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Class #1: Notes on Inductors

Here is a link to inductors that might help. Don’t fret over the formulas, read the first paragraph and see the pictures.

An inductor will store energy in a magnetic field and later release it as a current. It actually wants to keep the current going through it continuously. Therefore once the field is built up if the current is interrupted the voltage will rise very high trying to restore the current. This will cause a spark of much higher voltage than the applied voltage. If you put a 1.5 volt battery across an inductor and then release the battery while holding the wires you will get quite a shock. Anyone using a traditional ohm meter has likely experienced this when measuring the resistance of an inductor. This same principle is what allows the 12 volt battery in your car to make a good hot spark in your engine. It is interesting to note that the spark-plug fires when the points disconnect the battery, not when it is connected as the collapsing field is much faster (stronger) than the building field.

Using Iron or some other magnetic material will enhance the effect many times. That factor of enhancement is called permeability. The permeability of air is one. The permeability of transformer iron is many thousand, there for it increases the inductance of a coil of wire by that amount. See the explanation and table of materials in this article.

Of the examples I gave last night the most applicable to audio is the speaker crossover where there is an inductor in series with the woofer. In this application the inductor’s reactance (think resistance) increases with frequency thus reducing the current going to the woofer as we go up in frequency. Not all speakers have this inductor and let the woofer roll off through cone breakup, though this is not the best way to make a speaker. Better speakers roll the woofer off before cone breakup and hand the signal off to the tweeter.

Good speaker inductors, also called chokes, use a lot of copper and are thus expensive. The best ones are air core which uses even more copper. One can calculate the inductance needed for a woofer by knowing the impedance of the woofer and the cut-off frequency desired. This is usually 8 ohms and 1 KHz. In that case the inductor can be found L=8 ohms/6.3 x 1000 hz. (6.3 is 2 pi) or one can use a online calculator.

Using those numbers an inductor of 1.27 millihenries will roll off the woofer at 6 dB/octave starting at 1000 Hz. If the desired crossover point is 2000 Hz the inductor would be half as large, 0.64 mHy. Here is a 1 mHy air core inductor typical for a speaker. It also has 0.48 ohms resistance which will hurt the damping of the speaker somewhat. Thicker copper wire is needed to reduce that.–255-250. Here is the same inductor with heavier wire but still 0.3 ohms.–255-422

Please ask if you have questions or suggestions.

RIAA Preamps

Phono cartridges need both gain and equalization to make their output suitable for an auxiliary input or direct into a power amp.  A typical moving magnet cartridge has an output of 2 to 5 millivolts. Typical Moving Magnet (MM) gain at 1 KHz is 40 dB. The RIAA recording curve adds about 20 dB of boost at low frequencies and 20 dB cut at high frequencies to compensate for the RIAA recording curve and the fact that a magnetic cartridge is a velocity transducer. When displacement transducer is used (strain gauge cartridge) the required EQ is only 12 dB starting at 500 Hz and ending at 2000 Hz for a decrease of 12 db in the high end. This was done to reduce surface noise on records by boosting the high frequencies in the record cutting process so that they could be reduced by the same amount on the playback end.

When Moving Coil (MC) cartridges hit the market in the last 1970s their superiority was quickly recognized. However they required 20 dB more gain in the phono stage to bring them up to standard level as their output is 0.2 to 0.5 millivolts (20 db lower than MM). Early adopters of MC cartridges had two options. Buy an external transformer or a pre-preamp (head amp). Of course there was much discussion about the merits of these two very different ways of solving the low output problem. Both had typical gain of 20 dB and the better ones gave you choice over both gain and cartridge loading which affects the sound of the cartridge. When not loaded many MC cartridges sound bright and edgy because they are undamped. Most cartridge manufacturers give an ideal load resistance (typically 3-10 times the coil resistance is a good place to start if none is given). This load electrically damps the cartridge mechanical system for best response. If a cartridge has too much loading (low resistance) is will start to sound dull and lifeless. There were many high quality pre-preamps made up to the point of the popularity of the Compact DIsc. With the resurgence of vinyl, pre-preamps and complete MM/MC phono stages are coming back. A complete phono preamp will have sufficient gain (60+ dB) and RIAA EQ for MC cartridges along with loading. However it is an excellent, and sometimes better solution, to split the phono stage into a separate head amp and RIAA stage to isolate those functions as the very small, delicate output from a MC cartridge is best amplified in two stages.

The first product from Music Reference ( was the RM-4 pre-preamp which was designed to sit in front of a standard phono preamp and raise the gain of an MC cartridge to suitable level to go into a standard phono stage such as the RM-5, Modulus, CJ, ARC or other standard preamp at the time. It is still a fine solution to implement MC phono systems. What makes the RM-4 special is that is has low noise comparable with solid state units (which is not easy), cartridge loading and gain selection of infinite range through plug-in modules. The gain can be from 0 db to 30 dB. Along with a 40 dB standard phono preamp this combination will give a total of 70 dB gain, more than most MC phono stages and in some cases can go directly to a power amp via a passive preamp (simple volume control).

Although I prefer the active (pre-preamp) solution transformers are also available. Some transformers allow independent choice of gain (step up ratio) and loading, however many do not. In that case the load presented to the cartridge is determined by the chosen step up ratio and may not be the ideal load for that cartridge. With a good pre-preamp one has independent control over gain and loading. While some pre-preamps add noise, a well designed one will have noise below the surface noise of a good record. Transformers can pick up hum and often have problems at the frequency extremes where a good pre-preamp can have bandwidth from a few Hz to 50 KHz. Needless to say, Be aware that some complete phono stages use a transformer to amplify the MC input rather than to do that function electronically. I prefer the all electronic method as long as the design is low noise.