[sdiy] Chris MacDonald "Balanced Modulator"

[Tom Wiltshire]

On 13 Mar 2009, at 06:14, alfred.pear at gmail.com wrote:

> Could I convince anyone to go through the initial circuit and describe
> what parts should be doing what and maybe get an explanation as to
> why?
> I'm pretty certain that I understand how a ring modulator should
> sound, but I am still learning about the intricacies of how circuits
> are designed.
>
> -A

There isn't too much to say about that circuit, since most of it is  
inside the LM1496. If you want to know more about that, you can see  
what you can get out of the datasheet:
http://www.datasheetcatalog.org/datasheet/nationalsemiconductor/ 
DS007887.PDF
However, there are a few things that are worth pointing out;
The chip expects differential inputs, and provides differential  
outputs. In this circuit, the inputs are single-ended, so the other  
input is tied to a trimmer to ensure that it sees zero volts. This is  
done for both the carrier and the modulator inputs.
The final op-amp stage is a basic differential amp which converts  
from the differential output to a single-ended output. The two "arms"  
of the amp have different resistor ratios which is unusual (33K/390K  
Vs 33K/330K). Usually these would be the same so that both signals  
have equal influence on the output. Consequently, one of the first  
things I'd try in this circuit would be to replace one of these  
resistors with a trim - perhaps replace the 390K with a 500K trim.
Once you've trimmed it, you can measure the value of the trimmer, and  
then put in a fixed resistor and a smaller trimmer. This gives you  
more "resolution" on your trim.

The way that I understand the action of a balanced modulator is as  
follows;
Imagine a simple VCA controlling gain of a signal. A 0V to +5V  
control signal controls the gain of a non-inverting amp.
Now imagine that you've got another VCA that uses a 0V to -5V control  
signal and controls the gain of a inverting amp.
Stick these two VCAs together, and it's a balanced modulator! A -5V  
to +5V control signal will control the gain and give either an  
inverted output or a non-inverted output depending on the sign of the  
CV input.

Hope this helps,
Regards,
Tom