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Is Filter CV IN - TRS type ?

Posted: Wed Nov 13, 2013 6:16 pm
by jupiter
Hi all,

I'd like to control Filter CV using a pedal, but unsure as for what kind of input the Phatty's expects in the Filter CV Input... Should I connect a TRS plug into it ? or plain mono ?

I have a Korg MS-01 clone pedal (-4v to +4v ) and also not sure if the out is TRS or mono... If I connect a mono plug to its output I get 0 to +4v and not -4v to +4v...


Re: Is Filter CV IN - TRS type ?

Posted: Thu Nov 14, 2013 1:43 pm
by Stuka
Did you ever say which Phatty you're referring to?

The Little (LP) and Slim Phatty's (SP) are essentially the same in features & function, minus the keyboard on the Slim.
The Sub Phatty is a totally different beast.

You can use either cable (TS or TRS) with the LP / SP -
You'll need to use TRS if connecting an expression pedal, but TS works fine if feeding CV from an external source.
As long as you don't exceed the specified input voltage range, you should be fine.
(Just keep in mind what I mentioned in your other thread ~ with a 4V CV input instead of 5V, you are going to lose some control range).

I'm guessing the CV functionality is pretty much the same with the Sub Phatty...

Re: Is Filter CV IN - TRS type ?

Posted: Fri Nov 15, 2013 2:14 am
by Portamental
I don't know about the MS-01, but I have a Korg MS-04 pedal and it correctly generates -4 to +4 volts lfo waves.

That's not the moog capable voltage range of -5 +5 but that's a lot. The CP-251 LFO output is -2.5 to +2.5 and that's already too much for "expressive" modulation and it has to be scaled down through the attenuators. But may be you are looking for "extreme".

Use TS cables to plug into the cv input of your moog. Works just fine with the MS-04.

Now which Phatty? The LP and the Slim are full range as far as the CV inputs are concerned, ie. they process negative voltages. The Sub-Phatty (and the Minitaur) only accept positive voltage. The negative voltage part of an incoming LFO wave is leveled at zero volts. That means you can only half modulate (half the cycle) the filter or pitch CV for these.

Re: Is Filter CV IN - TRS type ?

Posted: Fri Nov 15, 2013 3:46 am
by radioland
I tried out the cp-251s triangle LFO out into the Sub Phattys pitch in, and indeed I only got the top half of the waveform. In addition, the pitch change was only minimal (compared to what happens when you do the same with the frequency input of a FreqBox). How can I fix that? Putting the LFO out through an attenuator (+5) or through the mixer (offset and master at the maximum) didn`t help much.

Thanks for your help,


Re: Is Filter CV IN - TRS type ?

Posted: Fri Nov 15, 2013 5:47 am
by jupiter

It's a Little Phatty Stage II. I'm not sure I understand...

If I use TS as Filter CV Input:
Should the Sleeve be the negative lead and the Tip be positive lead, reading the range of -5v to +5v between the two ? My pedal shows only 0 to +5v when I use TS cable.

If I use TRS as Filter CV Input:
Should the Sleeve and Tip be positive, and the Ring be negative lead ?
reading the range of -5v to 0 between the Sleeve and Ring, and 0 to +5v between Ring and Tip ?

I don't want to fry my Phatty :?

Re: Is Filter CV IN - TRS type ?

Posted: Fri Nov 15, 2013 11:48 am
by Portamental
Its a misnomer to call the two wires of a DC signal negative and positive. DC can be negative voltage.

The so-called negative wire is actually the ground and the positive is the hot lead but that can be negative voltage if it is fluctuating DC (like in an analog synth) vs a power supply voltage, which is steady positive voltage (there are negative power supplies too).

It is not recommended to use TRS cables on expression type equipment. With an expresion pedal, the ring carries a positive voltage that is sent to the pot of the expression pedal. The pot acts as (is) a voltage divider. The divided voltage shows at the tip. When you insert a TS cable in an expression jack, the ring voltage is safely brought down to ground by internal circuitry of the host.

Now if you use a TRS cable, the ring voltage supply is sent to the client device, which itself may carry a voltage. It is probably not harmful, but still, you need some mixing circuitry to properly mix two live voltage sources. Actually, if the client device is TS only, you are screwing up the ground level.

Measuring fluctuating DC is hard on a digital multimeter because the averaging time of the meter is higher than the usual LFO rate range. Use an analog needle meter and a slow lfo rate. If there is negative voltage present, it will bump the needle below zero on the negative portion of the LFO wave.

Using the offset knob to raise voltage on the CP-251 will do just that, raise voltage above zero. It will not sound the same since you are modulating the internal voltage level of the filter or pitch cv upward only.

You will not fry your LP or SP. It is built to safely handle any voltage in the -10 to +10 volts range.

Although the Minitaur and SubP seems to share a similar design, the literature is confusing. The Minitaur lit states that the CV inputs are positive only, while the SubP lit actually warns against negative voltage. All is safe with the Minitaur regarding negative voltage handling. If I had a SubP, I would not be afraid of negative v's and go right along, but that's just me. I can not officially recommend it.