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sawtooth to triangle like on the minimoog
MUFF WIGGLER Forum Index -> Music Tech DIY  
Author sawtooth to triangle like on the minimoog
Dimitree
hello,
I'd like to use this simple circuit
http://tinyurl.com/ycjcorq9
taken from the Minimoog, to convert the sawtooth waveform to triangle waveform in the Oberheim SEM I'm trying to build.
I can't do any prototype at the moment, so I'm trying to understand
1) what's the voltage of the SEM sawtooth output (after the 1K resistor)
2) if I should tap the signal from there or elsewherer, to run the converter
3) if I need an opamp buffer after the triangle converter
4) how should I adapt the resistors for +/-15V instead of +/-10V

SEM schematic: schematic

thanks!
guest
1. not 100% on this, but it looks like its +/-7V or so after the 1k.
2. you can tap the signal from there, but it might work better if you take it from opamp directly.
3. that second transistor acts as a buffer, if you want you could replace it with an opamp buffer.
4. it should work the same on +/-15V rails with no modification.
Grumble
very frustrating
Dimitree
thank you!

in the meanwhile, I noticed that the Oberheim OB-1 used a OTA to morph the sawtooth to triangle wave, and since OB-1 and SEM share almost the same VCO, I thought why not try to implement that to my clone, instead of a plain triangle converter.

I don't know it I can use it on the SEM without modifications.
The input would be tied to the sawtooth output on the SEM (before the 1K resistor I think), the output would go to the existing SEM mixer, before the VCF.
my doubts are:
1) would it still be possible to get the raw sawtooth like on the SEM?
2) amplitude and offset are still fine for the next stage?
3) what is the usefull range of PW cv in?
4) is R24 really needed?

guest
so, i have no idea what R25,26,21 and D6,7 do. i cant see how they would effect the circuit in any way, assuming PW and SAW-IN are all low impedance voltage sources.

1. yes, you should be able to use th raw saw
2. amplitude and offset should be fine for next stage
3. PW goes from 0V to ~5V (depending upon VR1)
4. the output resistor isnt strictly needed, but 100ohms is good to have if its an output to the outside world.

R20 should be 10k. i was wondering why none of my equations were working out, so i checked the original schematic. and now i see what all those other parts are for. its a different waveshaping circuit activated by the switch, U10.
Dimitree
Thank you really much!!

Is the other circuit activated by U10 the one that provide pulse waveform with variable pulse width? If so I will add that switch and remove the existing pulse waveform generator on the SEM, if they give the same results
guest
yeah, that does PWM, with an output from -0.6V to +7.5V.
Dimitree
just to understand it better, looks like the 4035 switch is powered from -1.1V and +10V. Is that because the signal passing thru it is within that range?
Can I supply the +10V by using a simple voltage divider on +15V?
and what's the purpose of VR2, after the switch?
guest
the 4053 has limited supply voltage range, and the inputs are within the -1V to 10V range. it also depends upon what the other switches in the package are connected to. they make analog switches that can handle higher rails, or if you dont need digital control, you could use a toggle switch instead. you will need an actual 10V supply rail for the 4053, so a 7810 would work. or a single transistor or opamp after a voltage divider. same for the -1V. you could probably get away with a zener and small value resistor.

VR2 sets the DC offset of the triangle wave.
Dimitree
so I tried the schematic on LTSpice,
connected SAW-IN to "Sawtooth Out" on the SEM (0.5V to 10.5V sawtooth), then PW to IC A5 output on the SEM (0V to 8.33V if offset trim is null).
In any of the PW setting, as output I only get a distorted sawtooth, not a triangle.
Any idea?
guest
the input needs to be centered around 0V.
Dimitree
tried both circuit I proposed..
the actual sawtooth from the SEM appears to be above 0V, so none of them worked as triangle converter..
I guess I'll just keep saw+pulse
guest
its pretty easy to shift the DC level, if you want to make it work.
Dimitree
I ended up using the minimoog triangle converter that I proposed on the first message, with an opamp in front of it, so that the SEM sawtooth (that according to my sim, is 0.5V to 10.5V) is shifted to be 0V centered. Then I modded the PW cv mixer opamp so that it accepts 0V/5V (instead of 0V/-15V) and routed its output to the triangle converter too, so that I can go from saw to triangle. I also added some trimpots to adjust simmetry and min/max pulse width.
Last, an electronic switch to choose between Saw/Tri and Pulse
On the simulation, the results are almost perfect.

schematic:
https://i.imgur.com/h3RPZh0.png
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