Switching multiple volume knobs...

[OverDriven]

Hi everyone. I'm very new to building, and I have what is probably a stupid question for you all. Say I have a single channel amplifier I've built. I would like to give the amplifier 3 switchable volume knobs that you can footswitch between, giving 3 preset volume levels. It would just switch each volume knob in or out of the circuit one at a time using a relay. What exactly would be the best way to wire this up, in the simplest terms possible? Thank you in advance for any help!

[Don Moose]

'Simple' is not really going to happen - 2 channels is simple, more than that ...

I would suggest a set of 3 2-coil latching relays and a diode matrix. You would use 3 momentary-contact normally-open footswitches - one for each channel - and arrange the diodes so that each switch fires the 'SET' coil on its relay and the 'RESET' coils on the other two relays simultaneously. The relays need to be signal switching types. They can be either single- or double-throw. Whether you need single- or double-pole depends on whether you switch just the wipers (SP) or both wiper and gnd (DP) - more on that later.

Normal practice with relays is to connect the (+) end of the coil(s) to the supply rail, and use a switch to ground the (-) terminal of the coil. It is also good practice to connect reverse-biased diode across each coil (anode to (-) cathode/bar to (+)). This traps flyback currents in the coil and helps prevent pops. It is also essential that the summing junction - where the outputs of the relays meet - have a DC path to GND - 1Meg or bigger is fine - this also helps prevent pops.

In an ideal setup, you would switch both wiper and gnd on each of these pots, but switching just the wipers should be fine if all of the pots are high enough value (1Meg), and it's a whole bunch simpler.

The diode matrix has all of the diodes connected such that the anodes are connected to the relay coils and the cathodes are connected together for each channel switch.

( A drawing would really help here, but I'm not up for one this evening - sorry)

Each SET coil gets a diode to that channel's switch. Each RESET coil gets a diode to EACH of the other channels' switches.

The signal wiring should be shielded cable - use a good, low-capacitance cable - with the shield grounded at ONLY one end (probably at the pot end).

You will also need to come up with a power supply to operate your relays. There are a bunch of ways to do it, but for minumum complication, consider a complete supply - separate transformer, rectifier, filter, optional regulator. Hook this transformer's primary to the same wires your PT's primary gets.

Hope this helps!

[OverDriven]

Thank you so much! I need to digest it a little, but that helps a TON. I really appreciate the help

BTW, would swapping controls really be considered multiple channels? I thought that multiple channels really means switching other gain stages in and out. I guess what I'm asking is, what is the definition of "multiple channels"?

Also, is there some other way I can do this switching besides conventional relays that would make it simpler? Obviously if there was such thing as a DP3T relay, it would be ideal, but I can't find such a thing. Is there any other device that would work like a DP3T relay though?

[Don Moose]

Is there any other device that would work like a DP3T relay though?

No _single_ device. The latching relay approach has several advantages over the other approaches I'm going to get around to listing.

1) power is only required to execute the channel change - no holding current is required, just a pulse

2) good relays are about as close to hardwired as you can get

3) the number of channels you can switch this way is limited by the panel space for the pots and by the current capacity of your relay power supply ... and I guess by the size of footswitch array you care to tote around.

The other approaches - regular relays, JFETs and LDRs have advantages, too. But:

While cheaper, regular relays require holding current and latching switches or logic to decode, latch and hold your desired state if there are more than 2 states.

JFETs are pretty much only good for shunt switching - shunting unwanted channels to GND. That almost requires physically separate, complete preamp channels - not really suitable just switching master volumes ... as far as I know.

LDRs are nice - the switching is popless, but they require 'holding' current, and they have finitely low 'on' resistance - a couple hundred ohms at best. They can also take a significant part of a second to turn adequately 'off'.

Hope this helps!

[Enzo]

In defense of holding, the current through an LDR can be just a few mas. And really the current through a relay coil is not a lot, unless you use big old honking relays. Modern amps are full of common relays.

[Don Moose]

All true, but the simplified switching logic is the real advantage to the latching relays.

