Adding an active/passive switch to onboard preamp

jaguarcat311 · #1 02-23-2005, 04:49 PM

I have a 3 band bartolini preamp NTMB-918F, and i know how to add the switch (on a push pull pot replacing the volume), but the volume pot currently in the circuit is 25K and the only push pull pots i have are 500K, and the only other ones i know about are 250K. Im not sure if this will affect the tone or anything in a bad way. Can anyone help out with this (maybe i could add a resistor or something). i really dont know much about that stuff

jaguarcat311 · #2 02-23-2005, 07:19 PM

im considering adding a passive volume (or tone) control with the active/ passive switch on that (500k for the humbuckers in my bass). when i originally put the preamp in my bass, because of the original holes, i put volume - blend - and mid in a line, and made new holes for the bass and treble and mid selecter switch under it. Id like it better to have all the tone controls in one line and the volume/ blends in another.

FBB Custom · #3 02-24-2005, 10:21 AM

The current Bartolini diagrams for active passive put the preamp (and the switch) after the volume and blend pots (both passive @ 250k audio) in the signal chain. Get a 250ka push-pull and rewire it that way.

Clark Dark · #4 03-05-2005, 07:12 AM

The 25K value is because it's an (or was?) active circuit. You should call Bartolini. If they don't have it then call EMG. I installed my Aguilar OBP-1 using a concentric pot from EMG rather than the pots Aguilar supplied. I think you should check with the active circuitry manufacturers when using 25K or 50K pots for push pull switches.

FBB Custom · #5 03-05-2005, 07:31 PM

25k pots are appropriate for low impedance signals and 250k pots are appropriate for high impedance signals like the output from a passive pickup. If you try to use a 25k p/p pot you will have a mismatch when in passive mode. That's why for this application you should use a 250k pot and do the volume passively all the time (before the signal hits the active stage).

jaguarcat311 · #6 03-06-2005, 02:44 PM

I added a separate volume control for the passive option, so i used a 500 K push/pull. when the bass is active, it uses the 20K volume, when passive it uses the 500K. I may change the passive volume to a tone, but i dont use tone in passive basses, i actually rarely use volume on any bass. i could go for a dead cut switch though........