Is there a way to passively cut a selectable frequency?

[daz]

I know how to use caps to shelf frequencies, but in this case i don't want to cut from some point all the way to one end of the other of the bandwidth. I want to be able to passively cut a certain low mid frequency in a guitar. It would be ideal if the width of the cut was controllable but it's not totally necassary and since i doubt theres even a way to do the main thing i'm asking i'll leave that one alone for now. But can anyone point to a schematic that shows ho i can take a signal and pass it thru some components with a pot that will select a center forr the area to be attenuated?

[bbsailor]

I know how to use caps to shelf frequencies, but in this case i don't want to cut from some point all the way to one end of the other of the bandwidth. I want to be able to passively cut a certain low mid frequency in a guitar. It would be ideal if the width of the cut was controllable but it's not totally necassary and since i doubt theres even a way to do the main thing i'm asking i'll leave that one alone for now. But can anyone point to a schematic that shows ho i can take a signal and pass it thru some components with a pot that will select a center forr the area to be attenuated?

Try using a Gibson Varitone type tone control that uses an inductor and a variety of capacitors to dip out various frequencies. See this link for a comprehensive discussion about this circuit. Varitone - Gibson Guitar Board

To obtain a compact inductor look for commercial transformers such as 600 ohms to 600 ohms. Wire the transformer primary and secondary in series adding and you should get an inductance of about 3H to 4H. If you need a larger value, look for other commercial audio transformers such as anything on the low side to something like 10K to 50K on the high side. Just wire the two coils in series adding to make a higher inductance inductor for the Varitone circuit. Remember this is a passive circuit and there will be some output level losses. To make this work, Gibson uses about a 150K ohm resistor in series with the signal to allow the LC part to work at cutting various frequencies. The other alternative is to use something active like the EMG active midrange control, usually designed for basses but it has a selector switch to adjust the frequency range. Check it out.

Joseph J. Rogowski

PS Check out this link also:http://music-electronics-forum.com/a...4-varitone.png

[52 Bill]

Please take the information on the Gibson board with a grain of salt. I just read through it and some info is correct and some is nonsense.

The second link is a schematic of Craig Anderton's passive tone control, which uses a transformer for the inductor.

As for the value of the choke, the original Varitone used a GA-90 choke. This was used on the Varitones as well as on the EB series basses. I have measured 4 examples of '60s inductors and all have measured 15 Henries.

Other values of inductors will work, but will have different tone characteristics from the original Varitone.

[J M Fahey]

Ouch !!!

That user "cheek" sounds like SGM's twin brother ... even claims being related to the "military Navy"

Loved the "reduce capacitor value to lower resonant frequency" bit

Wonder why he's labelled as "banned"

[Jazz P Bass]

I think what Daz is refering to is a 'notch filter'.

Band-stop Filters : Filters - Electronics Textbook

[rjb]

...take a signal and pass it thru some components with a pot that will select a center for the area to be attenuated?

I think what Daz is refering to is a 'notch filter'.

The Gibson Varitone is a notch filter with center frequencies selected by capacitors on a rotary switch.

It seems a twin-T notch filter would require a custom triple-gang pot, if you want to sweep the notch frequency.
Some possible (old-ish) leads: Source for triple-gang potentiometers?

- Dr. Obvious

[rjb]

Woo-hoo! (Maybe).

Rediscover The Truly Tunable Hall Network | Analog content from Electronic Design



EDIT:
Just looked the article over a little closer.
No idea how well the passive circuit will work without an "ideal voltage source" input and high, purely resistive output.
The active version does have notch width adjustment as well as frequency sweep.
Anyway, it might be a fun circuit to play with.

EDIT II:
One possible disadvantage- practical tuning range (lowest to highest notch frequency) is about 1:4
With the Varitone, you can pick caps for whatever notch frequencies you want.

EDIT III:
Another possible disadvantage- the notch filter would be "in" all the time.
I guess you could disable it by simultaneously shunting R3 and floating the pot wiper with a DPDT switch.

[Mick Bailey]

The killer with the Varitone is the volume drop, the same for most other notch filters due to a combination of insertion loss and the (lack of) steepness in the filtering resulting in a fair chunk of audio energy being removed form the signal.

