Mid Cut at 700 Hz

[Guitarist]

Using my Digitech modeller's equalizer I discovered that scooping 700 Hz was very musical. Especially for lethal metal tones. A wide Q was preferred.

I assume that an inductor/coil in series with a x Ohm pot to send this to earth would be simple.

What value in Henrys would do this?

[cbarrow7625]

I don't knwo what the exact Q will be but here are some general numbers that should get you in the ball park..

5uF Cap / 10.35mH coil
10uF cap / 5.17mH coil
22uF cap / 2.35mH coil
47uF cap / 1.1mH coil

For a passive notch filter the Q will INCREASE as you reduce the size of the cap & increase the size of the inductor. The Q will DECREASE as the cap gets larger & the coil smaller.

The 5uF version will be sort of "mid-Q", everything else will be getting wider. If you want it narrower, just keep cutting the cap in half & double the inductance.

Hopefully somebody else here can model these in pSpice or something and show you exactly what the resultant Q's will be.

[Steve Conner]

1 over 2 pi root L C
Is the resonant frequenc-eee

I went down this route a while back, and discovered an op-amp circuit that emulates a series combination of a cap and coil, with adjustable resonant frequency and Q. It does make some lethal metal tones, too

[Robert M. Martinelli]

1 over 2 pi root L C
Is the resonant frequenc-eee

and (1/R)*sqrroot(L/C) is the Q factor

The bandwidth is the resonant peak frequency divided by the Q factor

The lower freq is the resonant peak freq - (bandwidth/2)

The upper freq is the resonant peak freq + ( bandwidth/2)

[chipaudette]

Check out the Graphic EQ built into the Mesa Boogie Mark seriers (Mk I, II, III, or IV). I'm pretty sure that the middle band is tuned to 750 Hz or so. That EQ has certainly been used in a lot of metal playing over the decades. It looks like you've rediscovered that formula.

You can check out their Inductor/Cap/Resistor combination. Be careful how you hang it off your circuit, though. The Boogies use some transistors to drive and recover from their RLC circuit. If you just hang an RLC off the plate of a tube stage, it might shift the resonant frequencies.

An alternative option is to use the op-amp method mentioned by Steve.

A third option is to use a normal Fender/Marshall tone stack setup. Using the Duncan tone stack calculator (http://www.duncanamps.com/tsc/), you can tweek the cap and resistor values to make the mid notch show up right where you want it. Of course, having no inductor nor any op-amp, the mid notch might be a little wider than desired. If you do find values that you like, you can use fixed resistors in place of the "Treble" and "Bass" knobs. You can keep the pot for the "mids" and use it to tweak how much mid-cut to employ.

Chip

[Guitarist]

Very helpful advice. For one, I didn't realize you could dial in a typical tone stack so. I am on a Mac so can't use the Duncan calculator. But I am leaning towards the inductor as I always, on every amp I've own put the bass on max. Even my first lineup, (stereo) Ampeg VT-22s. Now I am penciling out a design to pot the (dual) bass pot in series with the cathode capacitor and maybe switching in a 5 uF from a .47 uF. Also another approach would be to switch in bigger blocking cap someplace. I have never used a variable capacitor but that might be good for the trebel control frequency sweep.
So, still have to figure out those inductor + cap values.
I hate to admit it, but transistors give me a mental rash. Maybe not IRF 820s.

By the way, nobody is making basic 1U tube preamps anymore. As every Lee Jackson I see come up on eBay goes for more than $500 I don't see why nobody has met that niche and improved on that design. Mmmmm......

Thank you gentlemen, or good and dangerous men.

[coxster]

Wide Vs Narrow Q

The wider Q is always more musical, but most folks dial up a narrow Q on a parametric so they can experience the change more readily, just my experience.

You want to watch going with too big an inductor. While a larger inductor can give you a larger Q, it can also give you a smaller Q based on the resistance of the coil. The formula is Xl/R, where Xl is the inductive reactance of the inductor, calculated as Xl=2piFL ( in this case 2*3.14*700*inductor of choice ) divided by the series resistance of the inductor. I've built some interesting FET tone controls using inductors as load resistors, or using an inductor and cap in series around the source resistor, but the series resistance always gave me too much Q to make it as defining as I had hoped for.

Does any body remember the Gibson ES series Vari-tone guitar, and what inductors they used??

Good Luck
Coxster
www.myspace.com/howawesome

[booj]

42TL021-RC
Xicon Ultra-Mini Aud
XFMR 4KCT/600CT

From Mouser. This is what I use for my varitones. Cut the center lead on the "P" side and use the end leads. You should measure about 150 ohms. It has the correct inductance. They're about $1.70