Changing a A50k pot to a B50k pot question

Thread Starter

musicalavtech

Joined Mar 23, 2012
72
My guitar has a factory preamp/mid-boost board installed on it. It has always sounded fine. I do wonder about the mid-boost potentiometer. It's connected by a coule wires to the preamp. It only controls the amount of mid-boost into the output signal. With it turned all the way down, there is zero mid-boost. When I turn up to about 30% or so, the mid-boost seems to be maxed. Turning up any more than that makes little difference. The sound is never bad, just that only less than half the pot movement has all the gain. A small turn of the mid-boost knob goes a long way.

I am wondering about swapping that pot out for a B50k, with a linear taper, to alter the usefulness of the physical movement of the pot from 0 to 100%. I'm hoping to still get the full range of mid-boost, but spread out over the whole travel of the pot. Would this seem to be the path to take?

The mid-boost pot is the last component of the preamp, before the signal goes to the output jack. I can maybe understand why one thinks it should use an audio taper, but that isn't the best choice. The guitar volume pot is mounted on the preamp board, and is a A50k as well. It's never had issues. It's taper works quite well for the guitar, as anyone would expect.

I can see how the circuitry involving the mid-boost pot can allow the IC AMP output into the guitar output. I just need better control.

Apologies if the image is doing ridiculous things. Thanks for anyone's knowledgeable advice.

charvel-jackson_active_mid-boost_je1200_sch.png
 
Last edited:

Alec_t

Joined Sep 17, 2013
14,318
just that only less than half the pot movement has all the gain.
I suspect it would be worse than that if you used a linear-taper pot. Audio-taper (logarithmic) is normally used in audio kit, to compensate for the human ear's non-linear response to sound level.
 

Jobit Joseph

Joined Aug 23, 2017
3
My guitar has a factory preamp/mid-boost board installed on it. It has always sounded fine. I do wonder about the mid-boost potentiometer. It's connected by a coule wires to the preamp. It only controls the amount of mid-boost into the output signal. With it turned all the way down, there is zero mid-boost. When I turn up to about 30% or so, the mid-boost seems to be maxed. Turning up any more than that makes little difference. The sound is never bad, just that only less than half the pot movement has all the gain. A small turn of the mid-boost knob goes a long way.

I am wondering about swapping that pot out for a B50k, with a linear taper, to alter the usefulness of the physical movement of the pot from 0 to 100%. I'm hoping to still get the full range of mid-boost, but spread out over the whole travel of the pot. Would this seem to be the path to take?

The mid-boost pot is the last component of the preamp, before the signal goes to the output jack. I can maybe understand why one thinks it should use an audio taper, but that isn't the best choice. The guitar volume pot is mounted on the preamp board, and is a A50k as well. It's never had issues. It's taper works quite well for the guitar, as anyone would expect.

I can see how the circuitry involving the mid-boost pot can allow the IC AMP output into the guitar output. I just need better control.

Apologies if the image is doing ridiculous things. Thanks for anyone's knowledgeable advice.

View attachment 315177
As @Alec_t use logarithmic pot for audio applications.
 

MisterBill2

Joined Jan 23, 2018
18,535
For this application, where the pot is not adjusting gain, but only a mix, (balance) a linear pot makes much more sense. In fact, I would call it a production error to have installed a log pot in that location. The two pots probably look alike and it would certainly be easy to put in the wrong one. It IS NOT a volume control, but much more like a stereo system BALANCE adjustment,
 

MrChips

Joined Oct 2, 2009
30,814
I would have expected the VOLUME control potentiometer to have a log taper (B) and the MIDBOOST pot to be linear (A),
Maybe someone got the two pots interchanged.
 

Thread Starter

musicalavtech

Joined Mar 23, 2012
72
I would have expected the VOLUME control potentiometer to have a log taper (B) and the MIDBOOST pot to be linear (A),
Maybe someone got the two pots interchanged.
The (A50K) pot IS a LOG taper (A). The (B) being a LINEAR taper per simple research, and I tested the pot for ohms as I turned it from 0-100%. I can confirm a LOG taper. Maybe there was cornfusion back in the 80s design. BOTH pots are the (A) version, per the schematic, and actual build, not interchanged between each other. So the volume pot is at least correct for the design. The mid-boost pot A50K seems to be incorrect even as designed and built.

