Where are all the inductors?

[MoltenVoltage]

I'm curious as to why inductors are rarely used in stompbox designs when they are so common in other audio designs.

[wavley]

I'm gonna guess because they are relatively expensive compared to other passives and the trickiness of resonances and whatnot.  Personally, I just want to build the best thing that I can.  It's funny because for the last two days I've been searching this very same subject to use as power supply filtering and low pass/band pass filtering.  Not to say that money is not an option, but I'm willing to pay a few more bucks solve problems/build something awesome.

[R.G.]

Wavley is correct.

Inductors at low impedances and low frequencies require big inductances. That requires lots of wire and ferromagnetic cores. That means lots of resistance, self capacitance, magnetic field leakage, magnetic field pickup (including hum), large tolerances, expense for good core materials, bobbins, insulation, weight, fragility (especially ceramic cores), non-standardization, you name what might be a bad characteristic of an electronic component, inductors have it.

Over the last century, it has become the hallmark of the skilled EE who considers all sides of a problem to design out inductors. Because of their limitations, inductors is what you do when you can't (or can't figure out how to) do anything else. A design without an inductor as compared to with an inductor, unless tiny things for RF or high-current things for power supply chokes, will be smaller, lighter, cheaper, more consistent, lower inventory cost, easy to stock and procure, have lower warranty costs, you name it.

The only real uses for inductors in pedals is either historical (Vox wah circuit) or very modern (switching power supply filters). Probably the majority of the rest are used in graphic EQs, and everthing else is very small indeed.

The success of the EE practice in getting rid of inductors has been so thorough that many EEs don't even remember the reasons they're not used. They just don't use them.

Of course, I may have missed a point or two.

[PRR]

> Inductors at low impedances and low frequencies require big inductances.

Inductors at HIGH impedances and low frequencies need bigger inductance. The physical size is largely independent of impedance from 1 ohm to 100K or so.

> they are so common in other audio designs.

Common?

Solid-state preamps "never" have inductors.

Solid-state power amps have one cheap inductor to isolate the load above the audio band. (When cost is crucial, this can be avoided.)

Wall-power supplies usually use a transformer which is an inductor. 50/60Hz is a low frequency and means a large hunk of iron. A 50,000Hz inductor is ~~1000 times smaller than a 50Hz inductor so old-fashioned line-frequency PTs are going out of style.

Power lines are full of crap from other appliances. A small inductor is often used to soothe the incoming AC.

When filtering DC you can use inductors or capacitors or both. Older DC power supplies use inductors. Cost of electrolytic caps has come down SO much that "nobody" uses a DC power choke any more. (Exception: 1950s plans which get re-built verbatim.)

A happy moving-coil speaker is about 10 ohms resistance. A pretty beefy tube is about 1,000 ohms resistance. Efficiency would suck if we did not use a transformer to gear-down the impedance. Transformer is an inductor. Audio extends to low frequencies which means big iron and big cost.

Back in the 1920s, inductors were used to separate DC from audio in a tube's plate circuit. By 1931 this was old-fashioned, resistor-coupled gives better results and by then tubes were cheaper than chokes. The idea has come back again in the hi-fi-fad world, but everything different gets re-invented and gushed over.

Odd corners:

Tape decks often have top-octave coils to peak the record and notch the bias tone. At 15KHz a coil is reasonable.

IIRC, the Dynaco transistor preamp used a C-L-C scratch filter.

Bandpass equalizers are simpler with coils. Into the 1980s, the coils for the higher audio frequencies were cheaper than the transistor and caps to make a fake coil, but already the lower frequencies were cheaper without an actual coil. Today you see coil EQ only in restoration work. (Note that the oldest "graphic" EQ, the Blonder Tongue, uses no filter coils.)

The Jensen 990 opamp uses inductors as part of a patented compensation scheme.

Audio circuits in the same box with digital systems often like a few small coils to block digital hash.

Radio systems, if they even still have coils, are not audio systems until they demodulate. (There's radios now which feed directly to an ADC and work it out on computer.)

[Taylor]

The Bartolini onboard instrument preamps use inductors for the mid range EQ. Seems weird when an opamp gyrator could be used instead.   

[PRR]

> onboard instrument preamps use inductors ... an opamp gyrator could be used instead.

Hmmm. An active inductance eats power, which is an issue in this situation.

[Taylor]

Ah, very true, although programmable opamps - even in quad packages exist...

Does a gyrator really act just like an inductor to ground? Can a gyrator only be used to replace an inductor to ground or voltage reference, but not in series?

[PRR]

> Does a gyrator really act just like an inductor to ground?