Setting up various filters in PSpice gives a good overall picture of the filter action. At least it allows some effective 'breadboarding' before committing to an actual physical design.

[daz]

Well, as simple as that is and w/o needing a inductor, i may have to try that. thanks. As to volume drop, no problem. The pickups are hotter than i want anyways.

Woo-hoo! (Maybe).

Rediscover The Truly Tunable Hall Network | Analog content from Electronic Design

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EDIT:
Just looked the article over a little closer.
No idea how well the passive circuit will work without an "ideal voltage source" input and high, purely resistive output.
The active version does have notch width adjustment as well as frequency sweep.
Anyway, it might be a fun circuit to play with.

EDIT II:
One possible disadvantage- practical tuning range (lowest to highest notch frequency) is about 1:4
With the Varitone, you can pick caps for whatever notch frequencies you want.

[Richard]

How is it connected?

[rjb]

How is it connected?

With wires.

[J M Fahey]

I guess Richard asked for a schematic

As of the original problem, why not consider adding some active stuff?
A TL062 is a dual op amp, allows a ton of cool things to be made, including a pot sweepable notch/boost parametric, eats some 0,4mA and a 9V alkaline lasts a year so in practice you don't even need a dedicated battery access door, unscrewing the pickhuard or pot access cover once a year is no big deal.

Or use the mythical LM4250 , the ultra low current op amp which powers music man basses: batteries die of old age, some 3 to 5 years after installed (full shelf life).

http://bitsavers.trailing-edge.com/p...es/AN-0071.pdf

[rjb]

A TL062 is a dual op amp, allows a ton of cool things to be made, including a pot sweepable notch/boost parametric,

Or use the mythical LM4250

Happen to have a schematic?
I searched some TL062 & LM4250 Data Sheets (which contain sample application schematics) and TI Op Amp Applications Manual - no joy.

[g1]

As of the original problem, why not consider adding some active stuff?

If memory serves, BC Rich Eagle uses some kind of active varitone circuit. Not sure if schematics are available.

[J M Fahey]

Ok, what do you exactly want to build?

I'll search some schematic for you , but the basic idea is to find a generic op amp schematic doing what you want, and use a TL062 there ... "an Op amp is an Op Amp", by definition they are generic and freely interchangeable, except in some reaching-the-limits use.

Anything doable with, say, T072/82 , LF353, etc. , can be made using a TL062 instead; it will need 1/5 the supply current, work better with just 9V compared to others, only problem is that it has somewhat less highs if used at max gain,say 100X ; but for most purposes it's the same.

In fact I keep a stick of TL062 in my parts shelf for active guitar or pedal projects/repair but if run out of TL072 on Saturday afternoon and my customer is playing that night, I slap one of them and none complains.

Heck, I even have a stick of lowly humble LM358 low power dual op amps, which achieve low consumption (0.25mA per op amp) by biasing them class B .
In theory not suggested for hi fi audio, but for "auxiliary" duty, such as compressor active diodes, comparators, DC amplifiers, clipping Led drivers, short circuit protection, DC at speakers detection, **servo bias** for Fets and MosFets, etc. , yet one fateful Saturday night I replaced a TL072 by an LM358 (all I had at that moment) in a balanced input mic preamp, I warned my customer: this is just a band aid, sound was noticeably duller (gain dropping 6db/oct above 5 kHz) , bring it back on Monday ... that was 3 years ago, he loves the "smooth, sweet sound" he gets at that mixer channel , go figure.

In a nutshell most op amp circuits are valid with basically any op amp

Search fo active electronics for guitars (bass too) , some of them might do exactly what you need and minimize experimenting.

Here's one free sample the active Jazz Bass preamp.
It shows a dual frequency adjustment pot, not sure what it does, maybe one of our simulator gifted friends can trace it .
It uses a TL072, will work just as well with a TL062, giving 5X the battery life (think one year or more):


this very complete tone control should be converted to 9V single rail and used with TL062.
Or you could ditch the bass/treble stage and leave just the parametric mid ...you should use the first op amp as a buffer anyway:


the sky is the limit