Changing potentiometer sensitivity OBP-1 bass knob

[Clemouze]

Salut all

I have a bass equiped with an Aguilar OBP-1 preamp. I bought the "STK" version , which comes with stacked potentiometer.
It works fine ok , big lows at center shaft and cristal trebles at outer shaft BUT when I use the bass knob here is what I got :

0% of knob turn : nothing
5% : almost nothing
Something like 6 to 9 % : usable bass boost
10 % : BROUUUUUUUUUM super giga low 40HZ in the face
11 To 100% : the same

The treble knob doesn't act like this !

==> Can something be done to extend the usable range of bass boost ? Like adding resistor ?

I hate the mega subby tone past 10%
I set up the knob past 10% and then I have to use a high pass filter to get a correct tone. ( if I leave it between 6 and 9 percents it sounds good but if SOMETHING touch the knob I have to spend time again finding the sweet spot )
So currently I don't Touch the knob, it is always on..

Also Ive got the same pickup and the same preamp on a modulus flea bass, no issue ! Usable bass knob ! In fait it is not a devily concentric knob from outer space in it..

 

[Clemouze]

Ok I know I should try to buy an EMG 25k concentric knob and instal it to see if the issue is the same

 

[NKBassman]

I can't help you, other than to say my OBP-3 has the same "problem". Turning the bass knob just past the center detent is about all I can stand. Anything beyond that is pretty extreme low end boost.

 

[RobbieK]

Hmm. Interesting problem... I'd say the short answer is to turn down the bass on your amp, so you can set the bass control higher on the bass where the action is more gradual.

AFAIK, the Agi obp1 has a scaled fender tone stack for treble and bass, so make sure your pot is the correct value and taper. (From memory, it should be a 25K log taper.) If it's a linear taper in there, then I think this would explain what you are hearing. Or even if it's audio taper, it could still be too abrupt because "audio taper" seems to vary and it is really just two (or sometimes three) linear tapers that approximate a log response.

There should only be two wires going to the bass pot for a fender tonestack, so I'd remove one of the wires, then measure the resistance of the pot with the control knob in the centre. (Without a centre click, you may have to remove the knob and mark the shaft and nut to get it accurately to the middle of its rotation.) Measure between the wiper (centre) and anti-clockwise lugs. (These should be the two lugs being used.) For a typical 25k audio pot, you should get 3-5k.

Ideally, you want a pot that is on the low end of this scale. (A true "log" taper pot would be 10% of the full value at half rotation. IOW, 2.5K.) For a tonestack, when the resistance is zero, the bass is at lowest, so having a smaller number at half rotation will mean you have less change spread over more distance.

 

[Clemouze]

Thank for the answer

I use a LOT of pedals and several basses so if I play on a bass knob for this particular bass it has to do before the pedals ( which I do by turning ON the high pass filter just before the bass guitar output )

Concerning the pot it is strange because this pot was sold with the preamp by aguilar.
I Will see if I am brave enough To take measurements because the wiring of this bass is hell ( I had to let this to a luthier )

 

[Clemouze]

Ok it is a 50k pot and the only stacked pot that Aguilar sells in their " for OBP" section

 

[Clemouze]

Hm ok the 50k stacked is written as for OBP-3
They DO sell a single 25k written as for OBP-1 and the OBP-1 schemes show 25k also

But no way To find something about a 25k stacked
I Will email them to understand what I have installed

 

[RobbieK]

With those green pots, there will be a code on the side, written on the metal part. If your pot is 50k, then it is most likely linear tape, and it will say B50k or 50kB. That's because the agi obp2 and obp3 use a different type of filter. Using these with an obp1 will certainly change the sweep but also the frequency response of the treb and bass controls.

Also, make sure you have an obp1.

 

[Clemouze]

UPDATE

Until today I was OK with the setting and the issue : some tape to lock the bass knob at the ultra precise position that works fine with the pickup and NEVER touched it again.

But today I made a big discovery.
Looking at the pot, it was eventualy well a 25k and not a linear one. So the pot was fine.
BUT THE WIRING was inverted !!

It was wired as if the following view was from the TOP of pots

Initially I thought it is nothing, the pot rotation will just be on the other way but..
What about the logarithmic thing ?

I wired it the correct way and.. issue seems to be gone !
I can know use the whole bass knob, ( even if I usually stay in a low zone ) at least one degree of rotation is not dramatic for the boomybass !
The knob now works like my modulus OBP-1 bass knob !

Is there some logic here ?