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Old 17th June 2004, 05:36 AM   #1
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Default ALPs blue velvet question - new to DIY audio

Hello everyone,

this is my first post here... I have some experience with electronics, but DIY audio is fairly new to me. I have an older amp that I am attempting to upgrade with a new potentiometer for the volume. This is mostly due to the previous pot breaking.

The original pot was a 70K very basic dual unit alps. 70k seems to be impossible to find so I have decided to go with a 100K instead.

So i was able to track down a Alps rk27112 series pot at Mouser which seem fairly well regarded and relatively cheap... however, the exact part number that they carry seems to be undocumented and has some aspects that I cannot figure out. The full part numer is RK27112-s20c1mn104. This does not seem to correspond completely with all of the part number breakdowns in the data sheet, however, I can say with certainty that it is a dual unit, single post, slotted 20mm shaft, with one center detent (unfortunately, this was a surprise to me when I received it), and 100K Ohm. aprox. 300 degree rotation.

I've soldered the thing in place which brings me to my confusion and question:
when rotated all the way to the left no volume comes out (as expected). as you turn the pot towards center only one channel increases in volume so that at center that one channel is at peak and the other channel has no volume. then as the pot passes center the other channel comes up as well, so that at full right both channels seem to be aprox. equal.

This seems like very strange behavior to me. At first i thought maybe the part was for channel balance, but on second thought that doesn't make any sense either.

Does anyone know if the pot is working correctly and I am just using it for the wrong purpose? or does it sound like I have hooked something up wrong? I can't find any documentation from alps for any part variations that sound like they would work in this way. and I can't come up with any sort of misconnect that would explain the odd behavior...

I am stumped.
any help is greatly appreciated and my apologies if the answer is stupidly simple.


best regards
b. scandanavia
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Old 18th June 2004, 12:45 PM   #2
dhaen is offline dhaen  Europe
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A centre detent pot is most likely intended for use as a balance control, and will have a different law (taper) than one intended as a volume control.

Even so, dropping one in place of a volume control, should not give the effect that you are experiencing. Most likely, a wiring error has occurred, either because the original had unconentional pinout, or a mistake while soldering.

You will need to buy a proper log-law volume control. While you are waiting for it to arrive, I suggest you examine the original pot with a meter, and see whether the pin-out is conventional.
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Old 18th June 2004, 01:53 PM   #3
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This is certainly a balance pot. The effects you describe would be produced by the sort of balance pot that in the centre position puts out L and R at no attenuation. As you rotate left, the L channel stays the same but the R channel gets attenuated, while for a rotate right the opposite occurs.

No use at all for a stereo gain control though!
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Old 18th June 2004, 01:56 PM   #4
dhaen is offline dhaen  Europe
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Ouroboros is right. I had not considerd the "zero attenuation" type.
Just get the right type, and all will be well
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Old 18th June 2004, 02:48 PM   #5
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Default thanks

thanks to everyone for your replies - they were really helpful... I guess I should have checked it with a meter before going through the trouble of installing.

Any suggestions on US suppliers for the alps rk27112 pots? They seem to be fairly common in the UK but only at 50K Ohm - so far I have had no luck finding a 100kOhm (which is why i was a little over-excited to find that other one at mouser) type.

Best,
b. scandanavia
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Old 18th June 2004, 06:14 PM   #6
rabbitz is offline rabbitz  Australia
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Default Stupid question?

Might be a stupid question, but what's wrong with a 50K pot? Actually volume controls have caused me more pain than any other device....... grrrrrrrr.

You can still use your other pot as a balance control if you need one.
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Old 18th June 2004, 07:16 PM   #7
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Default what's wrong with 50K pot...

Hmm- well the main problem is that with a 50k pot I am unable to turn the volume completely off, in fact it is still pretty loud at the full 50k resistance position. i guess it would be possible to put in a couple of resistors in series at the outputs of the pot, but the Alps unit fits perfectly in the existing PCB, so it would get sort of messy to add in more components.
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Old 19th June 2004, 03:29 AM   #8
amb is offline amb  United States
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I suggest going with a 50K ohm audio (log) taper pot. It's still high enough not to become a problem with loading down the previous stage, but offers a small advantage in noise performance over a 100K ohm unit. Only some tube gear require 100K or more for the volume control. SS designs should be happy with 50K.

If you want a really nice dual pot (Alps, Noble, etc), check out the offerings from Michael Percy Audio. These are pricey but premium quality.

http://www.percyaudio.com

-Ti
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Old 19th June 2004, 09:26 AM   #9
amb is offline amb  United States
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Default Re: what's wrong with 50K pot...

Quote:
Originally posted by bscandanavia
Hmm- well the main problem is that with a 50k pot I am unable to turn the volume completely off, in fact it is still pretty loud at the full 50k resistance position.
Then you have a wiring mistake. Is your pot's ground connection not wired up? You should be able to turn the volume down to complete silence no matter what value pot you use.

-Ti
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Old 19th June 2004, 04:09 PM   #10
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Default Re: Re: what's wrong with 50K pot...

Quote:
Originally posted by amb


Then you have a wiring mistake. Is your pot's ground connection not wired up? You should be able to turn the volume down to complete silence no matter what value pot you use.

-Ti
of course! I don't know why this went over my head before... at any rate I have since removed that 50k Ohm pot which was not very high grade anyway so I can't test what the problem was.



Thanks, again.

By the way, if anyone is curious, I have decided to order the Alps pot from thlaudio.com in hong kong which was the only vendor that seems to carry the serrated shaft style that i need..
I read a couple seemingly positive reviews about them... I hope they are reputable...

best
b
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