I am new here so hello everyone! before i begin, i think its really cool that there is a forum dedicated to circuits. I am going to enjoy this forum alot as I am just getting into circuits now. anyways, here is my problem.


I am building a boom box. I am using 741CN op-amps to build my active analogue filters.

My arrangement is below:

as I am building it, the first time the whole thing worked completely. I was very pleased with it. afterwards I got crazy and introduced some audio amplifiers and a summing amplifier to do a karaoke mode. I guess my audio amplifier in front of the summer blew a few op amps so I started replacing them.


now my first low pass filter barely spits out any music. i can BARELY hear it. before it was loud and audible. that weak weak signal from the first lpf goes into my hpf, and hpf picks it up, amplifies it, and I can hear it coming out of audio amplifier.

however! the weak weak signal goes even weaker after the second lpf, which is confusing me because, 1. never happened first time I built it 2. I do not know why my lpf has blocked the signal completely.

I checked all of my connections. they are 100% correct, trust me I spent 5 hours trying to figure out why today.

since weak signal goes into second lpf, a even weaker signal comes out of second lpf, and my audio amplifier doent seem to pick it up. I do not have music coming out of my bass channel. my treble channel works flawlessly as hpf are somehow good at picking up, amplifying, and spitting out very music from very small signals.


somebody help! I have been working on fixing the problem for more than 10 hours!!!!

 

all of my filters are 3rd order bessel active filters with 741CN op amps.

 

Can you post up a schematic?

 

What are the gains and cutoff frequencies of all of your filters?

If the first lpf, which drives both the second lpf and the hpf, is not functioning correctly, do you notice any difference on the hpf side? I would think that volume would be down, or out, on the hpf side as well.

How is the circuit constructed? Is it a PCB, protoboard, ...?

Which op amp parts did you replace?

Did you completely isolate the different filter stages when you tested them?

What type of test equipment to you have?

 

hey thanks for the replies. I am posting schematics I drew in SPICE of the three filters. I have not done a whole board. I have to scan one in.


first lpf cuts off frequencies at 10,000 hertz. no gain.
second lpf cuts off frequencies beyond 5900 hertz. no gain.
first hpf cuts off frequencies below 2900 hertz. no gain.


as soon as I figured some op amps blew, I replaced all of them. I got new ones and put it in.

I have built all three and probed three filters. the first lpf has very very weak signal coming out. my first hpf is working fine even though its using the weak signal from my lpf.

my second lpf is not working, even if I connect the signal directly to it, skipping the first lpf. the signal is yet again too small to be picked up by my audio amplifier.

my hpf is working if I skip the first lpf. which is good.

I think the problem lies in the first or second lpf. I have no idea why though! I tried replacing everything, even the wires. i took a picture of my circuit when it was working. I rebuilt it referencing the picture, node to node!!!


I am building the entire thing on a breadboard. today I will try replacing the breadboard. tell me what you think. I don't think lpfs are supposed to lower signals at all. i have not set a specific gain, so they should not be amplified either.

my volume is turned all the way up on the ipod.

 

The output of the IPOD is still working nicely, right?

The first place to start is after the first op amp in the first lpf. Do you get what you expect there? Check after each op amp until you find something that you do not expect.

 

the first lpf has output signal. my voltage meter indicates that there are identical strength in AC voltage coming out of my first lpf. yet when i probe it with a speaker I do not hear any music.

second lpf, voltage is almost none and no music.
I will try rebuilding it today, i dont know what else to do after that fails.
do you have any triggers as to why my lpfs are messing up?!

 

Are you trying to drive the speaker directly from the 741 op amp? I don't think they typically have enough power for most speakers.

 

A 741 opamp can drive 2k ohms. An 8 ohm speaker is like a dead short to its output. You need a power amplifier to drive an 8 ohm speaker.

Your opamps are not powered.
Do they have a dual-polarity supply? What are its voltages?

 

my hpf can make the speaker go off. my lpfs use to make the speakers go off as well, before the audio amplifier.

there is a definite possibility my op camps are not powered. my vcc+ and vcc- are set at 12 volts. current is limited to 0.1A. im going to try powering the op amps directly and see what happens today. will keep you guys updated. hopefully it works. fingers crossed.

 

ITS WORKING!!!!


Somehow out of the blue it started working. I have no idea why. Maybe I nudged something. still has no clear reason why. i am really glad though. no design faults. it has alot more noise than before but hey, what can I say I am just glad I have something. Thanks for all the help!