Pickups - Noise and Hum Characteristics

[randyc]

Some measured spectral characteristics, selected as a cross-section of typical guitar pickups:

Gibson L-5CES


Heritage H-575


Fender Stratocaster


Gibson ES-330TDC

[randyc]

Guild SF III



Note that the lowest noise level depicted in any of the above is about -90 dBV (decibels below one volt) but that the maximum levels vary. The scale changes from curve to curve depending on the maximum measured level of noise. In the first two curves, for example, the peak noise level is about -67 dBV while the third curve indicates a peak noise level of -48 dBV. In other words, the third set of data is 19 dB NOISIER than the first two sets of data at very low frequency offsets.

We aren't too concerned about the noise in that area, however. We care about audible noise. We can integrate the noise in each curve from about 30 Hz to about 2 kHz to obtain a more meaningful comparison. The noise characteristics start to "flatten" past the 2 kHz offset (the noise is similar for all pickups beyond 2 kHz).

Note that both the Guild and the Fender "appear" to have low "hum" characteristics, based on the appearance of their spectra. That is deceptive, however, because the high noise of these two pickups hides the "hum" contribution.

Test conditions for all guitars were: neck pickup selected, volume control maximum, tone control maximum (home environment). There are discrete A.C. line frequency components shown in the spectra (60 Hz and harmonics of 60 Hz). The following lists the order of "hum" resistance, worst to best:

ES-330TDC and Stratocaster (both single coil): worst
H-575: 3 dB better
L-5CES: 5 dB better
SF III: at least 5 dB better but high noise floor at 60 Hz

Then I integrated the noise of each guitar and rated them, worst to best:

Stratocaster: worst
SF III: 5 dB better
ES-330TDC, L-5CES and H-575: 9 dB better

The two best pickups were the ones on the L-5CES and on the H-575, from the combined standpoint of noise and 60 Hz pickup. Although the H-575 guitar was purchased for about 1/3 the price of the L-5CES, this doesn't directly relate to the cost of the pickups. Since the sonic qualities of guitar pickups are determined by opinion, there is no rating assigned.

The worst pickups for "hum" naturally were the guitars with single-coil pickups.

[randyc]

I should have mentioned this previously: the measurements were made with the spectrum analyzer tool in "Audacity" (SourceForge freeware).

I'm starting to evaluate this interesting (- free -) software package in detail although I downloaded several years ago. Reconciling the "virtual spectrum analyzer" results with real (and "real" expensive) spectrum analyzer results are interesting.

The utility of the "Audacity" spectrum analyzer is related to the sound card in your computer and other less important variables. Sampling rates are not too important - the normal CD rate of 44.1 kHz seems to work OK for the frequencies with which we are most interested. The maximum significant bit (MSB) of your analog-to-digital converter (ADC) selected by your sound card manufacturer) determines the resolution (and the noise floor) of the virtual spectrum analyzer capability.

I appreciate this free virtual instrument immensely. I have a Hewlett-Packard 8570A spectrum analyzer ($18,000, new - Ebay-ed for "peanuts" and personally repaired for "peanuts"). It's a wonderful instrument for RF/microwave applications (my specialty). I've constructed outboard converters that allow me to use the H-P analyzer for audio applications but I'm appreciating the "Audacity" freeware analyzer a LOT since the measurements are direct and much more convenient. Also more dynamic range for the ADC in my sound card!

I'm adding comments about the "Audacity" spectrum analyzer tool to the "Design of Vacuum Tube Amplifiers" book. The current - and always FREE - downloadable version doesn't include some of this. I've added other chapters and will soon upload the most recent version of the book.

cheers,
randyc