On a modular how is the Lin / Exp. rocker switch used on the 902 VCA for audio signal? Also if processing a voltage, say from 911 eg what would it be switched too? Are there any general rules?
Taken from the site listed below:
“The control may be either exponential or linear, depending on the setting of the switch. In either position, the amp is full on when 6 volts (total) is applied. This doubles whatever signal is input. In exponential mode the signal decreases 12 db for each volt below 6. In linear mode the signal amplitude is porportional to the control voltage.”
Keep in mind that for each -3db decrease in signal, the resulting audio level will be reduced by about half.
So whether you want a linear, or exponential behavior of your VCA is entirely up to you, depending on what you hope to accomplish with the signal level.
Most hardwired analog synths, but not all, have an exponential VCA by design.
Here’s the link to the source of my citation:
http://artsites.ucsc.edu/ems/music/equipment/synthesizers/analog/moog/moog.html#904
The most common use for linear position is VCA control of modulation signals IE LFOs or EGs.
So for audio signal the setting on a VCA should be Exponential? Is this normal ? How do modular owners decide which should be used…for audio and control voltage?
Is it that Linear has a slow attack phase whereas Exponential is faster?
If I understand it correctly, in Exponential Mode, if the CV is 5V, the signal drops 12dB. If the CV is 4V, the signal drops 24dB. Such large decreases in signal strength are detrimental in audio signals. That is, they are better suited for use in subtle modulation, rather than audio. The attack phase is controlled by the envelopes, not so much the VCA.
He’s not talking about phase, he’s talking about response of VCA with respect to a uniform attack transient time, assuming a linear attack slope.
Exponential mode will “sound” faster than Linear. But both have their applications for VCAs passing audio. Percussion sounds are best with expo, while bowed strings and brass may work better with linear.
FWIW, vintage Oberheims have linear response VCAs, vintage (non-modular) Moogs are expo. Voyager has linear attack but expo decay and release 
It’s just a matter of personal preference really. But an exponential response is faster than a linear response using the same CV.
A typical EG/VCA setup normally would have an exponential curve on the envelope voltage output and linear response on the VCA. The 902 gives you the choice of either voltage resopnse.
The 902 has inverting inputs, positive on top and negative/inverted on the bottom. So feeding both inputs the same signal should almost cancel each other out. The outputs are inverted on top and non-inverted on the bottom. So the 902 can be used as a signal inverter, audio or CV. The 902 is an amplifier and at full voltage, 6 volts, will amplify the signal by a factor of 2. (Many other brand VCA’s only give a unity gain at the maximum output.)
The lin/exp switch will only effect the control voltage inputs. This is the sum of the three input jacks, input node, and front panel fixed voltage vernier. In linear mode with an audio signal patched through the 902, the fixed voltage vernier will have an even gain from 0 to 6. In exponential mode turning from 0 to 6 will have a steep volume rise toward the end of the knob. This would be the same type response from an envelope , LFO, or any CV, where the linear response is an even line. The exponential response would be a curve with more increase toward the high end of the voltage applied.
Here is a chart from the 902 brochure showing the response from a triangle wave applied to the CV input.
First the tri wave CV, the linear response and the exponential response.
Exponential response could be thought of as a more percussive or struck articulation. Even though the 911’s are very percussive and fast, in expo mode they would be even sharper. And the sustain level would be different, where a 911 sustain of 5 would be heard in linear mode. At 5 the 902 would be barley audiable in expo mode, due to the sharper curved response. The Minimoog seems to have a similar control range as the 902/911 combo in linear mode.
A small example of a sequence played first in linear mode, then switched to expo mode:
902 sequence lin/exp MP3
The expo mode is much more percussive. This sharp cutoff can be very usefull depending on the application.
Another example similar to the chart above where the 902 is being modulated by a triangle wave. First in linear then in expo mode.
Triangle modulating 902 linear then expo response MP3.
In expo mode the tri modulation gives more of a chopped on/off effect.
So it all depends on the application, where you would need or want that sharper percussive response, or a smoother one.
With this one simple module there are dozens of uses from applying a delayed vibrato to a VCO, to amplification, to inverting signals, subtraction, ring mod, etc. The expo/linear switch helps in shaping the CV response of the 902 in that particular application.
Here is a patch I used where I wanted to only patch in the Moog 904B high pass filter and needed an extra 902 to invert an envelope. The 904B or most high pass filters respond the opposite of low pass filters. So to get a percussive envelope that would normally be a fast decay to zero sustain, I needed a fast attack to full sustain, then inverted that signal via the 902 to feed into the 904B. There is a third 911 that is delayed by a 911A that sweeps the high pass up when sustained. The 904B has a positive feedback loop through the CP3 to make it squeal like it has resonance. (I know the 904B is not a favorite of many, but I think it freekin’ screams!)
Moog 904B high pass resonance.
960 sequencer in background controlling Minimoog.
Just another use for the 902 in a Moog system.
Thanks…particularly to CZ ! Learned more in these replies than from any manuals …good or what?
Yes, thanks for the in-depth explanation, CZ! 