Electrically, trimmer pots behave the same as full sized pots.
The only difference is going to be that trimmer pots cannot handle the same wattage as full sized pots, but you are dealing with a very low current to begin with, so you should be fine.
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Build your own onboard preamp
- Thread starter rojo412
- Start date
Electrically, trimmer pots behave the same as full sized pots.
The only difference is going to be that trimmer pots cannot handle the same wattage as full sized pots, but you are dealing with a very low current to begin with, so you should be fine.
Looking over that circuit: if a source-follower stage was added using a FET of fairly high gm, you could redo the tone controls using low impedance (10k to 25k) pots. You could also set up a virtual earth mixer stage and have a nice blend control where each pickup could be mixed together without effecting the loading on either one.
I'm going to hack up a circuit that does it and see what it might look like.
I'm going to hack up a circuit that does it and see what it might look like.
adam.beauchene
Commercial User
Actually, this isn't a good idea because trimmers are only meant to be cycled a few times, for setting resistance values in a circuit somewhat permanently. You'd wear them out in a typical bass volume/tone pot application, they'll go all scratchy and eventually quit working much sooner than a normal pot. Alps makes some tiny pots though, they are very nice and have a very small footprint.
I'm guessing they won't be adjusted too often?
Also, there are small PCB mountable pots available too, rather than trimmers.
just got 2 PCB's. one for an enclosure and the other for the 7 switch "kitchen sink" bass. one board has component markings and other has holes for a custom design. adding the J201's and other components for a total of $10. a lot of bang for the buck. let the games begin! 
they are actually tiny, 2.00" x 1.25". the pics are zoomed. my ears will be the final judge but I have a feeling if I tweak it right it's gonna kick some major ***
thanks to line6man for the inspirationWhere in the signal chain are people installing these preamps? After the controls and before the output jacks?
You can go either way, but blends/two volume setups have to go before the preamp.
- Nov 15, 2006
- 2,219
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- Disclosures
- El Presidente of Sanford and Sonny Audio Weapons (Bluebeard Fuzz)
agreed and then you could also have a master volume after the pre that controls overall output
and/or you could have a master volume before the pre that controls the input gain of the pickup(s) to the pre.
Wow. The Frankenstein bass I am working out on paper now has theoretically 14knobs & 9switches (6knobs & 3 switches are for two fuzzes) if I don't mount something internally it will look like the spawn of two 70s BC Rich basses.
Has anyone encountered the tone control circuit from the Fender Jaguar basses? What does it work exactly?
What's obvious from the external controls it has:
Does anyone have a full-blown schematic?
What's obvious from the external controls it has:
- A neck pickup on-off switch
- A bridge pickup on-off switch
- A series in/out switch (judging by its described behavior, a series/parallel switch)
- Active 2-band EQ of unknown type and construction.
- Passive V/T controls.
Does anyone have a full-blown schematic?
just built this basic circuit. before I test for a signal can I test the J201 with a DMM with the power on to make sure its in spec? should the drain to ground be +5 volts? before I go any further I want to make sure I have good transistor
You can make that board much smaller than that.
I can fit 9 preamps on a 2.5"X3.8" board.
You can save even more space by getting rid of C2....it basically does nothing in a battery powered class A design.
Adding the source-follower stage brought down the output impedance from 6.8k to about 300 ohms. If you lower the input resistor R1 down to about 150k to 200k you can place this preamp immediately after the pickups and before the tone network. This allows use of low impedance Baxandall style feedback networks with two additional FETs.
Why not just test it in the circuit?
Adding the source-follower stage brought down the output impedance from 6.8k to about 300 ohms. If you lower the input resistor R1 down to about 150k to 200k you can place this preamp immediately after the pickups and before the tone network. This allows use of low impedance Baxandall style feedback networks with two additional FETs.
Why not just test it in the circuit?
Yeah, I was thinking of how useless that cap was.
It's not like we're filtering ripple from a rectified power source or anything, it's just a battery powered circuit with a single active component. You can add it or skip it, it really doesn't matter, but I kept it in there for good measure.
H*** YEAH! it worked! changed R1 to 200K. installed it in the 7 switch bass and WOW!
thanks for all the help. let the tweaking begin! that guy Albert Kreuzer said putting a 100uf cap in parallel with R2 (2.2K) will make it sound better for bass. we'll see! 
H*** YEAH! it worked! changed R1 to 200K. installed it in the 7 switch bass and WOW!
200K for the gate-ground resistor? Since the impedance is close to being purely real, it's accurate enough to say that the impedance at the input is equal to the value of R1.
You want a high input impedance, and 200K is relatively low.
I would say that an acceptable input impedance is above 1M ohms.
Adding a capacitor parallel to R2 will increase the gain.
I'm not sure how you jumpered across C2?
Since it's negative is grounded, and it's positive is connected to the battery's positive, shunting it would short the battery out.
Whatever you do, don't short out C1. That's a coupling capacitor used to block the 9V DC power from reaching the output. Without that cap, there is a risk of frying whatever the bass is plugged into.
Also, even though the voltage and current is low, it's never a good idea to short out electrolytics. It decreases their performance, and can even cause them to fail.
I accidentally shorted out an electrolytic capacitor that was charged with 526V by trying to drain it with a resistor whose value was too low, a few months ago, and that spark scared the hell out of me! The capacitor was still ok afterwards though.
R1 should be changed to 200k if it goes immediately after the pickup leads....wasn't sure if I was clear about that. 3M is too high to load down the pickups properly, and it will sound really bright if placed right after the pickup. 200k is reasonably close to a volume pot and tone control in parallel (from a loading perspective).
If you place this preamp after the volume and tone controls you should leave R1 high (1M+).
The cool thing about adding a cap across R2 is that you can introduce a gain step wherever you want it to emphasize higher frequencies....that's the secret to the tube screamer kind of gain controls. If the RC time constant is below the lowest note (e.g. the capacitance is large enough) then gain at all audible frequencies is increased. This is a good way to increase the gain of this circuit as the only other ways are: decrease R2 (moves the quiescent operating point), increase R3 (which increases output impedance), or use a higher gm JFET (more noise).
If you size the cap just right you can emphasize the resonant bump of the pickup for a very bright and punchy sound (in the 2-4kHz range).
