Dumble FB circuit-22 M & .05 mF

[rackandroll]

I have been checking out the NFB applied locally on the second stage (non HRM). The 22M and .05 mF circuit. It is interesting to note that this technique was used in the 1940's and 1950's in phono preamps to get rid of turntable rumble. (Ryhmes with Dumble!...sorry).

Anyway if anyone is interested in this, I can try to post the original schematic. I think it had .01 mF and 15 M as values. Also there was an additional feedback resistor for another frequency.

Understanding this may help in achieving similar results by other methods and deciding whether to leave it in or dispense with it. I suspec HAD didn't use this on HRM versions to get a more aggesive sound. Also with the HRM having high frequency pre-emphasis, I suspect that this may not have as much of a pronounce effect as on non HRM. Any ideas or comments?

Regards,

Alex

[heisthl]

I would love to see the original schematic - what was the other freq they were trying to attenuate?

[rackandroll]

I don't have the ability to post a schematic right now, but here is a description the circuit: A .01 mF and and 15 M ohm as local feedback from anode. Then this plate has a 100K resistor, which is coupled to the output via a .05 mF cap to a 100K pot. The .05/100K node is connected (fedback) to the grid (input), via a 1.5 M resistor. That's it.

Is is part of a preamp called "The Brimar SP55 Preamp."( 1.)
The stage in question is the last one, and is labeled as a rumble filter. For low-frequency turntable garbage.

If someone could draw out the schematic I would appreciate it. It may be of use.

1. Valve & Transistor Audio Amplifiers by John Linsley Hood.
Newnes Press. 1997
Figure. 5-16 page 89.

[Dai H.]

http://books.google.com/books?id=YbzBWf ... &ct=result

[rackandroll]

good job posting the pages from the text.

Alex

[Tavda3172]

Hey rackandroll,
Not trying to be nit picky here but you probably want to use the right nomenclature when referring to your capacitors. Above, I assume you are referring to a 50nF capacitor when you say .05mF, however .05mF is 50,000nF. I believe you mean .05uF which is equal to 50 nF. That is a major difference!

[rackandroll]

Yes thanks.