Inductors - where do you start?

[Gabsonjackellis]

Hello everyone.
Ive come across a few guitar related circuits which use inductors. Theyre a new thing for me completely...

like this on the end of the varitone circuit: http://i5.photobucket.com/albums/y15...r/varitone.gif

I've also think ive identified one in a Fender active strat. there's a weird dummy pickup coil. could this be an inductor?
Wasn't expecting that.... a weird extra dummy pickup/inductor under the scratch plate! | Flickr - Photo Sharing!

And also part of this modification:
a mid boost circuit:
Rothstein Guitars • Serious Tone for the Serious Player





What do inductors actually do soundwise?

[Steve Conner]

Any coil of wire has inductance, and so could be called an inductor. But it's impossible to generalise about "what inductors do to the sound": the answer depends on how much inductance, and where it's placed in what kind of circuit.

In varitone-style circuits, or guitar amp tone stacks, inductors allow more tone control options: the classic example being a more powerful and focused mid boost or scoop than you could achieve with resistors and capacitors alone. A mathematician would babble something about "complex-valued poles in the transfer function".

Sometimes they are used in guitars as hum cancelling coils: in which case they're not supposed to affect the sound at all, simply get rid of the hum, by picking up an equal and opposite hum but no music signal.

Pickups are inductors too, and their inductance already colours the sound in a similar way to what the Varitone style circuits do. This is why, say, a Duncan Invader sounds duller and heavier than a Strat pickup.

[Chuck H]

FWIW the inducor used by most DIYers is a small audio transformer made by Xicon. Part # 42TL021 available at Mouser. The larger winding is used at it's ends with the secondaries and CT clipped off. Inductors with values in henries are hard to come by but this little transformer seems to do the trick.

[Mark Hammer]

Place a capacitor in series and frequency content below a certain point (determined by cap value and other things in there) will be attenuated, relative to content above that point. Place an inductor in series, and content above a given frequency band will be attenuated.

The classic tone control uses a cap and a variable resistance to determine how much the content above a certain frequency range will be attenuated. If you place both a cap and an inductor in series with the signal, and their respective ranges are well-chosen, you can create a notch by combining a little bit of attenuation of stuff above frequency X and attenuation of stuff below frequency Y. Most often, this is used to produce an audible midrange scoop.

Since an inductor is "just a coil", inductance suitable to the task of midrange cuts can come in a variable of physical packages. One of these is to use small transformers. Since many miniature transformers can come with a centre tap, it is common to see circuits where only the portion of the coil between the centre tap and one outside tap of the coil is used. Other circuits may allow the user to employ a toggle or other switch to select between "half" (up to the centre tap) or "full" (end to end) coil to result in two different inductance values and two different frequency ranges of operation.

[Gabsonjackellis]

Thank you everyone for your responses.

Awesome, that's some real food for thought.

That makes sense now about the Dummy pickup in the active strat

Chuck c - ill be ordering some of those Ive read that as well as the usual 42TL021, the 42TL019 is handy too. Annoyingly the UK Mouser store is on backorder for the ...21s


Time to buy a mulitmeter that measure inductance now hey!

thanks again everyone

[kg]

one way to consider inductors is that they are the opposite of capacitors:

cap - block dc, pass ac
inductor - block ac, pass dc

[jrfrond]

As Steve said, any coil of wire is an inductor. In fact, all electronic components and wiring have some inductance, referred to as "parasitic inductance". Same goes for capacitance and resistance. Ironically, though a component is built for a specific purpose (i.e. L, R or C), it sports all three characteristics. It's all pure physics.

A pickup, though it is electromagnetic generator, is also an inductor. A dummy coil is just a pickup without magnets, so it cannot read string, and is used as a passive humbucking coil. However, ANY coil, in parallel with another pickup, will add inductance to the circuit and change the sound accordingly. There's no free lunch in passive pickup systems. Any humbucking scheme will change the sound of the pickup by adding parallel inductance. In the case of split pickup systems, like the P-Bass, etc., the inductances are in series. Like capacitors, inductors add in parallel and divide in series.

When you throw capacitance into the mix with inductance via tone controls, you can form bandpass and notch filters, depending on how they are arranged (series or parallel). The Varitone kicks it up a notch by employing a dedicated inductor, along with resistors and selectable capacitors to form an adjustable notch filter.

In many tube amps, a choke, which is an iron-core inductor, is used in a scheme called a Pi Filter to filter out hum components from the rectifier. The same principles apply, but it is used in a non-audio application.