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Old 28th January 2010, 01:05 AM   #1
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Default gain

Gain pots on car amps. Are they linear or audio taper or both depending on brand?
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Old 28th January 2010, 03:04 AM   #2
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Do you have a working pot and an ohm meter?
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Old 28th January 2010, 01:18 PM   #3
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Yes I have an old pyramid amp that i use to experiment on. Practice my soldering teckniques etc. Also a fluke 75 multimeter. I had 5 broken amps. 4 now cuz I fixed one. A PPI PC-2150 that needed a mosfet and gate resistor in the PS and also had a 15% power difference in the channels that was fixed by changing the gain pot. Hence the interest in the different pots. Now onto the KAC-600 (two blown mosfets) an A-200 (brown sugar and bumps on one of the PS caps) and the one that's a mystery. An Orion xtreme 500 that has a working PS but won't turn on. It's the one with the "made in USA" sink and the "Made in Korea" red board".
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Old 28th January 2010, 04:15 PM   #4
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Gain pots depending on how much fiddling was done to them over the years can be a source of issues like you describe. I manage to keep a healthy supply of replacements around even now. The older PPI and Orion amps has some of the worst Phier pots I ever saw and almost all of them needed to be replaced within a short period of time, and they caused all sorts of issues like scratchy noise issues to complete fall outs of audio and what not.
Sounds like you having fun though....
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Old 28th January 2010, 05:46 PM   #5
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Yeah I'm having a ball. 3yrs 8mths to retirement (freedom 55) and I decided I needed to get a hobby that i could do sitting down. Working on old cars is too labor intensive (unless it's my own). The PC-2150's pot had a large difference between the left and right channel and the difference was progressive. When it was at near zero they would be almost the same and the difference got wider as you turned it the other way. When i set the gain on it using the "hot" side (I think it's the right channel) the left channel had about 15% less voltage. There was no audible difference but I'm anal about these things and had to correct it. Most amps I've worked with using a single dual-gang pot have a diff of less than 5%. Around here no repair shop will touch a car amp for less than $100. I had a shop remove and hard wire the (flimsy piece of crap) right phase switch on an Orion Extreme 800 and it was $100. So now I do it myself.
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Old 29th January 2010, 12:53 AM   #6
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A 20K linear taper pot will read near 10K from wiper to each of the other 2 contacts when the shaft is adjusted to the dead center point. When turned to 25 percent of it's range, it will read 5K from wiper to one contact and 15K from wiper to the other contact. An audio taper pot when adjusted to center of it's stroke, will probably read 15K from wiper to one contact and 5K from wiper to the other contact.

The dirty Orion and PPI pots are very easy to repair. Remove the pot from the board, go to a hardware store and purchase a 1-1/2 inch pipe nipple about 3 inches long. Purchase 3 PVC end caps to fit on the pipe. Drill a hole in one of the PVC ends. Screw one end cap on the pipe. Spray WD-40 into the pipe, deep enough to cover the pot, Drop the pot in and screw the end cap with the hole drilled in it. Now using a rubber tipped nozzle connected to an air hose, pressurize the pipe bomb. Do this a couple times. Remove the end cap with the hole, remove the pot and turn it from stop to stop several times. Screw the 3rd endcap onto the pipe to save the WD-40 for the next time. Reinstall the pot.
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Old 29th January 2010, 01:01 AM   #7
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You need to check the taper on the pot because they can be designed both ways. Typically you put a log taper pot across the signal source, output from wiper. But you can also put a lin pot across an op-amp and make it logarithmic.

Sometimes log pots are marked with 'A' for audio taper e.g. 10KA is a 10Kohm log taper pot. But there;s really no standard. Best to measure as described above.
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