Hello!
I used to have a multiband saturation module called Trident.
It was very nice but way to big for my cough Eurorack, at 16hp.
Out of possible 3 bands, I ended up using just the top one, for adding some crisp post-filter.
Lately I have been missing it, and since the schematic for the original module is available, I was wondering how hard it will be to just remove anything that I don’t need out of the schematic so that instead of 8 potentiometers and 8 connectors at 16hp, there’s just the top band with 2/3 pots (crossover frequency,drive and maybe a post drive attenuator) and 2 connectors (in/out) at 4hp.
Since I posses no knowledge or experience in PCB / circuit design, I was wondering if anyone here might agree to take a look at the original module’s schematic and advise on how to make it happen. It’s basically adding nothing to the original design, only stripping it down.
How to make it happen: JFDI! Download Kicad, watch a tutorial video or two, start drawing copies of simple schematics, make them into PCB layouts. Learn by doing, it’s really not that hard. Ask questions here. Then you can design this and anything else you want.
I love this approach. The ol’ kick in the backside
So, first question (out of many more to come, you have been warned), is there a quick way to get the schematics from PDF to KiCad, or do I need to start from scratch and draw the schematic one component at a time?
The component symbols themselves are pre-drawn (mostly; if there’s something you need that isn’t in the symbols library or if you want it drawn differently you can design your own, but for most parts you don’t need to). You need to click and drag them into place and draw the connections between them.
Taking a quick look at the PDF, some quick notes that might help:
It looks indeed like it should be safe to chop off the middle and bottom path. Looks like it’s also safe to skip the U3D op amp, so it can fit all on a single tl074.
When adding a TL074 KiCad will make you place 5 symbols: the power section then the 4 gates. They will all be attached to the same chip.
For the power rail in KiCad: you will need a symbol for a power header, AnalogOutput got one in his library, or you can just use a 02x05 pin header, but it’s easier with a custom symbol. As drawn, the original schematic might not pass the Electrical Rules Check unless you add a PWR_FLAG on the rails.
For RV2, you want a potentiometer named “dual gang” in the library, don’t use two separate symbols.
When designing the PCB, you’ll want to start by adding a ground pour on both sides, and increasing your default trace width, i think KiCad defaults to the smallest tracks you can get away with, i prefer to go bigger at 0.25mm to make sure things will work well.
Don’t forget to place the 100nf decoupling capacitors as close as possible to the power pins of the TL074. The 10µF ones are for the whole board and can go wherever.
Don’t hesitate to send us a completed project for review! I’ll be happy to take a look if you ping me.
Thanks for the thorough explanation! There’s a lot to take in, and I’m a total beginner, so I’ll take it one step at a time. One question: You said that I can get rid of the bottom and middle parts.
Although I don’t need to saturate the middle and bottom bands, I do want to keep the ‘dry’ portion intact. In other words - I want to saturate the high-passed signal and then mix is back with the rest of the unsaturated signal - as in the the original design.
So having that in mind, can I still remove the complete middle and bottom parts or is there a mixing section somewhere that I need to keep?
Also, at the output stage, there’s another OpAmp, U7D, should I ignore it as well?
I think what i said about the op amp was me misunderstanding the dry signal is desired, but speaking of them, this circuit will end up with 5 op amp gates, so you will have 1 (or 3 if you go with two 074’s) unused gates, those should not be left disconnected to improve stability
What if I don’t need the attenuators, RV3 and RV4, can I just remove them or do I need to remove additional resistors around them as well?
About the unused OpAmp, I understand the general idea, but I don’t know exactly which pins should be connected to the GND/Ref, and where to find the Vref symbol. can you help?
One more question, if when moving it all to the PCB editor, I find that I run out of space, can I just replace the THT footprints with SMD footprints?
I do not fully understand the original circuit so i’m only focused on whether it was copied properly, you’ll probably want to breadboard this before sending it to print!
Yes. Keep the downstream 100k fixed resistors though.
Take a look at the datasheet. Unused outputs (pins 1, 7, 8, 14 on TL074) should connect to corresponding inverting inputs (pins 2, 6, 9, 13) and unused non inverting inputs (pins 3, 5, 10, 12) should connect to ground.
Vref is only relevant with single sided power supply.
Carefully: Some SMD components have different pinouts than the corresponding THT, and therefore would need different symbols in the schematic.
This is a pretty simple circuit though. Should fit in Eurorack, I would think. Then again, I build Kosmo pretty much exclusively.
