What is a sum-difference amp / What does it look like?

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Ethan

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I'm curious exactly a what sum-difference amp is, and if there are any schematic examples floating around?

Thanks.
 
An MS converter is a classic example. It takes two inputs (Left & Right) and adds them to gether to give you a mono (sum) output and also subtracts one from the other to give you a Side (difference) output.

Example schematic here:
msmatrix.gif


Keith
 
Wait... that schematic of mine is drawn wrongly.. R2 should go to the inverting input (pin 2) of IC3, NOT the non-inverting input (pin 3)

I haven;t built it yet to test it, but that's the idea anyway...

Keith
 
Thanks Keith
Are the sets of outputs then identical in amplitude?
 
Surely that depends what goes in? If you feed it two identical signals then one channel should be silent?
Stewart
 
'Zackly. If you feed a "WIDE" stereo signal in, then the 'S' (difference) output may well be larger than the 'M' (sum_ signal.

Incidentally, this is how it's done in FM broadcast to remain mono-compatible. The stereo signal is first filtered with a brickwall filter at 15kHz, then split into L+R (mono, 'Middle', or sum) and L-R (difference or 'Side") and the L+R mono signal is broadcast just as in the days before stereo radio, so if you have a mono bedside radio, it receives the mono and stereo stations in mono... no problem with compatibility.

The L-R (difference signal) is lower sideband encoded onto a 38kHz carrier. The carrier is out of the hearing range, and is attenuated significantly by old radios, sue to the pre-emphasis and de-emphasis that was always part of the mono FM standard anyhow, so luckily helps out. (like your old 1930s radio speaker will reproduce 38kHz anyhow!) Since the difference signal never has any frequency in it which is higher than 15kHz, the lowest it will swing the LSB is 38kHz minus 15kHz, so 23kHz. -theoreticaly inausible, and only when there's significant HF content in the signal, when it would be masked by the HF content in the 15kHz region anyway...

Anyhow, the stereo radio locates the 38kHz carrier, lights a red "STEREO" light to indicate the prescence of the carrier indicatinfg a stereo transmission, surpresses it, decodes the LSB difference signal and adds and subtracts it.

(L+R) + (L-R) = 2L
(L=R) - (L-R) = 2R

Violà! -Stereo!

Keef
 
A theremin is a sum and difference amp. One oscillator is fixed at some high frequency, and the other is adjustable by the tiny amount of capacitance between your body and the antennae. The sum of the two frequencies is discarded, while the difference (a much lower frequency -- somewhere between 20 Hz and 20 kHz) is taken as the audio.
 
Although the process of adding and subtracting frequencies requires a nonlinear element, like a multiplier---it's a lot different than summing and differencing voltages.

I love Theremins. My father spent a year making a fancy one (all-tube) with organ-style stops, which he hoped to market to the same customer base as Hammond chord organs. He later referred to the effort as "Wood's Folly".
 
I love Theremins. My father spent a year making a fancy one (all-tube) with organ-style stops, which he hoped to market to the same customer base as Hammond chord organs. He later referred to the effort as "Wood's Folly".

What did the organ style stops control? Just the timbre of the signal? Pitch? Harmonics? Maybe a schematic?

adam
 
Timbre. He had some pretty decent formant filters for a variety of "instruments".

I have the schematic buried away in a file somewhere. Alas, I let the chassis itself go years ago.

I was getting pretty good at playing it for a while.
 
I designed and built an all-tube theremin about 8 or 9 years ago. I never got around to drafting a schematic of the final (?) version of the circuit. But I still have the theremin, and reverse-engineering my own work is somewhere on my to-do list, albeit somewhere between the middle and bottom of said list nowadays. :wink:

The design was pretty conventional save for three things. The oscillators used a shunt-fed Armstrong arrangement, which is a bit unusual (the "tickler coil" is usually series-fed). Also, I concocted a wave-shaping circuit that allowed one to change the character of the output waveform by simultaneously varying the bias and a bit of positive feedback around the two cascaded AF VCA stages. Also, the oscillators were fine-tuned by 1N4007s configured as varactors.

The oscillator frequency was pretty low, somewhere around 200kHz for the tone oscillators. I think the volume control oscillator may have been up around a Meg. I used a low Fo for the tone oscillators for a number of reasons, including better stability, better playing action and yes, better sound. There were fairly large, high-Q inductors (tens of mH) in series with the antennas to act as "capacitance multipliers" permitting the miniscule hand capacitance to shift the variable oscillator by several kHz. This was the approach taken in the original Theremins, by the way.

The volume control circuit was comprised of one variable oscillator feeding a slope detector circuit. Again, this is conventional. The control voltage was fed to the grids of the two cascaded AF stages I described earlier.

I'm pretty sure I used a diode-wired triode for the tone detector, but I can't say for certain until I get around to looking at it again.

The box used four 12AU7s and a B+ of 100V, crudely shunt-regulated by zeners. I was even more of a minimalist then than I am now!

Sorry to take the thread even further off-topic, but the mention of a tube theremin brought back memories of this old project of mine.
 
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