Variable Frequency Cutter idea

[Elijah-Baley]

Hello. I have a (new?) idea.
After I heard an amp of a friend of mine with a very middie sound I though about a schematic.
The schematic I create (but not the circuit built) just cut a wide range of frequency. I'd like to know what you think about it.



As you seen the range of the frequency is pretty wide and it cuts about 10-11db. Maybe I'll work on it again, because to go over 10kHz is not so useful nor noticeable, I think. And I'll try to go a bit lower than 140Hz.
Even with the cut control to zero it scoops something (you can see that the flat is not so flat), but I don't care, this is what this circuit have to do. Anyway I added a gain control to get a minimum range of volume to cut or boost, how much depends even from R7 or the approach of the output, I'll say it now.
In the emulation it seems to work fine, but I have some doubt about R7 (470k), because if it's smaller, for example 100k, it cuts a bit of bass, something about 1-2db, depends the value of the resistor. I can see in the graphic, but I don't know if I could hear it. It's a bit annoying to see the line not so straight, I am just a bit maniac, I prefer more flat possibile, not considering the frequency I cut, of course.

My madness aside, in your opinion which value or R7 would be fairer?
There's something else you would change?

Thanks!

[antonis]

in your opinion which value or R7 would be fairer?

Our opinion can't outguess next stage input impedance...

[kraal]

There's something else you would change?

Use an equalizer ?
EQ700 for instance ~25 USD (new), might be more cost effective.

[Elijah-Baley]

It's not something I need or a real issue. Just a point to start to build something useful. I don't buy pedals.

What would the options?
None resistor. (Can we do it? The software give me an error, so I used to don't do it).
Smaller value. 47k, 100k... But some bass are slightly cut.
Big or very big resistor. 1M, 4.7M. Loss of bass is irrelevant, but... what?

I would like to find a solution to avoid this bit loss of bass. What I can do?

[FiveseveN]

What would the options?

Are you asking how to isolate the filter from the next stage's input Z? Use a buffer (or a bootstrap):



See RG's Peaks and Notches: http://www.geofex.com/article_folders/eqs/paramet.htm
And Jack's variations on the bridged T: http://www.muzique.com/lab/notch.htm

[Elijah-Baley]

An output stage?
It should be easy even to make a volume control.
Too bad, I hoped in something easier. I worked for a veroboard layout and the board was very small, 11 x 11. I have to use a dual op-amp, maybe I can contain the size.
Thanks!

Edit: Do I need input and out cap with that opamp? Or something else?

[Fender3D]

...
Too bad, I hoped in something easier. I worked for a veroboard layout and the board was very small, 11 x 11. I have to use a dual op-amp, maybe I can contain the size.

Dual op-amp has the same size as single op-amp, it's just 3 tracks more...

Edit: Do I need input and out cap with that opamp? Or something else?

No cap, but you should bias it

[Elijah-Baley]

Thank you.
Yes a dual opamp. Just I have to move something on the board and add mabye some parts. I already saw that the output stage solve the issue.
Anyway, I'll tray a thing: I'll make it more complex than I can, and then will decide which parts we can leave out.

[Rob Strand]

The schematic I create (but not the circuit built) just cut a wide range of frequency. I'd like to know what you think about it.

The bridge-T circuit you used is good.   

There's a few different ways you can achieve variable depth.  The SWR Bass amp's are more or less similar to yours.  There's also the Mark-bass and Eden methods.   

For interest, check out the Yamaha NE-1 Nathan East bass preamp.   Schematic in the web.   The same circuit was used on one of the Fender Jazz basses.

[Elijah-Baley]

Thanks!

This is the Yamaha:



It's very similar to my schematic! Except the low pass switch, that in my case, trying it, cuts a lot with low frequency setting, but for the rest of the range is nice.

The notch control is a bit different, it works almost the same, but my control is a bit more flat at min and cut a little more at max. I want to make some other tests to be sure which of those I want to use.

In the end I think I could use the veroboard layout of the Yamaha NE-1, it's just bigger than mine. In case I'll redraw it by myself.

I'll post later my schematic just to learn something more.

[Fancy Lime]

The schematic I create (but not the circuit built) just cut a wide range of frequency. I'd like to know what you think about it.

The bridge-T circuit you used is good.   

There's a few different ways you can achieve variable depth.  The SWR Bass amp's are more or less similar to yours.  There's also the Mark-bass and Eden methods.   

For interest, check out the Yamaha NE-1 Nathan easy bass preamp.   Schematic in the web.   The same circuit was used on one of the Fender Jazz basses.

Out of curiousity: what be those Mark-Bass and Eden methods thou speaketh of?

Andy

[Rob Strand]

Out of curiousity: what be those Mark-Bass and Eden methods thou speaketh of?

The essential idea, is to start with a passive band-pass filter, like a wein-network,



To get a notch you want,

    notch =   1 - band_pass

If the peak gain of the band-pass filter is G0 then the attenuation of the notch is 1-G0.

The way it's implemented is an inverting version of that,  so you feed the input signal into an inverting
gain stage.    Then you feed the bandpass into the non-inverting input of the same opamp to get the
subtraction required to create a notch.

The next step is to make the notch depth variable.    This is done by simply feeding the input of the
band-pass filter from a "volume" control.   When the control is on minimum, the gain is just the inverting
gain.   When the control is on full you get the deepest notch.


It's very much like the George Giblet parametric equalizer,

https://www.diystompboxes.com/smfforum/index.php?topic=95483.0

The difference is instead of the boost/cut pot connecting to the output of the opamp you connect the boost-end of the pot to ground.
That way you only get cut to flat adjustment, and no boost.

That's the basic idea.    The real circuits add a few caps so the high-frequency gain isn't level with the
low-frequency gain.   The SWR aural enhancer has the same sort of "tilted" response but it's basically a notch.

[Elijah-Baley]

I played around my schematic, and the Yamaha NE-1, too. In my schematic I added a big cap from the feedback of the opamp to the ground, I added the output stage, biased, with its input and output caps, I added a low pass filter to cut the ultra high frequency before the Volume pot. I included the low pass switch of the NE-1. I changed some values of the Notch section to get a wider range until I reach 74Hz - 7.5kHz. I tried the Q pot like the NE-1 and my Cut pot.
In the end I found this schematic imperfect. Both the two options of Cut/Q pots don't work in a well balanced way on the whole frequency. I don't want to write all the setting, but in a few words there are tape or value problems to make one of this pot work fine at any frequency setting.

With this schematic the better thing I can do, and that is not really good, is to use the log 100k Q pot like the NE-1 and get a good setting just with the mid frequency. With higher frequency setting the Q pot cut at max unless is at zero; with lower setting the cut is just around 5db instead 10db. I'm thinking of add a switch to cut the bass, changing the cap named C12 in the NE-1. I'll get a low cut mode, a different thing, but it could work nice.

Or, I could abandon the idea to get the wider frequnecy possibile, keeping just the mid frequency where the Q pot works better. Not a bad idea, acutally, but I'm a little sorry.
In case I still can help the handling of the sound with the low pass and the low cut switch mentioned before.

[Rob Strand]

The actual Yamaha NE-1 is a bit different to the one above, which is a DIY version,

[Elijah-Baley]

I'll see how it works.

[blackieNYC]

I think 75 Hz to 7.5 kHz is a very wide range for one pot on an eq.  the slightest touch on that pot will result in rather drastic changes in frequency, in terms of notching a guitar signal.