Hot-Rodding the STC Preamps

Posted on by Jon Moody

When you’re a gear-head, nothing is ever – EVER – completely perfect. Sure, you’ll throw in the Steve Harris Signature Pickup into your P Bass, and enjoy the tone (and for my impressions, click here). But then you’re thinking, “Wouldn’t an STC preamp completely unleash the beast?” and you’d be right. But you’re left with a quandary; keep the passive vibe or go active. Well gents, I’m going to tell you how with hot-rodding your STC preamp, you can have BOTH.

The STC-2 and STC-3 Preamps are amazing upgrades to your current bass, whether you have passive or active pickups (and to review exactly what they can do for you, click here). The issue arises when you either put in an active preamp with passive pickups (thus making the bass an active bass) or you replace the OEM preamp that may have previously had an active/passive switch. Thankfully, with a little soldering and either an SPDT or DPDT switch, you can have it all.

Seems easy, right? Quickly, here’s what you need to do if it’s not clear:

1. Solder a wire that goes from the hot output of the blend pot (that would be the same prong as the grey wire in this diagram). Solder this new wire into the “hot from pickup or blend pot” prong on the active/passive diagram.
2. Unsolder the white wire from the volume pot that is connected to the preamp, and solder it into the “hot from preamp out” prong on the active/passive diagram.
3. Solder a wire from the middle prong of your switch and connect that to the volume pot (where you unsoldered the original white wire). You are done.

And for those guys with active pickups and an STC preamp, you can benefit from this too. Wire it in the same way, and it will act more as a preamp-bypass, for those few times when you’re in the middle of a gig and your bass starts sounding like crap because the battery is dying, that bypass switch may save your butt because it’s pulling far less of a draw with just the pickups. Don’t ask me how I know; I’ve heard things.

Passive Tone Knob STCBut wait! There’s more!

For the TRULY brave, why stop with just the switch? Let’s wire in a passive tone knob as well, and really open this thing up! Again, it’s a pretty easy project. Using this standard jazz bass diagram, you can see how the tone knob fits into the equation. All you have to do is wire the tone knob in between the wire that goes from the blend pot to the switch (that would be the wire you made in step 1, listed above).

By doing that, when you switch the bass to passive, you’re running the signal through the passive tone knob. When it’s in active mode, it’s running through the STC preamp. So for that hot-rodded P Bass in the beginning paragraph, you can still have the original passive bass with complete features, and then with the flick of a switch, make the heavens open up and rain down flames.

Passive Tone after the STCCould it get any better? Sure it could! You could wire that passive tone knob into the signal path leading from the hot output from the active/passive switch output into the input of the volume pot, essentially putting the passive tone AFTER the active tone.

With a little time to study, a soldering iron and a handful of parts, you can compound the capabilities of the STC-2 and STC-3 Preamps by adding some tonal options. With the best of both worlds, you can really dial in your personal tonal signature and make that bass sing.

Written on April 27, 2014, by Jon Moody

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  • CJ Broz

    I use 500K pots on my passive basses (when I build or modify). The author recommends 100-250K pots. How would the preamp respond to 500K pots?

    • Jon Moody

      That’s definitely something to look at. The preamps themselves already come pre-wired, so I’ve never had the inkling to completely rewire the preamp to find out. For me, they sound great out of the box, so aside from adding the bypass switch, there’s nothing else I need.

  • Hi! So, I have this bass I’m making (still waiting for the necessary pots). My goal is to have a standard 2V+1T passive setup plus the STC-3 and the same two volumes in active.
    The way I figured the active-passive switch for the 2V active diagram ( is depicted in the schematics I uploaded. The signal goes from the volume pots either to the jack directly (hooked to the tone pot), or via the preamp and bypassing the tone pot.

    My question -as I have limited knowledge/experience- is this: does the battery circuit get cut when the switch is set to passive, or does the battery keep draining always, as long as the audio cable is plugged to the jack?
    Thank you!