(Unit E of the TL074 symbol, or unit C of the TL072. If you use a TL071 the power pins are shown connecting directly to the op amp symbol and you’d need to put the bypass caps there.)
The sleeve should connect to ground. I don’t normally use that symbol and I’m not sure about the possible footprints but whatever you use, make sure the sleeve terminal doesn’t float in the PCB layout. I use a jack symbol that has no separate ground pin, and connect ground to sleeve.
Thanks @AriaSalvatrice and @analogoutput for the helpful comments!
I have made the changes, and am now in the process of getting yelled at by the ERC and fixing the schematic accordingly.
Regarding what you suggested, in order to route the unused output of the ICs, do I simply add them to the schematic and route, as seen in the below image, circled in red? (I’m asking because I don’t think I ever seen this done)
Also, can I just delete the components marked with a red X if I don’t need this functionality? I know I have asked this before and got an answer, but I’m just making sure.
Edit: should have also crossed the opamp near the output, U3.
I am sure others who are more knowledgeable can be more helpful but here are my thoughts.
If you delete the two potentiometers you will have no control over the wet/dry signal. I am not familiar with exciters, but it sounds kinda useless to me if you cannot adjust the wet and dry signal. Also, I’d say that you definitely need that op amp that you deleted. Otherwise I cannot see how you can combine the differential voltage output of the two transistors.
It is also strange that you have three unused op amps. If you use a TL074 for the three top op amps and a TL072 for the bottom two then you should have one unused op amp, not three.
In addition, the unused op amps are not properly terminated in your schematic. The non-inverting inputs should not be left floating but connected to ground.
The crossed out op amp and its feedback resistor are an integral part of the distortion circuit.
U3 is needed to sum the two signals going to the output.
You need five op amps. It would make sense to use a TL074 and a TL071 (five) or TL072 (six, one unused). What you have here in fact is two TL074 and a TL072, with U1B, U2C, and U2D not used and shown and U3B, U3C, and U3E not used and not shown.
The pots can be omitted.
The unused op amps should have their non inverting inputs connected to ground.
I just realized… this isn’t going to do what you want and what I carelessly thought it would do. In the original the signal is split into three frequency bands, each has distortion applied, and the results are summed. Here the upper frequency band only has distortion applied, and is summed with the input signal. Basically the upper band appears in the output twice, distorted plus undistorted. Except I think the high signal is inverted, while the input signal is not! Which means the distorted signal is subtracted from the input. That’s very much not what you want.
You could add back in the original mid and low filters and just omit their distortions. But a simpler thing I think you could get to work would be to take the inverted output of the high filter and sum it (with inversion) with the input signal to get an undistorted low+mid signal, and then sum that (with re-inversion) with the distorted high signal. That would add one op amp and a couple resistors, I think, to what you have. I think it’s just something like this: U1A pin 1 to a new 100k resistor R19; other end of R19 to say U2C pin 9; U2B pin 7 to a new 100k resistor R20; other end of R20 to U2C pin 9; U2C pin 8 to a new 100k resistor R21; other end of R21 to U2C pin 9; U2C pin 10 to ground; U2C pin 8 to RV4 or to R18 if you’re omitting RV4.
This is definitely something to be breadboarded before proceeding. Use a scope if possible to verify the signals are being added in phase (U2C pin 8 and U3A pin 1 should both be inverted compared to U1A pin 2).
More or less true, but as noted above this wet+dry sum isn’t what was in the original S&C design and is incorrectly implemented here (at my incorrect suggestion). The original intent was that you’d have something like an equalizer, in that there are pots to control the amount of low/medium/high frequency content. With the revision I suggested above there could instead be two pots to control low+medium and high content. Essentially bass and treble knobs. But they don’t really affect the amount or character of the distortion. Those are controlled by the frequency and drive pots, which have been left in place.
Regarding the Dry/Wet knob, it does makes sense to have it, however in practice when I owned the original full module, I’ve never used it. Since I’m trying to fit my ‘minimal’ edit into 4hp, I’m trying to free up as much real estate as I can, hence the omission of the dry/wet control.
Yes, you are absolutely right. The signal is split at the cutoff, distorted and then mixed back with the original. I followed your instructions, hopefully I didn’t mess up.
As for bread boarding, I have never done that before. I will look for some guides on Youtube for doing that. Do you have any recommendations on the size of board to buy or any other parts that are essential? (apart from the components and jumper cables)
This is an affiliate link recommending a specific brand so take it with a good spoonful of salt, but low-quality breadboards are often a huge pain, and people recommend buying quality stuff from the start:
I still only have low-quality junk myself though :^)
