Bass Bench: Passive Interplay

As we saw in my previous column [“Exploring Passive Tone Controls,” December 2015], there are a lot of things one can do with passive circuits. It’s possible to create passive circuits to alter all portions of the frequency spectrum, but because their elements interact, these circuits often behave very specifically and rarely have the independent nature of an active EQ system. It turns out most passive circuits will only work as intended for a defined purpose.

Many players reject active electronics and favor passive circuits, calling them “pure” and tonally “natural.” And like a drummer’s snare, our basses are the most personal and emotional part of our rigs, so it seems reasonable to opt for simplicity. But in the end, you have to look at the complete signal chain, and even with a passive instrument, there are a lot of active elements that appear after the output jack. Insisting you want to keep your instrument simple and don’t like to deal with batteries is an argument that’s hard to refute, but—as always—this choice comes at a price. And an active element in an early stage of the signal chain can spare you from a lot of other “passive interactions.”

Let’s assume we have the common scenario of passive pickups running into passive volume and tone controls. It’s a simple and familiar wiring, but once you nose into the details, things can quickly turn less “natural.” Say you’re one of those bassists who often grabs the volume knob to adjust it for different parts of a song: When rotating the pot counterclockwise to reduce your volume, you’ll probably notice a corresponding loss of higher frequencies.

Is this acceptable? Well, that depends. Usually you turn down the volume because you want to get quieter. If that’s the case, then losing some timbral edge works to your advantage. But what if the whole band gets quieter? In that instance, you might want or need a more uniform tone between 1 and 10. For electric bassists, it’s a simple demand that should have a simple solution.

Our guitar friends are extremely sensitive about how their upper end behaves, so when playing through a fuzz or overdriven tube amp, they often use a treble bleed circuit (Fig. 1) to compensate for this loss of highs. Remember that in a passive circuit, everything influences everything and our system of pickups, pots, and cables forms a low-pass filter—the lower frequencies pass and the higher ones get cut off at a certain point. If either resistance or capacitance goes up, the cut-off frequency goes down. So rolling down the pot, and thus raising its resistance, causes the loss of treble.

To avoid this, the classic solution is to add a parallel capacitance right between the pot’s input from the pickup and its middle lug (as shown in our diagram) to form the opposing element of the low-pass filter. This high-pass filter compensates for the raised resistance, but as with everything passive, there is now a side effect: There isn’t an absolutely uniform tone over the full volume range, and this addition often causes too much treble at full volume.

To better assimilate the circuit to our tender ears, we throw in another parallel resistance to compensate for the amount of treble in our compensational circuit. Again, a bassist’s treble range isn’t as critical and sensitive as an overdriven guitar, but it’s worth noting that this parallel resistance affects the taper of the volume pot. In short: Having a more uniform tone while rolling down the volume results in a less uniform volume roll-off. Also, as the high-pass moniker implies, it cuts the low end, and if you don’t carefully choose the electrical values of these extra parts, it can cut a lot of lows.

The possibilities don’t end here, as there are also treble bleed wirings that use the cap and resistor in series, and another one that includes dual pots to add a variable resistance while turning down the volume. As bassists, we can usually ignore the complex versions, yet the message is clear: The more we tinker with the details, the more side effects we have to deal with.

This is nothing you’ll have to worry about in a humming, busy, loud stage environment, but remember, we only asked for a simple thing—a uniform tone at different volumes—and we’re already dealing with the side effects. Granted, we’re really digging into details. But even if their implications are actually more theoretical than not, it’s easy to imagine how broader passive tone controls might create other interdependencies and limitations ... something to explore in the future.