SSM2044 question/s

So I got my SSM board finished and it all worked first time. But im not sure if it is 100%…
Could someone tell me if this is normal? With no cut off (IE, filter at 127), If I increase the resonance from minimum, upwards, I get a significant drop in level. Probably at least half. Once it starts to self oscillate, the level (of the oscillation) seems about right again. But by that time, the original audio underneath the oscillation is almost gone.
Looking at the circuit, it’s very simple… Q off the digital board, straight into a 6.8k resistor, that goes to the 5k reso trimmer, then it’s right into the SSM chip. Not a lot to go wrong there surely!?
No such issue with the SMR-4 board…

I’ve tuned it as best I can according to the specs, but the build/tune instructions for the SSM are rather more vague than they are for the SMR-4. Still, I think it’s been tuned well enough. And further tweaking doesn’t appear to affect the level drop anyway.

Thanks in advance for any assistance! Paul

I#d like to help, but my SSM Filter is still unassembled… but i will have a look if i can do it tomorrow to check.

Yeah, I figured not a lot of people had one of these yet, so help was going to be limited, particularly while Olivier is away…

Incidentally, it is a v0.1 board, built as per the BOM with Farnell sourced parts. Including a TLO74 (instead of 84). And R36 is 330K (Instead of 1M, that is printed on the PCB)

Anyway, help whenever you are able, would be appreciated :slight_smile: However, if it turns out to be normal behaviour, then I think I prefer the SMR-4 filter…

I cant decide wether i like the CEM3379 or the SMR-4 better… The CEM3379 is more Vintage and a Beast, the SMR is more elegant and musical…Maybe ill try to build a CEM3399 Filter (must*not*giggle).

Is there a reason i am missing for that R36 is changed in the BOM? And: im about to order some 084s - should i throw in another one for you?
If yes, drop me an eMail (adress is in the Profile)

This is a standard “feature” of the SSM2044 filter, unless the design includes some sort of gain compensation, like the Tiptop Audio Z2044 eurorack module claims it does:

I ‘think’ that resistor change relates to this, as seen in the v0.2 release notes “Increased VCA gain bias range.” It is the only component change between the 2 revisions that I could see… All the other changes were mechanical, like hole size/spacings etc…

And thats kind of you to offer the 084 :slight_smile: Do you think it could play a part here? (I’ll have a quick squint at the schematics again, although im no electronics wizz). I guess I would like to try one. I did read your post where you discussed this with Olivier, but again, I couldn’t be sure if it would change anything here?
I don’t suppose you need any 4066’s do you? I’ve got a tube full of the things, ready for part exchange! haha
The SSM board otherwise appears to function perfectly though. It’s just this gain drop when the res goes up that seems odd to me.

If you are building yours soon, then I guess you’ll be able to tell me if your findings are the same. And perhaps if you have some TLO74’s knocking about, try one in there to compare?

There is no hurry though, I still have my SMR-4 board of course. And yes, Olivier did a great job with the SMR-4, so it’s not so easy to choose between it, and the established classic alternative designs.

Thanks again! Paul

[EDIT] Thanks for your post too Glitched. Perhaps it is normal after all then. Although im curious what other folk with the SSM filter in their Shruthi’s think of it.

I have no clue what to use the 4066 right now but if you have some spare i gladly will do some trading - maybe i use them in my Weirdo Controller

you’ll find projects! hehehe

R36 change makes the tuning easier.

SSM2044 has a loudness drop, it’s normal. Since the VCA controlling the resonance is “internal” to the chip (contrary to what’s done with a SSM2040 or without VCF ICs and straight OTAs), there’s no way of messing with the circuit to compensate resonance loudness. SMR4 has the compensation, this is also why it screams a bit at self-oscillation.

Indeed this compensation would be possible with an extra VCA or OTA, but none of the synths with SSM2044s I have studied use this trick, so by looking at the schematics, I somehow know they would all have this problem. Polysix owners could confirm.

Thanks Olivier! That saves me going crosseyed looking at the schematics again, or endlessly tweaking trimmers…

fcd72. I guess this means I won’t need the 084. However, if you decide you need some 4066’s for something, then i’ll gladly send you a bunch anyway! I’ve got over 20 of them, and I really can’t see me using them now… (They are left over from x0xb0x projects)

FYI: SSM2044 LP Filter Designs