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:
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
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:
An externally hosted image should be here but it was not working when we last tested it.
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
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.
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.
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.
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.
KBK said:
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
panomaniac said:
Well, different people will have different subjective preferences so I would never fault someone for preferring shunt attenuators on sonic grounds.
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.
I'll point them to this thread, now.
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
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
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
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
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
Isn't it obvious? The voltage divider is set with two resistors - ideally both should be of high qualtity. If one is compromised the end result is still better than if both are.
As an example i use some dirt cheap Alps pots which normally would be borderline. By replacing half the pot with a nice tantalum resistor i get a significant improvement in sound against a very small additional cost. I would argue that the same cost invested in a better pot will not bring the same improvement.
Apparently AN do the same but using a stepped attenuator. It is a solution maximising value but not absolute sound quality.
Btw, in a stepped attenuator you can have a single shunt resistor per step, thus minimising not only series connected resitors but also contact junctions.
As an example i use some dirt cheap Alps pots which normally would be borderline. By replacing half the pot with a nice tantalum resistor i get a significant improvement in sound against a very small additional cost. I would argue that the same cost invested in a better pot will not bring the same improvement.
Apparently AN do the same but using a stepped attenuator. It is a solution maximising value but not absolute sound quality.
Btw, in a stepped attenuator you can have a single shunt resistor per step, thus minimising not only series connected resitors but also contact junctions.
I agree with Steve though, that the shunt resistor is equally important, if not more important than a series resistor. I didn't use Ohm law to figure that out, but my listening tests indicate just that.
However, the shunt attenuator has obvious economical advantage over the regular attenuator type of the same type.
If you consider your listening habits, you may notice that your average volume setting range varies within 10dB or so, any other settings are very rare.
So why not choose a single, properly chosen fixed resistor for series position, getting rid of a switch here (which is certainly a good thing) and choose similarly good quality resistors for most often used shunt positions with regular resistors for all other positions?
Such attenuator will be almost as good as a ladder type, considering input/output resistance variations and certainly much better if highest quality parts are being used.
This is exactly what I'm doing now: Shallco switch with two decks, 2 nude Vishays for series elements, Caddocks for most often used shunts and Mills for all other.
I choose Caddocks in shunt as they sound best here with GC. I choose nude Vishays for series as they are most neutral.
IMHO, this is the best attenuator you can built using stock parts. If you know a better one, I would like to hear about it
However, the shunt attenuator has obvious economical advantage over the regular attenuator type of the same type.
If you consider your listening habits, you may notice that your average volume setting range varies within 10dB or so, any other settings are very rare.
So why not choose a single, properly chosen fixed resistor for series position, getting rid of a switch here (which is certainly a good thing) and choose similarly good quality resistors for most often used shunt positions with regular resistors for all other positions?
Such attenuator will be almost as good as a ladder type, considering input/output resistance variations and certainly much better if highest quality parts are being used.
This is exactly what I'm doing now: Shallco switch with two decks, 2 nude Vishays for series elements, Caddocks for most often used shunts and Mills for all other.
An externally hosted image should be here but it was not working when we last tested it.
I choose Caddocks in shunt as they sound best here with GC. I choose nude Vishays for series as they are most neutral.
IMHO, this is the best attenuator you can built using stock parts. If you know a better one, I would like to hear about it
Hi,
what if it never ocurred to us dim wits and we went ahead and fitted the same type of resistors into all positions?
Does that make the sound better or worse?
Steve,
Source sees Rx + (Ry//Zin).
If Zin is > 10* Ry then the error is small, but as Ry increases the errors become very significant.
what if it never ocurred to us dim wits and we went ahead and fitted the same type of resistors into all positions?
Does that make the sound better or worse?
Steve,
you do realise that your formula is incomplete.
Source sees Rx + (Ry//Zin).
If Zin is > 10* Ry then the error is small, but as Ry increases the errors become very significant.
Nordic said:
You probably won't find a better value than those (that is if switch does not do the popping):
http://www.diyaudio.com/forums/showthread.php?s=&threadid=31327&highlight=
http://www.diyaudio.com/forums/showthread.php?s=&threadid=27499&highlight=
That one here was pretty good too, but far cry from the one I described in my previous post:
http://www.diyaudio.com/forums/showthread.php?s=&threadid=3279&highlight=
it appears that most are agreed that AN has placed emphasis on the wrong (series) resistor.Peter Daniel said:
However, there appears little consensus on whether both resistors are equally important or if the quality should be placed in the shunt position.
I'm the dim wit that put ordinary 1% metal film in both positions.
Tell me I did it right or wrong, or "room for improvement" as the schools now like to say.
AndrewT said:
Generally speaking, there are 2 types of possible tweaks: the general purpose and the green tweak, as explained in a previous attenuator thread.
http://www.diyaudio.com/forums/showthread.php?postid=914699#post914699
Regards,
Milan
AndrewT said:
It all depends on your objective. Without knowing that, I can't say anything.
I wanted to build the best possible attenuator (using stock parts) for a particular application (my GC amps and my sonic preferences) and described how I went about it.
What do you want to accomplish with yours?
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