Preamp low pass filter
 

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

Preamp low pass filter
 

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

Preamp low pass filter
 

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.

Preamp low pass filter
 

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.
--
Chris Morriss

Preamp low pass filter
 

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

Preamp low pass filter
 

Plenty of "Pro Audio" crossover that are perfect for what you ask. Not
expensive, either.

Preamp low pass filter
 

Ian Bell wrote:

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.

Cheers.


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

Oh well - I was hoping there might be some clever chippery that could do
that sort of thing.


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.

I dare say - I want to be able to match the slope to whatever response I get
out of the upper bass / mid speakers.


Can a subsonic shelf be incorporated?

No problem, in fact hard not to do this.

Bypass is easy too.

Good-o.


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

Preamp low pass filter
 

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

Preamp low pass filter
 

Chris Morriss wrote:

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).

Cheers for that - downloaded.


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

Preamp low pass filter
 

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