Wally wrote:

I have just acquired a slightly beat-up Cambridge A75 preamp (thanks
Rob!). Aside from some tidying up, this will form part of my intended
system upgrade to a bi-amped setup with a low-pass filter on the bass end.
I'm attracted to fitting a second stereo output stage and filter to the
preamp - that would let me try various amps for driving the bass speakers
without having to deal with incorporating the filter into each. Although I
can operate a soldering iron and a multimeter, I'm not terribly clued up
on silicon electronics...

I'm sure there are chips that can take an input from the PCB and drive a
second pair of outputs to the same spec as the existing outputs - but what
ones would be suitable? Those low noise mosfet 741-thingies?

NE5532 would be my choice.

Ian

Is there a chip that I can use to construct a filter which doesn't
introduce the sort of phase problems that a speaker-side crossover would?

Very difficult to design a filter that changes amplitude with frequency
without affecting phase. Laws of physics type stuff.

How much scope is there for being able to tailer the slope of the filter,
and make the slope selectable?

No problem provided steps of about 6dB/octave are acceptable.

Can a subsonic shelf be incorporated?

No problem, in fact hard not to do this.

It
would be good to be able to switch the filter off and have the second
output provide full bandwidth.

Bypass is easy too.

Ian