Understanding an active low pass filter - beginner's question

Discussion in 'General Electronics Chat' started by ogg, Sep 25, 2009.

  1. ogg

    Thread Starter New Member

    Sep 23, 2009

    I'm working on learning about filter design and have seen this guide (http://alpha.science.unitn.it/~bassi/Signal/TInotes/sloa049.pdf) to active low pass filters recommended in the forums a few times. I captured and simulated the "original" filter on page 15 (see attached image) and found that changing the value of R1 changes the frequency response. So I built it on a breadboard and put a variable resistor there, but no sound comes out. I am just starting out and would be grateful for some help figuring out why and understanding the circuit.

    For the audio source and output, I have stripped the cover from a jack cable plugged into a keyboard, connected the ground wire directly to a small speaker and the other wire to the left side terminal on my circuit. The right side terminal connects to the other speaker input. Connecting the two jack cable wires directly to the speaker plays a sound, so I'm inferring that these connections are fine and the problem is with the circuit.

    I changed C1's connection to the negative battery terminal rather than ground used in the pdf because I'm working with batteries and don't have a true ground. I'm surprised that this made no difference to the output of the simulator, but then the op amp works without power in simulation so perhaps it's not modelling this correctly. Will this make a difference?

    I'm using the maligned 741 as it's the only op amp I have to hand. From what I've read about it here, I understand that it might not work so well at higher frequencies, but I don't know at what frequencies it could cause problems. From my simulations, I find that the filter's peak is at 2.5kHz when R1 is 200k, and 100kHz when R1 is 200k. Given this range, I'm assuming that it may not be ideal but I should at least hear something, is this right?

    Also I'm only supplying the 741 with 9 volts. I see that it should have 15V, but I'm guessing that it would still do something with 9? Will it work on 9V, or do I have to go to 15? Will the performance and amplifying range degrade with lower voltages, or is there a minimum voltage requirement for it to do anything? My suspicion is that this is why there is no sound, am I right or is there more to it?

    Any help appreciated, thanks.


  2. flat5

    Active Member

    Nov 13, 2008
    Start with a data sheet for the 741.
  3. hgmjr

    Retired Moderator

    Jan 28, 2005
    If I understand your verbal description of your circuit hook-up, you have connected the speaker directly to the output of the 741. If so, then that is why you are not getting any output since the 741 is not capable of driving a speaker directly.

    Last edited: Sep 25, 2009
  4. Wendy


    Mar 24, 2008
    Problem is, the data sheet for a 741 doesn't include minimum voltage, and some 741's will not work with 9V. If you must use a 741 then use two 9V batteries for a ± power supply. You can also use a LM324, which is a quad op amp that is speced down to 2V minimum. Both these op amps have pretty bad specs for audio, notibily crossover distortion, so if this is for something real (other than learning) you may want to bump up to a better chippie later.

    After I posted this I saw hgmjr's post, I missed the speaker reference. You will need to build up a separate audio amp, or make a driver for the op amp. We have had many posts on the subject, so there is lots of information out there, I'm pulling up the first convienent schematic that would work.



    There is another chip, the LM386, that is an audio amp on a chip.
  5. SgtWookie


    Jul 17, 2007
    Even with a very light load (>10k Ohms) the 741 can't get within around 2v of the rails. With a 9v supply, that would mean the output could range from 2v to 7v, for 5v swing total.

    But that's with a 10k or higher Ohm load on the output. A speaker is typically 4 to 8 Ohms impedance, which looks like a dead short to the 741's output.

    You will need an audio amplifier circuit on the output of the 741. An LM386 IC is an inexpensive low-power audio amplifier.

    Don't expect anything approaching decent audio sound quality out of a 741. If you want a much better opamp, use a TL071, TL072, or TL074 - single, dual and quad opamps.
  6. ogg

    Thread Starter New Member

    Sep 23, 2009
    Yes I was connecting the 741 directly to the speaker (also hgmjr, the schematic is attached to my post). I connected it to an amp and now I hear sound, thanks! Curiously it sounds the same with power to the op amp and without, could this mean I haven't powered the op amp correctly? I set up 15V of batteries in series and used them to power it.

    It's also hard to tell exactly what the pot is doing. When I adjust it, I hear a low pass filter sweep for a while then it sounds like a high pass when the wiper reaches the other end. I am using a very cheap amp so this could be affecting the sound too. Is there a way to check the effect of the filter without using an oscilloscope, or will I need to get one to do this?

    This is all for learning at this stage btw, so I'm not after hi-fidelity but I do want the components to be good enough to learn from.
  7. Audioguru


    Dec 20, 2007
    Your opamp is not working because its DC input voltage is not at half the supply voltage.
    This filter must be fed from a very low impedance like the output of an opamp.

    Lookup Sallen-Key Butterworth lowpass filter in Google.
    You will see that a Butterworth response is flat then sharply drops above the cutoff frequency. Usually a Butterworth response is made with equal-value resistors then the value of C2 is double the value of C1 or both capacitors can have the same value then the opamp has a gain of about 1.6 times.

    Try it like this:
  8. Wendy


    Mar 24, 2008
    To really see the effect you'll need a variable frequency source. You'll be able to hear the volume change as the frequency goes up that way. A really quick and dirty source would be something like the 555 Hysteretic Oscillator. Use the signal at pin 2 and 6, it is a triangle wave, so approximates a sine wave that much closer, though it is easy to kill the oscillator if you overuse it.
  9. ogg

    Thread Starter New Member

    Sep 23, 2009
    Thanks for all of your help. I am reading up on the impedance and op amp voltage details. I've made a simulation of the circuit to see its effect - in theory at least! It's online here if you'd like to take a look, there are some other simple music circuits there too.

    Best wishes,