TwinT or Bridged-T and Why?

[Bill Mountain]

I'm working on a simple notch filter for my bass guitar.  After lots of experimentation, I've settled on around 700Hz as a good frequency.  Now I can't figure out what type of filter sounds best. 

I tried a Bridged-T first and it had a nice scooped sound but I worried that I was losing too much low end information as it felt a little thin.

I then tried a Twin-T which was fuller sounding but I felt didn't give the desired effect (still too much mid-range "clack" in my signal).

I both cases I needed extra equalization to get the sound I wanted.  I know I can make either work for me but I was wondering if there was a reason to choose one over the other.

The Bridged-T would be nice because I can vary the frequency easier but I don't plan to do much of that.

I was also considering a gyrator but I was hoping to avoid too many parts but it is of course an option.

Thoughts?

Ideas?

Criticisms?

[dschwartz]

The twin tee has a higher Q factor, that means a narrower scoop, that's why you gained body, but also gained mids, in contrast to the bridged t..

You can get somewhere between by using a twint tee, and putting a resistor between the cap to grnd and ground, that will lower the Q.

[Transmogrifox]

The twin tee has a higher Q factor, that means a narrower scoop, that's why you gained body, but also gained mids, in contrast to the bridged t...

As a passive RC network, "Q factor" isn't very meaningful.  The twin T uses a relative phase shift relationship to create a very narrow and deep notch, but not quite as narrow, as say, a high-Q second-order notch filter can do (although the Twin T can get a deeper notch with well-matched components).  The bridged T also uses the phase cancellation to create a notch, but is unable to line up for perfect cancellation as in the Twin T.

This small technicality aside, this is absolutely right:

You can get somewhere between by using a twint tee, and putting a resistor between the cap to grnd and ground...

As often is the case, RG has a nice page on this topic:
http://www.geofex.com/article_folders/eqs/paramet.htm#bridged_t
Particularly of interest is this:

Notice how by changing the resistance in the leg of "2C" that it approaches illustration #2, which becomes a bridged T network.  That said you can put a pot in that capacitor leg and adjust for any amount of scoop in between. The notch frequency will move down in frequency slightly as you change the resistance in that leg, but it stays less than 20%.

In the "2C" leg place a value of 2R and you will get a notch of about -12.5 dB, which you will probably find is nearer to a natural sound.

Keep in mind the twin T is a really wide-band notch even though it might be considered relatively narrow band if measuring the -20 dB bandwidth.  The -3 dB to -3 dB bandwidth is about 4*Fc.

Interestingly the -3dB to -3 dB bandwidth of the bridged T is about 2.6*Fc, so you can see that the Twin T is effecting the range far out of band more than the bridged T.

If you want to narrow the bandwidth but make it deeper, but still keep it passive you will probably need to consider using an inductor.  My guess is you will be able to find a decent scoop sound by adjusting the value of an R placed in series with the "2C" leg.

Also remember both the Twin and Bridged T networks expect to be driving an "infinite" impedance, or in the practical world of circuits, an impedance that is "much much" higher than the resistors used in the bridge network.

[Bill Mountain]

This is some good info.  Thanks everyone!  I looked at R.G.'s article a bunch while working on this design.

I though about it deeply and what I want is the Bridge-T (for the wider scoop) but with more bass response so I'm either going to make it slightly variable in frequency and adjust it by ear for the best response or keep it where it is and add a Bass boost control (active or passive).

[Transmogrifox]

I thought about it deeply and what I want is the Bridge-T (for the wider scoop)

Per my post above, the Twin T actually has a wider scoop, but it's also much deeper, and gets increasingly narrow so you pretty much have to center it right on the stuff you want to get rid of.

That said, I agree using the bridged T with some flexibility to tweak the notch frequency a little bit sounds like a good decision to me.

Your mention of maybe adding a bass boost made me think of the BMP style tonestack. You can put the scoop wherever you like and as wide or narrow as you like by adjusting the relative high and low cut-offs. Especially if you cascade stages you have a good deal of control over both width and depth by adjusting the relative high/low cut-offs.  You can get more narrow and deep by adding more filter stages:
http://www.muzique.com/lab/tone3.htm
http://www.muzique.com/lab/dtone.htm

Particularly taking this idea into consideration:


I actually stuck a BMP tonestack like this into a friend's bass that had a mid-range hump that made it really muddy sounding.  I adjusted the high and low cut-offs by ear to get the scoop where I needed it.  I just rewired the pots to Front/Rear Mix -> BMP tonestack -> master volume.

Sounds like an entirely new Bass, not so "thuddy".