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,

[david]

As I read the above Richard is agreeing that the shunt mod does use the other end of the log scale. John does not agree. Testing-using the ear and an ohmeter-reveals that you are using the other end and if you think it through that is the only way it could work. If you substitute a linear pot for your log pot with the same shunt resistors you get a pretty similar rsult, demonstarting again that we are using a pretty linear part of the log scale at its other end.

This site gives some further info on log fakes and different ways of wiring

http://www.geofex.com/Article_Folder...s/potscret.htm

David

[Richard]

Hi David,

No, the descriptions and getting across the points we (all) are trying to make is difficult, but I completely agree with John and it was just coincidence I was posting pictures after he wrote his post last night.

All the info is in the 2 threads running about this at the moment. It may be tricky to follow and my drawings may not be clear but it, and John's comments, are correct.

It would be good if you can confirm you have tried the wiring I posted for the TDK above or give it a go and see if it fixes your problem. The problem you describe of none or fierce control despite massive 1M resistors is consistent with having the pot the wrong way round, or the resistors in the wrong place, and the TDK numbering doesn't help by being different to other pots.

I have sought the info for you from the TDK data sheet and used the TDK numbers on the drawing so it should work. Any differences between a TDK pot and an Alps (apart from the numbers) is not shown in the data sheet drawing of the pot. If the pot is log type there may be small changes in the taper but given its overall value being similar there cannot be much difference and certainly not enough to cause the issues you and Jan seem to be having.

For an illustration/confirmation of which way round the track taper is look at my last drawing and the K values on it. At the 1/2 way position you will see that one half of the track is 40K and the other 7K. When you then look to the dB attenuations under them you will see they progress in a linear way. This is correct and would not happen if the track was the wrong way round. The dB values show as linear number changes as dB is a logarithmic value and hence the log-like effect.

[david]

Hi Richard,
I started this post because I wanted to separate this new thread from the connections one. As I said on the other post, it is not a connection issue and please stick to this thread.The Alps and Panasonics have all worked for me and I am completely familiar with the connections and variations on these. It is only the TKD with its different slope characteristic that does not give an effective range for Jan and I with our sensitive 300b clones and in my case 2.3v rms source.

OK, if you agree with John then rather than John's 'your reasoning is incorrect' it would be good to hear the arguments as to why you don't consider the simple concept that we are using the other end of the track connecting it in the way you describe. The very connections we are changing and way it is connected establish this case.
David