Simple audio band-pass filters with sharp cut-offs

[[email protected]]

I want to split an audio signal into 5 bands by frequency range (top
and bottom can be low-pass and high-pass) and send these into speakers
with moderate power, say 10 watts. I can use amplified speakers if a
5-way audio amp gets complicated.

This is not for quality audio, but for educational demo purposes, so I
don't need an even response and distortion is okay. I'm really looking
for the sharpest cut-offs I can get, in as simple a box as I can build.
Can anyone suggest how to accomplish this?

I haven't made many coils before (rather embarassing for a ham op), but
I'm fine with simpler circuits. I found a post about an IC that might
do the trick, but I'm doubting it'll handle audio frequencies.

 

[Charles Schuler]

I want to split an audio signal into 5 bands by frequency range (top
and bottom can be low-pass and high-pass) and send these into speakers
with moderate power, say 10 watts. I can use amplified speakers if a
5-way audio amp gets complicated.

This is not for quality audio, but for educational demo purposes, so I
don't need an even response and distortion is okay. I'm really looking
for the sharpest cut-offs I can get, in as simple a box as I can build.
Can anyone suggest how to accomplish this?

I haven't made many coils before (rather embarassing for a ham op), but
I'm fine with simpler circuits. I found a post about an IC that might
do the trick, but I'm doubting it'll handle audio frequencies.

Don't use coils. Use active filters. I think there is an "Active Filter
Cookbook" or somesuch floating around out there. You might be able to get a
copy for a few bucks.

 

[[email protected]]

I want to split an audio signal into 5 bands by frequency range (top
and bottom can be low-pass and high-pass) and send these into speakers
with moderate power, say 10 watts. I can use amplified speakers if a
5-way audio amp gets complicated.

This is not for quality audio, but for educational demo purposes, so I
don't need an even response and distortion is okay. I'm really looking
for the sharpest cut-offs I can get, in as simple a box as I can build.
Can anyone suggest how to accomplish this?

No coils are required if you go to
http://www.linear.com/designtools/filtercad.jsp and get this program.
Just answer a few questions and you will be presented with a fully
designed circuit. The IC's are probably going to be surface mount
ones, but there are easy ways to handle that.

Jim

 

[Roger Hamlett]

I want to split an audio signal into 5 bands by frequency range (top
and bottom can be low-pass and high-pass) and send these into speakers
with moderate power, say 10 watts. I can use amplified speakers if a
5-way audio amp gets complicated.

This is not for quality audio, but for educational demo purposes, so I
don't need an even response and distortion is okay. I'm really looking
for the sharpest cut-offs I can get, in as simple a box as I can build.
Can anyone suggest how to accomplish this?

I haven't made many coils before (rather embarassing for a ham op), but
I'm fine with simpler circuits. I found a post about an IC that might
do the trick, but I'm doubting it'll handle audio frequencies.

There are several IC's on the market that do exactly this. You could also
look at active filter circuits based on op-amps. A lot will depend on the
nature of the signal, and what you want to actually 'do' with the signal.
A sharp cutoff, will probably involve a several stage filter, and this
will be much easier with an op-amp, or an IC. This is why inductors have
almost vanished for this type of application...

Best Wishes

 

[Ben Bradley]

I want to split an audio signal into 5 bands by frequency range (top
and bottom can be low-pass and high-pass) and send these into speakers
with moderate power, say 10 watts. I can use amplified speakers if a
5-way audio amp gets complicated.

This is not for quality audio, but for educational demo purposes, so I
don't need an even response and distortion is okay. I'm really looking
for the sharpest cut-offs I can get, in as simple a box as I can build.
Can anyone suggest how to accomplish this?

As in so much of engineering, you have competing demands which
means you have to decide where the tradeoff is. Sharper cutoffs means
higher pole filters, which require more components with closer
tolerances. ISTR Lancaster's Active Filter Cookbook discusses this.

The 'best' solution technically would be an A/D, DSP, and five
D/A's, as the DSP can be programmed to do quite sharp filters and the
result will be much more stable than you could possibly get with
analog components.

I haven't made many coils before (rather embarassing for a ham op), but

You wouldn't want to do this with coils.

 

[Walter Harley]

[...]
Don't use coils. Use active filters. I think there is an "Active Filter
Cookbook" or somesuch floating around out there. You might be able to get
a copy for a few bucks.

You'll be well served by Don Lancaster's "Active Filter Cookbook", 2ed.,
ISBN 075062986X.

Or, google "Sallen-Key bandpass filter" and you'll probably come up with
something.

 

[Michael Black]

[...]
Don't use coils. Use active filters. I think there is an "Active Filter
Cookbook" or somesuch floating around out there. You might be able to get
a copy for a few bucks.

You'll be well served by Don Lancaster's "Active Filter Cookbook", 2ed.,
ISBN 075062986X.

Is that still being printed by a traditional publisher, or does
the poster have to visit Don's site, http://www.tinaja.com to get it?

Michael

 

[[email protected]]

Beautiful! I'll check out the Active Filter Cookbook (my library has
it!), but if there's a good IC for it, so much the better. I just want
to take a line-level or microphone input (not powered) and have it put
the sound out at line level (I'll feed them into amplified "computer"
speakers), each representing a different frequency range (probably
0-200Hz, 200-400, 400-800, 800-1600, 1600+) so I can turn channels on
and off, and kids can experience fourier decomposition concepts and
filtering in real-time. Again, quality is not a big issue (although
reliability is).

Can you suggest where to look nowadays for ICs like this, or an IC that
would work well? I used to use the old radio shack semiconductor
reference. It's been a while. I'm guessing the Active Filter Cookbook
doesn't include ICs like this, so are there any "cookbook" schematics
for these handy ICs?

