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Some noob questions about component substitution
MUFF WIGGLER Forum Index -> Music Tech DIY  
Author Some noob questions about component substitution
aragorn23
Hi all,

I'm building a module and I'm short of some components, so I was wondering how wise it is to do some substitutions.

First, is it okay to substitute ceramic for film/poly caps or am I looking for trouble?

Second, if I only have 100k pots and need 50k pots, will a 100k resistor across the two outer legs be a passable solution?

Thanks!
mskala
Substituting ceramic for film: that depends. There are two major types of ceramic, ferroelectric ceramic (X7R, Z5U, and similar), and non-ferroelectric (NP0, C0G, and similar). These two types are quite different. Ferroelectric ceramic caps usually have wide tolerances to begin with, and change value a lot with temperature, and sometimes also with applied voltage. They usually will not perform acceptably in situations where film was called for. Good-quality non-ferroelectric ceramic caps often will work as replacements for film if the value and tolerance is right - but to really make this call accurately you need to know why the original design called for film in the first place.

As for potentiometers: quite often a circuit that called for a 50k pot was really just using it to generate a voltage, or divide a signal, and you can substitute in a 100k pot and it'll work fine. But if the circuit was using it as a variable resistor, then you need the right value of pot - trying to fake it with additional resistance in parallel will change the curve. I would not put a resistor in parallel across the ends of the pot (as you describe) because it's the resistance to the wiper that matters and a parallel resistance across the entire pot won't change that. There would be very few circuits in which an end-to-end parallel resistor would give any benefit; in almost any case where the 100k pot will work as a replacement for 50k, you can just substitute the 100k pot directly without needing to add anything else.

For both questions, there is no simple always-correct answer; you need to look at the specific circuit.
aragorn23
Thanks mskala, this is very helpful :-)

The specific circuit is the Toppobrillo 281 with Barcode panel: https://www.thonk.co.uk/shop/buchla-dual-function-generator-281/

I'm not good at reading circuits, so I'm not sure whether I can sub the ceramics here. I have seen some photos online where it looks like people have used ceramic caps on this circuit though. These are the 1.8nF caps I want to substitute for the film ones on the BOM: http://www.mantech.co.za/ProductInfo.aspx?Item=72M0984
mskala
I can't find a schematic on that page. Hate when kit vendors do that. The kits my company sells have exhaustively detailed technical information including schematics posted in the Web store; but until customers are willing to insist on published schematics, most manufacturers won't provide them. There's a lot of misplaced fear that publishing schematics will result in people "ripping off" the design - which is pretty funny for a kit that is itself cloned from Buchla's design to begin with.

That said: the capacitors you link to are Y5P, which I would not normally want to substitute for film. They will have much less temperature stability compared to film, and much more loss; suitable for power supply bypass applications, but not for audio, in general.

However, I dug around a bit on the Net and although I couldn't find a schematic for the exact board you're looking at, I found another Buchla 281 clone schematic which uses just one 1.8nF capacitor (not two as in your BOM) and uses it for some sort of coupling associated with the mode switch - where the exact value may not be critical and there's even a note that the value isn't critical. That suggests, if your board is using it the same way, maybe you really can get away with using the Y5P ceramic caps in this circuit. I'd say try it, they'll probably work, but only if you're willing to deal with the possibility that you may end up having to unsolder those capacitors and replace them with film if they turn out not to work.
aragorn23
Awesome, thanks again mskala. I think I'm going to take the sensible route and just order some film caps.

PS: I really respect your ethos when it comes to schematics, etc. I do wish all kit vendors held similar views.
flts
mskala wrote:
I can't find a schematic on that page. Hate when kit vendors do that. The kits my company sells have exhaustively detailed technical information including schematics posted in the Web store; but until customers are willing to insist on published schematics, most manufacturers won't provide them. There's a lot of misplaced fear that publishing schematics will result in people "ripping off" the design - which is pretty funny for a kit that is itself cloned from Buchla's design to begin with.


You might be reading too much into this - Toppobrillo is usually pretty good at providing schematics for his projects, so instead of "guarding secrets", it may be it's more having overlooked something, or simply not having thought about "packaging" the board project nicely for sale at Thonk, as it's a pretty old thing that went through several revisions and was once upon a time sold directly via forums from hobbyist to another (the first Toppo 281 boards I bought I think I got via E-M).

FWIW http://www.sdiy.org/toppobrillo/DOWNLOADS/ has schematics for the older single (A/B) board versions, so it may be the dual PCB is just those combined. Unfortunately the dual PCB directory appears to just have the same layout without schematics, the best we can do is to assume that it's just the two separate boards combined without the need to do A/B wiring.

Also, http://www.toppobrillo.com/twoeightyone.html contains some information and a link to the original EM build thread which has quite a bit of discussion on the project.
mskala
flts wrote:
mskala wrote:
I can't find a schematic on that page. Hate when kit vendors do that.


You might be reading too much into this - Toppobrillo is usually pretty good at providing schematics for his projects, so instead of "guarding secrets", it may be it's more having overlooked something


It's quite possibly Thonk's omission, not Topobrillo's - that's why I said "vendors" instead of "manufacturers," and I hope it was clear that my comment about there being a lot of misplaced fear of rip-offs was about the entire industry and not specifically this kit. But whatever the reason, I think not having the schematic easily available to whoever's looking at buying the product, is a significant issue.
emmaker
mskala wrote:
I can't find a schematic on that page. Hate when kit vendors do that.


As far as I know there are no real schematics for this. I don't think that there are any real schematics for any of Toppobrillo's projects. Not having good documentation is a good reason not to buy DIY projects which don't.

You're supposed to use the single 281 board schematics which are different and not all that great.

That said the B amount pot is spec-ed at 50K-100K. For the attack and decay pots there is a 47K resistor across the wipers and B+. This is done to change the curve of the pots from linear to reverse log tapers. Those pots are spec-ed at 10K-50K so the 47K resistor would probably need to be changed.

Potentiometer Tapers
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