I created this circuit to amplify the frequencies the bass has problems of reproducing (frequencies under 100 Hz). This was part of an university project. This entire design is based upon three things. The filter (the sallen key filter shown left above), the sum operational amplifier and the equaliser. The equaliser is the capacitor in parallel with the resistor, just behind the filter. The final product is listed in the picture. I simulated this circuit in CircuitLab and I got a desired result. However while testing this circuit physically, it didn't work. I used as opamp the TL072. I would like to know if there are some improvements, you think should be made or if there are parts wrong. This circuit was made by scavanging some parts on the internet, with an exception to the equaliser, which I thought of my self.

Side information
Filter is a second order buttersworth filter with a cutoff frequency at 100 Hz.

The equaliser will at 100 Hz have a +90 degrees phase. This has been made to equalise the -90 degrees the filter will output at 100 Hz.

 

A couple things... A sallen-key filter doesn't quite look right when compared to the wiki website... it looks like the filter requires some positive feedback, but I haven't fully analyzed your circuit so, maybe it's okay.

http://en.wikipedia.org/wiki/Sallen–Key_topology

Have you included decoupling caps? This is so low frequency that it probably doesn't matter, but it's worth considering still just to make sure that it's not the problem.

Finally, have you taken a scope and function generator to see what's wrong and see if the output of each amplifier is giving you the output you think it should? Have you done any troubleshooting? Usually a second or third look at everything usually reveals the solution to me.

 

Also you didn´t show the power supply to the opamps, they need dual supply to work in this configuration.

 

the bass has problems of reproducing (frequencies under 100 Hz).

What sort of problem? Insufficient amplitude? Distortion?
Your schematic shows no power supply at all. Are you using a dual (i.e. +/-) supply?

 

Here is some clarification.
What I meant by problems reproducing low frequencies, is a low return in acoustic sounds in the lower frequency range, which according to my lecturers happens in all speakers. So to combat this low return, I increase the voltage over these frequencies.

This is a pre-amp so I'm working with voltages around 0.8 volts. My source however is -18 to 18 volts. Which I believe is enough.

Audio_In->BassAmplifier(Increase bass voltage)->NormalAmplifier->Filter{low, mid, high}->speakers

 

However while testing this circuit physically, it didn't work.

This is what is meant by what sort of problems are you having. Mind you that 3db is not much, so you need to measure the response and not test it by ear.

 

Got any Bass problems?



Get an Ampeg, and a Rickenbacker 4001 or 4003.

 

This is a pre-amp so I'm working with voltages around 0.8 volts. My source however is -18 to 18 volts. Which I believe is enough.

Audio_In->BassAmplifier(Increase bass voltage)->NormalAmplifier->Filter{low, mid, high}->speakers

The statement in red makes me wonder,..
Where is your filter circuit is inserted in the signal path?

 

Got any Bass problems?



Get an Ampeg, and a Rickenbacker 4001 or 4003.

My Bucket List includes a B15N (original)

 

Just boosting all the bass frequencies generally just results in boomy bass. You need to view the response of the particular speaker you have and then boost the frequencies below the rolloff frequency of the speaker for proper equalization.