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Hi,
I'm working on a balanced (differential) preamp with 600 ohm drive
capability. At various places I need capacitance or resistance between
the two legs, for example volume control with a series and a shunt
resistor. There are two ways I can do this, put a series resistor in
each leg, with *one* shunt resistor between the two halves. The other
way is I can have the same series resistance, but have *two* shunt
resistors, of half the value, going from each leg to ground. Which is better? The floating version means more tolerance to component variations, and the second menas the circuit isn't allowed to float anywhere it wants to.
It seems like, as a general philosophy I should let it float everywhere, except the outputs which I reference to ground.
What do you think?
-Dan
The floawing circuit is not used much for volume control for good reason - it creates high source impedance. While some circuits might be OK with it, other might complain.For example, at low volume point, there the signal is very low, most grounded volume control circuits will have source impedance of close to zero. Therefore low noise. The floating circuit will still have high impedance, and will most probably have too much noise at that point.
You always need to consider the volume control's resistance vs. position.
Floating circuit is best used for small amount of gain trimming, like in balance adjustment. Can also be used nicely in a floating RIAA or any other filtering circuit.
Use the grounded one.
BTW, your circuit would not "float everywhere", most likely. It is common to establish some DC reference, either by default ot intentionally.
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? In either case then don't I need to worry about the two legs being very well matched?
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.So I guess the questions for the volume control are 1) L-pad or series/shunt and 2) referenced or floating for either choice?
Thank you!
-Dan
***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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