Variations on the Shunt Pot Mod

[Richard]

Hi David,

In your previous posts in this and the other thread you say your vol control is not working correctly and I was trying to help see why that might be.

What you say here, "but means you are using the 'correct' slope or right end of the track of the log and therefore you get a better range than with the Tweaks version of the shunt mod." is simply not the case.

If it works for you and the Tweaks ones doesn't there must be another reason as the attenuation law will be the same in both cases.

Yes by all means wire it this way but noise may be an issue and when we explored it a long time back came to the conclusion it was better to earth the wiper and connect both ends of the track, hence inverting the pot.

[david]

Hi Richard,
I appreciate you were trying to helpbut there's a difference between a pot with a quite different slope and wrong connections!

But my contention goes back to one basic point which you seem not to agree with but it's a) easily measurable and b) very obvious if you look at the slope on the link on my post of 8.23 am today.

Do you agree that the shunt pot mod means we are using the other end of the track to the normal arrangement? If so, you can easily calculate from the graph of an audio log slope that the resistance changes more slowly at the 'normal' end compared to the shunt pot end.

If you don't agree that we are suing the other end to normal that's a different argument but again, there is consensus elsewhere and among some here that that is the case.
David

[John Caswell]

David have you wired the pot as shown in the shunt drawing below. If so then it will work with a normal logarithmic low level start.

John

[Richard]

Originally Posted by david

Hi Richard,


Do you agree that the shunt pot mod means we are using the other end of the track to the normal arrangement? If so, you can easily calculate from the graph of an audio log slope that the resistance changes more slowly at the 'normal' end compared to the shunt pot end.


David

Hi David,

Here is The Shunt Pot Mod with its track reversed, followed by your pot which has its track connected to ground at the normal end as you say.

However they both use the same series resistors and the same end of the pot track is acting as shunt. See how although the track is reversed for the SPM the wiper still travels towards the same end of the track and the same part of the track is the shunt in both cases.

In both cases, using these pin numbers, the part of the track acting as shunt is between 1 and 2 and the part doing nothing/shorted out is between 2 and 3.

As you say the attenuation can easily be calculated and is the same result in both cases shown in dB under each fraction of rotation which confirms this is the case.

(Note that these are for Alps pin numbers.)

[Richard]

Originally Posted by John Caswell

David have you wired the pot as shown in the shunt drawing below. If so then it will work with a normal logarithmic low level start.

John

Hi John,

I agree with your drawing except to point out that the numbers you've used for the pot pins are the opposite way around to the ones printed on the Alps pot.

(I too numbered them your way the first time in the FAQ then someone pointed out the Alps actually has them printed on so I re-did the drawings for consistency with the pot and photos of it.)

Here's the photo of the Alps, you can just see the numbers in the chequer print of the label

[John Caswell]

Hi Richard et al,
Well Sods Law says I would get it wrong for the ALPS, I didn't look at the ALPS in fact I never do, familiarity I suppose, I just picked up any old pot and labelled it accordingly. What with your excellent pictures and my c**p drawings we should be able to sort this out for all and sundry. I still cannot understand why David is not getting the correct results, everyone else has.

John

[colin.hepburn]

Hi All
Yes my alps 100k pot works just fine as per the F.A.Q with a 82k Resistor on my 12B4A Head phone Amp and my JE Labs EL34 however the very first time I wired it up a sudden shock then panic as it worked in reverse i.e. full volume at zero not good for the headphones or my ears
I had wired it up wrong but after I sorted the wiring all was fine and has worked correctly for the last five years or something like that as yet I have not yet tried another types of pots

[david]

Hi All,
Still on hols-last day! I will try and set out next week why I don't think the resistance that the signal sees when used in shunt pot mode follows the same log function as when wired conventionally.

Looking at your (Richard's) Alps pin nos, the resistance between pins 3 and 2 when wired conventionally is the log function. What I'm saying is when the pot is wired as per tweaks the shunt resistance between pins 1 and 2 does not follow this same log function.

That's the only explanation I can come up with as to why the TKD behaves so differently when wired conventionally as compared to in shunt mode. Some measurements will help demonstrate this.
David

[Richard]

Originally Posted by david

Hi All,
Still on hols-last day! I will try and set out next week why I don't think the resistance that the signal sees when used in shunt pot mode follows the same log function as when wired conventionally.

David

Hi David,

Enjoy it!

Now then, this may be where any misunderstanding lies. Your wiring does connect the track to ground at the same end as a conventionally wired pot but that doesn't make it behave like one.

In both shunt pots the series resistor is a fixed value whereas in a conventional pot the series resistor changes as the wiper moves. Hence the attenuation slope of a conventional pot is different to the shunt pots.

Here's a conventionally wired 47K pot, look along the dB figs and compare them with the shunts

[A Stuart]

This is giving me a very sore head!!!!

What letters follow the 2CP2511 on yours?

How about measuring all combinations of resistance between C, 1, and 2 at, say, half a dozen equal rotational angles and plot them on a graph like the one in http://www.acoustic-dimension.com/tkd/2CP2511.pdf so we can see how similar to or different from usual is the law on your TDK, as this will clarify whether the TDK's law is the issue.

Have we got this interpretation of your wiring correct (below)?
Both your version and the FAQ version deliver the voltage developed across the "gentle" end of the slope at low volumes (anticlockwise end of the knob on a normal pot). Neither uses the voltage across the "wrong end" and both appear they ought to give close to standard wiring results until you reach the loudest 10dB fraction of rotation.

Your (David's) problem, one might suspect to be different from Bearded Fiddler's as he got measurements totally at odds with Ohm's Law (29+68=47 ) if we are to assume two resistance tracks totally isolated from each other and a sound wiper contact. Perhaps we cannot assume these.
(- and Bearded Fiddler, I presume, did not have more than one ohmmeter attached at the same time )

You don't by any chance have shares in TDK and just want us all to rush out and buy one to find out ?

Alastair
Have collected together what seem the most relevant details in attachments below