VC Noise Source and LFO

[Thomas_Henry]

Hello Everyone,

Here's a new/old take on a VC Noise Source using the SN76477. What's old is clocking the pseudo-random generator by the internal VCO of the chip. You'll see me doing that back in 1981 in the SuperController (see the reprint collection recently posted).

What's new is:

-- no range switching on the noise
-- exponential control of the noise source; 10 octave span
-- no range switching on the LFO; 1500:1 sweep range
-- no +5V supply needed; uses internal Zener
-- transistor current sinks are way more elegant and versatile
-- 2 chip design

The range is insane. On the low end it sounds like the last gasps of a Geiger counter. On the high end like the shrill scream of a radiator boiling over. Sweeping between the two extremes under ADSR control, say, is great for explosions or percussion. All this without tying up a VCF. And of course, everything is DC coupled, so it can also be used as a control voltage, too. This is particularly nice on the low, sub-sub-sonic end as a random source.

The LFO is excellent and can run from 0.083 Hz to 125 Hz without range switching. Those are periods of 12 seconds to 8 mS.

You might wonder why I ignored some of the features of the SN76477. Well, it has a really stinky envelope generator suffering from truncation, the VCA has limited dynamic range, and the "mixer" is not all that useful. Instead, I chose to use the parts of the chip that work very well and jettison the rest. The result is what I think is a good quality circuit, yet quite elegant (if I do say myself).

I've only built it on a breadboard so far, but offer it here in hopes someone else might breadboard and test it to confirm chip-to-chip variation is all taken care of. So, beta-testers, please step forward.

One final thing. I've done a few unusual things with the power, so be sure to read the schematic very carefully! Watch out for the -15V on the LFO rate pot, and also observe how the +5V is generated by means of the internal Zener.

Hope you like it,

Thomas Henry

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VC Noise.JPG
Description:	Breadboard
Filesize:	106.65 KB
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VC Noise.pdf
Description:	Schematic	Download (listen)
Filename:	VC Noise.pdf
Filesize:	27.8 KB
Downloaded:	1447 Time(s)

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[Thomas_Henry]

Hi again,

And here are some sound samples, recorded flat. No EQ, VCF, reverb, etc.

1. sweeping the noise manually; at the end I throttle down the internal filter so you can hear the deep low end

2. under control of an ADSR, going through a VCA; the noise is swept by the ADSR

3. under control of a keyboard which might be good for tuned percussion

Thomas Henry

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Manual Sweep.mp3
Description:	Manual control; internally filtered at the end
Filesize:	794.69 KB
Viewed:	1635 Time(s)
▶ Play    ⇩ Download

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By ADSR Control.mp3
Description:	Under ADSR control (through a VCA)
Filesize:	294.28 KB
Viewed:	1653 Time(s)
▶ Play    ⇩ Download

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By Keyboard.mp3
Description:	Under keyboard control for tuned noise
Filesize:	379.59 KB
Viewed:	1582 Time(s)
▶ Play    ⇩ Download

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[Thomas_Henry]

One more thing. You might note from the spec sheet that the Zener can only accept a max of +9V. Yet here I am using +15V. The secret is R3, a dropping resistor computed to match the current drain of the circuit. Never take "no" for an answer!

If you decide to modify the circuit in any way, R3's value will need to be recalculated.

[cementimental]

This looks great, I am going to have to build one, hoarded a few SN76477s and this looks like a more achievable step towards a full noise-synth using all the features

Will it run on 12v OK, or would I need to change some values?