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Erica Synths DIY Multimode VCF
MUFF WIGGLER Forum Index -> Music Tech DIY  
Author Erica Synths DIY Multimode VCF
CRD
hello dears,
i'd start this topic originally to find an alternative to the film capacitors which are designed in the BOM by erica synths manufacturers..

these are :

470 pF (C20-21-17-23)
2.2 uF (C15)

I have actually some disk ceramic caps with the same value available and thought I want to finish the building soon without ordering other caps.

There are some threads and web searches around which specify I could have some issues with disk capacitors when used in audio modules..
Some say they can act as "piezo microphones" and also they are less reliable after some time than ceramic ones..

Does anybody made this module using other caps than the film ones?

help
groove
Someone else can comment on the electrical characteristics - I think there are a few reasons why ceramics aren't usually used for this application.

If you're curious, though, why not cut out a few pins from a dip/sip socket and solder those in so you can swap around the cap(s) in question to test? It shouldn't harm anything to try a few different cap types and values.
mskala
Which module are you building? The schematic and BOM linked from http://ericasynths.lv/en/shop/diy/diy-kits-1/diy-polivoks-vcf-2/ don't contain these capacitor values.

In general: there are two different kinds of ceramic capacitors, ferroelectric ones often designated X7R or Z5U, and non-ferroelectric ones often designated NP0 or C0G. These two kinds are quite different from each other.

Ferroelectric ceramic capacitors change their value a lot depending on temperature and applied voltage. They're cheap and physically small. These are usually fine in power supply bypass applications where the exact value doesn't matter, but I would not substitute them for film caps because people usually only specify film caps in cases where the exact value does matter.

Non-ferroelectric ceramic capacitors are usually very well behaved electrically and in most cases they can be safely substituted for film caps if you can find the right values - which may not be possible at the larger end. I don't think there are many 2.2µF NP0 capacitors on the market. I wouldn't worry too much about microphonics with these, though using surface mount would certainly help reduce any such issues.
CRD
mskala wrote:
Which module are you building? The schematic and BOM linked from http://ericasynths.lv/en/shop/diy/diy-kits-1/diy-polivoks-vcf-2/ don't contain these capacitor values.


hello, i did a mistake by posting the wrong filter.. this is the module i am bulding :
http://ericasynths.lv/en/shop/diy/diy-kits-1/diy-multimode-vcf/
CRD
groove wrote:
Someone else can comment on the electrical characteristics - I think there are a few reasons why ceramics aren't usually used for this application.

If you're curious, though, why not cut out a few pins from a dip/sip socket and solder those in so you can swap around the cap(s) in question to test? It shouldn't harm anything to try a few different cap types and values.


good idea i'll try that before..
mskala
Looks like C17 and C21 are integrator capacitors and you don't want those to go off-value. Decent-tolerance C0G/NP0 ceramic should be fine but you don't want ferroelectric (X7R/Z5U/etc.) ceramics. The other two 470pF capacitors look like they're just to filter control voltages and those would be less critical. It looks to me like C15 is for AC coupling and its exact value may not be critical. But do you really have a 2.2µF ceramic disc, anyway? That's awfully big for a ceramic disc.
CRD
mskala wrote:
But do you really have a 2.2µF ceramic disc, anyway? That's awfully big for a ceramic disc.


you're right, i just read wrong the specs of that disk caps bag !
thanks for the schematic read by the way
CRD
I completed the build adding these caps:
C17-C21-C23(470 pF film caps.. i found some lately!)
C22-C20(470 pF ceramic caps -- not disks)
C15(2,2 uF ceramic cap)

When I made the first test it sounds not as hoped.. The input level is low, i had to turn both VCO amplitude and VCF input at maximum level to hear something.. Also there is a kind of white noise from each output (it looks like it's not effected by the filter, kind of post noise!)..
The only clean and loud sounds happen when I let it self oscillate by putting resonance at full level..

Will start soon for troubleshooting.. Any help welcome!
Faustgeist
Hello CRD,

A few thoughts.. Might your central capacitor location 'C-18' be populated from the other side? Hard to tell from the photo but solder?



More obvious however is the capacitor placed @ location 'C-26'. This is marked 'NU = not used'. That one certainly could be upsetting the performance. SlayerBadger!



Somewhat beyond my expertise is your use of alternating capacitor types. My full parts kit was harmonious in its supply of 470pF capacitors as the square pink films. Where I see you are using some of that style along with ceramic packages (ie : C-20 vs C-23). This is not my realm, but there could be an issue there?

And finally, I would double verify the value of the cap in location 'C-15' as it is different from the rest (2.2uf) and indeed in my parts kit a very different looking packaged film/tantalum (?) (big white square). See Erica's populated board here :



Hope this helps!
~Robin
Faustgeist
Now a question of my own.

I am placing the 5 potentiometers and noticed one is an "A-100k" while the other four are "B-100k". The A version pot is an audio (logarithmic) while the Bs are linear.

Neither the BOM or build doc mention this and I'm wondering if I need make them all Bs, or...?

I am asking here and have written Erica.

Response from Erica :
"Hi Robin ,
Thank you for your question. A100K potentiometer might go on the Audio Level only.
Sincerely, Kirils."

I am hoping, of course, the word 'might' is a translation thing, lol. Rockin' Banana!
-Robin


For anyone interested here is a copied bit of info regarding pots of this nature :

"Linear is linear, Audio is Logarithmic.

Basically, let's say for a volume knob you use a Linear taper pot, you'll get a huge range of volume for the first tiny bit of the turn and as you go up the knob the increase you get plateaus off, so the last half of a turn hardly does anything. If you use a Log pot it's more natural of a sweep for volume. For distortion amount, as an example (I think) A Log pot would be terrible, as you'd get the opposite effect, very little change for the first part of the turn, then a huge jump at the end.

Now, depending on what you're doing with the pot you might want a linear taper, meaning that the resistance increases in an / angled line as you turn it, rather than a J style swoop for the Log style.

So it depends on what the pot does and how much control you need over that feature. "
oilpanic
Hello all.
I have built 2 of these and they both are working. However I find the resonance to be a bit too much and gritty. Im wondering if some modification can be made to tame the res? Im thinking maybe different values around r21,r30 or r28, but I am really just guessing. Any ideas?[/img]
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