Variations on the Shunt Pot Mod

[david]

Some, including Jan and myself have experienced issues of a very short usable range when using the shunt pot mod with for example the TKD pot, which has a different taper. or log slope, to most others eg Alps.

My variation on shunt mod tweak as described in the tweaks section gives you the advantages of the hard-wired signal connection, 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.

What you do here is connect the earth as for a conventional pot arrangement, but then you connect the incoming signal from your shunt resistor to the slider AND the outgoing signal lead. This works quite well for me with the TKD. You can connect the 'free' end of the track to the slider as well or not as you wish.

I am on hols at the moment so cannot add a diagram but will do so later.
David

[Richard]

Hi David,
The reversed track in the Tweaks section is correct and works with a very similar feel to a regular pot.

[John Caswell]

I am sorry I am not understanding this.
If the pot law is correct when wired conventionally, then if it is wired as per Richard's drawing in tweaks it will work exactly the same.
Using the shunt mod it is good engineering practice to have the wiper connected to the earthy end of the pot. Richard's drawing makes it all clear. I have used this mod with many varieties of pot and they all work the same.
I reiterate what is stated in "Langford Smith" taken from the RTMA specs that "for a standard clockwise pot 50% rotation = 10% resistance for a 10% pot ditto 50% rotation = 20% resistance for a 20% pot. I think that nowadays these are classed as "A" laws.
Looking at the TKD spec they all state "A" log law so I really cannot see them varying from world norm

John

[david]

I am not saying the Tweaks version of the shunt pot is incorrect: quite the contrary. But what it does do is means you are using the pot in reverse so not the gentle increase at the 'normal end' of the pot but the other end. The shunt acts as a pseudo log against this and normally works well in practice.

The way I'm describing gives most of the benefits of the shunt mod-hard wired signal-and means you are using the 'correct' end of the pot and the gentlest slope.

Think through which end of the log slope you are using in the shunt pot mode. It is a brain strainer but there is only one conclusion as discussed in posts a couple of months ago.

John, I am afraid whatever the spec suggests the reality is a very different slope as confirmed my Michael Percy who supplied the post for Bearded Fiddler and myself.

As described, this is only an issue for a very sensitive amp and doesn't come into play with normal sensitivity.
David

[John Caswell]

No your reasoning is incorrect.
With the pot in reverse if wired as shown then the gentle increase is at the "Top" of the pot.
Looking at the pot wired normally with the wiper at 50% rotation and a 50 K pot, the resistance from the top (input) to wiper (output) is 45K and from bottom (earthy)to wiper is 5K, so when you turn the pot upside down the resistance values will not not change it will still be 5K from top (input) to wiper and 45K from bottom (earthy) to wiper, exactly as Richard has drawn.

John

[Richard]

David, assuming the data sheet is correct wire the TDK this way.

[Richard]

Below is a comparison of dB attenuations between Standard and SPM at 1/4 1/2 3/4 Max points of rotation using Alps pin numbers.

See from the lower drawing that the pot track is reversed for the SPM but the wiper's effective direction of travel (relative to signal and ground) is also reversed. (In the Standard one the volume increases as the wiper travels up toward pin 3. In the SPM the volume increases as the wiper travels down towards pin 3.)

The dB numbers show a similar progression of volume against turn. These are maintained in a similar fashion compared with the standard pot and even when the SPM is being used with different values of series resistors to attenuate the max signal,