A More Dramatic Bass Cut On My Twin

[batflash]

Hey there,

I've got a '93 Fender Twin Amp that I've been dying to do a tiny lil' mod to. It seems like the bass knobs on either of the channels are not cutting as much as I'd like.

How would I go about modifying my amp to make the Bass cut a bit more dramaticly. Any suggestions or link to help would be great.

Thanks!

Jason

[JohnH]

Jason - Instead of messng with the amp, how about an EQ box before the amp? It could be a simple home brewed one for bass cut, or a graphic EQ pedal from a manufacturer.

John

[batflash]

Hmm... you might have something there. Though... won't that just take the low end off my guitar and not off the amp-affected signal? So, in theory, the amp could still add a little unwanted low end.

Also, the bass knob on the amp really has little effect to the signal. I mean, I turn it to '0' and I'd expect a significant amount of bass cut. But, nope. So I figured it'd be best to just eliminate the root of the problem than put a bandage on it. Plus who want to carry that extra pedal around with them? )

Any suggestions?

Thanks,
Jason

[RandomHero]

Why, oh why, would you want to destroy that beautiful in-your-chest presence? ;_;

In all honesty, an EQ pedal seems to be the route to go, to me. Yes, it's an extra pedal, but think about it. With an EQ pedal, you'll be able to:

1.) Do a lot more than just cut the bass, to suit your tastes as time goes by.

2.) Will be able to cut the bass out of ANY amp, including ones you may own LATER...

[ChrisK]

If I recall, the treble control has the most effect on a Fender tone stack (that crappy, non-feedback 50's approach to cheep tone controls). The middle and bass have a lesser effect.

These most certainly aren't active tone controls, just a lossy variable cut "stack of pots 'n caps.

I may well be wrong, sumgai, unk???


Unless there are some inter-modulation or sub-harmonic issues, I don't think that the amp is going to add bass to the signal.


One can pickup Eq pedals for not much. Try one first. (Or fool with different caps in series with the amp input.)

[UnklMickey]

in general, the Fender / Marshall / Vox tonestacks look about like this:



it's (IMHO) a crappy way to do things.

it's passive, lossy, and although it can only CUT,

in the case of the Fender,

relative to the midrange, the bass and treble can only BOOST.

set all your controls at "5", and you have a curve with with dip in the midrange.

the closest you can get to "flat" is with the bass at 0.5, mid at 10 and treble at 0.




ENTER THE MIGHTY AMPEG!

Ampeg's approach was to use a passive version of the Baxandall circuit, similar to the one shown below.



still passive, but it allows for both boost and cut ot the treble and bass, relative to the midrange.

they also use a midrange control, in a separate portion of their preamps, that is essentially passive when cutting, and active (reduces negative feedback) when boosting.


even without the midrange control, the Ampeg's scheme is much more to my liking.

set the treble and bass to their midpoints, and the output is flat.

you can cut or boost the treble and bass.

and there is much less interaction of the controls compared with the Fender / Marshall / Vox.


unk

[ChrisK]

Thanks unk,

relative to the midrange, the bass and treble can only BOOST

Well, cut less.

still passive, but it allows for both boost and cut of the treble and bass, relative to the midrange.

Well, less cut or cut as compared to the mids (until the mids are boosted)

Yeah, I know "relative to the midrange". But these circuits are all lossy and noisy. Ain't nothin' like an active circuit with noise reducing feedback.

Makes me want to find my old National Audio Applications Handbook and get back into "Floobydust"

[UnklMickey]

chrisk said:

...Well, cut less.....

yup, all depends on where yer standin'.

chrisk said:

...But these circuits are all lossy and noisy. Ain't nothin' like an active circuit with noise reducing feedback....

right again.

the bax adapts nicely to a simple active configuration.

(actually, i think this was the original design, and the passive version was derived from this.)

[ChrisK]

You CAN add a middle cut/boost to this active (?and passive??) topology as well, but anything beyond requires a multi-band EQ circuit with LC's, or gyrators and C's for each band (the gyrator being a single OPAMP sub-incarnation of the Generalized Impedance Converter).

You'll note that the active Baxandall circuit makes use of a capacitor optimization of the passive Ampeg approach. It's not as definitive, but good enough (the art of engineering is all about "good enough").

[UnklMickey]

chrisk said:

...(the gyrator being a single OPAMP sub-incarnation of the Generalized Impedance Converter).

gee Chris, it was better before you explained it.

Elvis and/or dancers in the Gentlemen's Club.