Master volume - switchable

[txstrat]

Hi,

the amp I'm currently building is a two channel amp with switchable master volume controls. I built another amp with this feature already and decided to use a relais for the switching between the master volume pots.
When the relais is off the clean channel is on. When the relais is on the gain channel is on.
When the volume is at a very low level, you can hear the hum of the relais in the speakers (of course only in the gain channel). It's not audible when you play with a band cause of the volume level of the signal, that is louder than the hum.
I'm now trying to amend the switching process (also with relais, cause the vactrols are hard to find for me, and if so they are very expensive).
I attached a schematic with this post how I imagine the switching could be done. Am I right with this and what should the values for R1 to R4 be.

[imaradiostar]

why not just switch between the two channel outputs and use the other pole of the relay to mute each preamp somewhere further up in the circuit? Correctly setup with pulldown resistors this shouldn't cause much of a click or pop.

you may want to consider the dumble type channel switching- it uses the clean preamp in front of the distortion section so it requires less tubes and less gain. They used a simple relay arrangement.

Look at schematic heaven for some examples.

jamie

[txstrat]

Thats what I do with the channel switching. I think I didn't put it right.
I'm looking for a switching method between two master volume controls just to have two different switchable output volumes.
What I think is: I divide the input into two via the first two resistors and set either one or the other volume pot to ground using both poles of the relais. After the pots I put the signal paths over the resistors 3 and 4 together while either one of the volume controls is still at ground. I just don't know if this is gonna be working out right the way I think. Does anybody have a suggestion?

[Don Moose]

Since you mention HUM, not pops, I have to ask if your relay coil is using an AC voltage or DC?

Going to a DC type should help - you do have to be careful how you go about getting that DC voltage though.

Details on your control circuit, please.

[Enzo]

I'd suspect that the relay contacts are not where the hum comes from. I'd be betting on a shared ground path for the relay coil current and the signal circuits.

One experiment - turn on the relay with a battery and see if the hum remains. It might be that the hummy channel is hummy all by itself.

[txstrat]

Enzo, you win. I indeed share the ground paths of the signal and the relais' coils. I didn't think of it as problematic.
How could I place the ground for the relais that no interference would occur?

The battery experiment you suggested made me think. I'll try that as soon as I can. Sure, when filtering the dc current there should be no hum (at least not that bad). Just as using a battery.

I added a schematical drawing of my circuit to let you see what I mean.
The ground wires are yellow, orange and pink (for better differentiation) and lead all to the same star ground (black arrow).

Yellow is used for pots (where necessary) and the shielding of the signal input wire as well as "the ground for the relais'".
Orange is used for cathode resistors and caps and filter caps of the voltage supply of the preamp tubes.
Pink is used for footswitch ground, reverb ground and OT ground.

The red wire provides the voltage for the relais' by an IC7812 with filter caps (sort of 22u/35V). The light green wires are the grounds for the relais' leading to a manual switch. The manual switch is connected to the footswitch jack (dark green wire) to leave the manual switch with no function while using the stereo jack.
The manual switch connects the light green wires to ground causing the relais' to pull in. Same happens with the footswitch when a stereo plug is connected.

I thought of "switching" the ground wires would cause less problems (pop, hum etc.) than switching the hot wires. And I don't have to lead the hot wire through the footswitch.
Not to forget and most important I can use a stereo cable for two switches (channel switching and master volume switching) in the footswitch.

[Enzo]

Just run the ground return for the relay circuits back to your star point with a separate wire.

[txstrat]

I will. Thanks.
Yesterday I pulled out the relais and switched the channels with the manual switch and the hum was gone. Not completely , though, but the remaining hum was the circuit itself.

But back to my origin of this thread. Would it be suggestive to switch the two different master volume pots the way I showed in the schematic I added to my first post? Or would it be OK if I just go and switch the signal, like I did before?
Maybe the hum would not be there cause of the separate ground wire.

[Enzo]

Switching pots with relays is done all the time. Look at recent PV amps for example - 5150 series.