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Advertising Index Altronics..................... 13-14, 27-28 Blackmagic Design....................... 7 Dave Thompson........................ 111 DigiKey Electronics....................... 3 Emona Instruments.................. IBC Hare & Forbes............................. 11 Jaycar............................. IFC, 41-44 Jim Rowe Test Gear Sale......... 111 Keith Rippon Kit Assembly....... 111 LD Electronics........................... 111 LEDsales................................... 111 Melbourne Society of Model & Experimental Engineers..........OBC Microchip Technology.................. 9 Mouser Electronics....................... 4 PCBWay....................................... 65 PMD Way................................... 111 Product Showcase..................... 12 SC Pico W BackPack.................. 95 SC Programming Adaptor.......... 85 Silicon Chip Back Issues..... 81, 89 Silicon Chip Binders.................. 64 Silicon Chip GPS Clock........... 109 Silicon Chip PDFs on USB......... 70 Silicon Chip Shop.................... 107 Silicon Chip Subscriptions........ 15 The Loudspeaker Kit.com............ 8 Wagner Electronics................... 101 Notes and Errata 180-230V DC Motor Speed Controller, July-August 2024: in the parts list (July issue, pages 76 & 77), the Altronics part codes for T1 are correct but the transformer is a 12V + 12V type, not 15V + 15V. Also, element14 no longer sells the RURG3060 (D1). You can get the new version, RURG3060-F085, from DigiKey or Mouser. Next Issue: the October 2024 issue is due on sale in newsagents by Thursday, September 26th. Expect postal delivery of subscription copies in Australia between September 24th and October 14th. 112 Silicon Chip will present another Class-A amplifier again. For example, compare the performance of the Ultra-LD Mk.4 (August 2015, p37) to the 20W Class-A amplifier (May 2007, p36). You will see that the Ultra-LD’s distortion level is lower at 100W than the Class-A amplifier at 20W! Calculating guitar pickup inductance I am wondering if you can help me find out how to calculate the inductance of electric guitar pickups. I have been looking online, but most of the information is written by guitar people and is more like alchemy than electronics. What I do know is that the inductance of a guitar pickup is related to the shape of the windings. A pickup can have thousands of turns of wire, usually 42-43 AWG. The resistance usually ranges from about 3kW up to as high as 12kW. In a nutshell, this usually translates into ‘hotness’ or the amount of output. The reactance is affected by the shape of the coil; short and fat coils sound different to tall and thin ones. This translates to the ‘tone’ of the pickup, or the frequency response. If I know how many turns are in the coil and the resistance, and I can measure the height, width and length of the coil, can I calculate the inductance and determine what does it do to the frequency response or ‘tone’? Also, does the actual distance from the pickup to the strings makes a difference? What effect do things like the bobbin material, presence of a metal cover over the pickup or pickups with two coils with opposite windings (called a humbucker) have? How do I measure the shape of the magnetic field of the pickup? What will paraffin or beeswax potting of the pickup do to any of these measurements? The only information I can find on this is usually very light on detail and explanation. The only other stuff I can find is so technical that I need a Nobel Prize to decipher it. What I need is a good source that aims at us mere mortals with a little electronics education at about intermediate level. I also want some equations or a method to calculate this stuff. (A. P., Wodonga, Vic) ● We put this to Brandon Speedie, as he has an article on modifying Australia's electronics magazine electric guitars in this issue, and he responded as follows: Calculating the inductance of a guitar pickup using geometric methods is difficult to do accurately. The common inductance equation L = N2 ÷ R (and its derivatives) will get you in the ballpark, but it is difficult to know permeability with any certainty, and (as you say) shapes and sizes differ widely between types. A better way is to simply measure with an LCR meter. Silicon Chip has published many L meters in the past, but I’m unsure if any would read up to 10H. Commercial LCR meters that can do many 100s of henries are available for less than $200. Given that most guitarists don’t have an LCR meter, despite its pitfalls, DC resistance has become a popular way to compare pickups. It is broadly true that a pickup with more turns of (the same gauge) wire will have higher resistance (and inductance), and thus will be ‘hotter’, ie, have a higher output level. This is most relevant when fitting a guitar with multiple pickups. A common mistake I see is to load a guitar with a ‘hot’ bridge pickup, reasoning that it will be more suitable for solos, while the neck pickup will be used for rhythm work. But when both pickups are active, the bridge pickup will swamp the neck, providing less versatility from the instrument. Therefore, the DC resistance of the pickups is usually kept broadly similar across the instrument. Having said that, inductance is much more important to overall ‘tone’ than DC resistance. The pickup itself acts as an LC circuit, where the pickup inductance and parasitic capacitance form a resonant circuit. The peak frequency, and its Q, are the largest drivers of ‘tone’. To give some examples, a Stratocaster single-coil might measure 2.2H and have around 600pF of capacitance, which gives a peak a bit above 4kHz. By comparison, a Les Paul humbucker might measure 6.6H, which gives a peak closer to 3kHz with the same parasitic capacitance. You might be interested to read my article in this issue (starting on page 86) on replacing the onboard electronics on many common electric guitars and basses. There is some more information in that article on pickup types and their operation, which you may find interesting. SC siliconchip.com.au