[sdiy] Multiplier vs VCA

[Tim Daugard]

From: "Derek Holzer" <derek at umatic.nl>

> 1) Ability to use LFO as modulator for slow AM.

This module was created after reading my latest ARRL manual discussion on AM 
and ring modulators. The waveforms out of it resemble that of a ring 
modulator. This module avoids the cost of transformers. It also has a wider 
bandwith than a transformer coupled ring modulator. It works at DC (checked) 
to the end of the audio range and should work to the bandwidth limit of the 
opamp!

> 2) Ability to produce complex timbres from faster VCO as modulator.

I use all my modules to get complex sounds from my bass. U1a with Rv1 and 
U1b with Rv2 allow you to predistort waveforms going in to the modulation 
section. U1d is a free buffer that you can connect any of the outputs to for 
additional gain. It can also be used as an inverter for one of the inputs.

I checked my stored sounds, I don't think I recorded the sound of this 
module yet. When I built it, I had all kinds of fun making spacey radioish 
ring mod sounds. I haven't tried it with voice yet, I'll have to try that 
for a late 60's sound.

> Clean sine waves aren't my highest priority in the second case. I'll get 
> to breadboarding Timothy's modulator soon, I just wanted to know if I was 
> way off base by getting into it.

If you don't have the pots for Rv1 through Rv4 then you can sub 10K 
resistors. Connect a 10K resistors between: U1a pins 1 and 2, U1c pins 9 and 
8, U1b pins 6 and 7. Don't bother doing any thing with U1d - it was just an 
extra opamp in the quad. R1 C1 junction is connected to pin 2 of the opamp, 
R2 C2 junction is connected to pin 6 of the opamp. Do that and you're ready 
to play.

> As for the diodes in such a modulator, as long as they are matched is the 
> type critical?

I just scanned the article again. I noticed I didn't discuss the parts. The 
diodes are 1N4148 - cheap silicon diodes. Almost any diodes should work. You 
may not even have to match them.

The opamp is a standard - TL074. I generally only use three opamps: - 074's 
when I want a clean signal with no cross over distortion, - LM324's when I 
want to avoid anyu chance of the signal inverting when it hits the rail, and 
NE5534 for low noise microphone and instrument preamps.

Tim Daugard
AG4GZ 30.4078N 86.6227W Alt: 12 feet above MSL
http://members.cox.net/synthfred/h_toctop.htm (Fred's online edition)