Strange "NFB" on Stereo Turntable Phono Suitcase - How it acts ?

[kagliostro]

Looking in detail to the schematic of the Telefunken Stereo Turntable Phono Suitcase I realized that what I considered as an NFB looks like something with a different purpose

the 400R pot seems to act as an "NFB BALANCE" but I'm not really able to understand




Someone can explain me what is happening there and which can be the pourpose ?

Thanks

Franco

[PRR]

https://www.radiomuseum.org/r/telefunken_musikus_5062.html

The 400r pot is on the front panel. Therefore it is a user control to adjust left/right balance. It will not adjust a side to zero, only raise the other 10dB-12dB up, but that is enough to seem like all-one-side when the two speakers are on short leads.

[kagliostro]

Thanks PRR

So the NFB is used to limit the excursion of the signal in one channel at benefit of the other channel

.... that is so odd to me ...

why don't adopt a standard balance on the signal path instead of that unusual circuit ?

do you see particoular advantages on this architecture ?

Franco

[HotBluePlates]

why don't adopt a standard balance on the signal path instead of that unusual circuit ?

do you see particoular advantages on this architecture ?

2,2kΩ cathode resistors for the triode section of the ECL82 are needed regardless.

Telefunken probably wanted some power-section negative feedback to control bass-resonance in the speaker, so the 1,5kΩ and something a little-larger-than-150Ω would be needed for each channel.

A "standard balance control" requires an expensive dual-gang pot (a single-section potentially messes up the stereo image). 

But this feedback-adjuster uses a single-section pot, does the same job of varying the loudness of each channel (by changing the gain of each channel), and doesn't throw additional resistance in the main signal path.  It is probably the more elegant solution.

[kagliostro]

Many Thanks HotBluePlates

Very interesting info in your replay

Franco