stereo expander, part 2

The pots in this circuit do not affect feedback, neither negative nor positive. They are introducing a portion of the opposite channel's signal into the final stage's inputs, which are acting as linear mixers. In this situation, a linear taper would be used, because the center position of the pots should result in zero signal going into the opposite channel. A log taper pot would not give zero output in the center position.

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Dave M
MasonDG44 at comcast dot net  (Just substitute the appropriate characters in 
the address)

Never take a laxative and a sleeping pill at the same time!!

That's why I replaced the 10K pots to 5K and adds 5 resistors in series. This way, only the negative feedback (of the adjacent channel, you're right to spot this) will be used.

Funny but, a friend of mine just tried a quick and different approach for that circuit. He picked two small 600 ohm phone line transformers to get the negative out-of-phase feedback from the adjacent channel. Only passive parts. Because these transformers have a (very) bad frequency response at low frequencies as well as high frequencires, bass frequencies are almost unaffected. Same for treebles. The end result seems to cancel mostly vocals.

Indeed! Jim

"J, David"

** Just about all this device will *actually* do is REMOVE the deep bass from your music.

Typical stereo recordings have the deep bass in mono ( ie similar level and phase in both channels) - pretty much nothing else in the mix is like that.

Better it did not work full range and left the bass frequencies alone.

BTW

The designer is a friend of mine.

........ Phil

You keep talking about "negative feedback" in this circuit. This is NOT negative feedback, or feedback of any kind. It is simply the injection of in-phase or out-of-phase signal into a mixer stage. The pots simply allow the user to adjust the amount of signal fed into the mixer. That's it. Your idea to split the pots into 5K fixed and 5K pots will indeed do what you want, but your terminology is misleading.

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Dave M
MasonDG44 at comcast dot net  (Just substitute the appropriate characters in 
the address)

Never take a laxative and a sleeping pill at the same time!!

"J, Davi"

** Yawn.

Wota f****it.

......... Phil

My choice of words is bad no doubt. But my original question is still unanswered. Logarithmic or linear pots? I guess I'll have to experiment...

Regards! Jim

| ^ Yes I did answer your original question.. If you'll go back to one of my previous posts, I advised you to use linear pots

--
Dave M
MasonDG44 at comcast dot net  (Just substitute the appropriate characters in 
the address)

Never take a laxative and a sleeping pill at the same time!!

Back in the mid '60s I designed a stereo amp that had this feature. I've been trying to remember the details without success and the circuit is burried in a box in the basement. :-( However the principle was simple and the implementation only required two op-amps, one linear pot and a few resistors.

The principle is: Use the two op-amps to form a differential amplifier with variable common mode gain. If the differential and common mode gain are equal, there is no change in separation. If they are not then either blending (ultimately producing mono) or increased separation occurs.

I expect one of you geniuses can refresh my memory re what I designed 40 years ago. :-)

Ted

If the circuit is considered as a mixer, using linear pots do make sense of course.

Jim