Near enough for audio uses. It is always a "lossy" inductor, but real inductors are lossy, and if they weren's we'd have to add some loss.

Above the audio band, an actual coil has rising impedance to some very high frequency and then it drops (stray capacitance). A fake coil depends on active gain which probably won't rise so high before it falls again. I think I have seen extra-cheap over-stretched EQs where this error caused sag somewhat above the audio band. Even using 2N5089 or TL072 with reasonable values, this should not be a real problem.

> Can a gyrator only be used to replace an inductor to ground or voltage reference, but not in series?

There's a bunch of ways to do it. IIRC, a floating fake coil needs two amplifiers, grounded fake coil only one (and may be unity-gain). And in many-many useful cases the coil can be grounded with no ill effect. So why make it complicated? Offhand I'd say floating fake coils are very rare.

[MoltenVoltage]

The reason I was wondering is that I was working on a Whammy 4 and noticed that the first thing the input signal hits is 2 inductors in series, and the last component before the output jack is another inductor.

From what Paul said, I assume the output inductor is to block "digital hash" from leaving the box, but why would you need an input jack inductor, let alone 2 in series?

[Taylor]

The part about using 2 in series is probably just because it's cheaper to buy 100,000 1uh inductors than 50k of 2 different values. Today I was looking at the schem of Moog's theremin and they have several instances of placing 4 inductors in series. In fact, the Bartolini preamp I mentioned above does that as well IIRC.

The inductance adds in series, but if the coils are too close they interact and affect each other's value.

[wavley]

I think that RG was just trying to point out that inductors are imperfect and expensive and thus replaced wherever possible for most applications especially consumer electronics and when they do exist it is only because nothing else will work or the sound coloration is intentional.

Personally, I enjoy the sound of a saturated transformer in a mic pre be it solid state or tube, I like the sound of a transformer output mic, or an inductor based eq.  The sound is very musical to me, but by no means pristine, reliable, or repeatable.  I have a pair of Neumann pres that sound quite lovely when pushed into saturation as a room mic on some drums but it takes a lot of tweaking to get it just right and if you leave it and come back, then you will have to tweak some more to get it sounding right again.  Speaker crossovers will use them for the foreseeable future but they are mostly air core and less prone to saturation.

You can totally do DI without a transformer, mic pres have pretty much gotten rid of them with the exception of expensive things, same with eq's.  I pretty much don't want them in the signal path of something reproducing music unless they are REALLY high quality, but I am the opposite when it comes to creating/recording music.

Everything in moderation or sometimes desperation, that's why I've been looking at their uses in power supplies recently.

[R.G.]

The reason I was wondering is that I was working on a Whammy 4 and noticed that the first thing the input signal hits is 2 inductors in series, and the last component before the output jack is another inductor.

From what Paul said, I assume the output inductor is to block "digital hash" from leaving the box, but why would you need an input jack inductor, let alone 2 in series?

They're there to block both RF coming in and digital hash going out. Now that I know what you're talking about, the issue is clearer.

Digitally based pedals, like the Whammy, have to pass the RF emissions tests set up by governments to prevent rf pollution. Those pedals generate RF noise as well as being sensitive to it, and generate even more noise because they use switching step-down power supplies inside to convert the incoming whatever voltage down to the 5V, 3.3V, or whatever they use. Simple analog pedals do not do this.

Internally-switching pedals can emit RF from both the inputs and outputs - as they makers of the Whammy pedal have clearly found out. 

As a point of interest, anyone selling a pedal which uses a microcontroller comes under the RF regulations, because any microcontroller uses switching frequencies or harmonics over 100kHz, and the seller is legally liable for any RF interference they emit. If they are not either individually or as a type approved, the seller is liable for up to $10K per day per unit if found in violation. The reality is that most modern microcontrollers are so low power that they don't emit RF emissions in excess of the legal limits, but frankly that's by luck, and clearly not by design.

I'm more familiar with the EMI emissions standards than I would like to be. 

[wavley]

I need to look at the schem of the Whammy.  I am soooo close to having a "noise free" guitar rig, I used the quotes because of the reality of tube amps and many effects, but I really am close to having a rig that sounds almost as good when running through pedals and space echoes as it does straight into the amp.  It's been quite a battle, I've learned a lot, I've also learned that noise is like layers of an onion... I keep peeling them away to find another layer and then it makes me cry.

Anyway, I have learning that my two worst offenders happen to be digital, an EHX 16 Second Delay re-issue and a Boomerang +, neither of which use inductors on input/output or power supply (excluding what may be in their respective wall warts), though the Rang uses them between the digital and analog sections of the board.

