Variable High-Pass Filter.... (Pro Question)

  • Thread starter Thread starter Blue Bear Sound
  • Start date Start date
Gotcha... I was thinking about blurting out the obvious myself, but I decided against it... looks like you didn't :)
A serious question, though... how could that POSSIBLY be a vintage design? Its BRAND NEW in a box! ;)
 
Its based on an old design - and there is absolutely nothing wrong with that.
 
That was just for those who think its wrong to buy something when its old, new, 'coz it shouldn't cost as much when its new when its old, as old isn't new, and old must be cheaper than new:confused: :D
 
Blue Bear Sound said:
Hey you without shoes! (hmmm.... I'm sounding like Green Hornet!)

Thanks for the info... just out of curiousity, what would you charge for your little box?

What I need -
- 2 linked channels (ie, 2-channel but controlled by a single set of controls.)
- variable frequency on the hi-pass filter
- level control on the amount of cut
- steep slope 12db/octave or better, 18...
- +4 dbu gain structure, unbalanced is ok...

I think that covers it!

Bruce
Click to expand...
Bruce,

Well, if I were to build one it would be for the fun of it and not money. The parts cost for something like I described would be about $500 or $600.

If you want a single frequency control for both channels, then I would stick with a 12dB/oct filter. A 24dB filter would mean either using an 8 ganged pot, or a voltage controlled amplifier design (18dB doesn’t easily lend itself to frequency adjustment). Either way you’ll sacrifice sound quality and/or reliability. Plus the cost really goes up. I haven’t priced the tightly matched Penny & Giles 8 ganged pot, but I’m sure it’s damned expensive.

I could pull off something for less money using a standard +-15V supply and one of the old chassis I have laying around. Nice metalized rather than film-and-foil caps would also cut the cost. We could likely get the component cost down around $300. It would still have outstanding performance, but wouldn’t be as “overkill” as the other.

I came up with this preliminary circuit design. Notice all those small 1k resistor values. The AD797 can easily drive these kinds of feedback loops with absolutely no distortion penalty. The advantage is that resistor noise is significantly reduced. To these same ends I limited the frequency adjustment range to about 10-30Hz for optimal resistance values in the crossover. The range can go as high as you want, but this requires a larger pot resistance value which eats away more of that 120dB S/N. I suggest sticking with the narrowest frequency range which meets your needs. I used the OP77 for the lowpass level buffer since it works much better than the 797 looking back into the higher source resistance. The OP77 is even quieter than the 797, but it’s lower slew rate (speed) makes it more appropriate for low frequencies.

Anyhow I won’t bore you with more tech talk, but this sounds like a fun project. I’d definitely be into working on this with you. For my trouble maybe we could just arrange for me to get a new soldering iron or something out of the deal. Mine is on it’s last leg. The only problem is my time is very limited (isn’t everyone’s). I can’t promise a quick turnaround. It would probably take a couple of months to finish. Let me know what you think. :D

barefoot

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Thanks for looking into that barefoot! Let me mull it over a bit....

Bruce
 
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