Any demand out there for a passive filter pre-ish system?

superfunk47 · #1 03-11-2009, 09:19 PM

Hey all,

I'm really not sure of the terminology to use to describe this, but would there be any interest out there for a sort of filter based preamp (of sorts)?

I'm talking passive and cut only, but of any number of pots cutting any number of frequencies of your choice - sort of an expansion of G&L's passive tone control on L2000's. You could do anything from just a single treble, mid, or bass cut to 5 bands or more of cut, or even a 3 band with a sweepable midrange selection. Come to think of it, you could have bass, mid and treble, each with their own dedicated pot for frequency selection. But it'd be entirely passive.

Does that sound like something anyone out there might be interested in?

SGD Lutherie · #2 03-12-2009, 07:39 AM

I had exactly that in my Rickenbacker back in the 70's. I had two differently voiced high cuts, an LC mid cut, and a bass cut.

I didn't find it as useful as an active preamp, but it was cool at the time.

Stealth · #3 03-12-2009, 08:47 AM

SGD - how the heck did the LC mid cut work? Do you have any schematics for it? In particular, how do you set the mid freq and what controls the amount of cut?

Brado · #4 03-12-2009, 08:55 AM

Sounds like it could turn into a pretty complex setup. Lots of knobs & switches... I know that Adler made basses with a completely passive preamp which allowed the kind of tone shaping most would think was done by an active pre. What about a varitone? They have some cool mid shaping voicings which also (as a result) shape the lows & highs.

scotch · #5 03-12-2009, 08:58 AM

Yes, I'm into the idea! (It's sort of been done already, though - search for "varitone" and "birdsong b-box") Birdsong Bbox

I like your idea of multiple pots & frequency options- that would be cool, but many will argue for an active multiband when you get that complex. Still, passive cut has it's advantages, for sure!

Bassamatic · #6 03-12-2009, 09:00 AM

The reason that this is not commonly done is because the passive filters cut out a lot of signal level.

Simple filters are quite broad and not too selective (think of the tone knob), and complex filters cut the signal down tremendously.

scotch · #7 03-12-2009, 09:06 AM

Quote:

Originally Posted by Gio S

The reason that this is not commonly done is because the passive filters cut out a lot of signal level.

Simple filters are quite broad and not too selective (think of the tone knob), and complex filters cut the signal down tremendously.

Great point! Still, I do get a lot of use out of my B-Box in the studio. Cutting too many bands at once would be problematic, but if the "q" could be narrowed I could get a lot of use out of such a device. Oh wait, that's called a graphic.

DanSwain · #8 03-12-2009, 11:27 AM

I'd be interested in something like this too. I wonder, rather than the cutting circuit going to ground, would it be possible to 'return' the unused signal back to the input stage?

allexcosta · #9 03-12-2009, 11:34 AM

Doesn't Dingwall use it on a regular basis?

SGD Lutherie · #10 03-12-2009, 01:44 PM

Quote:

Originally Posted by Stealth

SGD - how the heck did the LC mid cut work? Do you have any schematics for it? In particular, how do you set the mid freq and what controls the amount of cut?

You can either do a varitone circuit, or like they did on the Gibson Ripper, which was one setting from a varitone with a pot.

So you just need a 1.5H inductor, a cap and the pot.

Gibson actualy used a 1/4" bolt with three washes and a nut for the inductor core! Then it was wound as a humbucking pair of coils with 36AWG wire.

I'm actually about to make one for a project and might offer them on my site if it works well.

superfunk47 · #11 03-12-2009, 02:41 PM

Quote:

Originally Posted by allexcosta

Doesn't Dingwall use it on a regular basis?

Sort of, but theirs is strictly a mid cut, IIRC. This would take it to another level and give you the option to cut...well, really any frequency you like. The bandwidth cut and amount of cut would be variable, as well; lots of options out there for that. So unless you wanted a huge shunk cut out of your mids andthe ability to dial it down like 20dB, I wouldn't think the effect on the output would be problematic.

I have a bit of experimenting to do before this will be a feasible product, though. My goal is to get options out there for the cats who like the tone shaping options that an active pre gives you, but don't like the idea of running active for whatever reason. Shouldn't they have aftermarket pres too?

Stealth · #12 03-12-2009, 03:26 PM

SGD - awesome. Thanks for the schematic.