One thing that I was intrigued by was the ability to control the synthesiser with the guitar.
Guitar into ARP 2500 synth envelope follower.
Pete (December 1971 Crawdaddy interview with John Swenson, “The Who Puts the Bomp”):It’s become the instrument which is almost like a voice, there are different sort of sounds that are made from different ways of playing and it’s like . . . you can recognize guitarists.
So there’s always that bit of humanness, if you like, but it’s only a breaking off point because a guitar is not that much just a guitar, but once it becomes electrified it’s turned into a giant instrument which can play to 60,000 people. It can also do a lot of other things, a guitar can be the control center for a synthesizer. A guitar can go into a synthesizer and have its sound taken apart and put back together again in a different form, so that you’re playing the guitar, but the actual sound that comin’ out is a completely different thing. On the album, on Who’s Next, there’s a very simple one which we use with the ARP synthesizer called an envelope follower, where you plug the guitar in and you get a sort of fuzzy wah-wah sound.That’s on Going Mobile. So that’s how you get that incredible sound.
But the guitar itself was controlling the amount of filter sweep. When you hit the note the filter went Bwaaumm! And when the string stopped the filter closed so you got nothing.
I wondered this when I bought my Korg Volca Modular. However, after reading about this question in regards to Moog’s, I wonder if my initial issue related to my amp/preamp:
Many of our instruments have a 1/4 input that will accept guitar level signals. The most immediate and direct way to use a guitar or other string instrument (that has a pickup) with a synthesizer, is to plug it into this EXTERNAL INPUT jack, to take advantage of the built in ladder filter found on most of our instruments. Keep in mind, that in this scenario, you are using the synth like a guitar pedal or studio effects processor to do things like roll off the high frequency content, add resonant feedback, or achieve tremolo like movement.*