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Old 18th January 2007, 03:10 AM   #1
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Default Shunt Attenuator Myth

While I've been aware of shunt attenuators for some time, I never gave them much thought because I use input transformers which demand a constant load impedance to perform their best. However a post on another forum got me thinking more about them and their claimed advantages and I came to realize that the advantages of the shunt attenuator are based on a myth and thought I'd write a little something about it here.

The claimed advantage of the shunt attenuator over the series/shunt attenuator is based on the notion that the "signal path" is a series path and that series circuit elements are of the most importance and shunt circuit elements are effectively "out of the signal path" because a shunt circuit simply goes to ground.

Based on this, the idea is that all you need to do is use just one very high quality resistor in the series position and then you can use lesser quality resistance for the shunt element without any loss of overall quality as the quality is defined by the series element.

For example, here's a quote from some product literature on Audio Note's website:

The volume control/attenuator is a hand assembled shunt attenuator, where the signal only traverses a single extremely high quality resistor before reaching the input to the line stage. All switch contacts and other components are in the shunt leg of the attenuator.

The notion that it's the series element that's the determinant of quality and what's in the shunt element doesn't matter so much because it's a shunt element couldn't be more wrong. The quality of the shunt element matters every bit as much as the series element.

That's because the signal seen at the output of the attenuator isn't the voltage across the series element, but instead is the voltage across the shunt element. So in a very real sense, you could say that the shunt element is the signal.

Let's look at it schematically:

Click the image to open in full size.

We have V to represent our signal source, Rx is the series element and Ry is the shunt element.

The signal source sees an impedance equal to the sum of Rx and Ry and for a given signal voltage, a given current will flow through Rx and Ry. As per Ohm's Law, there will be a voltage drop (Vx and Vy) across each resistor as a function of the current divided by the resistance.

As you can see, the voltage drop Vx across Rx, the "single extremely high quality resistor," isn't even seen by the input that the attenuator is driving. Instead, the input sees only the voltage drop Vy across Ry, the shunt element, the one which the proponents of shunt attenuators claim doesn't matter so much in terms of quality.

However as can be seen, the quality of the shunt element matters every bit as much as the series element. And even though the voltage drop across the series element Rx isn't seen by the input, the current flowing through it must also pass through the shunt element so any deviation from the ideal of the series element will also manifest itself across the shunt element by way of Ohm's Law.

The bottom line is that the shunt attenuator has absolutely no technical advantage over a series/shunt type attenuator. And in fact it has a distinct disadvantage in the form of output impedance compared to a series/shunt attenuator.

The worst case output impedance of a series/shunt attenuator is 1/4 the end to end resistance of the attenuator. So if you compare a shunt attenuator with a 10k series resistor to a series/shunt attenuator with an end to end resistance of 10k, the worst case output impedance of the shunt attenuator will be 10k ohms, and for the series/shunt attenuator, only 2.5k ohms.

se
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Old 18th January 2007, 05:30 AM   #2
KBK is offline KBK  Canada
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In my expereince, the 'interference' to the signal created by the element of the given attenuator via series positioning to the original signal, is halved by the positioning of the variable as a shunt to ground, in a dual resistor 't' type arrrangement. Similar to a CLC type filter in terms of layout, where the 'C' is the resistor (two of them) and the 'L' is the variable attenuator, or switched. In the case of using a completly variable standard 'pot', this arrangement shows it's real capacity to deliver a better signal quality, while retaining a lower costing. Ie, if all you gots at the time, is a pot..then use the shunt configuration. Then there is the given circuity's stability in dealing with variable loads. This arrangement stabilizes such to a large degree. Properly applied, this can be used to great benefit in tube circuits. Just my experience. Careful tuning of resistor values is always a consideration. The danger is that a bad pot can dump full signal when it acts up. not a good thing. Potential resonant issues must be considered, etc. Specifically speaking, I have learned to always give a shunt attentuator a shot at making a given circuit sound better, if it has a standard configuration within it. I try it..if it works..I leave it in there. I have yet to come across a circuit where it did not give benefit, however, as stated, loading can become an issue in a given existing circuit.

