Dummy Plug/Negative DC Voltage (Devarahi Book)

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Niles Crane
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Dummy Plug/Negative DC Voltage (Devarahi Book)

Post by Niles Crane » Sun Aug 19, 2012 3:51 pm

I'm reading the book to understand modular synthesis, it's been great but a few times he has mentioned these two steps in experimenting with patches.
For example, for non-tracking of a vcf, you must insert a dummy plug, what is this and how is it achieved? Same for negative DC voltage, I assume they are very basic steps but would just like some info on them.

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Re: Dummy Plug/Negative DC Voltage (Devarahi Book)

Post by sempervirent » Mon Aug 20, 2012 7:03 pm

Niles Crane wrote:I'm reading the book to understand modular synthesis, it's been great but a few times he has mentioned these two steps in experimenting with patches.
For example, for non-tracking of a vcf, you must insert a dummy plug, what is this and how is it achieved? Same for negative DC voltage, I assume they are very basic steps but would just like some info on them.
A dummy plug is just a plug with no cable attached. I remember seeing one of Automatic Gainsay's videos on YouTube, where he was demonstrating the ARP 2600. Most of the internal connections on the 2600 are normalized (which is why it's considered a semi-modular). If you want to break one of those internal connections, but don't actually want to patch something else in, you just insert a dummy plug to bypass the normalized connection.

In your example, it sounds like a similar situation is being described. If a synth has internally-normalized VCF tracking, but also has an input for overriding that, but you also don't want it to actually track anything, you just insert a dummy plug. That will shortcut the internal routing, while sending a whole lot of nothing to the VCF.

Back to the example of the 2600: it has attenuators on nearly everything so I'm not really sure why dummy plugs were used in the demo video I mentioned, because you really only need to do this if there's no attenuator to cut down the level of CV input to zero. So this is not a universal concept for all synths/filters, it really only applies to a device with internally-normalized connections that you might want to break.

As for negative DC voltage, many (most?) synths accept CV input (and generate output) in the range of -5 to +5 VDC (volts direct current). CV = DC (in general). If you have an LFO modulating some aspect of a synth, it's typically going to be oscillating between +5V at the peak of the waveform and -5 at the trough (lowest point) of the waveform. The -5 VDC is a negative voltage. If the book is talking about applying a negative voltage in a non-oscillating way, it's probably referring to using a negative voltage as an offset (which is just a way of saying that the initial voltage value is set to a higher value, or a lower value in the case of negative voltage, than it would have as a default).

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Post by daverj » Mon Aug 20, 2012 8:06 pm

A negative voltage (or a positive one) by itself is not a common signal source on a synth. But there are some modules designed to supply an adjustable bias voltage (adjustable from -5 to +5) which are just a knob and a jack.

The purpose of feeding in a fixed -5 or +5 voltage to an input is that some modules have a knob and a control voltage that are set in such a way that the knob doesn't cover the entire possible range that the combination of knob and CV can cover.

For example when you adjust the frequency of an oscillator with a knob and have nothing going into the FM input, you can only adjust the knob to go so high and so low. But often by feeding a signal into the FM input you can go higher or lower since the CV and the knob are added together. So in this case plugging in a -5v to the CV might make the oscillator go to lower frequencies than you can get to with just the knob (or higher, if feeding in a +5v).

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Post by sempervirent » Tue Aug 21, 2012 1:58 pm

I decided to go back and watch those ARP 2600 videos I was talking about before. Start watching around 5:58 to see a dummy plug being used in exactly the way that your book describes.

If you look closely at a high-res photo of the 2600's front panel, you'll see that there's a keyboard CV input in the filter section, one of the few inputs on that synth that doesn't have an attenuator.

[video][/video]

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