multiple outputs from a 15K:600 pre OPT?

[keto]

I'm hoping to run as many as four 100K-ish-ohm-input power amps from a single 600-ohm preamp output and would like to do the output-multiplying on the pre-amp chassis, rather than daisy-chained RCA cables, for flexibility's sake. I noticed that in the FP3 circuit, a 100-ohm resistor is added between the two outputs and that the ground is shared. Should I daisy chain three 100 ohm resistor per side, to connect the four outputs? And as far as driving the four amps per side, I figure the 600-ohm out won't have a problem with a 25K total load, but should I lift the input grounds on the power amps, to minimize loops? Thanks! --keto

 

[JC]

In commercial installations, we used to do a thing we called 'bridging' with a 10kOhm resistor to each amp input.  Since all the amps were generally in the same rack and running off the same AC source, we did not worry about ground loops.  In any event, with unbalanced signal connections, the signal 'ground' is also the return path for the signal, so you cannot very well do without it.

For four amps, that meant 4 10kOhm resistors, of course; Wattage is not critical, since you would not expect there to be more than a tiny amount of current flow.  This keeps at least 20kOhm between any two amp inputs, which we normally found to be enough to keep any interaction between the inputs to a minimum.

 

[keto]

Thanks for that info JC. I now consider the idea a "go" and will experiment with those "bridge" resistors.  --keto

 

[JC]

Yes, experimenting is good.  You may find that you need smaller value resistors, although I don't think this is probably likely.  You also may find that you can use larger-value resistors, which would tend to isolate the amp inputs more.

Jumper clips and a small assortment of different values should let you dial it in.

I usually prefer metal-film resistors for uses with the signal passing through the resistor, for what it's worth.  They tend to be noiseless and very stable.