Why are inductors hardly ever used in stompboxes?

[commathe]

I was having a ponder about this recently. Other than in wahs and in those guitar pickup simulator circuits I don't think I've ever seen an inductor. Why is this?

[R.G.]

They are
(1) big
(2) expensive
(3) not easy to standardize
(4) avoided like the plague by industry for reasons 1-3
(5) and therefore not available for DIY as leftovers.

[armdnrdy]

http://www.diystompboxes.com/smfforum/index.php?topic=101516.0

[commathe]

Awesome! Thanks guys. I guess I had never even looked into buying inductors so I hadn't realized the size issue.

[PRR]

Inductors are hardly ever used in *AUDIO*.

R.G.'s reasons 1-5 would be enough.

But also: an inductor big enough to affect bass will want to pick-up all the electromagnetic hum in the room.

I would point out that guitarists use *more* audio inductors than all other audio combined.

The guitar pick-up is (usually) a large inductor. We mount it very-close to the strings and very-far from power lines and power transformers, so that it "hears" much-much more string-hum than power-hum. Of course "much-much more" is sometimes hard to get.

We use a long cable (capacitor) from pickup to amp. This forms an L-C filter which perks-up the upper overtones of mellow strings yet cuts-off the higher inharmonic overtones of strings. This function is built-in on violin etc: the large slender bridge brings-out some overtones then damps higher overtones. On pure electric guitar we need to get the same thing some other way. Inducty pickup and capacity cable does the chore neat and simple.

There are more single-ended tube amplifiers (Champs, Epi Jrs) in guitar-land than anywhere else (even SET hi-fi fanatics). These rely on a big choke (overwound as a transformer) to give the one power tube something to work-against and get both-ways signal.

Ampeg among others used coils in amplifier tone-controls, in VT-40 and some SVT. Never been a lot of SVTs but it is going-on 40 years intermittent production now.

And of course the simplicity of an inductor in a one-pot variable peaking filter (wah-pedal) is hard to beat.

Other inductors "in audio":

We usually get DC power from AC circuits. Power transformers (50/60Hz and also switchers) are inductors.

Many-many electronic toys have "RF filters" on the power inlet. Computers throw radio trash into the house wiring. TV sets and audio gear may pick-up this trash, cross-modulate, and have crap in the picture or sound. Chokes are part of the answer. Since nearly all our toys today are "computers" (even my LED clock throws digital garbage), power-inlet filters are used by the billions.

Transistor audio power amplifiers often put a small inductor at the output to isolate or define the supersonic load impedance. The amp could do audio fine without it, but could be extra fussy about what it is connected to. This inductor is often 13 turns of scrap wire around a clay resistor, cheap-enough, and too small to matter below 20KHz.

Various forms of tone controls use inductors, especially in 1970s mixers and graphic EQs, especially for the upper bands. The falling cost of opamps made R-C fake-coils cheaper than real coils.

Jensen 990 super opamp uses emitter inductors to refine the open-loop gain/phase. The patent on this is long expired, but the idea has not been picked-up by others (except cloners of the 990), because of size and also because IC technology (which does not do inductors well) has generally met/beaten most parameters of the 990.

[commathe]

Awesome! I really appreciate this insight also. I was aware of the fact that transformers are ultimately two inductors coupled together, but didn't realize they were also used as chokes in lots of commercial consumer electronics.

[teemuk]

...And of course many of these high frequency thingies (radio tuners, switch-mode power supplies, class-D amps, etc.) use boatloads of inductors.

Those tiny inductors, however, would practically be just pieces of wire at audio frequencies.  

...And when you can simulate a high inductance, expensive and bulky inductor with four tiny, inexpensive and incredibly generic components why bother to use the real thing if not absolutely neccessary.

[Mark Hammer]

As PRR/Paul noted in passing, once upon a time, inductor-based graphic EQs were more commonplace, until the bulky inductors were replaced by transistor and op-amp based gyrator circuits.  Thirty years back I bought a dual 12-band graphic EQ from a guy, and still have most of the inductors in a parts drawer and the partially-stripped board.  A single KA2223 5-band EQ chip and all the passive components (minus the pots, of course) is smaller and cheaper, and more reliable than one of the inductors on that board.

[tca]

But don't forget inductors are among those "electric devices" that anyone can build, a electromagnet is something that every kid should build (and a compass, btw). Not much new technology there, just math and physics: Maxwell's equations.

...And when you can simulate a high inductance, expensive and bulky inductor with four tiny, inexpensive and incredibly generic components why bother to use the real thing if not absolutely necessary.

That and the fact that the simulated inductor does not replicate all the flaws of the real thing and this, in my IMO, is something not to forget when making a distortion/distortion-free device (e.g. of absolute necessity).

Of course there is also, but not only, some aesthetic reasoning associated with the use output transformers/inductors in audio amplifiers (tube and solid state).

[duck_arse]

I love winding. theremins like a bit of inductance here and there, too.

[Digital Larry]

At one job I had in the early 90's we had an audio power amp where the designer had put some ferrite beads right at the output to the speakers to help control RF emissions.  And the audio distorted like mad above a certain level, but it was clear that the voltage output of the amp itself was nowhere near clipping at this point.  Turns out (transformer pun) that at some level of magnetic field strength (proportional to current), ferrite material "saturates" which causes the inductance to drop.  At the currents we were driving, this was happening, and the shift in impedance at audio frequencies was enough to cause considerable distortion at the speakers.

[DiscoFreq]

Just FYI (not an answer to the question): at least one of these has a lot of inductors (4 or more?), I should add gut shots one day

King Vox Octavoice I Ampliphonic Clarinet
King Vox Octavoice II Ampliphonic Brass
King Vox Octavoice III Ampliphonic Woodwind

[Mark Hammer]

The old Boss TW-1 Touch-Wah is one of the only stompboxes I am aware of that used an inductor.  It is an interesting amalgam of a Mu-Tron-like envelope follower circuit, driving an actual inductor-based wah circuit, via an optoisolator.

There was a time when many graphic equalizers used inductors, rather than gyrators.  But for all the reasons RG and Paul/PRR noted, those days are long gone.