Thanks for all the great advice!

 

[John Popelish]

Beautiful! I'll check out the Active Filter Cookbook (my library has
it!), but if there's a good IC for it, so much the better. I just want
to take a line-level or microphone input (not powered) and have it put
the sound out at line level (I'll feed them into amplified "computer"
speakers), each representing a different frequency range (probably
0-200Hz, 200-400, 400-800, 800-1600, 1600+) so I can turn channels on
and off, and kids can experience fourier decomposition concepts and
filtering in real-time. Again, quality is not a big issue (although
reliability is).

Can you suggest where to look nowadays for ICs like this, or an IC that
would work well? I used to use the old radio shack semiconductor
reference. It's been a while. I'm guessing the Active Filter Cookbook
doesn't include ICs like this, so are there any "cookbook" schematics
for these handy ICs?

In the cook book, look for a 4 pole bandpass filter. This produces a
double peak result (one near the low frequency end of the band and one
at the high frequency end of the band) with sharp enough roll off at
the edges for this application.

Of you can combine a 2 pole high pass (to cut the low end) and a 2
pole low pass (to cut the high end). If you choose a bi quad or state
variable design for these, you will be able to adjust the cut off
frequencies with a pot without altering much of anything else about
the response. Good for demonstrating what more bits in the Fourier
does for the result.

Or you can use just 2 pole bandpass filters (single peak response)
made as a bi quad or state variable and adjust both the center
frequency and bandwidth as desired. These take 3 opamps each, but
since you can get 4 opamps in a single package for less than a dollar,
this is not much of a problem. The LM324 is a good choice for cheap
quad opamp that runs on a wide range of supply voltages with low
supply current, if sound quality is not a big concern (it does produce
a bit of distortion at higher audio frequencies). Common as dirt.
http://www.onsemi.com/pub/Collateral/LM324-D.PDF

 

[Ben Bradley]

Beautiful! I'll check out the Active Filter Cookbook (my library has
it!), but if there's a good IC for it, so much the better. I just want
to take a line-level or microphone input (not powered) and have it put
the sound out at line level (I'll feed them into amplified "computer"
speakers), each representing a different frequency range (probably
0-200Hz, 200-400, 400-800, 800-1600, 1600+) so I can turn channels on
and off, and kids can experience fourier decomposition concepts and
filtering in real-time. Again, quality is not a big issue (although
reliability is).

Aha. In this application you don't neccesarily need the steep
cutoff slopes. You can use a fairly low-Q (perhaps in the 0.3 to 1
range) bandpass filters, one for each band.
Another idea comes to mind: a five-band audio graphic equalizer, as
made by just about everyone in the '70's and '80's. Used ones are
surely plentiful and cheap nowadays. Even 7-band and 10-band EQ's are
common and cheap. Look on ebay. In such an EQ, all the filters are
summed to one output, but you can remove the summing network and run
each filter output to a separate amplifier/speaker. This is easier
than making your own circuit.

 

[Walter Harley]

Is that still being printed by a traditional publisher, or does
the poster have to visit Don's site, http://www.tinaja.com to get it?

I bought mine from Powell's a couple years ago, and it was still in print
then. It's on the Newnes imprint.

 

[Joel Kolstad]

Good recommendation for audio range active filters.

I bought mine from Powell's a couple years ago, and it was still in print
then. It's on the Newnes imprint.

I got mine a year or so ago from Marlin P. Jones who, at the time, was selling
off a bunch that were autographed by Don himself for no more than the regular
price of the book.

 

[Joel Kolstad]

One other suggestion: At audio frequencies, switched capacitor filters are
quite good, and since you don't care about a little distortion anyway, you can
use a dirt simple anti-image filter at the output.

The beauty of switched capacitor filters is that 'everything' scales with the
input clock frequency. I.e., if you design the thing so that you end up with
a 5kHz Q=10 bandpass with an input clock of, say, 1MHz, if you halve the input
clock to 500kHz you'll have a 2.5kHz Q=10 bandpass filter. Another nice
benefit is that most of the tuning elemenets for active filter ICs are on-chip
and thereby tend to be matched (ratioed) to one another much better than you'd
be able to achieve by just randomly choosing components (of the same nominal
value) from your parts drawer; if you decide to start building significantly
narrower filters that what you've specified, this can become significant. (I
had a professor who claimed that 'all good IC designs are beautiful to look
at,' and while I think that's sometimes just wishful thinking and have even
read papers suggesting as much, his point was that you can derive a few simple
rules for IC layout that attempt to minimize component ratio tolerances and
these rules end up making symmetrical patterns that tend to be aesthetically
pleasing as well...)

The Linear Tech LTC1562 might be a good place to start...

 

[ehsjr]

I got mine a year or so ago from Marlin P. Jones who, at the time, was selling
off a bunch that were autographed by Don himself for no more than the regular
price of the book.

Good information! I looked - MPJA still sells them.
http://www.mpja.com/directview.asp?product=7245+BK

Thanks,
Ed

 

[Ben Bradley]

I bought mine from Powell's a couple years ago, and it was still in print
then. It's on the Newnes imprint.

http://www.bookfinder.com has many copies, a few are quite cheap, but
most prices are in the $20-$30 range, where the new copy prices start.

I found this amusing entry under the title "Active-Filter Cookbook by
Lancaster, Donald E.":

"TRUE COLLECTABLE, AUTHOR'S SIGNATURE Tight, straight, clean, cover
shows wear as well as edges of text. This book has been used but not
in your normal cooking. $19.50"