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.



--
Wally
www.artbywally.com
www.wally.myby.co.uk/music

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

Ian Bell wrote:

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


Easy enough to do with a digital signal.

A quick demo would be to low-pass a file with Goldwave or similar,
reverse the file, filter it again and reverse it again.

In the analogue world:

You can keep the low-pass and high-pass outputs in phase
with a 4th-order filter. I got a cheap Soundtech active crossover,
which does this, on ebay. It's easy enough to build one though.

There's some interesting stuff at Siegfried Linkwitz's site:
http://www.linkwitzlab.com

--
Roger.

Old Fart at Play wrote:

In the analogue world:

You can keep the low-pass and high-pass outputs in phase
with a 4th-order filter. I got a cheap Soundtech active crossover,
which does this, on ebay. It's easy enough to build one though.

I'll have a look for ready-made active crossovers and see how much the cost.


There's some interesting stuff at Siegfried Linkwitz's site:
http://www.linkwitzlab.com

That looks like a good read...


--
Wally
www.artbywally.com
www.wally.myby.co.uk/music

Old Fart at Play wrote:

Ian Bell wrote:

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


Easy enough to do with a digital signal.

A quick demo would be to low-pass a file with Goldwave or similar,
reverse the file, filter it again and reverse it again.

In the analogue world:

You can keep the low-pass and high-pass outputs in phase
with a 4th-order filter. I got a cheap Soundtech active crossover,
which does this, on ebay. It's easy enough to build one though.

There's some interesting stuff at Siegfried Linkwitz's site:
http://www.linkwitzlab.com



Which include some very nice graphs showing both the frequency and *phase*
response.

Ian

In message , Old Fart at Play
writes
You can keep the low-pass and high-pass outputs in phase
with a 4th-order filter.


Not without an additional all-pass filter you can't. And with an added
all-pass you can make a second order crossover have no phase difference
between the LP and HP sections if you want.
--
Chris Morriss

Chris Morriss wrote:

In message , Old Fart at Play
writes
You can keep the low-pass and high-pass outputs in phase
with a 4th-order filter.

Not without an additional all-pass filter you can't. And with an added
all-pass you can make a second order crossover have no phase difference
between the LP and HP sections if you want.

Perhaps you would like to refer to the Loudspeaker Design Cookbook
which has graphs of amplitude and phase for various filters
including the fourth order Linkwitz-Riley filter.

--
Roger.

Old Fart at Play wrote:
Chris Morriss wrote:

In message , Old Fart at Play
writes
You can keep the low-pass and high-pass outputs in phase
with a 4th-order filter.

Not without an additional all-pass filter you can't. And with an
added all-pass you can make a second order crossover have no phase
difference between the LP and HP sections if you want.

Perhaps you would like to refer to the Loudspeaker Design Cookbook
which has graphs of amplitude and phase for various filters
including the fourth order Linkwitz-Riley filter.

Remember that it's the acoustical 4th order Linkwitz-Riley that has the
so-called zero phase difference, not some speaker implemented with 4th order
crossovers. The acoustical 4th order is implemented with second-order
crossovers. The other second-order filters are the roll-offs of the speakers
themselves.

In message , Old Fart at Play
writes
Chris Morriss wrote:

In message , Old Fart at Play
writes
You can keep the low-pass and high-pass outputs in phase
with a 4th-order filter.

Not without an additional all-pass filter you can't. And with an added
all-pass you can make a second order crossover have no phase difference
between the LP and HP sections if you want.

Perhaps you would like to refer to the Loudspeaker Design Cookbook
which has graphs of amplitude and phase for various filters
including the fourth order Linkwitz-Riley filter.


OK, I've looked at that, and it doesn't support what you say at all. I
think you are getting confused between a crossover that keeps the
outputs of all the sections in phase at all frequencies (which can only
be done with even-ordered networks, and then only in conjunction with
all-pass phase-correction networks) and the family of crossovers that
attempt to sum to a flat frequency and phase response, even though the
individual outputs have phase differences between them.

(By the way, you can't get a passive filter to have LP and HP outputs in
phase with each other at all frequencies, as it's not possible to
produce the right sort of all-pass network with passive components.)
You can of course make passive networks that have a summed flat
magnitude and phase response, but this is a different thing entirely.

Although all-pass phase corrected active crossovers can be made, they
are not universally liked, as the extra group-delay added by the
phase-compensating all-pass networks mean that the total variation in
phase across the whole audio band can be very considerable. (Whether or
not this is audible on music is debatable).

--
Chris Morriss

In article , Chris Morriss
wrote:
In message , Old Fart at Play
writes
You can keep the low-pass and high-pass outputs in phase with a
4th-order filter.


Not without an additional all-pass filter you can't. And with an added
all-pass you can make a second order crossover have no phase difference
between the LP and HP sections if you want.

However you may not actually want that. :-)

Personally, for active filtering, I'd tend to prefer using a LPF, then
creating a HPF output by subtracting the LPF output from the input. The
result if you keep the levels matched is a LP and HP pair of signals whose
vector sum always equals the input. Thus the combined result shows no phase
errors due to the filtering.

For the actual filters I tend to lift the basic designs from the Active
Filter Cookbook by Don Lancaster.

Slainte,

Jim

--
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