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Volta - Cable questions TS/TRS? Virtual Instrument Plugins
Old 13th May 2009
  #1
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majool's Avatar
 

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Volta - Cable questions TS/TRS?

I'm wondering if someone can shed some light on the following:

Motu says that using Volta, you should use TRS to TS (floated ring) from your interface to your synth, otherwise using TS to TS can burn out your interface outputs.

Can someone explain this to me? I can't understand how using a unbalanced cable from your Motu interface to your analog modular could have any affect on the Motu. Isn't it just audio going in one direction at the end of the day?

I don't want to run out and buy a bunch of expensive balanced cables just to run out to my analogs as cv and have the TS cables lying around.

Thanks...
Old 13th May 2009
  #2
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don't know but there's a Volta forum at muffwiggler. answer might be in there.

volta forum
Old 13th May 2009
  #3
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wow, cool. never heard of that site. thanks!
Old 13th May 2009
  #4
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It's not truly audio that's coming out of the MOTU.

It's using the DA convertors to create a DC Voltage.

I'm no electrical engineer, but I would guess that it takes it bit more electrical ooomph to maintain a consistant DC output vs. a constantly fluctuating audio output. I believe this is also why MOTU have a restricted list of interfaces that will work.

If they say it might burn out your interface... I'd listen to them. I'd rather buy extra cables, over replacing an audio interface.
Old 13th May 2009
  #5
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Probably has to do with using something that's designed for dynamic "AC" (audio signals) for static "DC" (control voltage), and shorting the ring signal to sleeve (ground) with little or no resistance. Ohm's Law. Current. Heat.

When connecting a TS plug to a TRS jack, the "Ring" contact in the jack would be contacting the "sleeve" part of the plug ("ground"). So you'd presumably be shorting a the polarity-inverted copy of the 'balanced' TRS output to a real or theoretical / potential ground.

The 'tip' part running the DC control voltage that is actually being "used" is probably encountering some much-needed resistance or buffering at the receiving-end in the synth CV input jack, so things don't get too toasty (picture how a power amp runs hotter and hotter the lower ohm-load you're running it into, until when you go below the recommended minimum rated ohm (resistance) load for the amp, it's passing too much current, gets too hot, and overheats and/or shuts down to keep it from melting or catching fire).

With audio signals, the voltage is typically varying over time, swinging through positive and negative values ("alternating" current), and you're dealing with more reasonable / lower levels most of the time - the audio signal would (unless you've got real problems) typically be a periodic 'wave' of some sort, that does things like visit the ol' zero-crossing from time to time, and be a lower average voltage, rather than pegging the output circuitry at "full excursion" one way or the other for long periods of time.

Shorting a control-voltage level "ring" signal to ground with little or no resistance "full time" would be, for one, a much 'hotter' signal being shorted to ground (those output circuits on the interface getting hotter) - and unlike even a square wave, it's gonna be DC - picture a waveform graph with the line pegged "flat" at the top (positive DC voltage) or bottom (negative voltage) of the graph - like the top or bottom of a "meters pegged" square wave, but staying slammed "up" (DC) instead of going up or down through positive and negative cycles (audio waveforms, by nature a form of "AC").

I know speakers don't like being "pegged at full forward excursion" over long periods of time (like what happens if you "test" a woofer by connecting a battery directly to its terminals). I'd imagine op-amps and other components might not be too happy about it either - particularly if they're being continuously "pegged with positive DC voltage" and "shorted to ground with little or no resistance, which jacks the current way-high" simultaneously.

Probably makes things heat up quite a bit over time. Short a speaker output to ground or to too-close-to-zero-resistance for long, it gets hot. Similar thing on a smaller scale could be happening here.
Old 13th May 2009
  #6
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thanks for the explanations. i think i get the basic picture.

i've tried doing this with reaktor, sending audio out my MOTU and get the same type of control as with Volta. does this mean Reaktor is sending out DC control voltage, simply because the information being (gate/pitch) sent is pegged and not alternating? i would guess that the only thing Volta does differently from reaktor is calibrate using pitch detection.

either way i guess it is safest to buy some cables (where? it seems you have to make them), although the guy who designed volta said he's been using TS-TS the whole time during development without issue and that it is mostly precautionary....
Old 13th May 2009
  #7
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Yep, I'd imagine it's a "covering our butts so you don't call us screaming in the unlikely event you actually cook an output IC on your audio interface" precaution as much as anything.
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