Advice for wiring active preamp and tone roll-off (LPF)

[Warrior_Bass]

I enjoy the wide frequency response of a buffered pickup, but I find that I never use onboard EQ. Often, however, I do wish I could roll off the treble like a passive tone control.

I've searched for topics on this, and there are a lot, but they seem to be case specific, or, they seem to be geared towards operation in passive mode for an active/passive system. I want to use this operation in active mode, in place of typical EQ. I've seen that Audere has a tone roll-off option for active, but with such a seemingly simple concept, I feel I could do it at a better price point.

I'm looking for general knowledge on how to do such a thing, if it's even possible.

I'll start with an example: could I run two passive pickups with a Bartolini AGDB-918 Adjustable Gain Dual Buffer, run the two outputs of this to a 25K blend pot, the output of this to a 25K volume pot, and the output of this to a tone pot? What value would the tone pot be, and what capacitor would I use?

Thanks in advance for any and all advice.

[line6man]

Sure.

Try a 25K tone pot, and experiment with different cap values until you find what you want.

[Warrior_Bass]

Originally Posted by line6man

Sure.

Try a 25K tone pot, and experiment with different cap values until you find what you want.

Cool. It seems like such an easy concept I had to verify it to make sure I was right.

Are the standard cap values for a passive system going to have the same effect for the active system, or are they going to differ like the pots do between active and passive? (I don't mind testing, I just need to know where to start)

[line6man]

Originally Posted by Warrior_Bass

Cool. It seems like such an easy concept I had to verify it to make sure I was right.

Are the standard cap values for a passive system going to have the same effect for the active system, or are they going to differ like the pots do between active and passive? (I don't mind testing, I just need to know where to start)

The impedance is lower, so they aren't going to behave the same as in a passive system.

I believe EMG recommends 0.1uF caps with their active pickups, you could try that as a starting point.

[Rob22315]

Put the buffer after your existing passive tone and blend or v/v pots. Works great and actually makes the passive tone control work better because the passive tone control won't get loaded down by cable resistance/capacitance. Change your pots to 500K to keep the high end intact.

If you really want the pups to feed an active system and can roll your own circuits, add an active LPF after your pup buffers/blend. Lots of simple active tone filter circuits can be found on the net. Some folks on TB will build a custom preamp if you want one.

[line6man]

Originally Posted by Rob22315

Put the buffer after your existing passive tone and blend or v/v pots. Works great and actually makes the passive tone control work better because the passive tone control won't get loaded down by cable resistance/capacitance. Change your pots to 500K to keep the high end intact.

If you really want the pups to feed an active system and can roll your own circuits, add an active LPF after your pup buffers/blend. Lots of simple active tone filter circuits can be found on the net. Some folks on TB will build a custom preamp if you want one.

I would keep the controls after the buffer to avoid resistive loading against the pickups from the pots. The OP even specifically said "I enjoy the wide frequency response of a buffered pickup[.]"

Also, remember that the buffer he plans to use buffers each pickup separately, so you would need a dual ganged pot if you want a tone control before the buffer.

What do you mean by keeping the tone control before the buffer so it won't be "loaded down" by cable RC? The impedance is lower after the buffer, and either way, a standard passive setup would do the same thing.

[Rob22315]

Originally Posted by line6man

What do you mean by keeping the tone control before the buffer so it won't be "loaded down" by cable RC? The impedance is lower after the buffer, and either way, a standard passive setup would do the same thing.

Passive tone control will be affected, to a small degree by the capacitance and resistance of the cable connected between the bass and the outboard preamp/amp. Putting a buffer after the tone control eliminates that interaction. Having the buffer drive the cable helps to minimize the impact the cable has on tone. This assumes you've designed the buffer to have a high input impedance.

The first buffer I installed on my GSR200 was a small single JFET circuit. It did help restore the effect the tone control knob had on my output.

It's certainly easy enough, with the availability of really good and cheap opamps, to throw buffers in front of everything but at some point, it's overkill. If you're reasonably happy with a V/V/T circuit and don't want fully active equalization, you should be good with 500K pots and a small buffer right before the output. If you're uber concerned about preserving the full output of the pups, you could buffer each pup but then you don't really need to buffer every stage (blend, master vol, equalization) after that. An opamp at every stage is certainly doable and not real expensive with the availability of $3 and $4 quad chips but it's more work to design, build and troubleshoot. Access to a good SPICE program would help but I'm finding it almost as complicated and time consuming to learn SPICE as it is to just build/try.