CV output controlling modular equipment

[Benjamin AM]

I bought a Little Phatty a few years back when I heard that there was a CV Out mod in the works. It was a selling point because I knew that I would eventually build a modular system in the future and I needed a controller. Therefor the Phatty served two purposes: stand alone synth and a controller for a larger system. Once the mod was installed, I soon found out that the keyboard out CV was easily loaded down when connected to more than one source. To me this was very disappointing because if the Keyboard CV out is connected to multiple sources through passive mult jacks(Like the one on my CP-251), the V/Oct changes. This is a huge bummer if you are using this voltage to control external oscillators and filters. Anyhow, I was almost on the edge of getting rid of the Phatty for this reason until I decided to make a few buffered Multiple jacks. Needless to say, everything works wonderfully now I'm glad that I didn't get rid of the synth because it is awesome in so many other ways. In this case it just needed a little help from a good power supply.

[Sir Nose]

Good to know. Is it the same for the envelope and mod bus outs as well?

[David Smyth]

I've got the CV output mod for this very reason also - are you saying I can't run the Pitch CV through a passive multiple module to control VCO's etc. without the signals loosing their tuning?

If so that is somewhat disappointing..

A few questions:

- Does a buffered multiple mean it is powered? Because I'm going to need one/some when I start my modular system.

- Do buffered multiples still work normally (as passive mults do) for other modular CV signals?

- I assume you're only talking about the Pitch CV Output. Have you noticed any similar problems with any of the other four CV Outputs?

Anyway I'm glad to hear you kept the Little Phatty and the connection to modular is now working out for you!

Regards,
David Smyth
New Zealand

[Benjamin AM]

Good to know. Is it the same for the envelope and mod bus outs as well?

Before I get started here, let me tell you know that I am not an electrical engineer of any sorts. As far as the other outputs go, no noticeable changes. I drove 3 to 4 sources from all of them and didn't notice much when the sources were added or removed. I would say that the modulation, env. gen., and gate control voltages are a different game, because there are used(speaking for myself) approximately rather than in a precise fashion which is need for creating perfect pitch and tracking in a VCO.

I've got the CV output mod for this very reason also - are you saying I can't run the Pitch CV through a passive multiple module to control VCO's etc. without the signals loosing their tuning?

No not exactly, my experience is that whenever I add another module to the Keyboard CV chain I would have to readjust the Little Phattys tracking on the analogue out board. I'm saying that this recalibrating is not acceptable for me, so I decided to buffer the signal to prevent doing this every time I change an external patch. The buffer is used to prevent the additional circuits from loading down the Phattys analogue out board, thus mainting "perfect" pitch and tracking.

- Does a buffered multiple mean it is powered? Because I'm going to need one/some when I start my modular system.

Yes, it primarily consists of a Tl074 Op Amp with the negative inputs wired to the outputs and the CV voltage entering the positive inputs. One thing that you have to take into consideration is that you will most likely need a linear bipolar power supply. If you own a modular you should be good.

- Do buffered multiples still work normally (as passive mults do) for other modular CV signals?

I wouldn't see why not. There are no coupling capacitors.

[poladark]

Always a good idea to have a couple of buffered mults in your modular. I got my cv out modified LP last week and it's been non-stop fun with my 6U eurorack system. The passive mults are simply not enough for all situations, little phatty or no little phatty. My more immediate future eurorack purchases will be more utility modules like that.

[Sore_Knee]

from what i could see from pictures of the install, the PCB they install is a buffer between the actual CV generated by the LP and the CV out plugs.

(not my picture)
http://www.moogmusic.com/forum/download ... &mode=view

to confirm this, when you load up the CV out, does the pitch of the LP change too?

Also, it is a normal practice to put a resistor in series with CV outputs to protect against user abuse i.e. plugging in something of low impedance such as a short or a hot signal. It looks like they have these, though i can't tell the value from the picture above. the problem with this is, the CV voltage will drop as you connect more things to it.

i could say, you could just find this resistor and short it, but i'm not. ok i just did, though i'm not recommending it.

you can measure the output impedance (which will probably tell you the value of the resistor) by loading up the output with a known resistor. compare the open circuit voltage to the loaded voltage and do a little Thevenin equivalence calculation. at least if you know the output impedance, the behaviour will be a bit more predictable.

[Benjamin AM]

to confirm this, when you load up the CV out, does the pitch of the LP change too?

The internal oscillators don't budge. Good point.

[Benjamin AM]

I was just poking around and found what I believe to be the pre-attenuated modulation source. I moved the original connection to an unlabelled resistor in between Q7 and Q8(LP StageII). The wire was connected to the leg that heads to a via. The other(unused) leg goes into Pin 3 of the lm13700. So far it seems to work real nice. The Phatty can now be a LFO or clock source for modular equipment while using its own unmodulated voice in the mix. Sweet.
Warning: if you try this, you could potential harm your synthesizer.

[davydka]

Awesome.