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Switching between two seperate preamps

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5/18/2000 8:33 AM

Switching between two seperate preamps

Hi there,

In my current (now finished) poject I incoorporated two seperate preamps in one chassis. I switch between those two preamps by means of a 6V relay. I have both preamps attached to the input socket, thus the two grid-to-ground resistors (1M each) are paralleled. I currently only switch the output of the preamps and ground each (not used preamp) after the Volume pot. I plan on changing to a scheme, where I switch the input signal and the output signal. I can see (and that is why I went that road with both preamps attached to input and only the output switched) that I will get some switching noises.
Is there anybody out there that has done this or could explain the merits of switching the input and output simultaneously? What happens if the grid to ground resistor is 500k instead of 1M (both 1M being paralleld by my switching scheme right now).

Thankful for any input,

Dominik

5/18/2000 9:41 AM
Kremator	Hello, i'm not too sure, but i think that only one 1M resistor would be sufficient. When it is 500k, it loads the pickups a very little bit more. This shouldn't be a problem. Is your amp noisy with this setup? =you can hear one channel in the second. If not, i don't see any reason to change. i might be wrong, but this is what i think. K.

5/18/2000 10:21 AM

K:

    I agree. I think that most people will put a single 1M grid load resistor right on the input jack, while putting the 33k (or whatever) grid stopper right on the tube socket.

    For the second channel, I've seen it wired up two different ways. In one scheme, a single 33k resistor in mounted on the V1 socket and a jumper wire is run to the other stage (which doesn't use a separate grid stopper). In the other scheme, two separate lengths of shielded cable are run from the input jack to each grid pin, which has its own grid stopper.

    The input impedance of a preamp tube is very high and sending a signal to two stages instead of one does not seem to load down the guitar at all. So I see no need to switch the inputs with relays; switching the signal at the output seems to work just fine.

    BTW be sure to mute the unused channel, especially if it has a lot of gain. (Sometimes you can skip muting a low gain channel but if the contacts are available I'd do it anyway.)

--Just my 2 cents worth.

Steve Ahola

5/18/2000 11:00 AM
Dominik	Thanks to Steve and Kremator for the replies. So if I got you right, I have to put one 1M directly on the input socket, and go from there to the stages. Indeed I had some problems with one channel leaking into the other. I (for now) mute the channels. Thanks again for the input. Dominik

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