Design an Equalizer w/ LM833 Op-Amp & Combine Datasheet Filters

[Tesladude]

I am looking to design an equalizer using the lm833 op-amp.
I have used the lm833 to make an active crossover and pre-amp but what i really need it to use one of the op-amps in each lm833 as a bandpass filter to make the different frequencies
So that i can have 2 adjustable frequencies for each chip.

Is there a way to combine the 2nd order high and low pass filters available in the datasheet to make a bandpass?

 

[dlgoff]

I took this snapshot image from the Texas Instruments data sheet. You only need one of the op-amps to make a band-pass filter.

 

[analogdesign]

Sure it is possible (EDIT see dlgoff's post) If you insist on using a single op-amp, though, you will have to compromise since you can't independently optimize the roll off and the quality factor with a single-opamp topology. Check out this tutorial from Texas Instruments: http://www.ti.com/lit/ml/sloa088/sloa088.pdf I recommend the Salen-Key topology shown on page 16-30. I've used these for Audio filters and they have been used in this fashion in various analog music synthesizers over the years...

If you want good performance from an audio standpoint, however, the much better technique is to put a 2nd-order high-pass filter in series with a 2nd-order low-pass filter to implement a bandpass. This will use two opamps but you'll have sharper corners since both skirts will roll off at 12 dB/octave. In the circuit dlgoff proposes the skirts roll off at 6 dB/octave. That might be good enough. It's up to you.

 

[Tesladude]

Ok my plan was to have a 12db cutoff.
So I am infering that it is not possible to use one op amp for a 12db/octave bandpass filter, right?

 

[Svein]

Déja vu ... I designed an equalizer back in '75 using gyrators. I seem to remember that it had a very good S/N ratio. If somebody is interested, I will try to dig up the schematics and publish them.

 

[analogdesign]

Ok my plan was to have a 12db cutoff.
So I am infering that it is not possible to use one op amp for a 12db/octave bandpass filter, right?

That is correct. 12dB/oct implies two poles (or zeros). One of the poles is giving you the low-pass part and the other is giving you the high-pass. Since each pole or zero only gives you 6db/octave you're stuck unless you use another op amp.

 

[Tesladude]

Ok now how do i set it up to make +- 12 db?
It can't be just a potenciometer on the input because that would change volume from full to 0

 

[analogdesign]

Like I said before you need a higher order filter, so you need two op amps. The most straighforward way is to put a 2nd order high-pass in series with a second order lowpass

 

[dlgoff]

If you want good performance from an audio standpoint, however, the much better technique is to put a 2nd-order high-pass filter in series with a 2nd-order low-pass filter to implement a bandpass. This will use two opamps but you'll have sharper corners since both skirts will roll off at 12 dB/octave.

I totally agree. And they don't call you analogdesign for nothin'

 

[Tesladude]

No sorry i wasnt clear :)

Im talking about potenciometer used to adjust the db of this channel.
I have a highpass in series with a low pass now how do i make the output variable between -12 and +12db

 

[analogdesign]

No sorry i wasnt clear :)

Im talking about potenciometer used to adjust the db of this channel.
I have a highpass in series with a low pass now how do i make the output variable between -12 and +12db

Oh OK, I get you. The best way to do this is with a gain stage AFTER the filter. Make the gain of the filter as big as you can and then hopefully then you can just use a voltage divider (with a single pot and one fixed resistor) after the filter to attenuate. If you have to use an amplifier after the filter your design will be noisy but you may have no choice.

 

[Tesladude]

Ok awesome i know how to cut the voltage with a pot and resistor but here is my problem, what voltage translates to db?
For example with speakers half or twice the voltage to the speaker is -/+6 db,
Would this work the same on the low level equalized output we are creating?

 

[analogdesign]

Yep. Cutting the voltage in half is always a reduction of 6 dB. It doesn't matter where in the signal chain you do it from that standpoint (although you should do it at the end from a noise standpoint)

 

[Tesladude]

Ok great! I can calculate it then.
And your saying to reduce noise i should put the potenciometer after the 2 filters?

 

[analogdesign]

Ok great! I can calculate it then.
And your saying to reduce noise i should put the potenciometer after the 2 filters?

Yes. You should put as much gain as possible as early into the signal chain as you can.

 

[Tesladude]

You mean use the op amps to increase db and not just use them as buffers for the filters?

 

[Svein]

Searching the web, it's amazing what you can find! This construction (http://www.geofex.com/article_folders/eqs/paramet.htm) is almost exactly like the construction I did back in 1980. I may have used slightly different values, but for all practical purposes...

Observe that it uses only one OpAmp per channel and that the frequency dependent part is "outside" the amplification.

 

[analogdesign]

You mean use the op amps to increase db and not just use them as buffers for the filters?

That's one option. Another option is to put a preamp before the eq. I would build it with just the eq first and see if the noise is too high. If it is, put a preamp in front.

 

[Tesladude]

How much am i supposed to gain the signal by?
And I am a little confused on whybi need to boost the signal...

 

[analogdesign]

How much am i supposed to gain the signal by?
And I am a little confused on whybi need to boost the signal...

Try to use as much of the dynamic range of the eq as possible. The idea is noise is roughly constant (from a power standpoint) so you want the signal as strong as possible to optimize signal-to-noise. If your signal is really low (no gain) you may find it noisy.

So I would suggest not worrying about gain at first. If the eq is too noisy for your taste, put an amplifier before the eq. Gain the signal by as much as you can without introducing distortion. The distortion will be audible and will limit how much gain you can apply.

Good luck!

 

[Tesladude]

Ok great!
Sorry I havnt responded for a while.
This all had helped a lot and once i find time to experiment i may have more questions so ill make another thread, see you all later!