davepusey

I need to build an audio line-level band-pass filter.

It will have:
two connecters... audio in and audio out
three pots... lower frequecy cutoff, higher frequency cutoff, and
amount of attenuation of unwanted frequences.
one switch... bypass filter on/off

Now what i need is some circuit ideas please. thanks.

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Dave Pusey.
PIC Programming Tutorials: http://www.mstracey.btinternet.co.uk...al/picmain.htm
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Nigel Goodwin

Try looking for 'active crossovers', you might have a look at http://sound.westhost.com/projects-3.htm which contains a number of examples.

Wanting variable frequency controls makes life a LOT more difficult, multi-gang controls are probably essential - using switched capacitor filters may be easier.

Variable attenuattion isn't usually offered either, it's not normally a requirement. You don't specify how many db's per octave you want, this is the slope of the filter - the higher the value the sharper the filter drops off, but the greater the phase distortion.

What exactly are you trying to do?.

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stevez

A few years ago I had considered building my own audio system. It's unlikely that it would have outperformed basic commercial equipment however it would be an accomplishment. In the little research I had done I had concluded that my controls would be more like the graphic equalizer (not sure the word "graphic" belongs) because the ICs appeared to be readily available. This approach might be a less complex alternative - just a thought.

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stevez

Nigel Goodwin

You have just described "scratch and rumble" filters, commonly used to reduce problems with vinyl recordings.

These are totally different to tone controls and graphic equalisers (which is just an enhanced tone control).

If you are wanting to build a quality audio system, the first thing you do is leave out any tone controls! - you shouldn't need to be compensating for poor quality components.

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audioguru

Nigel,
High quality components are not perfect yet, especially speakers that can benefit from EQ to flatten their response, and even speakers that have built-in passive EQ. Very few, if any, speakers have a flat response down to the lower frequency limit of hearing (20Hz), where a boost helps a lot.
Many sub-woofers are simply a high-Q resonant circuit with resultant delay and ringing. Frequently an EQed "regular" speaker sounds better.
EQ can make a mid-priced low-frills speaker sound better than an expensive "gold-plated" one.
EQ is also useful to flatten a room's resonant response (but only at one "sweet spot").
Harmonic and phase distortions caused by EQ circuits are minimal when compared to the distortions of speakers.

davepusey

What i need is a circuit that allows only a specific frequency range (set by the front panel knobs) to pass through and the rest to be removed.

__________________
Dave Pusey.
PIC Programming Tutorials: http://www.mstracey.btinternet.co.uk...al/picmain.htm
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stevez

What I intended to describe were what I'll call modern replacements for the tone controls. On my modest cost stereo at home I have three sliding pots that boost or attenuate various frequencies. An add on device that my son left behind has 8 controls that appear to be spread across the band. My car stereo has programmable settings for the various octaves that have names - presumably setting the response across the spectrum to suit the type of music or the listener. Again, it seemed to me that modest cost ICs allowed the low cost implementation of this approach. To me it's much better than the base and treble controls.

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stevez

Nigel Goodwin

Your requirements are extremely vague, you don't specify the slope of the filters, or the degree of attenuation required - this makes an absolutely massive difference to the design.

It would be a lot easier if you mentioned exactly what you are trying to do! - it's quite possible someone has already done it, or knows reasons not to try it.

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stevez

I took a second on google searching on 'graphic equalizer IC' and the first thing to come up was a BA3812L IC. I've never applied one nor do I know if this is a good one, old technology, etc. It would seem that there must be many choices.

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stevez

Nigel Goodwin

They are usually just multiple opamps, most of them configured as gyrators (not too sure that that's spelt correctly!). This is a way of making a capacitor act like an inductor.

The Elliot Sound Products site I gave the link to above gives plenty of details about graphic equalisers and their design.

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audioguru

Dave,
Why do you want to bandpass filter your audio?
Do you like everything to sound like a telephone?
Do you have a very boomy woofer and a shrieky piezo whistle (tweeter)?

davepusey

Morse code is sent as a tone burst of a specific frequency (usally 1khz but it varies depending on the sender's equipement). Some of the transmissions we receive are rather noisy and therefore if a band pass filter could just allow the morse tone frequency to pass through and remove all the background noise then that would be great!

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Dave Pusey.
PIC Programming Tutorials: http://www.mstracey.btinternet.co.uk...al/picmain.htm
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Nigel Goodwin

Well it's taken a long time, but you've finally mentioned what you want!.

It's an extremely common device, used by radio amateurs for that exact purpose - you don't need variable frequency or bandwidth, just a sharply tuned filter at about 1KHz. Morse code is transmitted as CW (carrier wave), the actual tone you hear is generated in the receiver itself (by the beat between the incoming signal and a 'carrier insertion oscillator'). So you tune the radio to match your filter.

Try looking on amateur radio sites, I've seen hundreds of projects in magazines over the years.

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davepusey

So what is the name of the project i am looking for?

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Dave Pusey.
PIC Programming Tutorials: http://www.mstracey.btinternet.co.uk...al/picmain.htm
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Nigel Goodwin

Try "cw audio filter"

The following link was found at http://www.discovercircuits.com/front/list.htm, and is probably what you need http://w1.859.telia.com/~u85920178/use/filt1.htm.

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