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Rectification and negative voltages?

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Rectification and negative voltages?

Postby Arpangel » Fri Aug 09, 2019 8:16 am

I have a synth module and the outputs need modifying to output positive voltages only, I was told that a Schottkey Diode in series on the output jacks would do this, basically acting as a rectifier, but I'm unsure of the wiring, any help appreciated.
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Re: Rectification and negative voltages?

Postby Wonks » Fri Aug 09, 2019 8:31 am

Aren't you going to totally change the waveform if you then clip off the bottom half?

To keep it the same shape, aren't you going to need to somehow introduce a DC offset to get the waveform all working up in a positive voltage range, and then remove the DC element with a series capacitor?
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Re: Rectification and negative voltages?

Postby Arpangel » Fri Aug 09, 2019 9:02 am

Wonks wrote:Aren't you going to totally change the waveform if you then clip off the bottom half?

To keep it the same shape, aren't you going to need to somehow introduce a DC offset to get the waveform all working up in a positive voltage range, and then remove the DC element with a series capacitor?

Thanks Wonks, I see what you mean, I don't really know what I'm doing here, component wise, the capacitor makes sense, but I'm unsure of the function of the Schottkey Diode in all of this.
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Re: Rectification and negative voltages?

Postby Hugh Robjohns » Fri Aug 09, 2019 9:10 am

Arpangel wrote:I have a synth module and the outputs need modifying to output positive voltages only.

I think it would help a lot if you can be more expansive about what modules you're using, what the output and input control voltages current are supposed to be, and what you're trying to achieve.

Converting a +/-5V CV range to 0 to +10V will require a completely different solution to simply removing the 0 to -5V portion!

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Re: Rectification and negative voltages?

Postby Wonks » Fri Aug 09, 2019 9:15 am

A Schottky Diode is simply 'faster acting' and has a much lower forward direction voltage drop diode than a standard diode, so is more suited to signal applications where you want to preserve the signal shape as best you can. It will stop any negative voltages up to a lower limit than a standard diode, but as this is at least 50v, then it's not an issue for most low-level audio signals.

As the signal tries to go negative, the diode prevents this from happening, so the signal sits at 0v for the negative part of its cycle. Basically half-wave rectification.

Image

So you can see that whilst one would stop any negative voltages, it would also seriously change the sound.
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Re: Rectification and negative voltages?

Postby Folderol » Fri Aug 09, 2019 10:19 am

If it's a fairly continuous signal, fat capacitor in series followed by shunt diode to ground (stripe to cap).
If not, then the only sensible way I know of is with an opamp wired as a unity gain buffer.

Edit
Can we have a bit more info on what this is feeding. It's seems weird to require positive only, unless it is for a control voltage.
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Re: Rectification and negative voltages?

Postby Martin Walker » Fri Aug 09, 2019 10:57 am

Arpangel wrote:I have a synth module and the outputs need modifying to output positive voltages only, I was told that a Schottkey Diode in series on the output jacks would do this, basically acting as a rectifier, but I'm unsure of the wiring, any help appreciated.

Aha - that dratted Buchla module rears its ugly head again Tony ? ;)

A circuit diagram would really help here, as you could possibly tweak a couple of components to shift the baseline so that the entire waveform goes from zero to positive, rather than negative to positive.


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Re: Rectification and negative voltages?

Postby ef37a » Sat Aug 10, 2019 8:39 am

If you are talking control voltages (CV) then a couple of op amps wired as precision rectifiers. diode feedback per amp, + and -, should give a fairly linear 10 volt resultant which you could then attenuate to suit.

Any old op amp "will do" but if stability is important look for ones designed for low DC drift.

But like the others, in the dark a bit.

http://midibox.org/forums/topic/17552-c ... ttenuator/

^ any help?

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