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 Forum index » DIY Hardware and Software » Thomas Henry designs
The Sound One
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Thomas_Henry



Joined: Jul 24, 2009
Posts: 129
Location: N. Mankato, MN

PostPosted: Fri Mar 28, 2014 1:49 pm    Post subject: The Sound One
Subject description: A One-Chip Cigar Box Synth
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Hello gang,

This project is just for fun, but it gave me a chance to try an idea which really worked out nicely. And, I'll also pass along a few thoughts concerning the SN76477.

The new idea is this: the VCO, LFO, noise clock, noise filter, attack, release and one-shot all require a timing resistor. But it occurred to me that each of those resistors could be replaced with a one-transistor current sink if desired. This has several advantages. First, it gives voltage control for each of these parameters. But most important is the greatly expanded range.

For example, using the circuit of the TI data sheet, the VCO range is a measly 10:1. With the current sink, this becomes a staggering 50000:1!

If you look at the TI data sheet, you'll see that the designers there make extensive use of range switching. There are gigantic multi-position rotary switches to kick in a half-dozen capacitors all over the place. But with this current sink idea, they all get jettisoned. One pot covers the entire range.

For the purposes of this project, though, I decided to restrict things a bit, just to make the PWM effect more pronounced.

Anyway, attached is the schem. Here are the features:

-- VCO
-- LFO
-- sweepable noise
-- FM
-- mixer
-- AR type envelope generator
-- battery operated, 10mA draw
-- one-chip design
-- no range switching, hence a super-low parts count
-- internal Zener is used to create the +5V supply
-- internal preamp outputs a line-level signal

A couple other thoughts: the internal one-shot business in the data sheet to generate the envelope stinks. It's completely awkward. So this design uses the external logic affair that Scott Stites stumbled upon. I simply redesigned everything to use a single transistor to generate the needed gate and trigger.

I also used Scott's idea for the PWM. You might want to compare this with the SN-Voice that he and I designed several years back. You'll find it on his Web site.

Note, too, how I do the noise sweep with a single transistor. You can't get any simpler than this for a very wide range noise source.

Be sure to pay strict attention to the power supply. There are two 9V batteries. The positive one feeds the Zener input at pin 14; this is its only connection. But the output of the Zener, at pin 15, creates the +5V supply throughout. Total current drain on the +5V supply is about 1mA, well under the 10mA limit imposed by the data sheet.

On the other hand, the negative battery creates the pot voltages required by the current sinks.

Speaking of which, with the values shown, the sinks sink 200uA max, as specified by the data sheet.

While this particular circuit may not be what you are looking for, some of the ideas in it might prove useful elsewhere.

As usual, please don't republish this schematic anywhere, but instead link back to electro-music. And be sure to ask first if you intend to do a circuit board or anything commercial with it. Other than that, have fun with it and be sure to post any results here.

Thomas Henry


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elmegil



Joined: Mar 20, 2012
Posts: 1449
Location: Chicago
Audio files: 14

PostPosted: Fri Mar 28, 2014 6:34 pm    Post subject: Reply with quote  Mark this post and the followings unread

That looks really cool.

I think it might be interesting (though obviously it adds complexity) to add a simple VCF with a similar set up to control it like the noise sweep, EG/LFO/Manual....
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Thomas_Henry



Joined: Jul 24, 2009
Posts: 129
Location: N. Mankato, MN

PostPosted: Fri Mar 28, 2014 10:09 pm    Post subject: Reply with quote  Mark this post and the followings unread

Hi,

Yes, a person could do that but...the filter is only a one-pole affair, and hence quite weak.

On the other hand, the noise clock sweep sounds for all the world like a potent bandpass filter with heavy resonance. So my suspicion is that sweeping the lowpass filter wouldn't add very much to that. But don't let me stop you from experimenting!

By the way, I first hit upon this noise clock sweep = bandpass filter effect back in 1981. It's perhaps the best thing in the world about the SN chip. I don't recall hearing anything else like it.

Thomas Henry
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