Preamp Info and FAQ

• For the Moonshot preamp with a battery inside the control cavity, the 9V battery must fit flat in the bottom of the cavity. For most batteries, this means the routed slot needs to be about 1.02" wide. If you have a separate battery box then there should be no issues since the preamp requires less than 5/8" of width.

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!

A filter is a means of modifying a stream of something and turning it into a different stream. Streams can be mechanical, electrical, or even mathematical, and include audio or visual signals and digital media data. An obvious example is a coffee filter. You pour a mixture of coffee and water (the stream) into a filter, and it is separated into large wet particles on one side, and small particles with lots of water on the other side. How does that relate to sound? If we think of fine coffee particles as high frequency and coarse particles as low frequency, then the liquid below the filter has the high frequency coffee, and the sludge above the filter has the low frequency coffee. In this analogy, if we keep the sludge, we call it a “low pass” filter, and if we keep the clear coffee, we call it a “high pass” filter, even though there is really only one filter. Think of the filter’s “pass type” as which part of the coffee we put in a cup and pass to our friend. With this analogy, an electronic filter circuit includes the filter cone, us, and the cup. If we want a midrange boost or cut for our coffee, we can pour the stream through a stack of two filters: A coarse low frequency filter (a sieve) above a fine high frequency filter (a cone filter). Now, if we keep the liquid between the two filters, it is “midrange boost” coffee, containing mostly medium sized particles. If we throw that away and mix the sludge on top and the clear coffee on the bottom, we have scooped “midrange cut” coffee. Preamp tone controls are like a stack of coffee filters of various mesh sizes, with taps between the filters to siphon off the liquids and recombine them into the perfect cup of coffee! While all of these tone controls are actually filters, it is common to call them “bass boosts” or “sweepable mid controls” or other names that identify their type and how they are tuned. A standard passive pickup and tone control is also a filter. The circuit behaves as a low pass filter, where we keep the thick stuff on top. The tone knob changes how fine the filter is, so when it’s up full, it keeps most of the coffee, from coarse to very fine. As it is rolled down, it lets the fine coffee through, but since we are keeping the coarse side, we call it a low pass filter. Additionally, when the tone control is most of the way up the circuit “resonates”, making the highest frequencies somewhat louder than the rest. We could get the same result with coffee by adding more of the finest coffee we are keeping back into the cup. https://static.wixstatic.com/media/296ae1_b5dc04d786884b98a386862880bafc86~mv2.jpg Sometime in the 1970s, engineers were looking at electric guitars and realized that an active low pass filter circuit could be used to create frequency profiles similar to a passive guitar, but the parameters could be adjusted on the fly. So one would be able to effectively change pickup windings, volume pots, tone pots, and tone capacitors simply by turning some knobs. This became known as a “filter preamp”, though we now know that all preamps use filters. For convenience, Underhill and other companies have settled on the moniker “Filter Preamp” for these resonant low pass circuits, and “EQ”, “Boost/Cut”, “Standard”, etc. for other types. https://static.wixstatic.com/media/296ae1_b4bd3d09b6284e0d814e6c431134e51a~mv2.jpg The next development was to put a separate low pass filter on each pickup of a two-pickup guitar, and combine them with an active mixing circuit. Active mixing (blending) prevents the two channels from interacting, and retains the character (individual tone adjustments) of each pickup. With passive blending, the two pickups are connected directly and become a single new circuit that behaves differently from what we expect to hear when combining the two pickups (which you know if you play a ‘61 Jazz or a Les Paul!). With the active arrangement, we have a kind of two band EQ, with one channel coming from the neck pickup and the other from the bridge. Since the filtering happens before the signals are combined and go out the single cable, this type of preamp can only be done on-board the guitar, or with a separate cable and outboard preamp for each pickup. These “dual filter” preamps were made famous by Alembic and Wal in their high-end basses, and offer a truly unique tone. Underhill preamps use the latest developments in low power and low noise components, and a “state variable” filter topology that has historically been restricted to outboard preamps running on line voltage. You won’t find a more precise, controllable, or quieter preamp anywhere!

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.

Contrary to popular belief, a preamp won’t die mid-gig if you use alkaline batteries (lithium batteries work longer, with more headroom as they wear out, but they can die a sudden death!) As the battery runs down, at some point you will start to hear clipping distortion, so just lower the boosts a little and you will be fine for a few more days. Turning the onboard volume down will not reduce the distortion, but turning filter or EQ boost down will. The best way to check battery level is by cranking up all the boosts and playing very hard before you go out. If you hear distortion, then think about changing the battery. Every bass has its own headroom requirements depending on pickups, pickup height, playing style, etc. so some basses will work fine with a battery that is down to 6V, and others will need it changed below 8V or more. The “play test” is the surefire way to know where you are at. Battery test meters and such can’t measure actual headroom, so they only help if you already know the voltage where the bass starts to distort.

We work closely with Turner Pickups, and have been developing and testing our products with theirs since the beginning. They work together like Brooks & Dunn, Loggins and Messina, Hall & Oates, The Captain and Ten... (okay, I better stop there.) For multicoil Turner SEP pickups, use the B4 or B5 buffers. For Turner SPPP pickups, use the BP buffers. Get in touch with them if you have other pickup questions. All other brands of multicoils we know of will work, but pickups with very low output are discouraged because of the noise increase when high buffer gains are needed with a multi-channel buffer. https://static.wixstatic.com/media/296ae1_dfb64447de6d4aa6b0b363733fa7eae4~mv2.png