series inductors...

[sniper1rfa]

New here. Found all the nice response plots and thought this would be a good place to ask.

Any reason why the cap to ground is the preferred method of treble cut, aside from cost and availability? Or is it just because the old school tone knob is a big cap, and the equivalent series inductor would be impractical?

How would, say, a 10mH series inductor (with a bypass pot, configured like a bass cut) compare with like a 4.7nF cap in a traditional tone control?

[JohnH]

Hi sniper1rfa, and welcome to GN2

Its a good question, in theory, most simple tone tweaks can be done with inductor-resistor circuits. But capacitors are much more compact and economical, in the ranges needed.

In a guitar, let's take the example you note, of an RC bass cut citcuit, changed to an LR treble cut circuit.

To get something effective, we might want an inductor that had an impedance of say 500k or more at say 5000hz. As an inductor, that would need 500000/(2Pi x 5000) that implies inductance of 16 Henries, which is huge. A cap in a normal circuit is much more feasible

We do see inductors in lower ranges like 1 to 3H for mid-shaping, but they aren't that common.

[sniper1rfa]

Makes sense.

Reason I ask is because I was looking into adding a single knob as a sort of "mid focus" tone control, which just rolls off the very low lows and the very high highs.

The simplest way to implement that on a single pot would be a small series inductor for the high cut and a small series capacitor for the low cut. Basically adding an inductor in parallel with the capacitor in a normal low-cut tone control. I feel like a mild high-cut wouldn't require much of an inductor - maybe 15mH or so would be plenty - which would be available in a plain axial or radial package for little money. Even up to like 100mH you can get something with a decent DCR for buck or two, which is no big deal when you're soldering it to a $10 push/pull dual gang pot.

That configuration also has the advantage of coming completely out of the circuit when the pot is rolled off. 

You say it would need to be 16H. Isn't it just an LR with a ~375k load (500k volume pot and ~1M input impedence)? Filter calculators suggest that a much smaller inductor would be effective with those parameters. Is the source pickup changing the math? Am I slipping a decimal somewhere?

[JohnH]

There's a couple of issues with that.

Anytime you put a cap and an inductor directly together you get some form of resonance or notch at a frequency 1/[2Pi.((LC)^0.5)]. In a parallel arrangement the LC pair becomes infinite impedance at this frequency, and in the filter you suggest, it will surgically remove this frequency.

With regard to a 15mH coil, at say 3000hz, its impedance is 2Pi x 0.015 x 3000 = 283 ohms, which is not enough to have much of a tonal effect combined with resistances in 100's of k


If you'd like to experiment, and I hope you do, Id suggest to get a SPICE sim program and there are several free ones. LTSpice, although old looking, is very powerful. Its easy enough to put in a basic model of a guitar, including pickup, pots and cable, and run scenarios.

I put a lot of that into a spreadsheet for instantly testing guitar circuits with many pickups and control options. You can download it here: (GuitarFreak 6.6)

guitarnuts2.proboards.com/thread/3627/guitarfreak-guitar-frequency-response-calculator

It covers every tone control arrangement I could think of, but it doesn't do the one you are suggesting! So SPICE is probably a better more general tool.

[sniper1rfa]

Well, I've fired up spice before and never got useful results, but I took another swing at it and seem to be getting somewhere. Makes more sense now. It's a *lot* easier to use on PC than on OSX, for starters.

I also tried a few other configurations, like placing tone controls before the xformer in a low impedance pickup, but at some point I realized I was basically trying to achieve buffered active EQ controls with passive components and the system was going to get real complicated and sensitive real fast.

So I took a look around and... why are there no active tone controls for guitars? Come on now. Does this forum contain 100% of the people who care about the tone knob?

There's plenty out there for bass.

[sumgai]

'sniper,


Hi, and to The NutzHouse!

Jul 17, 2020 13:02:22 GMT -5 sniper1rfa said:

So I took a look around and... why are there no active tone controls for guitars?

Errr, possibly because changing batteries is a pain, and after awhile the cost starts to add up. Not to mention, the extreme rise in your embarrassment quotient when a battery fails prematurely during a live performance.  Simple but effective deterrents to a wholesale shift away from passive components, I'm sure you understand.

HTH




sumgai

[sniper1rfa]

Sure, but that doesn't quite tell the whole story. Active *pickups* are all over the place, but with preamps built into the pickup itself and with passive outboard controls. Plus, active controls on acoustic and bass guitars are commonplace bordering on normal.

It's just electric guitars that seem to have avoided active controls. Go figure.

Somebody over on reddit put it succinctly I think - Leo Fender did it, and so by god that's how I'm going to do it.

I guess I need to fire up Eagle and learn something.

[JohnH]

I think that, apart from the simplicity of traditional passive circuits, the ways that they interact with the pickups, the cable and the amp have a kind of lucky magic which is inherent in the tone of an electric guitar, and it has become preserved over decades and countless players and recordings as the reference for what we want to hear. Ive tried active circuits, but they just don't sound as good (ie not the same)

[sniper1rfa]

Thanks for all the help guys. I fooled around in spice and this is what I came up with:

Two things worth noting: the voicing caps, which are switched in and out with a 4PDT switch hanging off the pickup selector (one RC per pickup) and the buffered output. Concentric pot handles volume and bass cut, and a push/push pot handles treble cut and the humbucker. I ended up wiring the push/push for parallel and split, rather than series/split or series/parallel, because I thought the crunch lab sounded pretty crappy in series. This yields 7 pickup combinations, two of which are humbucking.

All the various tone controls are subtle but effective, and you end up with what is essentially 21 shades of strat.

All stuffed into a Japanese "designed by schecter" 22-fret 7-string that the previous owner modified to HSS. I kindof took the guitar a step further with another SSL1 in the middle (instead of the single-sized humbucker that it came with) and all this tone control nonsense. It's a guitar with a pretty serious split personality, but I'm happy with it.