Postscript to the Pot Saga

[david]

So, I received a Tocos 20k pot and prepared to replace the TKD in my Aikido Pre. Before doing so, I measured the resistance for both as follows. The resistance I measured is the shunt resistance with both Pots wired as per the Tweaks section. By shunt resistance I mean the resistance between the slider and and the terminal that is earthed in the shunt pot mode or Pins 1 and 2 on Richard's diagram. Numbers on left refer to clock position of volume control.


Resistances as follows, all figs in kohms:

Tocos TKD
L R L R

7.30 0.41 0.46 6.87 7.28

9.00 0.72 0.77 6.75 7.21

10.30 1.42 1.44 6.88 7.53

12.00 2.99 3.04 6.98 7.48

These measurements prove conclusively that there is something very strange about the TKD pots supplied by Michael Percy to Bearded Fiddler and I. It certainly was not the connections.

Putting the Tocos in my Pre it works perfectly and gives a normal volume range. In fact, I have reduced the series resistor to 47k and this gives me loud music at the 1 o'clock position and a great graduation of volume down to silence.

So, TKD pots-at least those from Michael Percy-are best steered clear of, it seems!
David

[Ali Tait]

Off topic, but did you receive an email from John Broskie regarding a mod to the pre David?

[david]

Hi Ali, no I didn't receive any e mail. Can you pass it on to my e mail address?
It's an amazing pre amp-I'm more than pleased with it!
David

[Richard]

Strange one then David, glad you got it sorted anyway

[david]

Thanks Rich, these little things are sent to try us

[John Caswell]

Hmmmm. I am surprised that Michael Percy didn't notice this. Ah well glad it has all ended satisfactorily.

John

[david]

Hi John,
I did write to Percy, who said the following:

'Do keep in mind that a 20% spec log taper resistance element of 25K should not be expected to track precisely at the bottom or top 3% of its value (700 ohms) if that is indeed what you are describing. If you are referring to a 350 ohm imbalance in resistance value between channels, that is well within the 20% tolerance spec (about 1.5%). I rather doubt any pot would consistently maintain less than 350 ohm channel to channel difference, especially the TKD near fully CCW, which have a steeper curve than most others at the beginning of the taper. Manufacturing an element to that standard would be quite a bit more expensive. I will compare yours with other TKD we have on hand, but you may be expecting too much from a continuous element pot at the extremes of its rotation. It may be that you have just too much gain, forcing you to use it near the rotational extremes. A stepped attenuator with 1% fixed value resistors may be the only device that would work consistently in that situation. Can you lose some gain so that you are running closer to the 9:00 - 10:00 rotation minimum for critical quiet listening. Below 9:00 channel balance becomes more difficult.

I would not count on any pot tracking channel to channel properly at this almost off setting. It would be very unusual to operate a volume control this way for normal listening. I've never had occasion to use one this way on a regular basis in my 50 years of many different audio systems. You would probably have to sort through a good number of "A" taper TKD's to find one that would match at that volume setting. In this situation I would dispense with some gain, so that low level listening is at at least 9:00, preferably closer to 10:00.'

This of course sent me down the Cathode Follower and Aikido Pre route. I'm extremely happy with my Aikido now, but there might have been another solution en route!
David