op amp gain, woofer compensation?

[Joules]

Do you suppose its posible to add some bass gain to an input buffer op amp? I have a power amp that I use just for bass, this amp uses an input buffet op amp IC, with the typical feed back loop to set it's gain.
If I were to insert the appropriate resistors and capacitor in this feedback loop wouldn't I be able increase the gain at lower freq's to help compensate for the natural roll off of the woofer?

there is a little more to this but I think this is good for now.

[moamps]

The idea sounds fine. More useful info is to be found here: http://www.linkwitzlab.com/filters.htm

Regards,
Milan

[FastEddy]

I once built a chip amp with an op-amp in front and then put a notch filter in the feed back loop set around 15+K Htz (inverted band pass made of caps and resistors) ... and it worked quite well as a gopher chaser, oscilating nicely in the area that messes with the gophers' sex life ... got rid of all the gophers in the garden, which was the purpose.

My advise would be to keep anything in the feed back loop resistive only (non-inductive and non-capacitative) ... capacitors and inductors in the op-amp feed back circuit can cause oscilations in op-amps.

[unclejed613]

you could do something similar to this......

if you have a 10k resistor in the feedback loop of the op amp (from the output to the inverting input), put a .03uf cap in parallel with the resistor, this will turn the op amp into an integrator with a 520Hz corner frequency.
if the feedback resistor is another value, or you want a different corner frequency, go to: http://www.muzique.com/schem/filter.htm

and calculate the required cap value.

[Joules]

Thanks -
Using the feedback/gain formula, with Xc thrown in, It looks like I can get what I want. But I don't want to go ripping into my nice expensive amp to find out theres more to the story. These are some good points raised.

[unclejed613]

you could just put an RC filter at the input of the amp with the desired rolloff frequency, or build an active filter. a more expensive proposition is to use a speaker crossover with the mid and tweeter replaced by 8 ohm resistors. none of these would require modification of the amp itself.

btw, the reason that "gopher chaser" did what it did, was that the "notch" was actually close to open loop gain at that frequency, with 180 degrees phase shift....... sounds like an oscillator to me.....

[FastEddy]

" ... if you have a 10k resistor in the feedback loop of the op amp (from the output to the inverting input), put a .03uf cap in parallel with the resistor, this will turn the op amp into an integrator with a 520Hz corner frequency. ..."

As long as you don't try to increase that feedback resistor too much (like beyond 100K or 10 times current value) ... then chances are the op-amp won't go into oscolation ... maybe ...

(" ... "gopher chaser" did what it did, was that the "notch" was actually close to open loop gain at that frequency, with 180 degrees phase shift....... sounds like an oscillator to me ..." == exactly correct and that was the intent.)