This just dawned on me Re: VCO calibration

[elmegil]

My actual modular "rack" power supply use a well set power-one + / 15V supply (and +5 for good measure). I specifically calibrated it to be such.

My BENCH supply, however, is more like +15.69 and -14.69.

I do my calibration at the bench.

I've been frustrated that I don't seem to get very good tracking from my VCOs.

is the difference in supply going to affect the calibration? My intuition says it probably does.

[whomper]

I have placed the tracking trimmers on the front panel and am tuning them as needed.

There is a nice solution that I found a while back by Bourns that fit the longer trimmers and can hook to the front panel.

[isak]

Did you tried using Silent Way Voice Control?
I Got the best keyboard tracking results with The Voice Control using midi keyboards.
try using that and see if your collaboration is good or not.
hope it helpes you some how.

[Osal]

[creekree]

I suggest you do the calibration with the regulated power supply. It´s the logical thing to do.

[frijitz]

[elmegil]

The VCOs are an El Cerrito (which tracks pretty well) and an X4046 (which tracks pretty well also, but doesn't track much the same, so I can't use them in unison or fixed intervals).

Getting power from the rack to where I can access the trimmers will be a challenge, so I was just trying to do a sanity check as to whether this idea is correct or not. Changing the rack power to match the bench supply is really not my idea of a good practice. I've checked, unfortunately the bench supply is not adjustable.

I'll have to review the designs to see if I can understand that Ian. Thank you for what seems like a pretty definitive answer

[frijitz]

[elmegil]

Or I simply might not be very good at calibration

[marvkaye]

I've got several adjustable bench supplies that get used for various things, but module testing gets done with a fixed +-15 unit that I built myself (my first synth DIY project) using Ray Wilson's 337/317 PCB to which I added a pair of milliammeters so I can always monitor current. Anytime I go to use it I let it warm up for 15-20 minutes, then tweak the voltage trimmers to +-15V +/- 1mV (usually there's no adjustment needed.) Using that and an MFOS Volts Per Octave Calibrator also tweaked to a millivolt or two I'm usually pretty confident that things coming off the bench ought to work as long as my cabinet supplies are dialed in. Some pics are here.

I dunno, maybe it's a bit anal, but I figure if I eliminate as many variables up front as I can I've got a jump on figuring out what's wrong should something not work as expected during testing. Just my 2c.

<marv>

[elmegil]

I do use the MFOS precision calibrator, and I have taken steps to be sure it's as accurate as I can make it with my meter (3 1/2 digits, not 4+ unfortunately).

It's been on my To Do list to build my own supply for a while now, maybe this is the indication that I need to get it done sooner than later.

The good news being I have an appropriate transformer around here somewhere....

[Tim Servo]

One quick thought: Why don't you make up an extension cable that will allow you to power modules from your rack supply while they are sitting out on a bench. This will do a couple of things:
1) Allow you to do calibrations even if the designs have supply rail dependencies.
2) Allow you to check for problems that might occur if you are overtaxing your rack supply (since you will be using the rack supply, any problems should show up right away).
3) Even if the tracking and frequency range are independent of supply levels, you might have output levels that are dependent on the rails. If you adjust things like waveform centers or shapes, you'll find that these will change if you go to a different supply with the different offset 'errors' you listed. I know that would affect things like the centering of the triangle wave (and therefore the sine wave too) on the El Cerrito.

Tim (filing for an extension) Servo

[marvkaye]

[elmegil]

HA!

For now, yes, the "studio" is a jury-rigged rack about 4 feet from where I normally work.

And as far as it goes, the studio that I'm working on in the basement will have a workbench there also. So to use Tim's idea, I just need to grab one of these cheap-ass MIDI cables I've been sacrificing/cutting up to make 12" power cords, and make a longer one with it. Probably the best bet to be sure.

Then again the other idea I've thought about to resolve the whole unison business is to calibrate the two VCOs together at the same time; not sure how that's likely to work out either.

[marvkaye]

I suppose you could handle calibration in any number of ways, however, just getting the numbers right doesn't mean that the "instruments" are necessarily in musical "tune". Think about the pains that a piano tuner takes when trying to get the "temper" of the instrument so it is not only in "tune" but also musically pleasing. Generally there is a temperament or reference octave (usually centered on A440) that is precisely in mathematical "tune" from which higher and lower octaves are "tempered" slightly (teeny bit flat going down, teeny bit sharp going up) to get the tones musically pleasing. I guess the big question is whether you are after that zen realization of musicality or if you'd prefer to throw your ears to the wind and shoot for the numbers. Lots to think about, that's for sure.

[elmegil]

Very good points. Hm. I guess I was assuming that the scales I was looking at were already tempered, but now that I think about it, no, they pretty much are half frequency/half frequency/half frequency all the way down (aka turtles )

[marvkaye]

Sorry.... I didn't mean to overly complicate things... just thinking about one alternate approach. There's nothing wrong with attempting to achieve mathematical perfection... it worked well with the Hammond organ... all those spinning tone wheels with the proper numbers of teeth that generated very precise frequencies. Nothing tempered about them at all. Since most rigs have multiple oscillators you can always set one up to be tempered and patch it in when you want that color in your tone. Just a thought.

<M>

[elmegil]

I think right now the main thing I want is two oscillators that track together more than an octave. Ultimately I think that's more than likely going to be an exercise in having them side by side on the bench (easy with my bench supply anyway) than any of these other things, but they're all good info to keep in mind.

[marvkaye]