Two preamps feeding one power amp

Discussion in 'Amps and Cabs [BG]' started by Poele67, May 4, 2004.

  1. Poele67


    May 4, 2004
    I be able to switch between two preamps (using A/B switch to send signal from the bass to either of the two preamps) to feed one power amp. I used a little resistance mixer (DOD 240) to direct the two preamp outs to one power amp in. However, the DOD sucks away a lot of power from the signal, so I need another device. You guys have any suggestions? As little power and tone loss as possible....
  2. Petebass


    Dec 22, 2002
    QLD Australia
  3. For minimum power and tone loss, how about using a two pole changeover switch. One pole to switch the bass between the preamp inputs, and the other to switch the amp between the two preamp outputs.
  4. Poele67


    May 4, 2004
    Thanks for the reply. However, I only want to switch to send the signal from my bass to one preamp or the other. The two preamps should go into two inputs of some kind of device and both input channels should be always 'on' (so no switching) to feed the one output to the power amp. Basically I require a little mixer like the DOD, but without the loss of power and tone.
  5. nonsqtr

    nonsqtr The emperor has no clothes!

    Aug 29, 2003
    Burbank CA USA
    I agree with Mottlefeeder, it would make more sense to put the switch "after" the preamps. Here's what I'd do: jumper the two preamp inputs together. That way, your bass will be feeding both preamps all the time. There should be no problem doing this, and no signal degredation. You can even use a little "Y" cable, that should work perfectly. Then run the preamp outputs through the switch and into the power amp. The only thing you have to watch out for doing it this way is the possibility of a loud "pop" when you're switching between the preamps. To avoid that, you can use a "make before break" switch, those should be available at your local electronics store. As long as there's at least one connection to the power amp at all times, you should be fine.

    Here's a couple of other thoughts. What kind of power amp is it? If it's a stereo amp, like a Carvin or something, you can just plug one preamp into each input and run the power amp in "mono" mode, that'll work on most of their amps.

    Second thought is, in most cases you can probably just connect the two preamp outputs together. In the vast majority of cases, there is a blocking capacitor in the preamp that will prevent any DC from reaching the power amp, and under these conditions it would be perfectly safe to jumper the outputs. Especially if you're only going to use one preamp at a time. To be sure about this, you'd have to check the schematics of your preamps, and make sure there's no direct connection on the output (like some of the newer op-amp models run direct out, and if the preamp isn't well designed this can cause some problems).

    Third thought is, get a pair of 1k resistors and put them in series between the preamp outputs and the power amp. This will eliminate any direct interaction between the two outputs, and it'll provide enough of a load to keep the preamps happy.

    Fourth thought is, are you handy with a soldering iron? If so, you can build a little op-amp mixer for about five dollars in parts and about an hour's worth of time. It'll take a 9 volt battery (or two would be better), and you'll need a box to put it in, so probably the input/output jacks will be the most expensive part of the whole thing!

    And finally, many stompox-type footpedals have multiple inputs, so you can put one of those between the preamps and your power amp.