Esus wrote:Please elaborate--about the tuning, that is.
Sure, the quick answer is you not only tune the pitch like most oscillators, but you also want to tune the scale. So there are two tunings on the Moog 901's you need to adjust.
A longer explination and proceedure:
First thing you need to remember or be aware of is the voltage input range. The 901 spec is - 4 volts to +6 volts. So anything over +6 volts and they begin to crap out. Since there are adders on the 901A drivers, this +6 volt celing is including both 901A and 901B frequency venieres. So if both veniers are set high around 2 volts, you are already sending almost 4 volts and are only going to get another 2 volts of control before they crap out. So best to remember to start tuning with these closer to zero. Range switches on the 901B's do not add into this equation and are independent.
Tuning should begin with zero volts input on the remaning seven, 1 volt per octave inputs. On a standard Moog system there are three input jacks on the front panel, and an additional four inputs located via lighted switches below with 3 busses and the important 4th input with attenuator. So all switches off and nothing plugged in.
The CP-3 panel:
The next part may be difficult by ear, but with zero volts input you want to now tune the oscillators to the lowest "C" note in the range you are going to be playing. Do so by adjusting the frequency veniers. The 901's are now in tune. The rest is easy, now you want to tune the scale.
Tuning the scale can be done by a combination of either the busses 1, 2 or 3 and the 4th attenuated input. Or in a non Moog system without those buss switches a combination of one input and an attenuartor on another input. So basically you want to send the 901's two of the same 1 volt per octave CV with one straight and one attenuated. So send say, straight 5 volts out on your CV into one of the 901's , would be a high C on a keyboard and check for tune. If it is in tune the 901's are a perfect 1 volt per octave. But it will probably be a little flat or sharp and you will need to tune the scale. If it is flat, tuning the scale is done by adding a little bit of that same CV through the attenuator. The Moog 25K attenuators are very accurate and a tiny tweek near the zero range should be all that is needed to tune to the higher C. (This is now your high end fine tune knob.) If the 901's are sharp sending the 5 volts, then only the one attenuated input would be needed, again tweeking the 25K attenuator slightly down from the highest setting. Since the veniers were not moved during this process the low C is still in tune and the C five octaves above will be in tune.
The only tuning glitch may be the 901's are not perfectly linear and in that 5 octave range you tuned, one of the notes might be a little out. If this note is a problem you can adjust that bump or glitch to be in another spot by changing the 901A venier to a higher setting while lowering the 901B venier and retuning. This rarely happens but the there are little bumps in the 901's tuning range. Part of the magic!
This is all assuming the 901's are within specs and internally tuned properly, and your 901B's in a bank are internally tuned together. I have found they rarely go out of tune, but the scale seems to change with humidity and temperature. It is a PITA to open them up once a month to adjust the scale when you can do it on the front panel. Super eazy to tweek once you get the hang of it. And better than drilling holes in the front panel or modding them to adjust this. But 901's in spec should have no problem at all in a 5 octave range and can easily be coaxed to go an octave higher and lower to get 7 octaves. My 901's I calibrated myself and have them set so even changing the range switches are spot on.
Does that procedure make sense? I do it all the time? It is especially necessary if you have more that one 901A driver as that procedure is what you would need to tune each scale separately.
Couple of examples:
This was really an example of two 901's ring modulating each other. So I am constantly retuning one of the 901B's to get the ring mod tones. But you can hear I can tune these to many octaves range.
And another example showing the range switches being changed and still staying in tune. Mostly demonstrating what 901's sounds like through just a CP-3 mixer with no Moog filter. Still sounds Moogy.