Hello,

I've made the audio filter circuit shown on a breadboard and it works fine. Now I've built and soldered it on a veroboard and it's not working - the sound is coming out clean with P3 (the dual pot) affecting the volume, not the frequency, and the resonance pots are doing nothing.

I've been through a few debugging steps, some several times, and I'm not sure what to do next. So far I have:

• Checked the veroboard layout correctly follows the schematic
• Checked all resistor and capacitor values on the veroboard
• Checked all continuities and breaks on the veroboard (and re-soldered a few connections)
• Checked all power and ground points on the veroboard. All are connected properly.
• Checked the inputs and outputs to the op-amp. All are within a few millivolts.

I am bit stumped by the error and how to debug it - if there was no sound I would check along the circuit to see where it was getting to. I don't know what might be causing there to be no effect.

I could have a false positive on a check, but before I test everything again it would be great to hear your suggestions of any other tests I could do and general thoughts about how to debug this circuit. I'm quite new to this - this is my first soldered board - and I'm looking to learn how to approach this kind of problem logically, rather than just checking everything again and again. Where would you start?

Thanks everyone,

Oggz

n.b. I haven't built the capacitor bank on the veroboard - seems an obvious thing to leave out at the start and add later.

 

How is P3 wired into the circuit? You need to show it on the schematic as you did with the other pots.

EDIT: 2AA batteries will not give you 4.5V. Did you mean 3 AAs?

 

I notice that your power source is labeled as two aa batteries with an expected output of 4.5 volts both on the negatve and positive outputs. Two AA batteries only produce 3 volts.

hgmjr

 

My mistake with the batteries - the circuit worked on the breadboard with two sets of two AA batteries, providing +3V, -3V and a virtual ground. I'll change the schematic.

P3 is the frequency pot, its Y, Z and B pins shown in the pot diagram on the right are wired into the circuit at the points shown on the left. I could have made this clearer on the schematic too.

I'll upload a new schematic now with these changes.

 

The minimum supply for a TL072 is 7V when it barely works. They work fine with 9V but the input voltage and output voltage ranges are very small.

You have the pins wrong.

 

Ah, I'm sure it was working fine with 3V on the breadboard. I just tested it with 9V batteries instead of the AAs and there was no difference.

Actually I didn't notice much difference when the power was connected or disconnected, a slight volume increase maybe but not the clear filter effect I'd been hearing before. I tested the power pins and the ground rail again and all are as they should be.

I also tried adding in the capacitor bank, with 9V and 3V, but there was no difference with either.

Thanks for pointing out the pins - I will change the images and re-upload them now.

 

Are you using the amp to drive earphones?

hgmjr

 

The audio out is connected to a quarter inch jack socket, from which a lead goes into a small battery powered amp and speaker. The audio in socket is connected to a sampler's headphone output.

I have also connected the grounds on the audio in and audio out sockets, which isn't shown on the diagrams. This setup was working on the breadboard, producing a clear filter effect.

 

The audio out is connected to a quarter inch jack socket, from which a lead goes into a small battery powered amp and speaker. The audio in socket is connected to a sampler's headphone output.

I have also connected the grounds on the audio in and audio out sockets, which isn't shown on the diagrams. This setup was working on the breadboard, producing a clear filter effect.

We're waiting for the updated schematic, with the dual gang pot shown.
Are you sure you didn't wire the op amp on the Veroboard the way that Audioguru noticed on your schematic?

 

We're waiting for the updated schematic, with the dual gang pot shown.
Are you sure you didn't wire the op amp on the Veroboard the way that Audioguru noticed on your schematic?

Ok, I've uploaded the new schematic. I don't know what the symbol should be for the dual pot so I've gone for what seems logical, I hope it's clear.

I had the numbers wrong on the op-amp but not the pin locations; all of the pins I'm using, 1 to 4 and 8, are wired correctly. I just checked that again on the board. There are breaks on the right of 5-7 and down the middle of the underside of the chip, between all of the pins.

 

The input must be driven from a low impedance that is at 0VDC.

 

The input must be driven from a low impedance that is at 0VDC.

I believe the circuit does work, I built it on a breadboard and it worked fine using the same inputs, outputs and power. Now I have it on veroboard it doesn't work. There could be errors anywhere along the way of course, in the schematic, the veroboard layout, soldering etc.

I am thankful for your time looking at this and don't mean to sound ungrateful or expect anyone to solve the problem for me, but what I am particularly looking for is a logical approach to what to test and in what order. I have checked the schematic, veroboard etc. several times and don't know if there's a more efficient way to debug than this.

Perhaps there is just a certain amount of not knowing and being frustrated that goes with putting circuits together, and I should just keep testing?

 

I checked your schematic against your layout, and couldn't find any errors. I redrew the circuit, and recognized it as a tunable Sallen & Key low pass filter.
Here is a clue: P1 and P2 should both affect the low frequency gain, P1 much more than P2. P3 should not affect low frequency gain, except at high frequencies. Put another way: If you remove all the caps, it's just a variable gain amplifier. P3 should do nothing, P1 and P2 should affect the gain. Are you sure your pot connections off the board are correct?

A couple of more points: For op amp stability, you need to add 100nF decoupling caps from pins 4 and 8 to GND. Make the connections as short as possible. I would also make additional cuts to eliminate unnecessary long traces that go nowhere, to reduce parasitic capacitances. It may not matter here, but it's generally good practice.