Thanks Pete for the permission to post this here!
In this Tech Tips Newsletter I would like to focus on the electrical property known as inductance and how it relates to the performance characteristics of passive magnetic pickups.
Summary: Understanding the inductance values of passive magnetic pickups provides insight into the expected performance and may be a more reliable metric than the more commonly found direct current (DC) resistance values. Inductance is measured in Henrys [H] and ranges from 1.8 to 2.5 H for typical strat style single coil pickups and from 3.2 to 4.6 H for PAF style humbucking pickups.
One of the most common and well understood electrical properties used to characterize passive magnetic pickups is DC resistance (DCR). Typical values of DCR for strat style single coil pickups range from 5.5 to 6.5 k-Ohms; humbucking pickups often have DCR values in the range of 6.5 to over 12 .0 k-Ohms. DCR values can be useful in comparing similar pickups which are wound with the same wire gauge. In general, for pickups of a specific design and made with the same wire gauge, as the DCR value increases one typically observes higher output (as measured in mill Volts [mV]) and a shift in the peak of the frequency response curve to lower values (reduced output in the treble frequencies). In comparing the performance of pickups of very different size, shape or geometry or with coils wound with differing wire gauge, the DCR values may be less helpful. Here is a specific example of this issue: consider two otherwise identical strat style single coil pickups, each with 8,500 turns on their respective bobbins, but one wound with 42 AWG wire and the other wound with 43 AWG wire. The DC resistances were found to be different: approximately 6.2 k-Ohms for the 42 AWG coil and 8.1 k-Ohms for the 43 AWG coil. The inductance values for these two pickups, however were very similar: 2.41 H for the 42 AWG coil and 2.28 H for the 43 AWG coil. The output voltages for these two pickups (as measured at the peaks of their respective frequency response curves) were: 102 mV for the coil wound with 42 gauge wire and 103 mV for the coil wound with 43 AWG wire. From this we can see that the inductance values were a more reliable indicator of output voltage than the DCR values for these two pickups.
Let’s take a look at what happens in a typical passive magnetic pickup in an electric guitar or bass. When the strings of the instrument (which contain iron or nickel) vibrate, it causes the magnetic field of the permanent magnets in the pickup to rise and fall or fluctuate at the same frequency as the string vibration. This changing magnetic field induces an alternating current (AC) electrical field in the coil windings of the pickup and a voltage difference is generated between the ground lead of the pickup and the output lead of the pickup. This time varying voltage is typically in the range of 50 to 300 milli Volts. This is the output signal from the pickup which travels through the volume and tone controls of the instrument, through the instrument cable and ultimately to the input of the amplifier.
Inductance is the property of the magnetic pickup which relates the changing magnetic field to the AC current in the coil of the pickup.
Here are some important concepts to keep in mind regarding inductance as it relates to magnetic pickups:
- Inductance is directly proportional to the square of the number of turns of coil windings in the pickup. If the coil windings are doubled, the inductance will increase by a factor of 4. Inductance is helpful in predicting the output of pickups as the output of a pickup increases with the number of turns in the coil.
- The inductance of a pickup can be changed by placing iron or nickel containing parts inside the coil or near the coil. The property of materials which affects inductance is called Permeability. Permeability for empty space is vanishingly small (10-7 H/meter); permeability for magnetic materials can be 100 to 100,000 larger. An iron or nickel object within the coil of a pickup will increase the inductance of the pickup. In many single coil pickup designs such as a typical strat style pickup, permanent magnets made of alloys of aluminum, iron and nickel (hence the common name AlNiCo) are located within the pickup coil. In P-90 style pickups adjustable pole pieces made of mild steel are located within the coil and bar magnets under the coil. P-90 style single coil pickups often have inductance values of from 4.9 to 6.8 H. Jaguar style pickups have AlNiCo rod magnets inside the coil and an external “Claw” polepiece outside the coil with inductance values from 2.5 to 3.2 H.
PAF style humbucking pickups can have inductance values ranging from 2.8 to 4.5 H. Blade style single coil and humbucking pickups can have quite large inductance readings (6.0 to over 9.0 H) depending on the size of the blade pole piece and the number of coil windings).
Inductance measurements of magnetic pickups require an inductance meter and are typically made at a specific frequency (1 kHz). I use an EXTECH LCR meter (Model 380193); measurements are made in series mode setting of the meter.
Here are the important points regarding inductance as it relates to passive magnetic pickups:
- When comparing the data available for pickups the inductance values may be a better indicator of performance than the DCR values especially for pickups having different design features.
- The inductance of a pickup is increased by adding more turns to the coil and by adding iron or nickel parts inside or near the coil windings; both of these changes will increase the output of a pickup and quite often shift the peak of the frequency response curve to a lower value.
I hope you find this information useful.
The Engineering Handbook, CRC Press, 1995 pp 1117-1119