EQ State Variable Filter - Changing Opamp

[Razoumihine]

First I'd like to say hello  !
I've been reading this forum quite often, and would like to thanks every one for sharing his experiences and knowledges.

I'm on a state variable filter, inspired from fig 5 of the following link: http://sound.westhost.com/articles/state-variable.htm#s30

This circuit works very well with my electric guitar in, (with a buffer stage inserted before), but I would like to lower the noise of this circuit. Then I was thinking of changing TL072 with other Opamp for improving the circuit, but I don't know if it's a good idea, and if yes, I dont know how to choose an alternative opamp.

So if some of you have some simple clue to help me looking for a better opamp in this circuit, It would be great,
Thank you,

Lionel

[PRR]

Put a buffer/booster on front with a gain of 2 to 4. That gets guitar up above the universal hiss.

Then you probably need a volume pot on the output to cut-back what you boosted-up.

The input buffer/booster *may* need adjustable gain. Too much gain, the filter distorts. Too little gain, you are back in the hiss. With all the different pickups and picks and arms, it is hard to predict how much gain is good. Easier to trim and try.

The TL072 is not the core problem. It's that this filter (most filters) are designed for signals which have already been pre-amplified. Once you are closer to clipping than to hiss, you could consider '5532 opamps. The filter impedances are suitable, and the '5532 might have a hair less hiss.

[JFace]

I use the OPA2134 in a state variable filter pedal I made recently. Of all the opamps I have, it was the quietest. The TL072 is great, but it can be improved upon. The RC4558 is also a good choice-quiet and inexpensive. 5532 or 5534 are excellent, but you would likely prefer a JFET style opamp booster/buffer before it. If you haven't already, check out TI's sample chip offerings. I got a package of OPA2134 through that method.

[Razoumihine]

Well... Thanks for your advices!

It's amaizing how versatil is this filter. I roughly analyzed it thru Q-clone (I do with what I have....), and it is very nice. Changing the frequency really does not affect the Q or the gain like it does with a gyrator filter. Just amazing!
Following your advices I'll try to improve it, and also try to find out an acceptable ratio of input level / saturation / hiss.....
Also, beeing able to monitor and control the input level (clipping diode for example) seems quite important in order to use it with different instruments, having different signal levels...

I saw that OPA2134 is between 2 to 3€, where all the other discussed here are more like 0.30€. At least i'm curious to compare if there is a noticable difference... And so I will

To be continued....

Lionel

[JFace]

Look at the noise figure in the datasheets for these chips. Usually you don't notice this level of noise unless you start amplifying (like the resonance or Q of a filter). This is why I did my own experimentation to find a quiet substitution. TI has a free sample program, and incidentally you can get a handful of OPA chips for free