MFB Step64 CV circuit loading

In a Moog Mood? Here's a forum for discussion of general Moog topics.
Post Reply
User avatar
mayidunk
Posts: 639
Joined: Sun Oct 26, 2008 3:28 am

MFB Step64 CV circuit loading

Post by mayidunk » Sat Jan 23, 2010 3:39 pm

I have tried running the pitch CV from my Step64 through the CP-251, however it seems that doing so may be putting too much of a load on the Step64's output circuit, as the pitch ends up being about a half step below what it should be, and tends to wander when playing a sequence.

Has anyone else experienced this? Can it be overcome?

Thanks in advance.

User avatar
latigid on
Posts: 1579
Joined: Mon Jan 30, 2006 3:47 pm
Location: Auckland, New Zealand

Post by latigid on » Sat Jan 23, 2010 3:46 pm

Whereabouts are you patching it? Sounds to me like it shouldn't be doing this.

As a shot in the dark, could it be you are going through the mixer with the offset not centred?

Or is the scale within the sequencer itself not calibrated? (It works without going through the 251?)

acorkos
Posts: 219
Joined: Sun Nov 02, 2008 9:14 pm
Location: Redondo Beach, CA

Re: MFB Step64 CV circuit loading

Post by acorkos » Sat Jan 23, 2010 4:04 pm

mayidunk wrote:...may be putting too much of a load on the Step64's output circuit, as the pitch ends up being about a half step below what it should be, and tends to wander when playing a sequence.

Has anyone else experienced this? Can it be overcome?
my step64 behaves similarly with my modular under certain conditions...for example, i built a patchbay to convert RCA-1/4". If I connect the step64 CV plug into the patchbay and from the patchbay to a VCO exponential freq. volt/oct jack, i get wackyness. going straight into the VCO without the patchbay, but using the MFB-provided RCA-1/4" adapter plug, everything is fine. Also, plugging the step64 CV plug (adapted) into a mult and then from the mult to the VCO gives me wackyness. i'm thinking it might have something to do with adding cable length for the signal to travel along....causes the CV to become erratic. like the voltage is optimized for the length of cable provided by MFB. I plan to someday get a midi plug and some coax and experiment...

User avatar
latigid on
Posts: 1579
Joined: Mon Jan 30, 2006 3:47 pm
Location: Auckland, New Zealand

Post by latigid on » Sat Jan 23, 2010 4:10 pm

Hmm, that seems like an incredibly poor design.

For the DIY inclined, you might want to consider building a unity gain output buffer from an op amp and a few resistors.

User avatar
mayidunk
Posts: 639
Joined: Sun Oct 26, 2008 3:28 am

Post by mayidunk » Sat Jan 23, 2010 4:14 pm

Plugging it thru the mult on the CP-251 causes wackiness. Plugging it thru two 6 foot patch cables causes the result to be almost a step lower than it should be.

I think you're both right, the CV circuit is sensitive to slight changes in output impedance.

Oh well, too bad the circuit design didn't incorporate some kind of buffer on the output. It seems that would've been a no-brainer!

Thanks guys!

DontBelievetheHype
Posts: 148
Joined: Fri Feb 20, 2009 10:19 am

Post by DontBelievetheHype » Sun Jan 24, 2010 1:27 am

I just plugged a keyboard into my rig for the first time the other day and was trying to tune my oscillators, I just ran into a similar problem with my MP-201 + CP251.

I was comparing my Freqbox's to see how close they were and if I could use a single CV from the MP201 into the CP251's multi to control two osc's. While doing this I noticed that the CP's multi sucks voltage, regardless of cable length-- for instance, if I plug the MP into the multi and then into a single Freqbox, the pitch drops about 5cents from that of the MP directly into the Freqbox. I also tried using a long (4') cable directly from the MP201 to the Freqbox, and it kept a stronger signal than using two 6" cables+multi..

