Last time we did some (very rough) preliminary tests comparing an early version of the hex pickup with a Fender Stratocaster and a Gibson Les Paul. The initial test was done using a 2000 turns AWG 42 coil and the cheapo TL072 Op Amp in differential configuration. The goal basically was to see the frequency spectrum of lower impedance coils side by side with that of the single-coil Strat and Les Paul humbucker pickups. Here we will continue with the tests, this time focusing only on the Hex pickup installed on a Strat while continually improving on various aspects of the pickup design.
Note: This is the initial test and is continually being updated. See Let it breathe (part 3): Frequency Response for a latter installment.
What has changed from the previous test?
- Better electronics. We use lower noise Op Amps and better circuit design. Right now, we are using the low power LME49726. This Op Amp is quite capable and consumes very little current (0.7 mA). Soon, we will be trying out even better Op Amps.
- Lower impedance. This time, we tried coils with 1000 and 500 turns (150 and 75 ohms D.C. resistance). The gain is increased from 5 to 10 to compensate for the lower output of the 1000 and 500 turns coils.
- Better coil design. The bobbin is a bit taller at 8mm (before it was 5mm). The neodymium magnet remains the same at 5mm height thus making the bobbin and coil a bit taller than the magnet which is placed halfway from the top. With this setup, the magnet has lesser string pull.
Here’s the MP3 sample and the corresponding FFT graph:
With less string pull, the coil can be positioned closer to the strings giving it better focus (not to mention less cross-talk). A better focused coil gives the pickup a tighter response and fuller mid and bass without sacrificing the upper harmonics. Picking near the bridge, slaps, etc., bring out all the harmonics. The overall balance is noticeably better.
In this experiment, it turns out that the frequency graphs of the 500 and 1000 turns coils are more or less identical. It seems that going further low does not bring any more sonic improvement. It is very plausible that even with 2000 turns, we’ve already reached the optimum wide-band frequency response with a cutoff point well beyond 20kHz.
Next time, we’ll test 2000 turns again and probably go as high as we can —as high as the bobbin can accomodate (around 3000 turns). We’ll also get into actual frequency response measurements of the coils. Our goal would be to find the maximum number of coil windings with a cutoff frequency not lower than 20kHz. We want to max out the number of turns to optimize the output. By doing so we can lower the preamp’s gain which will then give us the best dynamic range and signal to noise ratio. It’s always a balance.