BAJA guitar preamp /buffer with switchable treble boost

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bajaman
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Post by bajaman »

It has been a long while since I posted any new projects - my apologies, sometimes life gets in the way ;-)
Anyway here is a little project that can easily be installed in your guitar and is extremely miserly with regards to current drain, so your battery should last for at least a year - probably longer!
If the battery goes flat during a gig, no need to worry just switch the preamp /buffer off and you are back to stock passive wiring in your guitar.
The idea for this little project came from the PRS guitar built for John Mayer to use with the latest incarnation of the Grateful Dead - Dead and Company. There are many concert clips on You Tube at the time of writing this, so if you are interested in the Dead, check them out :-) . There is a video on the PRS website where John Mayer describes the Super Eagle guitar design concept - very interesting little snippet. He talks about a small onboard preamp with optional treble boost function built into the guitar. He explains that the preamp is not neccessarily a gain boost preamp but rather a high definition buffer and that he uses it switched on most of the time. For the technically minded folk, placing a preamp / buffer immediately before the guitar's output socket serves to isolate the effects of the cable's capacitance (typically 500pf or more) from altering the guitar pickup's resonant frequency. Normally this capacitance shifts the pickup's resonance down in frequency and lessens the pickup's top end response. The buffer allows the pickup to sense a greater range of harmonics when engaged and this is the effect that John Mayer refers to as high definition.
The Super Eagle preamp also has a treble boost switch, further enhancing the bright top end response.
The project design presented here is my attempt at building a similar buffer treble boost - I have installed it in two of my guitars and am very pleased with the results obtained by such a small circuit, so I have decided to publish it for others to build and enjoy.
I built mine on a small piece of stripboard - I am sure someone will contribute a layout ;-) . The 2SK117GR is the only critical component - it must be the GR type for correct biasing and gain in this circuit, other fets will NOT work correctly and will either bias up wrong, alter the gain or draw substantially more current from the battery - experiment at your peril!
S1a and S1b is a miniature dpdt toggle switch, S2 is a spst or spdt miniature toggle switch. You will need to disconnect the output wire from the guitars jack socket and connect it instead to the input of the preamp /buffer board, the connect the output of the preamp to the guitar's output jack socket - you would be well advised to change this socket to a stereo type TRS and connect the battery black wire to the ring connection of the socket. This allows the battery to automatically disconnect when the guitar's lead is removed, further extending battery life. You will need to drill two holes in your guitar for the two miniature toggle switches OR you could use push pull pots to turn the preamp and treble boost on /off if you like! Do not expect more than a very slight volume boost when the preamp is engaged - this is not an overdrive boost circuit ! You will hear a bit more top end sparkle though, and this is what the design is all about. When the preamp is engaged you can further boost the top end with the treble boost - reduce R5 for a more extreme treble boost or increase R5 for less effect. If when built, you experience a drop in volume when the preamp switch is engaged try decreasing the value of R6 gradually until unity gain or a slight volume boost is achieved. Please feel free to ask any questions - i will try and answer them. cheers bajaman :D
Baja onboard guitar preamp and buffer with treble boost 090816.GIF
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Post by bajaman »

Ok - here is another version using the more common J201 fet with similar response characteristics and current draw :D
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bajaman
Baja onboard guitar preamp and buffer with treble boost 110816.GIF
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bajaman
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Post by bajaman »

but wait - there's more :roll:
a version with the 2N5457 :wink:
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Baja onboard guitar preamp and buffer with treble boost 2N5457 110816.GIF
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gsb
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Post by gsb »

Hi !
I will try to build one for Stratocaster and one für Les Paul ,I have 2n5457 and j201 .Is there any tonal diference ?.My question is what power the build drain from battery .Idea were to make small board with 3 cells ca. 2,5cm x 2.5cm (1"x 1") Duracell 2430 3v on the back .One cell had about 285mAh .With 3 cells hight will be ca.1cm
Thanks GSB

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Post by bajaman »

they all draw 44uA
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Post by gsb »

Hi Bajaman !
I,m :slap: it is all there in schematic .
Thanks gsb

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Post by toefish »

Hi,

What, if anything, would need to be changed to make this work with 12VDC instead of 9VDC?

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Post by plush »

toefish wrote: 24 Aug 2020, 14:21 Hi,

What, if anything, would need to be changed to make this work with 12VDC instead of 9VDC?
You don't have to change a thing in a current state.

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Post by bajaman »

hmmm - still using this in my two guitars - have not changed the battery as yet ;-)
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Post by toefish »

I just built and installed this circuit. Works great. I have it running off a 23A size 12V battery due to space constraints in my PRS' control cavity. Had a little distortion when strumming hard, but dropping the pickup height just slightly solved that. Very nice!

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