Thanks for your input.
 

Thread Starter

musicalavtech

Joined Mar 23, 2012
72
I suspect it would be worse than that if you used a linear-taper pot. Audio-taper (logarithmic) is normally used in audio kit, to compensate for the human ear's non-linear response to sound level.
I'm not sure how it could be worse.

I have always agreed that an audio taper is the right choice for audio, as heard by our ears. The mid-boost pot is blending 2 signals towards the output. That's why I'm (and others) thinking a linear taper could be a better choice. Both signals would have more of an equal strength towards the output if the pot was @ 50% for instance.
 

Thread Starter

musicalavtech

Joined Mar 23, 2012
72
For this application, where the pot is not adjusting gain, but only a mix, (balance) a linear pot makes much more sense. In fact, I would call it a production error to have installed a log pot in that location. The two pots probably look alike and it would certainly be easy to put in the wrong one. It IS NOT a volume control, but much more like a stereo system BALANCE adjustment,
This makes MUCH sense for this circuit. Thanks for the reply. The actual design (asuming pre-production per schematic) is faulty on the mid-boost pot taper value. The actual build follows the fault. Who knows how many units of this preamp were produced. Surely thousands. I've not heard of much complaints, but 35 years later, I had to step up and question this. Some production facilities will build to schematics and not question the design. (where I work, we frequently question the designers and ask what were they thinking).

This is most likely a design error (per schematic), in such a way that perhaps someone thought this particular segment of the design, still somehow was more of a single channel standard audio output, as opposed to controlling an actual combination of 2 amp signals (from the IC A+B) being blended, much like a stereo balance control. Different from the audio taper volume pot function.
 

MisterBill2

Joined Jan 23, 2018
18,535
The pots are the same resistance and my guess is they have similar appearance. I have seen unknowing folks do dumb stuff before, It would not be the first time the unknowing goofed things up.
 

MrChips

Joined Oct 2, 2009
30,814
If you set the pot to 50% and then measure the resistance (preferably out of circuit) between the wiper and the two ends, you will know if it is linear or log taper.
 

Thread Starter

musicalavtech

Joined Mar 23, 2012
72
It just occurred to me that the mid-boost output of the IC1B could possibly be a hotter signal than the output of IC1A. If that were so, would a certain pot taper be more useful over another. Would the chip make equal level outputs from pin 1 vs pin 7 (per schematic)? I know very little about how that is determined.

Thanks
Greg
 
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Thread Starter

musicalavtech

Joined Mar 23, 2012
72
If you set the pot to 50% and then measure the resistance (preferably out of circuit) between the wiper and the two ends, you will know if it is linear or log taper.
Sure. And more than 1 way to measure that taper. My measurement is solid. It concurs with the part spec of being a log taper.
 

MisterBill2

Joined Jan 23, 2018
18,535
It just occurred to me that the mid-boost output of the IC1B could possibly be a hotter signal than the output of IC1A. If that were so, would a certain pot taper be more useful over another. Would the chip make equal level outputs from pin 1 vs pin 7 (per schematic)? I know very little about how that is determined.

Thanks
Greg
The easy way to see if one side is louder that the other is simply to turn the pot one way all the way and note the volume and then turn it the other way all the way and compare the volume..Simple and easy and cheap to do.
 

Ian0

Joined Aug 7, 2020
9,822
Don't forget that a Log pot is designated A in Japan, but B in Europe, and a linear pot is designated A in Europe but B in Japan.
It is far better to write "Log" or "Linear" on the circuit diagram.
 

MisterBill2

Joined Jan 23, 2018
18,535
Does the designation elsewhere even matter?? If I specify "LOG TAPER" then I should not be getting a linear taper device. This is one more reason to avoid the cryptic designater scheme/language.
Because we know that the reason it is different in the various countries is primarily to create confusion.
 
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