Adjustable notch filter?

[Xander8280]

Working on a simple notch filter circuit and want a potentiometer that will bleed more un-notched signal. Was thinking a 2meg pot parallel to the notch filter(the pot would be a variable resistor 0-2meg).
Wondering if in theory this should work well?
I'm about to order capacitors online, don't want to waste my money if this is far off.

Thanks!

[roberto]

Use just an RLC filter.
LC to ground with a resistor in series to dampen the LC notch.

[Leo_Gnardo]

Use just an RLC filter.
LC to ground with a resistor in series to dampen the LC notch.

+1 !!! Done that successfully for the owner of a Hiwatt who wanted a deep scoop in the mids @ 650 Hz. Worked great! I used it following the standard EQ stack. You can use a switch to select different caps for a variety of frequencies. And if you can find a tapped inductor such as Ampeg used, even more choices.

[roberto]

Yes, or you can just use multiple inductors in series on a rotary selector (like Ampeg does with caps), keeping in mind Inductance sums when in series.

...or use gyrators with caps on rotary selector to change their L value, or...

[Xander8280]

Thanks guys! Wondering what's the power ratings needed for installing inside my tube amp? I see that a large cap is needed if a small inductor is used(500uH). Would a 5Hy fender style choke be over kill?

[Xander8280]

I'm looking to have the notch at approximately 180Hz-220Hz, most would just highpass but the circuit calls for a very specific tight notch. The RLC filter is perfect. Lot of calculators online, but inductors aren't my strong point.

The calculator reads that with a choke(5Hy) I could get away with using a capacitor as small as 0.22uF. But with a few series'ed small inductors(500uF) I had to get 100uF to get into the range. I could use a canned cap or a choke, that would be over kill, but sometimes you need protection!

[roberto]

A big choke can cost 20$, while a big cap costs 50 cents.
No power needed because it's in the signal chain.

Diezel uses a 500 mH choke (like the one you can find in wahs, or the bobbin of some relais) with a 3.3 uF to boost those frequencies. Use it as a notch with same components.

[eniam rognab]

radio shack sells a small audio output transformer that has been used successfully in wah-pedals, also taydaelectronics.com has a good selection of inductors on the cheeeps

[Leo_Gnardo]

Would a 5Hy fender style choke be over kill?

Mos' deafinitely overkill. What you need is along the lines of the "mini" cup core chokes, as used in Mesa/Boogie or old Acoustic graphic EQ's. The wire is hair thin, like pickup wire. Don't need to handle much power. Now . . . where to get one? Besides prying one off a discarded Mesa or Acoustic EQ.

Tayda's inductors are cheap indeed but range from 0.1 to 4700 MicroHenry. You need a lot more Henry than that.

[eniam rognab]

Tayda's inductors are cheap indeed but range from 0.1 to 4700 MicroHenry. You need a lot more Henry than that.

totally correct as usual Leo! i should have checked what they actually had, all i remember was seeing alot of them, but the values are pretty useless for this application.... i should think more before i type away, i blame the medication im on

[Merlinb]

Working on a simple notch filter circuit and want a potentiometer that will bleed more un-notched signal. Was thinking a 2meg pot parallel to the notch filter(the pot would be a variable resistor 0-2meg).
Wondering if in theory this should work well?

Yes, that will work. A common approach is to use a bridged-T, with the bridging resistor replaced by a pot. You will find this in Framus amps.
http://www.chasingtone.com/dvnator/wp-c ... em_V21.jpg

Gibson also used bridged-T, but with the pot in the ground leg.
http://www.6v6power.ru/inf/Amplifier/Gi ... -30RVT.jpg

I would avoid RLC filters unless you like spending money and dealing with hum.

[Leo_Gnardo]

I would avoid RLC filters unless you like spending money and dealing with hum.

But if you do go the RLC route, it may take some experiment to find an ideal location for your inductor to minimize hum. Away from the power supply and filament lines. The "cup core" inductor in the Hiwatt wasn't a problem*. I have found the Ampeg multitap inductor to be a hum picker-upper even in factory made SVT series amps. In one instance I spliced extra wire onto its leads, found a "quiet corner" of the chassis and placed it there on a thick cushion of RTV goop - that did the trick.

*Note Mesa/Boogie and Acoustic placed their RLC graphic EQ's cram-jammed next to the power supply, and they don't pick up significant hum. There's plenty of other hum sources in a Mesa though.

[roberto]

Otherwise there's always the gyrator option, as I wrote few posts ago.

[Xander8280]

http://sim.okawa-denshi.jp/en/RLCbekeisan.htm

The calculator doesn't except any parameters to be zero. But I imagine that at 0ohms the notch is completely gone? The graphs show some attenuation at the band when 1R is the resistance. I am wondering if the attenuation isn't noticeable? I'd like the control of zero notch to a deep notch.

[Leo_Gnardo]

But I imagine that at 0ohms the notch is completely gone? The graphs show some attenuation at the band when 1R is the resistance. I am wondering if the attenuation isn't noticeable? I'd like the control of zero notch to a deep notch.

IF you place your RLC filter from signal line to ground, you'll find the notch to be deepest with zero ohms. And one ohm isn't going to change that much. Keep in mind the inductor has its own built-in resistance that will limit the depth of the cut. Any R added to that will reduce the depth of the notch. If you would get your hands on an inductor and try it, you would see what's going on in less time than it takes to type this. By using a pot, say 100K, you can vary the notch depth from nothing to max depth. And most of the cut "action" will happen at fairly small R. I recall in the Hiwatt I modified, the max depth was 28 dB - that sure put a "hole in the mids" without a doubt.