Re: The finer points of a balanced circuit

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In Reply to: Re: The finer points of a balanced circuit posted by DrM on May 8, 2000 at 07:59:37:

***Hi Victor,
Thanks! I was hoping you would see this. So for my RIAA I should let it float, for the phono input should the 47k input impedence float or be tied? I've seen it both ways. For the volume control, Morgan Jones (Valve Amplifiers) advocates the one series/variable shunt method, floating. This has variable input impedence (not so good) but low component count (good) and doesn't need matched components (good).
The other choice, that I think your advocating, is to tie the volume control to ground. As a L-pad, or series/shunt?
I like the shunts. The do, however, require a pretty good understanding, or you might get screwy curves and strange effects.

***In either case then don't I need to worry about the two legs being very well matched?
It all depends on what you are trying to achieve. I think the matching you get with just standard parts is sufficient for audio purposes - you are not building a strain gage amplifier, after all.
***I don't understand what you mean by the grounded vol control will have zero impedence for low volume, it will for the series/shunt case, but if I use a L-pad it will have a constant input impedence at all settings.
You seem to be thinking about the input impedance, but I actually mean the output or source impedance, which in many cases is more important. As that source impedance goes up, so does your next stage's noise. To build a shunt control you probably will use 100k or so series resistors, so now your gain stage has to work from a 100k source. And your two input noise sources are not correlated, so they add together, there is no cancellation.
***So I guess the questions for the volume control are 1) L-pad or series/shunt and 2) referenced or floating for either choice?
I personally would use a grounded shunt. But it needs to be carefully optimized to work well.

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Follow Ups Full Thread

Topic - The finer points of a balanced circuit - DrM 07:26:21 05/08/00 (3)

Re: The finer points of a balanced circuit - Victor Khomenko 07:44:56 05/08/00 (2)

Re: The finer points of a balanced circuit - DrM 07:59:37 05/08/00 (1)

Re: The finer points of a balanced circuit - Victor Khomenko 08:27:26 05/08/00 (0)

Follow Ups

Re: The finer points of a balanced circuit - Victor Khomenko 08:27:26 05/08/00 (0)

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