resonant hump without high frequency dropoff?

[madzub]

it is well established that much of the character of a pickup comes from the inductive-capacitive resonant hump in the upper midrange, above which the high frequencies roll off at 6db/octave, iirc. alternatively, i've been getting fascinated again recently with low Z pickups and the high end extension they bring, albeit at the cost of having no resonant hump (or having that hump be out of hearing range), which gives them the common attribution of lifelessness or blandness. My question is:

Is it possible to have that resonant hump but without the 6db/octave roll-off above it? In other words, can you have a pickup that has the full bandwidth of a low impedance pickup that also has the resonant hump to give it character?

i suppose one way to achieve this would be to add an active parametric eq on a low impedance pickup. Another way might be with 2 pickups together, resistively or actively isolated from one another, one being high Z and the other low Z. any other ideas? is this something that is logically impossible in a single passive pickup? even if it could be done, would it possess the attributes of both, or would they somehow contradict and just not sound right?



Another question, related but should probably have its own topic: low Z pickups are usually made with a thicker wire, typically anywhere from 40 down to 26 awg. But from what i'm reading it would seem there's no need for the larger gauge of wire, it's only the number of turns that matter really. So would a small coil of 2000 or fewer turns of 44 awg be just as effective as a Low Z pickup compared to the same number of turns of 34 awg? obviously the coil size will be different, resistance different, and those factors would have some effect on the specific tone of the pickup, but would it have any actual drawbacks in performance compared to the thicker wire? would the bandwidth be compromised? the dynamic range?



Thanks everybody, i love spending way too much of my precious time scouring through the forum pretending to be learning important things

[Mike Sulzer]

If you have a low or medium impedance pickup you can make it resonate at a frequency which is normal for a high impedance pickup by adding a capacitor across the pickup. Then changing the length of the cable has little effect since the total capacitance, cable plus what you added plus a very small amount from the pickup, does not change much percentagewise. If you want to stop the roll off, you can put a resistor in series with the capacitor that you added. This will also have the effect of broadening the resonant a bit. The roll off from the cable capacitance would still cut in at a much higher frequency.

In general, using too small a wire in a low or medium impedance pickup will broaden the response of any resonance that you make with an added capacitor, but this is not too critical. I find that you can use #42 wire in a 1,000 turn pickup and it works just fine. Too small a wire can also cause a loss at all frequencies, depending on the load the pickup operates into. (Obviously there is no need to "match" load and source; you can run a low impedance pickup into a high impedance load.)

[bbsailor]

Madzup, and Mike

Winding a guitar pickup is a matter of compromise. These compromises relate to (1) desired output, (2) output impedance to match destination input impedance, typically 5 to 10 times higher than the pickup impedance, (3) physical space to put the wire coil in a strong enough magnetic field to get a reasonable additional output from the outer coil winds which are typically near half of the inner windings, (4) noise pick up by high impedance coils, (5) impact of coil capacitance plus coax capacitance and any added capacitors to form a resonant response and most importantly (6) a sound to please you ears, playing style and music type.

When you use a fixed impedance load on a pickup, such as a volume control plus the amplifier input impedance and coax load, it has the effect of damping or shifting the resonant peak (coil Q) of a guitar pickup which has a resonant peak typically in the range of 2 to 3 times higher than the coil inductance value as calculated by XL=2piXL. 500K pots in parallel with 1M amp impedance lowers the load to about 400K ohms plus the effect of cable capacitance.

When you wind a pickup with one tenth the number of turns, like the Les Paul Recording pickup, your impedance drops by the square of the turns difference or in this case 100. This puts the 15,000 Ohm impedance pickup into the microphone range of 150 ohms and now good for running directly into low Z mic mixer inputs that are typically 1500 to 2000 ohms to act as a bridging impedance for 150 to 300 ohm microphones. In this case, the cable capacitance has minimal impact on shifting the resonance and if a mic matching transformer is used directly at the amp input the transformer primary input impedance will be about 3K ohms when the secondary is loaded by 1M ohm. When feeding transformers you want the total resistance of the pickup coil plus the total resistance of the connecting cable to be less then one tenth ideally or down to one fifth of the input impedance to have minimal effects on the low frequency response. The Jack Casady low impedance bass pickup uses AWG 28 wire and has an impedance matching switch of 50, 250 and 500 ohm impedance selector to alter the input matching to obtain a variety of output levels and bandwidth limiting/peaking to act as an equalizer. The Les Paul Recording guitar uses a volume pot of 2.5K ohms which indicates that the pickup is in the 250 ohm range to feed a microphone matching transformer (Shure A95U 150 to 300 ohm input) remotely located at the amplifier input.


Madzub, I have used an EMG BQC Control (designed for bass guitars) on my neck-through-body Shecter Research Guitar (S-1 Elite) that has a toggle pickup selector switch, stacked concentric 25K volume controls for two active EMG pickups, the Treble/Bass controls are on another concentric knob set as well as a midrange frequency shift and boost or cut on the third concentric knob set. If you want to simulate various passive pickup resonant frequencies above 1Khz, you will need to alter some passive component values on the circuit board as it is factory set at a maximum high range of 1Khz. The bandwidth of resonance responses is usually calculated at .707 (-3db) times the resonant peak voltage on each side of resonance. It is interesting to note that most high Z guitar pickups resonate in the upper harmonic range of guitar pickups as the highest primary frequency is the high E string at the 24th fret or 1312 Hz. If you want to use inexpensive off-the-shelf transformers, read up about transformers, AL inductance value of the laminated core and number of turns on the primary and secondary to match a variety of input impedances and output loads being driven.

Here is a good web link to explain microphones and input circuits. This applies to passive low impedance guitars using matching transformers as well. http://myplace.frontier.com/~dwickstrom/well_con.htm

Joseph Rogowski