Best way to connect 2 preamps to 1 power amp

You can NOT connect two preamps to a common power amp using "Y" cables or splitters. The preamp that you are using at the time will not only see the amp as the load, but also the other preamp that isn't being used. You will definitely hear the difference in sound, even if the unused preamp is turned off.

You might be able to use the RCA's on one and the XLR's on the other, but this would depend on the design of the amp itself. If the amp is of a truly balanced design, there will be enough circuitry between the two different input jacks to help isolate the two preamps from each other.

The easiest way, albeit maybe not the best sonically, is to use some type of switch box between the two preamps and the amplifier. This electrically isolates the two, but at the same time, requires more cabling and introduces what would probably be quite a bit of junky low grade connectors, switches, etc.. into the signal path.

Short of manually connecting / disconnecting as needed, i'm not aware of any convenient yet good sounding approach. Sean
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Thank you Sean,
Do you know is my 2nd-hand amp (Lexicon 212) a truely balanced amp. Also, any suggestion on a Good audio switch Box. thank you once again from a newbie.
Best

The best thing would be if your 2 channel preamp has a HT or Processor pass through. The other thing that will work is to connect the front channel outputs of the Pro Logic preamp to one of the unused inputs on the 2 channel pre. The only downside the the latter is having two volume controls inline.