Master volume on/off switch not working

[markmalin]

I thought this would be an easy way to switch the MV in/out on this amp.  (the guy wants this feature, I realize you can just turn the MV all the way up).  Basically a SPST switch across the wiper and input of the pot, closing the switch would bypass the volume because the signal would take the path of less resistance.  I wired this up and the MV does work, and the switch sort of works, but engaging this switch the MV is still in the circuit.  In other words, if I close the switch, even though the regular volume takes over as though the MV is out of the circuit, if I dial the MV it will still change the volume somewhat, and MV on "0" puts the input to the PI to ground, so it turns the signal off.  Could I use a DPDT switch and when the MV switch is closed, open the connection to ground on the MV?


Here's a copy of the schematic with my original MV mod....


humbly
Mark.

[markmalin]

pardon the lame drawing, but this is what I was thinking would solve the issue.  Does that make sense?

[2deaf]

How about a SPDT with one contact hooked to the top of the pot., one contact hooked to the wiper, and the output of the switch hooked to C9?

[hesamadman]

I feel like the best way would be to use a DPDT. This should allow you to completely disconnect the wiper from the circuit and take the signal completely around the pot. Leaving only the pots connection to ground. Disconnecting it this way gives no path to ground at all from entering back into the wiper as the signal bypasses the pot.

[hesamadman]

I feel like this might work

[markmalin]

I feel like this might work

Interesting.  Thanks for the idea :)

[2deaf]

Two examples

[hesamadman]

Will the SPST example create a drop down resistor in the circuit depending on the way the pot is positioned?

I guess that would be the easiest way. The most it could do is 1ma. Which wouldn't affect the circuit a whole lot I guess.

[2deaf]

Will the SPST example create a drop down resistor in the circuit depending on the way the pot is positioned?

I am having some difficulty in understanding the question.  Is there any possibility that you could rephrase the question or maybe even expand upon it?

[hesamadman]

i think with the 1ma pot in the circuit the way it is with the SPST switch, it will have a path to ground via the pot. It will be 1m though so it shouldn't affect much. But I feel completely disconnecting the pot would be best.

[markmalin]

What is the purpose of the 1M resistor in the second drawing?  Why would it not be better to let the lower half of the switch be open (not connected to anything) when the master volume is bypassed?

[hesamadman]

Oh....maybe he's TRYING to create a drop down resistor here when bypassing switch. You don't have to do that here. You would if it was a PPIMV. I think anyway.

[2deaf]

What is the purpose of the 1M resistor in the second drawing?  Why would it not be better to let the lower half of the switch be open (not connected to anything) when the master volume is bypassed?

The only purpose of the 1M resistor is to make the circuit exactly the same as having the MV dimed, which is what I thought you were trying to get. When the MV is switched out of the circuit and there is no 1M resistor, the result is the same as having a classic non-MV amp.  Some people say that the non-MV version of a given amp sounds better than the MV version and if you put a calculator on it, they are not the same.  The difference is rather subtle, IMO. 

[hesamadman]

That's an interesting thought 2deaf. That makes sense. It would be something the builder could play with and see what he likes better.

[sluckey]

The only purpose of the 1M resistor is to make the circuit exactly the same as having the MV dimed, which is what I thought you were trying to get.

Your SPDT circuit accomplishes the same thing without having to be so clever. The only reason I can think of to use a DPDT would be if you want to totally remove the pot when  in bypass mode.

[markmalin]

How much voltage would this switch see?  Currently I'm using a Carling mini-toggle, which I think is rated at 6A at 125VAC.

[sluckey]

only a couple volts of signal. No dcv.

[markmalin]

Here's what I ended up doing.  This works well.  My only complaint is adding more wire to the master volume control - I ended up using shielded cable on the short run from the pot to the switch.


Mark.