Pickup Self Resonance Measurements

[bbsailor]

I understand that the self resonance of a pickup reveals something about the sonic character of a particular pickup. However, unless the pickup maker reveals how that measurement is taken, it could be meaningless.

I just received a Lace Alumitone, single coil size (to fit a Strat-like guitar) pickup and read the specifications on the Lace web site as follows; resistance: 2.5K; Resonant peak: 2343Hz; Inductance: 16.74H.

I put this pickup on my signal generator through a 1M resistor to isolate it from the signal generator output impedance, set my scope probe on the 10:1 setting for a lower capacitance loading and guess what... it's self resonance measured out at 9.5KHz. One decimal place is sufficient for this type of measurement due to the variables involved in the measurement process.

Is there a standard way that pickups resonances are measured such as alone; with the volume pot connected, a 10ft, 300 pf coax cable connected or some other standard loading?

If not, then all the self resonance measurements, while possibly being useful for comparing pickups within one brand, may be useless for comparing pickups between brands.

The Lace publication of a resonance being exactly 2343Hz creates a sense of accuracy that may not be practically achievable, duplicatable or verifible without a whole lot more information about how that measurement was achieved.

Does anyone have any insight into this matter?

Thanks

Joseph Rogowski

[Old Tele man]

...maybe (?) their spec is "in-circuit" with VOL, TONE and guitar chord attached?

[salvarsan]

Loading pickups for testing

I understand that the self resonance of a pickup reveals something about the sonic character of a particular pickup. However, unless the pickup maker reveals how that measurement is taken, it could be meaningless.
...
Does anyone have any insight into this matter?

No, but I'll give an opinion anyway.

The guitar electronics, cable, and amp front end all load the pickup.

I think you can simulate the entire load well enough for useful measurements.

Here is a quick and dirty page on dummy parallel loads for testing pickups.
Feel free to take issue with any assumptions there.

http://www.salvarsan.org/pickups/dummy.html

Here is a first approximation of a single coil dummy load:



-drh

[bbsailor]

It is even worse when you convert...

No, but I'll give an opinion anyway.

The guitar electronics, cable, and amp front end all load the pickup.

I think you can simulate the entire load well enough for useful measurements.

Here is a quick and dirty page on dummy parallel loads for testing pickups.
Feel free to take issue with any assumptions there.

http://www.salvarsan.org/pickups/dummy.html

Here is a first approximation of a single coil dummy load:



-drh

drh

I am not taking issue but I am agreeing in spades. However it is worse when you convert the capacitance in your example to the added reactance loading.

.02uf at 4KHz = 1.98K ohms
.005pf at 4KHz = 79.5K ohms

Use this web site to calculate capacitor reactance at any
chosen frequency. http://www.electronics2000.co.uk/calc/calcreac.htm

Now, put the IM ohm amp load in parallel with the 250K pot and the reactance of 1.98K cap + 250K (251.98K) tone pot and the reactance of the .005 cable equivalent capacitance and you end up with about 50K ohm total load on the pickup at 4KHz. At a lower resonant frequency the total effective load would have a slightly higher resistance but still alter the sonic quality of most passive pickups.

This is why putting a high Z input buffer FET close to the pickups eliminates this cumulative loading and gives players a few more tonal options. You can always switch in a 100K or 50K ohm resistor to simulate loading to take the edge off the sound. However, with passive pickups, if you want some edge back, you can't dial it back in at the amp without sounding noisy or harsh.

When you think of all this, its is amazing how the guitar pickup has traditionally sounded pretty much the same since it's invention. There are not many harmonics beyond the pickup's resonant frequency although the loading does flatten the resonant peak response somewhat and extend the higher frequency response a little.

The cumulative loading reflected by the diagrams on drh's web link suggests the sonic impact of using different types of magnets or bobbin cores. Pickups with ceramic magnets have a lower inductance and react less to the external loads placed on them compared to Alnico magnets or metal slugs through the bobbin that increases the coil inductance and make them react more to external loading.

I'm sure there are many more design considerations that affect the sonic quality of pickups?

Joseph Rogowski