I made this on breadboard today! I succeeded this time! 
Thanks, Moritz Klein!
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…just can’t get self oscillation. 
Any ideas on how to resolve this? Do I need to change the resistance in the feedback loop?
URGENT!: I need to pack (including the electronics assembly area) to go to another country and I need to get that filter functioning properly before I do! I finished building it on a protoboard (with similar layout to a breadboard, although an extra rail on top and bottom and some on sides), but it’s still not functioning yet. Sigh. That was as much work as building a small building with the right tools seemingly. And I know a building would actually function properly. Sadly the filter hasn’t functioned at all yet seemingly. There was a high pitch sound from some points on the board when had the dual supply a little low. Sadly no potentiometer has done anything to make it output sound from what is seemingly the output yet either. Was SOOOOOOO MUCH WORK. Sigh…
I have REALLY learned that I should ALWAYS have PCB made when making any electronics circuit that isn’t as simple as most guitar pedals. And then also prefer to have a proper PCB.
Anyway, here are the photos. Maybe there is something obvious simple and easy issue on it that will be easy to solve. The dual power supply is at maybe 11.87V.
Note: Swapped the ground and other rail on the side with the row of caps. Didn’t install the audio ports yet, simply using wires straight to speaker and a 555 synth. Disregard the other circuit on left of the breadboard, as that is a multistable vibrator that isn’t connected.
Thanks in advance for advising SOON on how to make it start functioning! Tested continuity all over the place, although didn’t go through maniacally methodically to confirm all continuity at every single point in that circuit. Underside has no bridged columns and that is a breadboard style stripboard. So it’s laid out based on the final breadboard schematic in that erika synths manual for that filter. (version 2.0 of that manual)
Again, do note that the power rails on the side with the row of caps were swapped.
I appreciate the recommendation, but I reviewed it again and again, A LOT.
I did use a poly cap instead of a film cap, but don’t think that would make it not function. Some film caps arrived today though, so maybe swap one in.
moritz: Some are claiming the breadboard diagram in the erika synths PDF manual is wrong. Sure it is correct?
Soldered side of the protoboard diode ladder filter that hasn’t functioned properly yet (Honestly any bridged looking columns with components on them don’t actually look bridged when actually looking at it from other angles. (There are some overhangs that aren’t touching and the photo is only one angle) ):
It’s hard to judge whether there are solder bridges just by looking at an image. Did you check all the adjacent rows and columns that are not supposed to be connected with a jumper wire with a multimeter?
Again, it’s hard to judge from the picture, but there might be a couple of cold solder spots that do not make proper connections. Perhaps you can reflow these and check again?
I would also check for faulty components. Perhaps you can replace the socketed componets (e.g. op amps) there are any improvements.
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SOME good news! (kinda) There is now “sound” outputting from where it is seemingly supposed to on that board. It’s very attenuated though, so it’s barely audible unless amplified and sadly the potentiometers don’t seem to change it whatsoever. Currently there is a 10K potentiometer instead of the 100K in the “resonance control” (seemingly) area next to the box style potentiometer, but all the other vals seem correct.
How can it be outputting sound, although the potentiometers not affecting that whatsoever?
How many opamps are you using? Just curious, because it looks like you have two quad opamps and that you are using at maybe five opamps total. Just seems like you may have done something wrong…
If you don’t have an oscilloscope, you can try using an audio probe as discussed here:
Starting from the signal input you can trace the path forward until you find where it (almost) disappears. That should give a clue what the problem is.
Main places to check would be outputs of op amps, + inputs where an audio signal should be, and before the input resistors on the - inputs where an audio signal should be.
Are you working from this schematic Designing a diode ladder filter from scratch - #17 by Dud or this one Designing a diode ladder filter from scratch - #34 by sharptrickster or something else?
I didn’t personally invent it. It’s literally the breadboard layout (although poly caps, and the power rails on the side with a row of caps are swapped) from page 40 of the erika synths manual here:
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Thanks to everybody that replied with good recommendations!
@analogoutput Sounds like educated advice. Thanks! I’m still learning how to deal with opamps. I prefer MCU based electronics. Also prefer single power supply circuits. That is based directly on the breadboard layout on page 40 of the manual for that filter: https://www.ericasynths.lv/media/VCF_MANUAL_v2.pdf
UPDATE: Some kinda good news. The long diode on the board was installed backwards, so installed a new one PROPERLY. It’s outputting sound from the correct location now, but the filter doesn’t seem to be functioning. Amplitude of output is really small, but maybe that is normal. At this point that circuit seems to sound basically the same as a long wire would. Also tried the listening from the jumper wire on the middle cap in the row of caps, but that sounded about the same. Maybe higher amplitude though.
