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I’ve just built a Cortado, and it went fine (build report here), but I have three questions and they may (and may not) be related.
First: The instructions regarding the piezo disc starting at Step 2, Page 14 confused me – I did misunderstand it, but still, I ended up following steps 1a-1e as well as 2a-2e. This is because when I skipped ahead to that line (as per instructions on page 13), I was already at the current step, so what was intended as “do these following steps” (which is implicit) became in my head “go back and do these other steps too.” So I misread the “2a-2e” as “1a-1e” and instructions to go back and do the things I’d skipped. This was true even after I re-read it a couple of times trying to figure out what was meant, and why it would’ve been arranged that way, as it didn’t make much sense to me. I suggest that be reworded.
So, the resulting question: having both the double-sided tape and the electric tape on the back of the Cortado – would that reduce bass response? Because I’m having issues with that. See also:
Second: When I attach the Cortado to the soundboard of my instrument using the suggested painting tape method, I don’t get any bass response. It sounds very tinny. I also tried using command adhesive strips, which I didn’t really expect to work well and didn’t, but did work slightly better.
However, if I just hold the pickup against the instrument with my hand – using ordinary “keep it here” pressure, not any strength – I have perfectly good bass pickup! (There is a playable short sample at the build report.) This implies the circuit is okay.
I’ve ordered a plastic clamp from Amazon to try that method, and hopefully, that will work. But: is this normal/in the expected range of behaviour? Would taking the piezo disc assembly apart and removing the double-sided tape layer be likely to help with the painting tape method? Or if that’s probably irrelevant, do you have any other attachment systems you suggest, or, more generally, not suggest?
Third: I saw the service report about the extra resistor when I went to download the assembly manuals. So I went ahead and just applied the modification from the beginning, as I have plenty of spare resistors, the service report didn’t indicate how often it was needed, and the circuit had been modified going forward to be 680 ohms rather than 1.5K. Now, I have LOADS of signal and a hilariously low noise floor, all of which is very good. (Well done, design team!)
But: if this is a unit that did not need the modification after all – only after going to the fora did I find this modification was only very rarely needed – would the modification reduce low-frequency pickup? If so, I can remove it easily, but if not, as I said, signal to noise levels are awesome, I’ll keep it.
Thanks!
Brach here, product designer.
Thanks for the post and the excellent build blog on your site.It’s true: the instructions are not very clear about which path to follow at that point. In our next revision of the manual I’ll make sure to clarify. Thank you for your honest feedback.
To be clear now: Step 1 on Page 13 applies ONLY if you intend to mount the PCB and the piezo together into a grounded shield container, like the Tin Can Microphone, or a garbage can. Step 1 directs you to wrap the piezo in electrical tape, then add the double-stick tape for mounting. AND YOU ARE DONE! SKIP STEP 2. If you are making the standard contact mic in the Altoids can, SKIP STEP 1; GO TO STEP 2, which instructs you to wrap the Piezo in electrical tape, THEN copper tape to shield it, THEN double-stick as one possible mounting method.
As far as the low-end issues you are having, coupling is everything (as you’ve found out). The painters tape method works for some applications, like a banjo or art installation or something that doesn’t have much low end anyway…but like you, I’m discovering that it usually just does not provide adequate coupling to pick up enough bass. (On our video when I mic’ed the bass and acoustic with the Cortados, I had to eq the low end up.) When we fitted a customer’s acoustic guitar with a Cortado pickup, we fitted the truss with a custom clamp to press the piezo against the inside of the guitar top. I also adjusted some component values in the circuitry to emphasize the low end a bit more. You can simply change C1 and C3 to 22nF to allow more low end to pass through. (In some applications, increasing the low end can really make the signal muddy and unclear, but it doesn’t sound like that’s a problem you’ll have.)
The bias mod you performed has virtually no effect on the frequency response. Please enjoy that high s/n in the best of health.
