dual channel design

[andmarti1424]

Hello everyone!
I have a question regarding this dual channel schematic I found somewhere on the net.
Shouldnt the 47K grid leak resistor (see green line) be switched with the relay as well, so the inactive channel is not amplifying a signal all the time?

Thanks!

dual ch design.png

[wpaulvogel]

I think it would be best to switch the input of either of the routes that were “offline” to ground because they would be amplifying if in the signal path or going crazy just floating. I’ve done it as I’ve described with a DPDT switch and it works. I’ve never tried it as illustrated in the schematic.

[hako]

+1
The 47k is a grid stopper, the grid leak resistor is the 1M at the input jack.
The main purpose of the gs is to prevent noise (rf) and i would use two of them, right at the grid of each preamp tube....

[Bergheim]

It won't make any sonical or functional difference if the unused channel is switched off or not as the two channels use dedicated tubes and b+ nodes. I'd run a shielded cable from the input jack to each of the two input stages, each with a 47k resistor right at the tube socket. I'd also reference the coupling cap on the lower channel to ground via a 1M resistor to avoid popping when switching to this channel.

[bal704]

The schematic you show is a common way to use a relay to switch channels. Use a relay to determine which channel goes to the PI. I don't ground the unused channel, and I've never had any issues. The weber channel switching PCB grounds the unused channel, and it works fine too.

[Lauri]

There needs to be a resistor to ground on the lower channel after the 0.022µF cap before the relay. Otherwise it will pop loudly when changing channel.

[attachment=0]dual ch design-popprevent.png[/attachment]

[roberto]

I think it would be best to switch the input of either of the routes that were “offline” to ground because they would be amplifying if in the signal path or going crazy just floating. I’ve done it as I’ve described with a DPDT switch and it works. I’ve never tried it as illustrated in the schematic.

IMHO it's better to ground the input of the second stage of each channer, rather than the input.
I agree on the DPDT: one side will ground the 2nd stages, the other will send downstream the signal of one channel or the other.
Please refer to ground the last coupling cap of the clean channel, on the relay side, or it will pop.

[andmarti1424]

I think it would be best to switch the input of either of the routes that were “offline” to ground because they would be amplifying if in the signal path or going crazy just floating. I’ve done it as I’ve described with a DPDT switch and it works. I’ve never tried it as illustrated in the schematic.

Thanks!

[andmarti1424]

+1
The 47k is a grid stopper, the grid leak resistor is the 1M at the input jack.
The main purpose of the gs is to prevent noise (rf) and i would use two of them, right at the grid of each preamp tube....

Ahhh. You re right!
Thanks for your tip. It would be one resistor for each stage then.

[andmarti1424]

There needs to be a resistor to ground on the lower channel after the 0.022µF cap before the relay. Otherwise it will pop loudly when changing channel.

dual ch design-popprevent.png

Thanks for your suggestion!!! I thought about popping problems as well, but didnt not where to put that resistor!

[andmarti1424]

I think it would be best to switch the input of either of the routes that were “offline” to ground because they would be amplifying if in the signal path or going crazy just floating. I’ve done it as I’ve described with a DPDT switch and it works. I’ve never tried it as illustrated in the schematic.

IMHO it's better to ground the input of the second stage of each channer, rather than the input.
I agree on the DPDT: one side will ground the 2nd stages, the other will send downstream the signal of one channel or the other.
Please refer to ground the last coupling cap of the clean channel, on the relay side, or it will pop.

Thanks! But why is better to ground the input of second stage rather that the input of each channel?

[andmarti1424]

Finally I got a pop when switching channels using 1M resistor in relay.
Will try 2.2M !!