After looking at everything on my scope and my meter it seems that I have two problems, one seems to be current flowing through ground from the amps into my pedalboard power supply or vice versa, and the scope reveals high frequency hash.  Thus prompted my recent exploration of the uses of inductors.  I've tried signal isolation with mixed results, some things worked really well (RG's hum free) and some things are hit or miss, like ground lifting via RC.

[Pigyboy]

I bought some 1.5H inductors from Kent Armstrong and built Vartitones in boxes for several local recording studios.

[PRR]

> Moog's theremin and they have several instances of placing 4 inductors in series.

That's a radio frequency system. Inductors are pretty practical when working well above the audio band. While I have built a coil-free Theremin, and just bought another, both would be better as coil-based designs.

[R.G.]

> Inductors at low impedances and low frequencies require big inductances.
Inductors at HIGH impedances and low frequencies need bigger inductance. The physical size is largely independent of impedance from 1 ohm to 100K or so.

Oh, sure, be picky about getting it right...

Sometimes I'm backwards. 

[Cliff Schecht]

The others have covered the big application of inductors in audio. EQ's, resonant circuits (wah's), chokes and power components are really all we deal with. Inductors at audio frequencies are big, clumsy, inaccurate devices that tend to change with temperature. So as your circuit heats up, the sound changes. Like R.G. said, most engineers are trained to avoid inductors, or stated another way they are never really taught them thoroughly. Inductors are obviously in the family of magnetics, a topic that is in itself poorly covered in most BSEE degree programs. If the EE's aren't doing it then the hobbyists tend to not do it and therefore the usage of inductors is a pretty poorly covered topic overall, at least in our field. Older texts really cover the topic better. Like with most basic EE topics, the older you go in past literature the more in-depth the coverage gets (less was assumed back then).

When you get into anything RF, inductors are your friend. They're used all over the place in forms like tuned circuits, LC filters, impedance matching, baluns, transformers, etc.. The heart of just about any RF circuit is the inductor. They have their share of problems, but they are essential to RF. Any RF designer worth his salt should have a pretty firm grasp of magnetics, electric fields and how they all play together (read: Maxwell's equations).

You also see inductors as one of the energy storage elements in power supplies. They are used as chokes (voltage can change instantaneously through an inductor, current can't) and as the energy storage element in the charging/discharging cycles of switching power supplies. As a switch flips on, current in the inductor ramps up to a certain point (usually monitored either directly or indirectly) at which point the switch flips off and the energy stored in the inductor dumps into the load. Pretty neat stuff and the inductor is the heart of all of this. Switching power supplies are the one place where inductors tend to act pretty darn good, if designed properly. A supply that has been optimized for the application at hand can reach efficiencies of above 95%!

[MoltenVoltage]

> they are so common in other audio designs.

Common?

Solid-state preamps "never" have inductors.

Solid-state power amps have one cheap inductor to isolate the load above the audio band. (When cost is crucial, this can be avoided.)

Seems to me speakers usually have inductors in the crossovers.

[PRR]

> speakers usually have inductors in the crossovers

You got me!! I wuz thinking inside the other box.

> Inductors at audio frequencies ... tend to change with temperature.

Voltage and Current.

Inductance of non-gapped iron rises significantly from teeny signal voltage to fairly large signal, then drops-off abruptly.

Inductance of iron with DC in the winding falls significantly from zero DC current to nominal DC current.

Inductance of laminated iron also tends to fall around 400Hz as eddy-currents urge flux out of the center of each lam.

With all this going on, the only temperature effect I can see at "stage or studio" temperatures is a slight increase in resistance when it gets too hot to hold. (Inductance may vanish before red-heat, but we hope we don't go there.) There may be an inductance shift but that's the least of the uncertainties.

Inductance can be more stable if the core is generously oversized, then gapped down to desired inductance. If inductance must be constant from 50Hz to 1KHz you may have to pay a lot for extra thin lams. Ferrites avoid this problem but only recently came in sight of iron size and cost.

> The heart of just about any RF circuit is the inductor.
> Switching power supplies are the one place where inductors tend to act pretty darn good, ...can reach efficiencies of above 95%!

The heart of classic RF design is the inductor-Capacitor "tank". And 98% efficiency was pretty routine in AM IF tanks.

Switching an inductor is just ugly. Your switch arcs-over, radio hash radiates like the spark-gap transmitters which were banned in the 1920s. Only the 50%-90% savings in size/cost makes coil-switchery look beautiful.

[MetalGuy]

I've seen inductors at the input of pedals that don't have uCUs and ot other digitall stuff. It looks like Marshall are using those ferrite beads everywhere from pedals to tube amps.