Thursday, January 13, 2011

A bit more vintage from your JH Living VCOs

JH: "I've never heard better drones than from three EMS VCOs running at almost the same frequency, and beating against each other in an ever changing pattern."

I've owned VCS 3s and Moog Modulars, and the early Moog oscillators, drifting away, outdo the EMS for a musically interesting varying tone. I don't mean out-of-control, but as he implies, something that is pleasing to hear.

If you want "more" of that from your LVCOs, consider installing a switch for the oscillator core capacitor, as the EMS (and even the Moog 901b) utilize vintage Mullard mylar capacitors, not polystyrene/polycarbonate.

C3, 2.7nf, is the polystyrene core capacitor, yes?

How to? This applies:

"Ideally, each Oscillator should have the two capacitors on the pcb joined at the current input end, to keep this sensitive point a short path. Fly a screened twisted pair cable wired to the other end of each capacitor to a panel switch such that it switches the larger capacitor in parallel. Ground the screen from a nearby panel control. "

"I've chosen the same noisy 4-transistor exponential converter that EMS used in the VCS3. I'm not overly scientific about it - I just like the sound of it."

Then why change anything at all about it if it sounds good as it is/was?

Adding the less-accurate cap may increase liking the sound of it- it's up to you. I remember Dean Hinton's original EMS mods page mentioned that its lowly vintage power supply had something to do with the "bad" behaviour of the instrument; you may wish to use a less-than-Kosher power supply to help regain the desirable character...I also note that when I sampled a VCS3 all those years ago, that the tip of each sawtooth waveform had a different overshoot, meaning that each cycle was slightly different. I don't know how that will turn out at audio frequencies but it has to be yet another aspect of Difference from the modern. I don't know how to re-create this, or if it is the same on all EMS oscillators. (I posted this at TGS many years ago; someone took my statement incorrectly, imagining that I meant that I could somehow hear each wavecycle at audio frequencies, and differentiate them...certainly not the case, but you can see the difference in a sample. I'm not sure about the cycle-to-cycle homogeny of the LOs, but it's worth considering.)

You may want to make sure that it has the same exponential .32V/Octave response (or so) for deep exponential FM.

Also, part of an interesting drone/its overall interesting character may be Oscillator 3. It doesn't, in the several images I've seen online, have 2C746 matched, single-housing trannies, which aid in heat transfer/stability. The 2N5172 pair which appear to be used in its core instead of the 2C, are sometimes (perhaps always) epoxied together without heat-transfer compound. This may produce more drift than the same oscillator with a 2C746 or other matched, single-housing pair.

"Back to the crow, back to the stone age!" -March Violets. Yes, I'm suggesting that on at least one of your LOs, that you install a matched but heat untreated pair. All of your oscillators don't have to sound like a bomber squadron, but one with a bit more drift will increase interest and perhaps get you closer to what sounds good about the original. Consider two-oscillator patches and use the third oscillator for FX. Sometimes, use all three for the different character. The Minimoog is also set up in this manner, however its Oscillator 3 is likely (haven't looked) of equal stability as the first two.

You can also put the Tempco resistor on a switch, DPDT, to select between it and a resistor of the same measured value but standard carbon comp or metal film. See the above link and considerations per switching core capacitors.

But of course, just ignore me; no one actually likes those old instruments or why they sound and act the way they do.

And to those who leap to fear any time I discuss the facts as I am learning them, get a life. Just get a life.