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Vintage threads from the first ten years

Re: Jen HF Modulator

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5/17/1999 6:05 AM
GFR	Re: Jen HF Modulator John J. Did you get the HF Modulator to work? Was the schematic OK or did you find any bugs in it?

5/19/1999 8:24 AM

I've just protoed the oscillator section of the HF modulator using CJ's schematic. The oscillator freq varies from approximately 60Hz to 140Hz, with something like 3V (at 60Hz) to 1V (at 140Hz) peak amplitude on the collector of the transistor. The schematic shows the modulator section (a transistor used as a variable resitor if I understood it correctly) connected to the base of the oscillator transistor, where the signal is much weaker than in the colector (just a few mV). Is it right?

5/19/1999 12:58 PM

Yeah, the signal at the collector should be small.

The Modulator gets ring modulator effects by doing amplitude modulation (tremolo) at audio frequencies. If you go through the math, this gives you the original frequency plus both upper and lower sidebands, where a true ring modulator gives you ONLY the sidebands, none of the original frequency in the output. Ring modulators are hard to play well because of this, as they always seem to be out of tune.

The transistor is indeed used as a variable resistor.

5/20/1999 6:14 AM

quote:
"Yeah, the signal at the collector should be small. "

I think you meant "base".

With 220K in series with the base I think you need a lot more than a few mV to get reasonable collector-emitter resistance variation.

quote:
"The Modulator gets ring modulator effects by doing amplitude modulation (tremolo) at audio frequencies. If you go through the math, this gives you the original frequency plus both upper and lower sidebands, where a true ring modulator gives you ONLY the sidebands, none of the original frequency in the output."

You're only changing the level of attenuation - that's a two quadrant multiplier. To get rid of the original signal you need a four quadrant multiplier and a modulating signal with no DC value.

Technically "ring modulator" refers to a specific implementation - the generic term would be "ballanced modulator" or "supressed carrier double sideband modulator" (SC-DSB).

Funny, a lot of commercial "ring modulators" feature some kind of "blend" control so that you can inject some of the original signal back (perhaps to make things more "normal").

The presence of the original signal doesn't bother me. What's really annoying is when the oscillator signal bleeds to the output (and you get that steady buzz even when you're not playing)

quote:
"Ring modulators are hard to play well because of this, as they always seem to be out of tune."

The concept of playing "well" may be a totally different thing with a ring modulator - what's good for one may be "noise" for others

Speaking of "noise", does the Ampeg Scrambler feature any kind of (explicit) modulation or is it just intermodulation products from distortion? The sound sample on Tonefrenzy sounds really amazing!

5/20/1999 8:05 AM

quote:
"I think you meant "base"."

Both. I was replying to the comment about the collector.

quote:
"With 220K in series with the base I think you need a lot more than a few mV to get reasonable collector-emitter resistance variation."

I have not measured it. The region of variable collector resistance is tiny for bipolars, not large at all. It's certainly small compared to the forward bias voltage, so maybe around 10-20mv. I'll measure it when I put the Proton2 together.

quote:
"You're only changing the level of attenuation - that's a two quadrant multiplier. To get rid of the original signal you need a four quadrant multiplier and a modulating signal with no DC value."

Right. That's another way of saying the same thing.

quote:
"Speaking of "noise", does the Ampeg Scrambler feature any kind of (explicit) modulation or is it just intermodulation products from distortion? The sound sample on Tonefrenzy sounds really amazing!"

There is no modulation in the Scrambler. It's just the intermod products. The Scrambler is a non-standard implementation of an unbalanced full wave rectifier octave up done with darlingtons.

5/20/1999 11:07 AM
gfr	Thanks.

5/26/1999 7:18 AM

RG says, "a true ring modulator gives you ONLY the sidebands, none of the original
frequency in the output. Ring modulators are hard to play well because of this, as they always seem to be out of tune."

The ring modulator that I manufacture (the Frostwave "Blue Ringer", named after the deadly Pacific octopus) overcomes this by having a signal/effect blend control. If it really is a problem, you could split the signal before the ring modulator and run a 'foldback' speaker near your ear!

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