tried "shunt" swtiching in their amps.

I have tried shunt switching with relays and JFETs and it "seems" to work great. The natural place to use it is for turning off the reverb. Most reverb circuits have a series resistance to a mixing stage so this works well. It also has the benefit of shunting the output to ground.

I have also used it for channel switching. This can be a bit tricky with amps designs like the dumble. There is a need to add some series resistance which can have an affect on the tone. But again, it shunts the unsed signal to ground which is critical for high gain designs.

I have never had any POPping problems, noise, or hum. It also makes the wiring a lot easier because you do not need to interrupt the circuit. It is also possible to keep the switch wires very short.

Kevin O'Connor talks about this a lot in his TUT series of technical manuals. It seems odd that it is not used in more commercial and boutique designs. I must be missing something.


-George

[UR12]

George

I have used jfets/mosfets for years to do switching. I built some 2 channel gates years ago for a car stereo system to mute the audio when it fell below a certain threshold. These thing do work great. They also have a built in resistance of usually less than 100ohms so actually just inserting one in the signal chain will add some small amount of resistance to the chain although I don't think my ears are good enough to hear any difference in a guitar amp application.

[Elcabong]

I have been considering to use JFETs to switch channels in one of my current project, in replacement of costly and somewhat obsolete LDRs.

What specific part number are you using? And how do you integrate those relatively smaller components in a "turret/eyelet" board environment?

[George Adzich]

Elcabong,

I use the J105 JFETs. They are fairly inexpensive and have an "ON" resistance of 3-4 ohms.

I also build with Terminal Strips, so I place the JFETs on a separate PC board along with a small RC time delay circuit and a connector. I usually can use the same holes that are used for my terminal strips to mount a couple of Stand-offs for the PC board "switch card".

From the switch card, I only need to run a single wire to the "Shunt" point in the circuit.

To turn "OFF" or "ON" one of the channels without affecting the other channel, I need 2 isolation resistors in the signal path. The JFET shunt point is in between the resistors and is sometimes called a "Shunt T" switch.

This is the series resistance that I mentioned in my last post which I beleive can have an affect on the tone of the amp.


Cheers,

George