aion SAPPHIRE Manual - Página 8

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BUILD NOTES, CONT.

Gain pot value

The original BD-2 uses a 250kA dual-gang potentiometer. These are not available from any of the usual

suppliers except for

Tayda

Electronics, who carries a straight-pin knurled-shaft type. These can be made

to fit the Sapphire PCB with some effort, but it's much easier to just use a 500kA dual potentiometer

which is readily available in the right-angle format used in this project.

Parallel resistors (R9 and R20) have been added to each half of the gain control to drop the 500k value

down to an effective value of 250k. The taper is affected slightly, but it's not noticeable unless you're

doing an A/B test against an original and trying to match knob positions. If using the stock 250kA value,

omit R9 and R20.

Q2/3 and Q5/6

It has been suggested that Q2/3 and Q5/6 (the JFET differential pairs that make up the two discrete op-

amp gain stages) should be matched in their V

value for for best performance.

GS(off)

This may be true in theory, but we took measurements of the JFETs in an original Blues Driver and the

two pairs were not matched any more closely than any of the other non-paired JFETs, which suggests

they were chosen at random. It doesn't appear that BOSS used any sort of matching in the manufacture

of the Blues Driver and so it's not something we recommend giving any consideration.

SAPPHIRE AMP OVERDRIVE