[Enzo]

A few diodes, perhaps the same ones decoding the latcher circuits, and you can have the same effect with conventional relays

[Don Moose]

I get the diode decode, but the hold ... and how do you get from one state to another?

... ... ladder logic with DPDT relays, use the second pole as a way to hold state?

... ... ... not quite seeing it, but what's left of my brain needs the exercise.

[Don Moose]

I think I could do it with 4-pole relays. Isolating the three hold/break paths is the tricky bit as changing any of the non-held relays has to break the current hold circuit and establish its own hold.

Unless you want to have to stomp two bistable switches to make any change. CH1ON->CH1OFF;CH3ON

Or I'm just tired.

[Enzo]

Oh, I'd have to think abuot it too. If I intended to MAKE this, I'd implement it with a little logic IC.

[Steve Conner]

I think latching relays are a really weird choice. They're normally only used in battery equipment where the power consumption of the coil is a real worry.

I'd do it with regular relays controlled by some 4000 series logic chips or a microcontroller, that contained all the brains to achieve what you want from three momentary pushbuttons. I once did this for a four-button footswitch, and included encoder and decoder chips (from a garage door opener IIRC) to send all four buttons down a stereo jack lead. I think I junked it in disgust when I started to appreciate the virtues of simplicity in amp design, but I might still have the schematic.

Like a previous poster said, switching between two states is a lot easier than switching between three. Two can be done with a latching stomp switch and no logic: the state is stored mechanically inside the switch. Three or more needs something like the preset buttons on old-school car radios, where pressing one in made the others pop out.

If you're under 30, you'll probably only know these as "radio buttons" in Windows dialogs, and have no memory of the original push-button radios that they were named after. Even if you did remember them, they don't make them strong enough to stand up to foot operation, so you need to emulate the mechanism with electronics.

[OverDriven]

Haha...I am under 30 (26), but I do remember the buttons you're talking about. I've had some old beaters when I was younger

Another question...how do you guys usually mount your relays? The amp is turret board based, and I don't want any PCB in there (regardless of whether it is more or less reliable). Do you somehow mount these to a typical turret board that is separate from the main board, or is there another way?

[Don Moose]

I think latching relays are a really weird choice.

Oh, I agree. And with your subsequent observations. The original challenge, though was 'simple'.

3 relays and 9-12 diodes is simpler, in my stupid opinion, than coming up with (and explaining) a logic circuit.

[BackwardsBoB]

Overdriven asked:

Another question...how do you guys usually mount your relays? The amp is turret board based, and I don't want any PCB in there (regardless of whether it is more or less reliable). Do you somehow mount these to a typical turret board that is separate from the main board, or is there another way?

You can glue relays to your turretboard upside-down, and make your connections with 24 awg or smaller solid wire. Same thing with those 4000 series ICs. "Glue" can be hot glue, epoxy, or even that foamy 3M double-sided tape. The technique is called "dead bugging".

I used to work with some Ampex camera guys, and they showed me how to build very complex high-performance low-power analog circuits. You get a piece of scrap copper-clad PC board, cut thin strips off it for power distribution as necessary, make the board cladding ground, stick everything down with double-sticky tape, and sky-wire all your connections. You're working on a solid ground plane, the power can be decoupled with 0.1uF caps with very short leads, and the results can be beautiful in a nerdy sort of way. As long as you're working with DIP ICs, TO-92 transistors, 1N914 diodes, 1/8W resistors, etc., the technique works great. You can stick the result onto a turrentboard with more tape.

[pdf64]

A simple way of achieving pretty much what you want using 2 relays and 2 on/off footswitches would be to have relay 1 switching between Vol1 and Vol2, and relay 2 switching a boost in - this would give 4 possible levels. The boost would be controllable from the 3rd pot, wired say as a variable resistor, in series with the ground return of the other 2 volume pots. The relay contacts would either short that boost pot out (no boost) or not (boost). Or the boost pot could be wired into a common hot feed to the 2 Volume pots.
Just an idea to save getting too bogged down in logic, and get playing your guitar - Peter.