Strange potentiometer, help identify

[the_badcliff]

I'm working on an old Pioneer stereo receiver with a faulty volume pot.  This happens to be a very strange pot and I am hoping I can identify it and find a replacement.

The pot is a 500k dual pot made by ALPS.  The serial number written on it is 2NC-ACV-105-0-500k.

Heres where its strange.  The pot has a 4th pin.  This 4th pin is at a fixed position where it is 165k to one side, and 335k to the other.  The pin connects to the center (wiper) such that with the pot turned all the way left resistence is 165k, centered it is O, and all the way right it is 335k.

Perhaps somebody here would be familiar with this type of pot.  In case it helps, the amp is a Pioneer Stereo Receiver Model SX-525.  Its a great little amp, and very classy looking so hopefully I can get it working.

[Mark Hammer]

Such 4th pins are taps for introducing frequency compensation.  My guess is that the unit does not have a "loudness" control, or if it does, that loudness control is connected to the 4th pin.

The reason it's on the volume pot is that you want the tonal compensation for the oddities of human ears to kick in at lower volumes where those ears need a little "help" and kick out when the volume is sufficiently high that the ear responds in a relatively "flat" manner.  All loudness controls work that way, and strangely enough so do the compensation caps on volume controls of Fender guitars and bright switches on Fender amps.  The major difference is that where loudness controls adjust both top and bottom end, the compensation circuits in fenders only hike up the top end at low volumes.  As with loudness controls on stereos, the bright switch on amps has no appreciable effect on frequency response at high volume.

[the_badcliff]

so it sounds like this is not an uncommon sort of part.  Any idea where I could order one?

[Paul Marossy]

Mouser.com perhaps?
Just a thought...

[Nasse]

I´ll see if I can find some schems, if you want "loudness" without special potentiometer for substitute. Maybe tomorrow

Loudness control works nice imho when listening at low level, but depends your speakers. If you have very good speakers, or add a subwoofer to your system you can live happily without it

[Mark F]

Funny you should mention ALPS Pots. I need a 50K dual log for my old Hafler and it took a long time to find this place. It is OCTAVE ELECTRONICS SDN. BHD. they are in West Malaysia. Their web add. is www.octave-electronics.com     They specialize in audiophile parts. As far as I can discern from what I've read @ a few sites ALPS pots for old pre-amps & amps are getting quite hard to find. For the audiophile quality stuff anyway. Blue Velvet series in particular. Anyway there is no minimum order at Octave. You can also GOOGLE ALPS POTS & you will get some results. I have found that there is usually a minimum order. Hope this helps.