In article , Chris Morriss
writes
In message , Wally
writes
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?

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? How much
scope is there for being able to tailer the slope of the filter, and make
the slope selectable? Can a subsonic shelf be incorporated? It would be good
to be able to switch the filter off and have the second output provide full
bandwidth.

I'd be grateful for suggestions of other ways to approach the filtering,
suitable chips, maybe some web sites that have circuit / schematics.




If you want an easy way to design and simulate low/high/band-pass
filters of either Butterworth. Bessel or Chebychev with a conventional
op-amp architecture with first to eighth order response, then go to the
Microchip website and download 'Filterlab' (it's free).

You fill in a table showing the -3dB points and the order and it does
the rest.

It even draws the schematic for you.

If you're attempting to put more modern IC's where older ones once went
make sure to run a scope over them to see if their not "hooting" in the
MHz region which they seem to be very good at!....
--
Tony Sayer