Sadly none of the 3 potentiometers seem to change the sound.
Any recommendations on how to solve that? It seems so weird that it’s outputting sound, but none of the 3 pots seem to do anything useful and the sound only seems attuenatued. Does that mean the opamps need to be replaced or…???
Thanks in advance for recommendations on how to solve that!
PS: Sadly the forum says new users can only post 3 replies to a topic, so I may be editing this multiple times or maybe deleting an earlier one later.
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I replied so you should be able to post again, fyi. Also make sure that the opamp power pins are hooked up correctly. Normally the power pins of a dual opamps are on opposite sides from there respective counter parts on the quad opamps. For example on Tlo 72 positive is pin 8 and the negative is pin 4. Pin 8 could be called the top right and pin for could be called the bottom left. On a TL 074 the positive pin is number 4 and the negative pin is number 11. Number four could be considered the left side and number 11 could be considered the right side. As you can see their respective positions have switched. Whereas the positive from the TL 072 is on the right and the negative of the tl074 is on the right… You mentioned not having much experience with OP amps so I figured I’d point that out
You mention 3 pots. The breadboard diagram shows 2 pots plus a trimmer pot. I presume those are cutoff and resonance, and the trimmer is in series with the resonance pot. The schematic shows an additional 3 pots which are input and CV attenuators. Not sure what your 3 pots are.
Check that voltage from the cutoff pot makes it to the two op amps it’s supposed to get to. I can’t tell you which pins because the schematic doesn’t show the pin numbers. Check not at the pins though (because they’re inverting inputs), but at the other sides of the 33k resistors connecting to them.
You say the audio signal is very low amplitude. I can’t see where you’ve said what you’re using for an input signal. Presumably this is designed for a 10 Vpp input and to make a 10 Vpp output. I also don’t see where you’ve said how you’re listening to the output signal, but 10 Vpp into a line level input on a sound card or amplifier would be plenty loud.
At the input the signal is divided down a factor of 50. Then before the output, at the second op amp from the right in the schematic, it’s amplified by a factor of 33. If there’s something wrong around there maybe you have an un-amplified signal getting to the output. You can use an audio probe to check whether the signal’s getting amplified there — again check on the upstream end of the 1k input resistor and at the output of that op amp. The second should be much louder than the first.
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@analogoutput: As I said before, I made it based on the breadboard diagram on page 40 of that diode ladder filter manual. There are only 3 potentiometers on that. Input and output aren’t pots, those are jacks. So is the cv.
I wish the schematic had been made in a CAD app and showed the pinout also. I maybe shouldn’t try to make anything that isn’t verfied to function and doesn’t have a proper CAD schematic with pinouts.
Anyway, either getting it functioning properly before the journey tomorrow or maybe smash that protoboard to bits to free Myself from any future hassles of trying to make it function.
Thanks, but it’s simply that layout on page 40 of that manual. Sadly the schematic in that manual is not from a CAD app and it doesn’t have pinouts. Not enough hours available today and early tomorrow to go too in depth analyzing the opamp data sheets and comparing those to the hand drawn schematic to decipher what is what.
What is main reason it would output sound, but not filter it? Why would none of the potentiometers do anything?
I don’t know much about what you are talking about, but I do know the pinouts for the opamps I use without much analysis of the datasheet. It seems like you may need to simplify your process a bit. I have no idea what a cad app is, but I use many of Mr. Klein’s hand drawn schematics with much success. It helps to be familiar with the components you are using for sure. This schematic worked for me with one exception; I couldn’t get it to self oscillate. Probably a resistor in the feedback loop I didn’t get right. It did work though with resonance.
I don’t have the circuit on breadboard anymore so I can’t really give you any pointers on it at the moment, but I can tell you I tried to build this circuit about a year ago and failed. It took me a while to get enough knowledge in the meantime to have a successful build on breadboard more recently. You should do some “analysis” because, in my experience, it’s important to get the pins connected correctly.
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This should help you out. Match the symbols on the schematics with the ones on this pinout diagram. This is literally how I learned about opamp schematics symbols.
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Thanks! I appreciate that. I literally built what is shown in the diagram from that manual though, so conceptually it SHOULD function. Still wonder why it would output sound, although none of the potentiometers modify that sound.
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