Lack of inductors used in pedals?

[Thecomedian]

I notice that almost no pedals use inductors of the small "in series" variety, such as an RLC circuit. Is there any problem with using inductors in pedals? It seems pretty easy to create very specific tone control ranges with them, or at least controlling "mid".

[Seljer]

Because to get the corner frequency of the RLC circuit into the audio range, the inductor typically ends up in in the range of 10mH to 1H. Inductors of that size are expensive to purchase and tricky to make. They're also rather bulky.

Gyrator based opamp circuits are cheaper and work just as well.


Its not such an issue at radio frequencies where a typical inductor is a couple of 10 turns of wire.

[brasiele]

Yes, they are quite expensive, bulky and with large tolerance.
But I think they are great for handcrafted equipment, short runs or one-off.
I buy them directly from Wilco Inductors, they manufacture shielded inductors between 1H and a few mH which are very cheap (3-4$ each)
I'm not related with Wilco at all, but I used to buy some piece and had great fun with those inductors!

[Thecomedian]

wow. 4-18$ is ridiculous.

guess it's not necessary when there's easy mid tone control schematics on the web.

[R.G.]

As a general observation, most professional electronics design does anything possible to eliminate inductors. They're expensive, heavy, and non-standardized in the way that resistors and caps are.

The exceptions are for RF work, where gyrators and RC active filters are not practical and the inductors are tiny, and for power supply work where the inductive storage and transfer of energy with isolation is not possible with Rs and Cs.

[Jdansti]

I notice that almost no pedals use inductors of the small "in series" variety, such as an RLC circuit. Is there any problem with using inductors in pedals? It seems pretty easy to create very specific tone control ranges with them, or at least controlling "mid".

Are you referring to this type of axial lead inductor?



The ones that I've seen in stores are less than 1mH (in the uH range). I found one locally that was 470uH and thought I had found a cheap wah inductor until I realized that it was a factor of 1,000 too small.  It also didn't make sense because there was no way the manufacturer could have wrapped enough wire in that small of a package to be the right value.

As an example, wah inductors to need ~400-600mH.  R.G.'s website offers some inexpensive alternatives to the expensive "wah inductors".
Mouser 434-03-154J 150mH chokes or Radio Shack's 275-1380 transformer (see http://www.geofex.com/ >Technology of the... >Wah Pedals). I imagine that an inductor to control tone as you suggest would have to be in a similar range.

[J0K3RX]

If you just need a couple every now and again you can usually find them inside old VCR's, TV's, computers etc... I cannibalized an old VCR not long ago and found all sorts of goodies inside But, I guess the real question was why are they not being used in pedals these days, not where to get them from...

[Jdansti]

^ What value range do you find in VCRs?  Just asking because I'm still far back on the learning curve as to the use of inductors in pedals other than for wah circuits.

Another related question: Can these axial sub-mH inductors be used for altering guitar signals in a useful way?

[J0K3RX]

^ What value range do you find in VCRs?  Just asking because I'm still far back on the learning curve as to the use of inductors in pedals other than for wah circuits.

Another related question: Can these axial sub-mH inductors be used for altering guitar signals in a useful way?

Not sure of the ranges, yet.. They are small, similar to the ones in the pic you posted. Some could easily be mistaken for resistors. There are a few larger ones in there as well but I have not checked those either...  

As for your question about the axial "sub-mH inductors be used for altering guitar signals in a useful way?" I would like to know this as well!
I have also seen a few SMD inductors on the controller boards on old SCSI hard drives... PC power supplies have all sorts of inductors inside too!

[Kesh]

I once used an inductor on the output of a chip amp. Air core, wound it myself.  I guess suppressing RF is the only possible use in audio work.

[Thecomedian]

Short answer, yes. I modeled it in LTspice and 400mh inductors in parallel with a cap and a resistor create band pass. If you have spares lying around, they can always be used.

R = 1k
C= 10u
L = 400m

11 dB at 1hz, then again at 6 khz.
-40 dB at 80 hz.

http://hyperphysics.phy-astr.gsu.edu/hbase/electric/imped.html

[R.G.]

I once had an epiphany back in school. Any inductor can (in theory) be tuned to any frequency by some capacitor. All that changes is the impedance of the overall filter on either side of the resonance.

So a 1H inductor can be tuned to 1kHz. So can a 1mH inductor. The 1H inductor is tuned to 1kHz by a 25.3nF capacitor, and the 1mH is tuned to 1kHz by a 25.3uF cap. The only differences are (1) the impedance of the filter is different by three orders of magnitude, as you'd expect and (3) the parasitics of a 1H and 25uF cap are way different, and may cause different amounts of imperfections.

If you can deal with different values of filter impedance, you can use any inductor.

If...

[Thecomedian]

Pretty much. The difference is that with a higher inductor the "band pass" range is narrower for a single RLC circuit of all parallel. It's quite possible that you could stick with small mH inductors and variations of resistors and caps in different placements of different sizes, the filtering effect could mimic any other filtering effect with a different size inductor.

my example is a practical 45 degree angle slope between the high point at 1hz and the lowest point at whatever I said the lowest dB level was. With a uH inductor, the range is much steeper and narrower.

[R.G.]

