How to wire up a relay to switch between two master volumes?

[GoodOleDW]

I'm planning on building an amp with two footswitchable master volumes I found this http://www.mojotone.com/kits/Mojoton...t-Switcher-Kit ,and I'm having trouble trying to figure out how to wire it up. Like where do I install the component I want switched in and out?

 

Edit: The Master Volumes are by default Pre-Phase Inverter. It's hard to be post-phase inverter when there is none, because it is a single ended amplifier.

 

I'm putting a second MV pot between the ground lug and ground wire of the first MV pot, and wire it up, so I can switch between the two MV's.

[WRGKMC]

I would think the kit comes with a wiring schematic for wiring it between two channels.

 

The input jack is for the switch of course.

 

The have to find the two terminals for the switched board outputs that would normally connect to the two channel inputs.

The third would be the guitar jack input.

 

You'd wire the guitar jack input to the line output of the preamp section.

 

The two switchable outputs will connect to your two master volume controls.

The volumes would then connect to the power amp in parallel.

 

Since you'll only be using one pot or the other, paralleling the pots to the power amp isn't a problem because the relay will select one pot or the other. Not both at the same time.

 

The path will be Preamp> Board input> Output 1> Pot 1 Center tap > Pot 1 Outer leg to Power amp. Other Pot 1 Leg is Grounded (most likely)

The same thing for the second output... Output 2 > Pot 2 Center tap > Pot 2 Outer leg to Power amp. Other Pot 2 Leg is Grounded (most likely)

 

cant give you any more details without seeing both schematics for the relay units and the amp but this is pretty simple stuff.

 

I do suggest you use shielded wires too and from the pots if the leads are long like across the chassis. This is easy to do and it will reduce hum and noise. One pot or the other will always be disconnected and hang in space so it can introduce unwanted hum to the power amp. If the wires are fairly short then you can use single wires but you need to keep them close to the chassis at all times to keep them close to the chassis to help shield them from hum.

 

Also be sure the Be sure the Input to the pot is connected to the center tap and the output is connected to the outer leg.

If you reverse these the pot will continue to attenuate the signal even if the channel is switched. You should also consider the fact you will have two pots bridging the power amp input. If the pots are 100K for example, you will be using two of them and the two in parallel gives the power amp a 50K load. because the hot on one side of the pot always remains connected, and the other end of the pot remains grounded.

 

If you know the amp should use a 100K pot for a single master volume, you'd want to double that for two pots. 2X200K in parallel = 100K total.

If you use two 100K pots you'll have 2X100K and get 50K ohms. That my cut your power amp volume way down.

 

 

You just have to be sure the master volume will properly taper at double the resistance. I believe they will using a linear pot. I've done similar things with higher value pots connecting a fixed resistor to the two outer legs before to bring down the total resistance. I just cant guarantee you you'll get the same taper when you adjust the pots. You may have most of your volume change within 50% of the pots turn.

 

Again I haven't seen the schematic so I'm just using some common sense here.

 

Of course you could always do what Neil Young does and stick a Motorized pot in there and just pan the pot up and down with an up and down switch. (or install an effects loop and stock a volume pedal in between the preamp and power amp and get any volume level you need. Less costly and much more effective.

 

 

[onelife]

… I'm putting a second MV pot between the ground lug and ground wire of the first MV pot' date=' and wire it up, so I can switch between the two MV's.[/quote']

 

I'm not clear on what you mean by this. Could you post a drawing of your idea?

 

 

… Also be sure the Be sure the Input to the pot is connected to the center tap and the output is connected to the outer leg.

 

I'm not sure why you would need to do this.

 

Why not put the output of the preamp across both pots and run the wipers of the two pots through the selector to the power amp?

 

 

 

 

If you reverse these the pot will continue to attenuate the signal even if the channel is switched. You should also consider the fact you will have two pots bridging the power amp input. If the pots are 100K for example, you will be using two of them and the two in parallel gives the power amp a 50K load because the hot on one side of the pot always remains connected, and the other end of the pot remains grounded.

 

That statement is a bit confusing.

 

 

If you know the amp should use a 100K pot for a single master volume, you'd want to double that for two pots. 2X200K in parallel = 100K total.

If you use two 100K pots you'll have 2X100K and get 50K ohms. That my cut your power amp volume way down.

 

That makes sense - but it is the preamp that is being loaded down by the Master Volume pots.

 

 

Since the OP is "planning on building an amp" ​my biggest concern is that the Master Volume controls be placed at a point in the circuit where there is no DC potential.