Alfa Rpar has come out with the AS3364, a quad version of their CEM3360 clone (AS3360). It drops the exponential input in favour of more VCAs in the same package.
One very unfortunate thing about the 336x is that it cannot be run from a +/-15V supply, which is what I intended to run my synth on. Now, I am considering switching to +/-12V anyway since it may save some power, but still.
Anyway, synths from the 80s, like the OB-8, used the CEM3360 with a +15V Vcc rail. As the chip can have a Vcc-Vee = 26V at max, they used Vee = -5V.
What I wanted to find out was how this affected the signal, especially, would the voltage still swing around 0V?
I breadboarded this tonight, and read the datasheet. The chip has a Vref connected to ground via a 100Ohm resistor and a 5nF cap, I presume that is for centering.
Here is what I figured out:
- The voltage swings around 0V even when Vcc and Vee do not have the same absolute value.
- The maximum swing is to within 1.5V of each supply rail, so with a +15V down to -5V the lowest the VCA can go is -3.5V, anything after this is cut off.
- The chip can be run at +15V/-9V, so a 18Vp-p signal is still possible.
- CV range is 0 (-80dB) to 2V (unity gain). Absolute max VC is 2.5, so be careful!
When connecting the output (as configured in the datasheet) directly to my scope, I saw quite a lot of low pass filtering on the output (but later testing showed that this went away when attaching a non inverting buffer).
I then connected the output to the negative input of an opamp, and put the 47k resistor as negative feedback. This got rid of the LP but introduced a lot of ringing/overshoot. This was removed using a 33pF cap. I got an even cleaner square output with a 15pF so I guess I should do a little calculation here.
I do however see that neither the crumar spirit, nor the OB-8 do any kind of filtering here, presumably it's taken care of later (or by using different op amps). The Jupiter-6 uses 22pF/100k feedback. The Prophet 600 uses a 20k resistor to ground plus a non-inverting buffer.
Update: While at it I've tested the following:
Increasing the feedback resistor
The output is a current that is fed to the negative input of an op amp. Changing the feedback resistor would have the same effect as in a regular inverting amplifier configuration, but there is no input resistor. However, we know that a 47k resistor gives unity gain, so for example doubling to 94k will double the output voltage, adding a 22k will almost give a 1.5x gain etc. I tested this and it's correct.
Using a non-inverting buffer
Using the circuit in the datasheet, and attaching a non-inverting buffer directly afterwards works very well. No need to filter the output it seems. Increasing the resistor from 47k to 47k+22k gives 1.5x gain. BUT - we still can't go below the negative power rail, so the output is clipped at -3.5V (it looks like 4 on the scope though).
CV Linearity
There is very little response for the first 250mV, the rest seems fairly linear.
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| Top: CV, 0 to 1.5V. Bottom: Response on a 3V constant input. Notice that nothing happens in the beginning, but then the rest is fairly linear (slightly dropping but not too bad). |
CV resistor voltage divider
The maximum CV input is 2.5V, and full range is 0-2V. By using a resistor voltage divider at the input we can transform a 5V CV to a 2V CV for example by combining a 33k from CV to CV input and 22k resistor from CV input to GND. This worked nicely. One could also use two equal resistors to get 2.5V which makes slight maximum adjustments possible.
There is no effect on the linearity so using a voltage divider seems perfectly fine.
Other interesting things
The chip doesn't seem to self destruct immediately if CV is > 2.5V (yeah, that happened, chip still works)
My chip seems to give max gain at CV = 1.5V, not 2V.
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| Same as above but with 1.8V max CV. Notice how gain maxes out before the CV reaches its peak. |
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