Underhill preamps are built from about twenty electronics modules that can be combined and arranged in many different ways. There are literally thousands of preamps that could possibly be built, but perhaps fifty that actually make sense! The preamp systems available in the web store have been tested thoroughly and enjoyed by customers, and are free from complicated interactions and other problems. But feel free to talk to us about your crazy idea for a fourteen knob setup; you can always reuse the modules for regular preamps if it doesn't work out!

Yes, every knob is a separate module that can be moved to different locations. Preamps are shipped with a set of cables that should work for most patterns. For unusual instruments like hollow-bodies with f-holes, cables can be custom made to any length.

We install each preamp in a variety of instruments to check the standard cable lengths. In most cases the preamps will fit in a variety of knob layouts like in-line, diamond, standard 5 hole with vol, blend, and 3 tone controls, etc. A few things to watch out for, that you may want to discuss with us: All of the boards will fit in a 7/8” (22.2mm) deep cavity. If your cavity is shallower than that, check the module dimensions for all of the modules in the preamp you are looking at. If you have an inline knob layout and the last knob (nearest the jack) is close to the end of the cavity, check the dimensions of the module. Typically the last position will be a filter or EQ module, which need at least 7/8" of space from the hole center to the cavity wall. If there are any questions, feel free to send us some photos, front and back with the cover off, and we can take a look for potential problems.

There is no simple way to use an active/passive switch with a dual filter preamp because the active filtering of each pickup has to happen before the blend and volume. The blend and volume pots are optimized for precise active mixing and low noise, and won't work with a passive signal. To add a fully functional active/passive switch, any passive controls like blend, volume and tone will need to be duplicated with passive-value pots (lots of knobs!) You can use a passive blend, pickup switching, tone and volume controls, etc. in front of a single channel (one pickup) preamp, but you will lose the advantages of active blending or dual filters. You can also wire a simple direct-to-jack switch in front of the preamp for an emergency option with no controls.

Set the gain trimmers on the buffers as low as possible for the quietest operation and longest battery life. Start with the gain trimmers all the way down (counter-clockwise). At that setting, the volume of the bass is the same as the pickups running passively. If you have two pickups, use one trimmer to bring up the volume of the weaker pickup for a more balanced feel. Only turn both trimmers up if the overall output of the bass is too low (i.e. lower than a typical P or J), or if you need a high output for tube distortion, etc. and don't mind a little more noise.

Solder your bridge (and string) ground wire directly to the sleeve terminal on the jack. Not only is it the best place for good grounding, it will forever be independent of whatever preamp you install.