Actually, that scales too. I think you're describing what the techies call "Q", a single number which describes the energy stored versus the energy lost per cycle in the LC resonance. High impedance LC circuits need low resistances to each energy so they can keep more energy per cycle. Low impedance LC filters can stand more resistive losses because the energy per cycle stored is larger.

However, it's easy to get outside the practical ranges of values for the rest of the circuit. And that gets back to practicalities - bigger inductors are heavy, expensive and non-standardized.

[Gus]

found a link to a Treble Boost with an inductor

http://rubidium.dyndns.org/cag/pdf/treb.pdf

[PRR]

> I found one locally that was 470uH

So do math.

FWIW: 470uH is 470/1,000,000 H. (If you find a scientific calculator, 470 EE -6.)

F * 6.28 * L = Impedance

Jus fer example, try 1KHz:

1000 * 6.28 * 470/1000000 = 2.9 ohms.

This is awkwardly low for most circuits.

Top of guitar band, 5KHz, 14.8 ohms.

We usually like values in the 2K to 10K range that opamps will drive happily.

Therefore we want (as someone here said) values 1,000 times higher than that 470uH part.


There's another problem. Small low-price 470uH coils are likely to have 13 ohms of DC resistance. So the 2.9 "ideal" ohms is more like 15 ohms, and hardly changes with frequency within the guitar band. A 15 ohm resistor does about as much and is a penny cheaper. Both are pointless as tone controls.


Overall there is a bigger problem. A coil has a winding which has length, and a magnetic path which has length. Trying to make a "high value" (audio-useful) coil means going to great lengths, and then greater lengths to get the resistance down.

This leads to "large" coils of good audio performance and "small" coils which are just useless for audio.

Same in power transformers. A 10-Watt wall-wart has really bad performance at 50/60Hz power frequency. It may waste half the energy going in. A mega-Watt transformer can work really fine, even at the older 25Hz power frequency.

Or in jargon: the "Q" of any inductor goes down with frequency but up with size. We audio-guys are on the wrong end of the frequency spectrum. We are also cheap, so don't want a 2-pound coil for a tone-knob.

It gets this good only with "iron" (or ferrite or Nickel) cores. But these will saturate. A typical 4-cent 470uH coil can only stand 25mA. If it only has 3 to 15 ohms impedance, we can only swing a small fraction of a Volt across it. Which means living close to noise.

Caps don't have silly-long paths and can be made of materials that have high overload. So as a practical thing, caps are usually the way to go. Even if you have to throw-in some amplification to "flip" the way it changes with frequency.

The major audio coils:
Pickup (guitar, dynamic mike)
Input transformer (now rare)
Output transformer (tube amps)
Stabilizing coil (transistor power stages)
Power transformer (or switcher supply)
Radio/TV hash reduction on power input

And of course the several Wah designs, where it is *maybe* easier this way (or the flaws-flavor is musically useful).

[duck_arse]

I pulled a couple of 22mH chokes from an old cassette tape-deck pcb not long ago.

the best thing about inductors is winding them.

[Jdansti]

> I found one locally that was 470uH

So do math.

FWIW: 470uH is 470/1,000,000 H. (If you find a scientific calculator, 470 EE -6.)

F * 6.28 * L = Impedance

Jus fer example, try 1KHz:

1000 * 6.28 * 470/1000000 = 2.9 ohms.

This is awkwardly low for most circuits.

Top of guitar band, 5KHz, 14.8 ohms.

We usually like values in the 2K to 10K range that opamps will drive happily.

Therefore we want (as someone here said) values 1,000 times higher than that 470uH part.


There's another problem. Small low-price 470uH coils are likely to have 13 ohms of DC resistance. So the 2.9 "ideal" ohms is more like 15 ohms, and hardly changes with frequency within the guitar band. A 15 ohm resistor does about as much and is a penny cheaper. Both are pointless as tone controls.


Overall there is a bigger problem. A coil has a winding which has length, and a magnetic path which has length. Trying to make a "high value" (audio-useful) coil means going to great lengths, and then greater lengths to get the resistance down.

This leads to "large" coils of good audio performance and "small" coils which are just useless for audio.

Same in power transformers. A 10-Watt wall-wart has really bad performance at 50/60Hz power frequency. It may waste half the energy going in. A mega-Watt transformer can work really fine, even at the older 25Hz power frequency.

Or in jargon: the "Q" of any inductor goes down with frequency but up with size. We audio-guys are on the wrong end of the frequency spectrum. We are also cheap, so don't want a 2-pound coil for a tone-knob.

It gets this good only with "iron" (or ferrite or Nickel) cores. But these will saturate. A typical 4-cent 470uH coil can only stand 25mA. If it only has 3 to 15 ohms impedance, we can only swing a small fraction of a Volt across it. Which means living close to noise.

Caps don't have silly-long paths and can be made of materials that have high overload. So as a practical thing, caps are usually the way to go. Even if you have to throw-in some amplification to "flip" the way it changes with frequency.

The major audio coils:
Pickup (guitar, dynamic mike)
Input transformer (now rare)
Output transformer (tube amps)
Stabilizing coil (transistor power stages)
Power transformer (or switcher supply)
Radio/TV hash reduction on power input

And of course the several Wah designs, where it is *maybe* easier this way (or the flaws-flavor is musically useful).

Thanks for the lesson, Paul!  I've saved it in my "Electronic Notes File".