4pole Tuning


just wanted to make shure that the tuning procedure is right for me.
I cant really hear a change in sound by turning the Range pot - what does the range pot do exactly? By decreasing the range, the sound gets affected by the filter at a lower cutoff freq?

What voltages are a good point to start for the Range and Tune pot? My range pot seems to have a voltage range between 4V and 5V on the middle pin - is this correct?

Thanks a lot

Since this is a 4PM, we shall call this The Snow Drift™ effect.

> Is it supposed to drift like that ?

Transistor circuits are temperature dependent. There are all kinds of complicated ways of making them relatively temperature independent (one of them is to stick a heater on the chip). Worth doing for a VCO, not for a VCF.

Drifting and such quirks are part of the analog sound that people are always after.

easy: nothing leads to more misconception than following forum advice and / or build instructions too precisely.

ha, ha, ha

yep, that’s what i learned when i tried to get the dual SVF filters in tune to each other. i never used it in a way that would have needed them in tune.
but i never had the impression with any of those shruthis that they were out of tune on one of the keys ends.

If you want a perfect following Filter maybe you’d try some kind of VA Synth? Oh, you cant trim the Filter there at all ?? :wink:

BTW this slight imperfections add a lot to the organic feel of your Music, on my first machine i also tried to match the filter as closely as possible, on the newer ones i did it just quick and dirty by ear :wink:


I just remembered a word from Albert Einstein matching this super precision discussion:
“Durch nichts zeichnet sich mathematisches Unverständnis so sehr aus als durch übergrosse Genauigkeit im Zahlenrechnen.”

Youll have to translagoogle this yourselves as my translation feels not that matching…

My two cents: trim to some value you can live with, screw the top on and forget about it. The tuning of the filter is not worth obsessing over.

i haven’t made the 4pm yet, but in recent builds it was quite easy to do. first was getting rid of the not usable resonance range at the higher end, then the intervals. my tuning requirements have never been milspec, so i did it by ear only (good enough for noise).
then i switched one of the oscillators on with a non piano sound and tuned the resonance to that pitch. checked again the intervals, but by that time the drummer is already playing usually.
good luck!

It will not be a perfect octave apart. At least that is my understanding. You are just getting it as close as possible, but I don’t think it has a huge influence on the sound.

i anticipated doing it but i almost never used the resonance sine to play “songs”

Okay okay, don’t blame me, just asking as i’m new to this.

I am going thru this procedure as well and i can’t get the tone to be perfectly one octave appart. But the weird thing is the tone drifts according to my tuner, like if i get C3 at 130.8Hz and keep that note going, it slowly increase to 130.9 then 131 and so on every 30 seconds or so …

Is it supposed to drift like that ?

Wow, it drifts 0,2 Hz/Minute :wink:

Are you disabling the oscillators and setting the resonance to 63 for tuning? You should hear a change in the pitch of the self-oscillation tone ; not in the oscillators’ pitch.

Ah yes, with the self-oscillation of the filter i hear a change, didnt really notice any when trying to vary the “spot” where the filter affects the osc sound.
But how to find a good value for this?
I cant really tell from the e-piano sound…

My tuning seems to be very near to perfect, do you have maby a reference note? For example: playing a C2 you should get a G2 or someting like this?

I’m really not getting this tuning thing… still…

I’m trying to set the interval, and I can’t even get it to produce the same note over two octaves, let alone the same note, in tune, it’s maddening.
I press a C2, get a B note, press C3, get a C, tune that back down to B, but by this time, 2 now plays an A#… I’m going round in circles…

In the past, I’ve just given up, to be honest. In fact, with the Dual SVF, I broke one of the pots trying to get it in tune and had to replace it. This time though, I’d like to crack it…


Incidentally, I’m using the tuner on my MOTU 828. I’ve not tried it before, but I’d have thought a pure sine wave would be pretty easy to track for any digital tuner…


I just twiddled the trimmer until octaves worked, so C4 was an octave higher than C3 etc. I don’t think you can make the tone produced the same pitch as the note you play. Just ensure the tone tracks the range of the keyboard.