My primary work tends to exist around modifications to gear as opposed to outright 'ground up' design. Which is tougher, is debatable. (design or mod, I mean) Fixing someone else's stuff can be just as fun as designing it from the ground up. Think of it as psychoanalysis for audio gear. To each their own. I've no time to do ground up work. Maybe down the road.

What this means, is I have gone from standard arrangements of attenuation to shunt attenuation in many an existing design, and have always managed to find sonic benefit in it.
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Old 18th January 2007, 05:45 AM   #3
Pano is offline Pano  United States
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Thanks for that, Steve. Very clear. There has always been something about the shunt mode volume controls that bugged me. Never thought they were much good, or at lest not better than normal types.

For example, there was a fad for using the poor quality pot of the Sonic Impact amp in shunt mode - because "the signal does not travel thru the pot so it's poor quality is not a problem." I've talked a lot of guys out of that one. Just buy a good pot and use it in a normal mode.

I'll point them to this thread, now. Sure makes sense to me.
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Old 18th January 2007, 06:35 AM   #4
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Quote:
Originally posted by KBK
What this means, is I have gone from standard arrangements of attenuation to shunt attenuation in many an existing design, and have always managed to find sonic benefit in it.
That's cool. I'm a firm believer in people going with whatever sounds best to them for whatever reason.

I'm simply pointing out that shunt attenuators have no technical advantage over series/shunt attenuators, contrary to the erroneous technical claims that have been made about shunt attenuators. If one is going to make technical claims, they should at least be true.

se
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Old 18th January 2007, 06:47 AM   #5
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Quote:
Originally posted by panomaniac
Thanks for that, Steve. Very clear. There has always been something about the shunt mode volume controls that bugged me. Never thought they were much good, or at lest not better than normal types.
Well, different people will have different subjective preferences so I would never fault someone for preferring shunt attenuators on sonic grounds.

Quote:
For example, there was a fad for using the poor quality pot of the Sonic Impact amp in shunt mode - because "the signal does not travel thru the pot so it's poor quality is not a problem." I've talked a lot of guys out of that one. Just buy a good pot and use it in a normal mode.
Yeah. If you're using a shunt attenuator because you believe "the signal does not travel through the pot so its poor quality is not a problem," then you're using one for the wrong reason as that is simply incorrect.

Quote:
I'll point them to this thread, now. Sure makes sense to me.


Lynn Olson by the way has on his website a presentation he gave at the 2004 European Triode Festival which also dispels the myth that shunt elements aren't in the signal path so you might want to take a peek at that as well.

se
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Old 18th January 2007, 07:01 AM   #6
Pano is offline Pano  United States
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Quote:
Originally posted by Steve Eddy
Lynn Olson by the way has on his website a presentation he gave at the 2004 European Triode Festival
Well.... I was going to mention current loops, but you beat me to it! Lynn is a cool guy, his site has lots of good info.
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Old 18th January 2007, 07:22 AM   #7
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Hi,

well, one thing to consider (in favour of shunt att.) is, that the series resistance that the following circuit sees is almoust constant, so the hf rolloff point does not change that much.

a negative is, that the input imp. can change quite a lot...
so the stage in front of the att. has to cope with it.

regards
Michael
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Old 18th January 2007, 08:32 AM   #8
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Quote:
I'm simply pointing out that shunt attenuators have no technical advantage over series/shunt attenuators, contrary to the erroneous technical claims that have been made about shunt attenuators. If one is going to make technical claims, they should at least be true.
It is always a question of the actual use IMO. With the volume turned up or almost up the influence of the "bad" pot is indeed reduced withthe shunt configuration.
But I agree with you that a general statement claiming the shunt configuration being superior is indeed BS.


Regards

Charles
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Old 18th January 2007, 09:56 AM   #9
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Hey, Steve. So, what are you boycotting these days then?

To bring Ohm's law into demistifying shunt attenuators seems incredibly lame but if you don't like them, don't use them.

For many of us shunting a pot is a great cheap-skate solution to improve the sound. If not by 100% then at least by 50
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Old 18th January 2007, 10:30 AM   #10
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Quote:
Originally posted by analog_sa
For many of us shunting a pot is a great cheap-skate solution to improve the sound. If not by 100% then at least by 50
Why? How? Honest questions.
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