[sdiy] Ring mod and op amp

[Aaron Cram]

Seb Carr wrote:

> Hi all
>
> i was just going over a ring mod schematic (Ring mod version 1 by Aaron
> Cram, sorry forgot URL) and being a newbie with no elctronics training past
> gcse i was confused by the out put of the LM13700: the modulator output
> (from what i guess is a sort of buffering op amp) meets the outtput of the
> LM13700 and on the schematic this is shown as two little circles
> overlapping. Is this just a normal join in the wires/tracks or something
> else?

  The double-circle-thing, along with the two diodes, are actually part of the
LM13700.  The line going into where the circles overlap (from the 4.7k
resistor) is a pin on the IC.  The 13700 is an OTA (Operational
Transconductance Amplifier) and works differently than a normal op-amp.  What
makes OTAs nice for synth stuff is the fact that the gain of the amp can be
controlled with a voltage (or current.)

Someone correct me if I'm wrong, but I think that two overlapping circles
represents a "current mirror."  (But you don't need to know that to use an
OTA.)

For the pinout of the 13700 or to learn more about it, grab the datasheet here:

http://www.national.com/pf/LM/LM13700.html

>
>     Also i came up with another thought: what would happen if you took 2
> different signals and connected 1 to the inverting and one to the non
> inverting inputs of an op amp?

Well, if you connected the signals directly to the inputs (one to + and another
to -) with no feedback loop, you would have yourself a comparator.  When the +
signal is higher than the - signal, the output will be +V, otherwise it will be
-V.

It's also possible to have an op-amp configured as a "voltage subtractor."  Any
"sameness" in the two input signals would then cancel out.  These are used alot
in "balanced" preamp inputs.  The balanced mic (or whatever) actually sends out
two signals of opposite polarity.  The idea is that if you pick up any noise in
your mic cable, the noise will infect both signals equally.  Then, when the
signals are subtracted, the original signal is boosted and noise gets cancelled
out.

>
> Thanks for any help.
>
> Seb Carr

Any time!

-Aaron