Bootstrapped Twin-T Would It Work And Where To Start?

[vanessa]

I started work over the weekend on this fuzz:

http://www.geofex.com/PCB_layouts/Layouts/Maestro_BB-1_Bass_Brass_Master.jpg

I noticed that it has a Twin-T filter section that most members of the forum have complained that both sections do not seem to change the tone of the circuit. Another forum member said they have an original unit (that the filter section works) and remembered that the "top right values were different than the values" from the values found in the schematic that's floating around (above).

I wondered if using a "bootstrapped" Twin-T in place of these top right values (both 6.8k) might yield a broader range of tone options? What would be a good value for a dual ganged pot to get a wide usable range? Would it even be worth having a switch (maybe just get rid of it?)?

Here's the bootstrapped article:

http://www.geofex.com/Article_Folders/EQs/paramet.htm

[Rob Strand]

There's something not right with that top right twin-T filter section.  I haven't tried to simulate the filter but as it stands it looks wrong to me. 

I believe the left arm where C10 and R25 join, which floats,  should be grounded.  If it's not grounded the filter will be radically different and there will probably be little filtering going on.  I think you should try that instead of modding the circuit.

One thing to note about that circuit is the input (from Q7) drives the central "legs" of the twin-T.  That forms a band-pass filter instead of a notch; the band-pass form isn't well known outside of filter-geek circles.  The more common twin-T arrangement drives the left arm and grounds, or bootstraps, the central leg.

Boostrapping can of course provide more options for filters but you don't want to go putting in notch Twin-T if the the original circuit was a bandpass and possible had errors!  I think there is a way to boostrap the band-pass form but see if you need to do it at all first.


(Ed: Another possibility is the C10 and R25 node connects to Q6's collector - I would have to look closer to confirm what this does).

[lowstar]

hi vanessa,

you misunderstood me. i don´t have an original, i built it using the geofex layout, but my switches work and do change the sound.
while building it, i noticed that two resistors on the upper right side (6K8x2) had no name, so i followed the traces on the pcb to see which numbers they were supposed to have, and found out on the way that what is r22 on the schemo is r27 on the pcb.

so what i figured out, look at the schem you´re linking too, upper right-hand-corner:
the two 6K8 resistors are r24 and r22
the 22K resistor is r27
the cap (0.01) between those two is c11

that´s how i built it.
the brass hi/lo changes the sound from bottom-heavy gnarly to bright-gnarly, and the harmonics hi/lo switch works only in the "brass hi" position, providing two different flavours of the bright sound.

now maybe somebody with a lot of experience could look into this and either conform this or tell me that i´m a total failure  , either way´s fine with me.

hope i could help,
cheers,
lowstar

[Rob Strand]

A quick look at RG's layout shows he has the floating arm going to the junction of R28 and C12.  That looks OK too.

[vanessa]

So maybe if the layout is set up to correspond to the schematic it might be usefull to have that "brass hi/lo" switch be a stomp switch after all (to stomp on the octave up)? Maybe not? I've read that the octave up really only works over the 12th fret. Is this true?

Thank you for all your help!

[gez]

I've read that the octave up really only works over the 12th fret. Is this true?

The octave part full wave rectifies the signal, so it's only going to be as good as any other circuit of that ilk.  Never built it/owned one, but I'd say true.

[lowstar]

imho the brass hi/lo switch is not worth a stomper, but hey, tastes are different...

just build it and see how you like it, and then, before you box it up, you can still decide if you wanna use a stomp switch or not.

cheers,
lowstar