Let me know if you have more questions/comments.
Oh okay, so this is something to work on! Fun.
I built a new pickup disc (I had many extras already, having ordered them for the boundary microphone project) and one interesting thing is that it did not help with the painter’s tape method, but did improve the situation with the plastic clamp I had. I was able to use it in concert with a plastic clamp. Sadly, I do not have a recording, because the sound crew had unrelated but severe equipment issues that show. 🙁
I have not yet tried this new pickup with the 3M Command Adhesive strips, but perhaps I should. Were those to work, it would solve many problems.
Also – and this I find interesting, but confusing – the new, correct-build pickup, while much better with the clamp, did not have as even a bass response as the errant-build pickup (with the extra adhesive and backing tape) did when the errant-built pickup was held in place by hand. Something in the mid-bass went on walkabout. This is also why I want to try again with the command adhesive strips – perhaps that foam will serve the same modulating purpose as the double-sided tape and layer of protective paper. It seems unlikely, but it would be extremely cool if it worked.
It would be interesting to adhese the pickup to a thin plank of material and then place that plank under the bridge. Since my bridges are floating, that could actually be done; the strings would anchor it tightly in place. But they’d have to be very thin material to prevent detuning issues… or perhaps I could get spare bridges and sand them down so they are the correct height with the pickup plank installed!
…that’s an idea.
What would you suggest as a good material for this plank? It should still be thin. We’d be putting it underneath a floating bridge, on the flat face of the instrument, held in place by the bridge which is in turn being held in place by the strings, pressed against the face.
Anything in contact with either side of the piezo is going to affect frequency response. You will always get the best response when the piezo is tightly coupled to the instrument. This is why our “start here” design calls for the piezo wrapped in one smooth layer of electrical tape, one thin layer of copper, and then bonded tightly to the sound board with the provided piece of very-sticky double-tape. Beyond that, everything is an experiment. When using clamping pressure from above, instead of adhesive from below, everything counts. We think you will generally achieve best coupling when the clamp pressure is distributed uniformly across the back of the disc. We once used a piece of thin, dense foam, then a disc of 032 aluminum between the disc and the clamp.
The board-under-the-bridge idea sounds great! Would this “paddle” extend over a sound hole, or off the side of a narrow-bodied instrument? Obviously, the material will strongly affect the tone. Ideally, the paddle should be the same material as the sound board. That’s easy if you are making your own instruments. Otherwise, I would recommend 1/16″ thick basswood, such as from Midwest Wood Products, distributed through Utrecht Art Supplies and many hobby shops:
Basswood from Midwest at Utrecht
This cuts with a very sharp X=Acto knife or a micro-coping saw. Experiment with shapes and sizes. I think the length of freespan tab (or tongue) would influence resonant frequency response. Let us know what you find out!What I was planning was to adhere the cortado to the end of the paddle, and then have the rest of the paddle just held down against the instrument’s face by the floating bridge. There wouldn’t be any soundhole interaction; it would all be pickup directly from the bridge and face of instrument. This lets the paddle run the full length of the bridge (underneath it) rather than requiring the very wide (and fairly long) board which would be needed to allow the cortado to reach over the sound hole.
This will also hopefully give me some useful data I can apply later on the boundary mic project. I want to be able to use these microphones outside, so I’m going to have a largeish resonating plate to which I will have attached 1-2 piezo discs (which I already have), which will be separated from a backplate and side-protective rim, possibly (particularly at first) just by paper tape. This will make the whole resonating plate into (effectively) a large microphone diaphragm, with the backplate used basically as a backside deflector, preventing pickup from that direction – or so I hope, anyway. 😀 Plus it’ll provide attachment points to hang the device, a place to attach the box containing the amplification circuit, etc.
Anyway, if this works well, I’ll at least consider rosewood for that resonating plate. If it’s weak in the high end (strikes me as unlikely) then I’ll consider a lighter material, and so on.
