Continue to Site

Welcome to

Welcome to our site! is an international Electronics Discussion Forum focused on EDA software, circuits, schematics, books, theory, papers, asic, pld, 8051, DSP, Network, RF, Analog Design, PCB, Service Manuals... and a whole lot more! To participate you need to register. Registration is free. Click here to register now.

OpAmp Filter not working as it should

Not open for further replies.


Junior Member level 3
Feb 13, 2013
Reaction score
Trophy points
Activity points
I'm making musical lights; lights flashing in rythm with music. I started this project last spring, and I'm making it at school. Ihad other courses in between so I haven't been on it for a while, but I'm able toreturn to it now.

It Consists of 4 stages:

First is the microphone, I haven't tested this particular design yet, just took a schematic off the internet.

Second is the Aplifier stage, which I have made out of 2 stages for some reason, I can't remember why, but there must've been a reason.

Third stage is the filtering. The frequencies are filtered into 4 channels:

LOW= 0-800 Hz,
LOWMID= 800-2400 Hz,
HIGHMID = 2400-4000 Hz,
HIGH => 4000 Hz.

However as I built these filters seperately and tested them, they turned out as follows:

LOW= 170-1000 Hz,
LOWMID= 450-2400 Hz,
HIGHMID = 1000-3000 Hz,
HIGH => 5000 Hz

While getting 100% precise results is difficult, these results are unacceptable. I don't have the material I used when designing these, so I can't tell you what formulas I used to calculate the size of every component. This is the part I need help with; getting these filters to match description.

The final stage powers the lamps through a triac with 230V.

If memory serves me right, then the amplification stage works more or less as intended. I haven't built the final stage yet, so that's just hypothetically correct. I'll build the last stage after I get the first 3 to work in unison with LEDs (for testing purposes)

All part numbers SHOULD be correct.

ALSO; if you notice anyhthing else wrong with the schematics, please tell.

Musical lights.jpg

I suggest you double check the filter designs with Texas Instruments Filter Pro (free). It will also tell you the recommended GBW for the opamp to get the desired response.

Also, what did you use for the capacitors? You need a tight tolerance and a decent quality capacitor for filtering. Large values of capacitor will often be made of unsuitable dielectrics unless you choose carefully.

Did you implement it in board physical?
It is being requested you to split it in small portion of whole circuit and try to run it in Proteus.
I think you have done already it.
Then check the capacitor value it is so sensitive to get multistage filtering.
Keep in touch.
Best of luck

Ok, got new designs with the help of TI, don't have time to test them out physically atm, so in the mean time, anyone with AC experience find that stage 4 possibly correct? Getting it wrong would give.. shocking.. results. J1 and J2 are where the AC comes in, the other pin pairs are for AC lamps

I have just noticed one thing that I think is not correct - you have created mid rail points with pairs of 150k resistors which you have then connected with precision resistors to part of the filtering circuit. Have your taken into account the effect of the 150k resistors on the precision resistors?


- - - Updated - - -

p.s. If that is the problem, decoupling the junctions of the 150k pairs to ground should fix it.

- - - Updated - - -

p.p.s I have checked one part of the filter - the "high pass" and you definitely have a problem with the 150k resistors. The values of 3.3nF/3.3nF with 8.5k/17k resistors are correct for a positive, single amplifier biquad at 4kHz. With the 150k resistors you will get peaking (and possible instability) at around 2kHz.

Firstly, I haven't considered the effect of the 150k resistors on the precision resistors since I don't know what effect they would have on them :D.

Secondly, I think I should go with the filters that TI app that you suggested for me compiled, since the filters I made look very messy, at least compared to the ones the TI app made. No need to even use parallel capacitors to get a value that we don't have in stock.

Thanks Mr. Keith

Not open for further replies.

Similar threads

Part and Inventory Search

Welcome to