I haven't checked for drift, only tested single notes with a tuner, but basically had the same result as you-- I'm not sure how it all relates to your step64 or the other guys step64+patchbay, but figured I'd put that out there at least.
Synth bass guitar rig including MS-20 filter clone, Paradox TZF, MF101, 102, 103, 107(x2), CP-251 and MP-201

User avatar
Portamental
Posts: 888
Joined: Fri Jan 23, 2009 8:53 am

Post by Portamental » Sun Jan 24, 2010 8:20 am

Huumm.... interesting but worrying? Maybe, or maybe not. I have not used the Step64 through CP-251, and i did not detect any pitch issue connected direct. Very aware (part of my day-time job is interfacing and calibrating voltage controlled scientific measuring devices) of resistance issues caused by long cables and connections in low current signal electronics, I never used the cables that were included with the Step 64 (too long, RCA-1/4 jacks adapters = not good) but made custom ones, shorter and soldered, right from the start.

The pitch CV is a very special CV. It has to be very accurate as it's level must correspond to a note into the frequency scale. An expression CV is not that critical. It replaces a knob (or is added to the knob value) for functions where full range means : lot of range (and never mind the small loss in resistance along the connections). The output current of the 5v ring feed on the Voyager's input jacks measures at .001 amp exactly (5 volts), that means that the amount of current generated by a CV for the lowest note (but the one at 0 volts) equals .001 amp divided by 60 (5 octaves at 12 steps per octave) or .00002 amps roughly. That's how much current there is available to be sampled for conversion into a different state. That's low, but enough to handle a good pitch CV in reasonable conditions (MP-201 and short patch cables) under Moog design parameters. I have tested both the Voyager and the MP-201 extensively for pitch issues.

If the CV output current of the Step64 is significantly lower than what is the 'norm' on the Voyager, that can spell trouble with long cables and multiple physical connections points. From simple math, my guess would be the lower the pitch, the less accurate the note will be if not enough current (.00002 amp vs .001 amp at 5v moog spec) is available to do the job. Can you verify that a step programmed 5 octaves higher than the starting note is actually closer in pitch to what is intended (more precise)?

By the way, it's important to realize that the CP 251 is an "expression" CV control device and is not intended to carry CV's with the precision required by the pitch CV. It is designed to handle full range expression CV's where a little loss of voltage here and there can be adjusted to taste with knobs. On that regard, the voltage supplied to the ring in the CP-251 is 5.6 volts (and NOT the precisely calibrated 5 volts of the Voyager's input jack or the equally well calibrated MP-201 CV output). I can only think the difference is a fudge factor added by design so that a "full range CV" from an expression pedal remains a full range CV (and maybe a tiny amount more) even with long cables and patching at multiples.

I never expected the CP-251 to carry pitch CV's with precision, and never will, but a add-on module buffered on input, with slight boosting and calibration features suddenly seems like a good idea for the CP-251 and a nice DIY project. The CP-251 has some of that (minus the buffering and boosting) through patching it's modules, but the fact remains the pitch CV is one that has to be carried with extra concern to precision. A single-in single-out dedicated module for pitch CV handling would minimize patching issues and provide tuning anyway.

The CP-251 provides it's own way to help with the situation: patch the CV to the Mixer and offset it by a tiny positive amount.

I'll put the MFB on the testing bench one of these days.

DontBelievetheHype
Posts: 148
Joined: Fri Feb 20, 2009 10:19 am

Post by DontBelievetheHype » Sun Jan 24, 2010 10:56 am

I thought it might have something to do with CV's in general passing through connections, but your explanation regarding the 'expression CV' capabilities of the CP makes sense- thanks 8)
Synth bass guitar rig including MS-20 filter clone, Paradox TZF, MF101, 102, 103, 107(x2), CP-251 and MP-201

User avatar
Portamental
Posts: 888
Joined: Fri Jan 23, 2009 8:53 am

Post by Portamental » Sun Jan 24, 2010 11:25 am

DontBelievetheHype wrote:I thought it might have something to do with CV's in general passing through connections...
Well it does... it means that all CV's are allways accurately-approximately handled depending on design and hardware issues and limitations. And I am sure the CP-251 is good analog circuitry within it's design: expression CV control. Current losses happen through connections and mults, but they are too tiny to be of concern for expression or wild lfo effects. Not so with the pitch CV.

Post Reply