Alps pot strange hook up

[Jrook]

Hello all, not sure if this is the right spot for this question but here goes. A few week ago I ordered a new 100K PEC pot for my tube preamp. The stock unit is a conductive plastic Alps that has begun to display some noise etc. When I had the new PEC pot in hand I set off to do the install. I opened up the chassis and found the hookup of the Alps pot to very odd. The preamp is stock I have had it since it was new so no one has ever modified the unit exept my self a few caps and some silver wire. The strange thing was that the input of the pot which is the top connection has a small 43K ohm resister soldered to it and a jumper from the input to the wiper (output) of the pot. I have never seen anything like this before and I can't understand how this pot is actually working. It seem to me if you place a jumper accross the input to the ouput then you effectively bypass the attenuator. I did replace the alps with the new PEC pot following the exact wiring of the alps and it works but I can't for the life of me understand what is going on here. I contacted the manufacturer and all they said was that my hook up of the new pot was correct. If anyone can tell me whats going on here I would be most indebted.
Note on picture. The top connection is input with small resister, center is wiper (output) bottom connection is ground.

[Rundmaus]

... strange hookup, yes, but it will work. Simply pulls your line level input down to ground to achieve attenuation.

Imagine the Pot turned fully cw: The input signal will flow via the small resistor and the jumper towards the amplifier circuit. In addition, the signal wire is loosely coupled to ground via the 100kOhms of the pot. So you have no or only minimal attenuation.

Now imagine the Pot turned fully ccw: The pot wiper ist close to the grounded terminal, your signal line is practically pulled to ground potential after the small resistor ---> high attenuation...

Greetings,
Andreas

[Jrook]

Thanks Andreas, I'm not sure I completely understand this. But after installing the new pot it does seem to work just fine.
I'm curious as to why the designer went to all this trouble to implement the volume contol this way. Is there a specific reason why this was used as opposed to a more conventional hookup?

Thanks,
Jrook

[Nixie]

Aren't Alps pots plastic? Plastic has significant parasitic capacitance; I've seen claims that good cermets are actually better for audio (though wear can be a problem depending on usage).

[AndrewT]

Hi,
the extra resistor from input to wiper modifies the attenuation rate of the pot.
For audio, a log law pot is normally used but there are difficulties getting it to match between channels.

Some will use a larger linear law pot and add a log faking resistor from wiper to ground.

I think your pot is a reverse of the log faking, effectively reducing the log law effect. Presumably the designer wanted the feel of the attenuator to be nice and usable and found full log law was too coarse around the most used positions. But I stand to be corrected if I surmise wrongly.

Edit,
just noticed the pot label:- 100kax2. Does the "a" mean antilog?

[Rundmaus]

...mhh you're correct about the use of an extra resistor between input and wiper.

But note that in the picture above, there's no extra resistor, but a direkt bridge instead... I understand how it works, but I have no idea WHY one chooses that kind of hook-up...

Greetings,
Andreas

[sreten]

Originally posted by Rundmaus
I understand how it works, but I have no idea WHY one chooses that kind of hook-up...
Andreas

Hi,

If I understand correctly the input is via the 43K resistor, and
the pot is wired as a variable resistance, forming a divider with
the 43K resistor.

This will always have some attenuation and have a greater
range of usable settings if its a log pot. I can't see a linear
pot working well with this arrangement.

/sreten.

[testlab]

This looks like a balance/pan pot. The resistors are being used to give a square law response to a linear pot. This results in a 3dB boost at either end of the control.

The control itself is a carbon composition type. Cermets are quite expensive by comparison. They can also have noise problems with wear but, they are very stable. I would find a good conductive plastic one if you are going to replace it. Or, bypass it completely.

[Jrook]

The alps pot is indeed a log pot. It is a 100k and and is of conductive plastic type. I replace it with a PEC carbon composition
100K Picture below When I e-mailed the manufacturer regarding the preamp's pot all he would disclose is that the signal which is passing through a 43Kohm rn55 resister does not electrically see the full attenuation of the control. He would not provide me a schematic of the preamp so I don't know what is actually going on in the circuit but I have a hunch that it is some kind of contour working with the 12AX7 output.

Jrook

[Jrook]

Here is another look at the wiring of the stock pot.
After a week of listening to the new PEC pot there is no comparison. The original alps conctuctive plastic seems lifeless and one dimensional. The PEC provide a much more liflike and fully fleshed out presentation. voices are tonally pure now and piano has the proper attack and decay that was missing before.
Just my opinion of course. But it sounds more like music to me.

Jrook