HP Shelving Filter Questions

Hi,

A client has asked me to add a sweepable HP shelving filter to an item of his. He claims that his previous tech made such a device with a passive / inductor circuit...(said tech is not available for comments...). I'm not aware of how such a thing could be done without an active circuit, i.e. a state-variable filter. Not only that, but even a passive circuit with greatly limited sweep range is a) going to need to be driven from a relatively low-Z source, and b) need some make-up gain on its output. By the time you've factored everything together, is there any point to the passive solution?

How many ways are there to make a passive HP filter? Can a passive be swept at all? Can anyone post links to examples?

Thanks in advance.

Justin

Maybe what he calls sweepable is in fact switchable?

Michael

A shelving filter requires one pole so should be possible with a variable R and fixed L.. I'm not sure why someone would want to use an inductor these days but the customer is always right.  Simplest would be use two opamps, one to make variable HPF, and second to add/subtract that to make shelf.

I have never used inductors to make audio filters, so I don't have any practical experience, nor desire any.

JR

JohnRoberts said:

A shelving filter requires one pole so should be possible with a variable R and fixed L..

JR

Click to expand...

You mean something like this: http://home.online.no/~jaeioluf/sound/eqpass/lshelcut.gif ?

The customer seems to want something with an adjustable shelving point, sweeping from the lowest sub freq to the highest, thus eliminating the whole signal if desired... I have no idea how you'd do this passively. Anyone invented the adjustable inductor? 

Maybe I should just make something like the above link and see if he's happy?

Thanks

Justin

thermionic said:

Anyone invented the adjustable inductor? 

Click to expand...

Adjustable inductors exist since the invention of radiotelegraph. There are several variants: one is similar to a rheostat, with a moving wiper on a single layer winding, another uses a moving rod into a hollow bobbin, another uses two bobbins whose coupling can be varied by modifying the respective orientation of the flux lines. RM type ferrite cores exist with an adjuster nut, nearly all FI coils in a superhet receiver are adjustable.
Neither of these are practically usable in a variable audio filter because the tunable range and/or the inductance and/or Q are not high enough.

I should've said 'adjustable inductor with sufficient range'. Last time I looked, the likes of Antique Electronic Supply still sold the finned adjustable inductors. Cheap they ain't. I think there was a conversation here a while back about trying them in an EQ.

J

thermionic said:

JohnRoberts said:

A shelving filter requires one pole so should be possible with a variable R and fixed L..

JR

Click to expand...

You mean something like this: http://home.online.no/~jaeioluf/sound/eqpass/lshelcut.gif ?

The customer seems to want something with an adjustable shelving point, sweeping from the lowest sub freq to the highest, thus eliminating the whole signal if desired... I have no idea how you'd do this passively. Anyone invented the adjustable inductor? 

Maybe I should just make something like the above link and see if he's happy?

Thanks

Justin

Click to expand...

You don't need a variable L.. the variable R with fixed L will give a variable pole frequency.

So Fixed L variable R to make the variable pole frequency then send the results of that to boost/cut stage to make shelving EQ.

You may not find many examples of this, because IMO it's not a great idea... but it will be different, which is attractive enough to some.

JR

Why 'shelving'? Does he need more storage?

Lo-Z out. Fixed series C. Variable shunt R. Hi-Z input.

0.5uFd, 10K pot, 1K fixed. 34Hz-340Hz.

It does not "shelve", but it cuts the mud, and that may be what he really needs.