NOTE: This is now in the FAQ section....
This is a rough draft of a new FAQ to describe what a "filter" is. Let me know what you think!
What the Heck is a Filter?
A filter is nothing more than a means of modifying a stream of something, be it mechanical or electrical or even mathematical, into a different stream. 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 the fine particles as high frequency and the coarse particles as low frequency, then the liquid below the filter is the “high frequency” coffee, and the sludge above the filter is 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 two filters: A coarse low frequency filter (a sieve), and a fine high frequency filter (a cone filter). Now, if we keep the liquid between the two filters, it is a “midrange boost”, consisting of all the medium sized particles. If we throw that away and keep the sludge on top and the clear coffee on the bottom, we have a scooped “midrange cut”.
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. 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 putting some of it through a very fine filter, and adding some of that clear brew back into the cup.
Sometime in the 1970s, engineers were looking at electric guitars and realized that an active low pass filter circuit could be used to create the same frequency profiles as 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 low pass circuits, and “eq”, “boost/cut”, “standard”, etc. for other types.
The next development was to put a 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 hear with the individual 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 has to happen 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!
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