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Tone control circuit is driving me nuts!


 
2/1/2000 1:32 AM
Carl Z
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Tone control circuit is driving me nuts!
I've got a tone control circuit here that's making me crazy. Here's the problem. The treble control doesn't have enough "action" at the lower settings. It's pretty dead from 1 to about 5 and then comes on like gang busters. It's loosely based on the Dumble tone stack. I'm using the standard CTS 250K audio pots. I've tried just about every reasonable cap combination and it doesn't really seem to help much. I'm running out of ideas here and any comments or suggestions would be greatly appreciated.  
 
Regards;  
Carl Z  
Summit Amps  
http://www.geocities.com/summitamps
 
2/1/2000 2:02 PM
Stephen Conner
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Hi Carl Z,  
 
Why not use a 250K linear taper pot instead of an audio taper one.  
 
Steve C.
 
 
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2/1/2000 4:08 PM
Doc
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Are you familiar with the interactive Tone Control program over on Duncan's site? This is invaluable for a visual "guage" of what happens to the frequency response when controls ar rotated.  
 
Now take a close look at what taper is installed in the program (as it comes up when you open the program). In most cases, it's NOT a 10% audio taper, but what he calls an Audio B taper (I think that's the 30% midpoint version that's being discussed as special order in another Ampage thread). In this program, the component values and the pot tapers can be changed. Go in and change the LogB taper pot in a familiar tone control circuit to a LogA audio taper. The response vs. rotation changes drastically!  
 
I found this out more than a year ago, and mentioned it to the AX84 site, because many builders over there were obtaining different tonal responses than each other, after building the "same" AX84 circuit. Response/tonality depends drastically on the pot taper.  
 
Didn't the early Fender amps use the 30% taper for volume and some tone controls? You can't achieve the same response when building these circuiuts using 10% log taper controls. The boost vs. cut extremes will change, and so will the general frequency pass band of the amp when controls are on "5".  
 
Check it out. Play with that tone control program. I was really surprised at the magnitude of error.
 
2/2/2000 3:12 PM
Stephen Conner
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Pots nuts
Hi folks,  
 
Here in Scotland you have a choice of 2 kinds of pots. If you buy the Japanese Alpha mini ones (my favourites) the 'A' ones are log and the 'B' ones are linear. If you buy British made pots then it's the other way round, 'B' is log and 'A' is linear. I didn't know there was a choice of different log tapers.  
 
Steve C.
 
2/2/2000 6:47 PM
Doc
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The taper "A" and taper "B" were the descriptions that Duncan uses on his tone control program. I only restated them that way in case anyone wanted to verify my observations with taper vs. frequency response. They are not the industry designations.  
 
As far as I know, the currently accepted convention labelling pot tapers is:  
A= audio taper, or 10% logarithmic taper  
B= linear, technically it has no actual "taper", but a straight line slope  
C= (I'm not certain, but think it's the 30% log taper. Sometimes the 30% log taper is labelled "1MEG A 30".)  
 
Old Allen Bradley pots had many different letter designations for their tapers. I seem to remember a type "J", which was a linear type.  
 
 
 
2/2/2000 8:03 PM
R.G.
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Any pot taper you want
You can taper to suit your needs. See "the secret life of pots" at GEO.
 
2/3/2000 12:34 PM
Stephen Conner
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"C"
Hi folks,  
 
C is reverse log, like log but backwards. They are the exact taper needed for the sweep control in sweep and parametric EQ circuits.  
 
I know you can bend the law of pots by adding an extra resistor, but I've never figured out how to turn a linear into a reverse log when connected as a rheostat, which is what you need for my favourite p-eq circuit. I don't think it can be done.  
 
Steve C.
 
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