Bridged T notch circuit

[bancika]

Hi folks...
I'm thinking about adding a notch circuit after 6021 gain stage to do mid scoop.
I found this explanation by stm http://www.diystompboxes.com/smfforum/index.php?topic=50531.msg378167#msg378167
Based on that I built a model in duncan tsc but when I change input impedance from 1k to 6k (which seems to be the output impedance of 6021 gain stage) lots of treble is lost. Anyone has formulae that involve input impedance?
Tnx

[aziltz]

make it atleast 10 to 100 times higher that the output impedance of the stage before.

have the same impedance in both causes your signal to get cut almost in half because you are loading down the output.

[bancika]

I realized that, but how should I adjust the values to compensate for higher impedance

[earthtonesaudio]

Multiply the resistors, and divide the capacitors by the same number.

[bancika]

thanks! that's what I needed 

[aziltz]

sorry i misunderstood the question.

[bancika]

no worries, I may be vague

[bancika]

Multiply the resistors, and divide the capacitors by the same number.

I just tried doing that for 10x the original impedance and it keeps notch and treble ok, but now I get lower bass response. Below are two versions with 1k and 10k loads.
Any ideas?

Thanks

[earthtonesaudio]

You kept R2/R4 the same...?

[bancika]

hmm, I though those don't affect anything.
20M is there just to make it act as n/c, so it didn't change the response, but 1M->10M made the difference.
Thanks!

[liquids]

That's quite a 'notch.'

Anyhow, I don't think you are really doing what you think you are doing... changing R2 isnt affecting the input impedance so much as controlling the freq response in conjunction with C1, like a simple low pass filter.  Check out the AMZ filter calculator: http://www.muzique.com/schem/filter.htm

You can always mess with the cap values.  Note that things are flat if you make R1/R2 the same and C1/C2 the same. You can go from there --22k/22n are typical starter values, from flat.

Note that the greater the difference between the Caps (if the resistors are the same value) the greater the mid scoop (or mid 'boost,' as it were), while simultaneous affecting the frequency of the scoop...

I'd try something more like what is in the superfuzz for an alternative...

But If you really want a true notch, I'd look into something more along the lines of an op amp-based active filter, or with an inductor, even...

[bancika]

actually, what I'm hoping for is to get frequency response similar to V shape on graphic EQ, something like this

[liquids]

I see - interesting.  I would study that specific, active EQ.  You can't necessarily follow the 'graphic' in such an EQ with 5 bands.  It may be 5 'small notches' with lots of untouches frequencies in between or have a lot of bleedover...

I'd really recommend building a graphic EQ, than...there's too much beautiful subtlety to be had in the slight adjustment of one of five or ten sliders, especially when you are scooping so much and so deeply...

IF you want to keep it simple, check out Mark Hammer's posts on the subject, along the lines of the superfuzz tone control..not sure it has an easy visual 'tone stack calcuation' but it's a bit simpler, and has some adjustment for depth of notch.

http://www.diystompboxes.com/smfforum/index.php?topic=75716.0

[earthtonesaudio]

If your tone control is getting overly complex and unmanageable when you try to fit it to your circuit, maybe the better approach is to change the circuit to fit the tone control.  Simply adding a buffer before and after the tone stack will make it perform better without having to resort to high resistor values (which can cause other problems like noise).  If the circuitry that comes after the tone stack is high impedance, then you can omit the output buffer.

Building a full graphic EQ to do this would be very ambitious and awesome, but perhaps over-doing it just a bit.   

[aziltz]

you would need to raise R2 accordingly, it forms the high pass filter with C1

[liquids]

If your tone control is getting overly complex and unmanageable when you try to fit it to your circuit, maybe the better approach is to change the circuit to fit the tone control.  Simply adding a buffer before and after the tone stack will make it perform better without having to resort to high resistor values (which can cause other problems like noise).  If the circuitry that comes after the tone stack is high impedance, then you can omit the output buffer.

Building a full graphic EQ to do this would be very ambitious and awesome, but perhaps over-doing it just a bit.  

Nah, with the help of RG's page and/or just getting the BYOC kit (or heck, find a graphic EQ used and plug it in), you can have a great 'graphic'/active EQ (even if it does use knobs instead of sliders) without much more trouble than is building any other effect.  I think they're worth their weight in gold, if you don't have a 'killer tone' from your amp, and/or want/need a way to get radically different tones happening.  I had two at one point, and it sure helped me do both of those thing affordably when I was very unhappy with my core amp tones, and wanted some extremely different sounds at the tap of a switch--endlessly variable EQ tweakability with only one source of overdrive.

True, it might be daunting to ADD a true graphic EQ to a pedal, but getting a big box and adding 5+ knobs to a build is not anything unheard of--see 4ms.  Its probably cooler to have it but not hard wiring it 'within' another distortion-type circuit, so it could be used independently/or in conjunction with other effects...or you could put two in one large box with close foot switches to make turning one or both on easier...etc, the possibilities are many. Following, most of those Mesa amps have the graphic EQ as a on/off 'stage' within the amp, so it fits the original idea if you seperate it in some regard.

Cheers!

[WGTP]

http://www.aronnelson.com/gallery/main.php/v/WGTP/BassBoostNotch.jpg.html?g2_imageViewsIndex=1

I have been using this notch filter on my bread board, for several years with various values of both caps and resistors and I love it.  You can get a vintage notch or a deep one.  I just stick it at the output of the distortion.  Op amp, CMOS, Mosfet, Jfet, works for all of them.

You can use the Duncan Tonestack, James version and cancel out some values and simulate this filter so you can see visually what is happening.  All resistors can be replaced with pots for a great variety of tones.  Or switches could be used for different resistor values.  Caps can be socketed.  

I usually end up with between 4.7n and 15n for the high mids of the notch and between 47n and 100n for the low mids.  Varying the resistor values changes the shape of the notch.  Changing from 10k resistors to 22k lowers the notch an octave.    Doubling the caps does the same thing.  Using a 1k resistor at the input maximizes the highs and increasing it lowers them.  

[liquids]

I've never tried it, but that's the one I was referring to as 'the superfuzz tone control' that Mark Hammer has mentioned from time to time. He has some interesting mods for it too. 

glad to hear it gets good reviews...now I'm interested.   

[WGTP]

Yes, this is the same as the one Mark mentions, and also the same one discussed by R.G. at Geofex.   

[liquids]

I've had some interesting findings recently regarding tone stacks that come back to this, per this post http://www.diystompboxes.com/smfforum/index.php?topic=79206.0  ...when you look at it, and add in Mark Hammers mod (which is puts a variable resistor between the cap and ground), this is really just a simplified Blackface fender/Marshall type tone stack, right?

Think about it...the input resistor is the loading of the stage and in some ways sets the insertion loss and some of the frequency response, like if it is fed an a cathode/emitter follower or fed from the plate/collector of a gain stage...

The first resistor toward the cap that hits ground is the tone stack 'slope' resistor, 100k in the fender.  The small cap across the top is like the treble cap. The second resistor to the cap is like the treble pot, which in the tone stacks is replaced with a pot wired as a voltage divider rather than a variable resistor, to vary the content of signal passed through the 'high frequency' cap just mentioned. The larger value cap to ground is the mid cap (for scoop)--in the fender, it's wired with a variable resistor to ground like Mark Hammer's 'mod'..

Linking my understanding of the bass pot is confusing to me, since it is in conjunction with the with the voltage divider treble pot....likewise, simplifying the circuit in TSC to reflect this simplified version I can't get a scoop for some reason.   Can some else help fill in the gaps there, or correct my understanding?