So I have started calibration of the SSM board but no matter what I try I can’t get it to self oscillate. It feels like i’m not even getting close. It responds fine to cutoff and resonance CV but I just can’t get the Q high enough. I’ve tried lowering R4 (the 6K8 in series with the 5K reso trim pot) to appx 1K but that only helps a little and I don’t know how low I can go without killing the SSM2044.
Is there anything else I can do to make it self oscillate?
To which filter setting is the control board set? Check that the Q> pin on the control board goes up to +5V (or close to it) when you set resonance to 63.
The control board is set to ssm. With resonance set to 63 the Q> pin on the control board goes up to +4.99V (close enough right?) and the Q input on the SSM chip (pin2) goes up to about 1.26V.
I do get some resonance and the filter does have that lovely liquid organic sound that I associate with SSM filters. For most sounds it would be just fine, it is just that with self oscillation I could get to those great ethereal whistely whooshy sounds that I remember from my old Polysix and that I would love to have again. There is something special about the sound of white noise through a filter that is on the verge of oscillating (If you know what I mean).
From the datasheet self oscillation occurs with 425µA of current on the Q pin ; so that’s +5V through a 11k resistor. In this particular instance it’s possible to measure the resistance between the Q pin and pin 2 of the SSM2044. What do you read? Maybe you’re not turning the trimmer in the right direction. I would avoid sending more than a few mA through the pin… You might try with another SSM2044 chip too - I’ve already seen a few chips with this behavior… Don’t know if they were bad / defective or if it’s due to very very wide tolerances.
The resistance between pin 2 and the Q> pin is 6.78K. I also measured the voltage over R4 in order to calculate the current and that goes up to 550µA so nothing wrong there.
Looks like I’m gonna have to go on the lookout for another SSM2044.
Okay, just ordered a new SSM2044 from Vintage Planet (who lives around the corner from my office, really nice guy too. Fixed me up with a new Eprom set for my MKS70 last year.)
Let’s see if I can persuade this chip to oscillate
New SSM2044 chip arrived today (Thanx Senso for the great service. Although much better than the one that originally came with the SSM board this one also doesn’t self oscillate) It does get very close though. I decided lowered R4 to about 220 ohm and being careful to not turn the res trimmer all the way up I can get the filter to whistle along if I input just some noise. This just enough to be able to tune the V/oct so that it is in tune over 3 or 4 octaves (sort of)
I do love the sound of the filter!
you got the last one with your board? then it’s supposed to be ok too. you now have reason to build a second one, as soon as you got this running!
I bought the SSM PCB from Mutable Instruments complete with the SSM2044 and V2164 chips because that was easier than trying to source the chips individually. The ssm2044 that came with the PCB is now a spare
Out of the “old masters” filter range the SSM sounded the warmest to me. The CEM is basically a more stable SSM, but many say they lost the warmth. The SSM signal level varies depending on the resonance, but that’s no problem as you can “fix it in the mix”.
To my ears this filter has a bit of that Oberheim SEM magic. Lovely sweet organic, wet, liquid, squelchy… nice! I have so much 80’s Roland analog stuff in the studio that it is a nice change from the CEM sound.
There’s a bit of a test of the two chips here:
CEM vs SSM
yes the SSM is my favorite. second is the CEM, great bass! i like the SSM’s screaming