Feasible to make a line-level 3rd-order infrasonic filter?

[noah katz]

Hi,

Is it feasible to construct a line-level passive circuit consisting of three cascaded RC filters?

I guess the showstopper, if any, would be insertion loss.

The amp is a Crown K2 with 10Kohm input impedance.

Could someone tell me where to look for design formulae?

Background:

I have two ported subs using Avalanche 12, tuned to 18 Hz.

I have occasional bottoming on aggressive soundtracks, which I'd like to fix with a 3rd-order (is Bessel the recommended type?) HP filter at, say, 16 Hz.

Thanks

Noah

[Tweeker]

Insertion loss will be very large, may need a preamp. You might try something like a second order high pass at say 20hz on such tracks instead.

PLLXO

[noah katz]

Thanks, Tweeker!

Looks good, 2nd order should be fine.

I guess I calculate the insertion loss by ignoring the caps and calculating the voltage divider affect?

Thanks

[Tweeker]

RC high pass filter is a nice little tool that might help you out.

[phase_accurate]

AFAIK the K2 already has a 3rd order HPF. If you are not afraid of tweaking that you could try to change its cutoff-frequency.

Regards

Charles

[noah katz]

Thanks for that new link, Tweeker.

That one doesn't have second order, and doesn't seem to account for the amp impedance the way the first one does.

Speaking of which, can I just use the 10K amp impedance as R2 and dispense with the extra resistor?

Thanks

"AFAIK the K2 already has a 3rd order HPF. If you are not afraid of tweaking that you could try to change its cutoff-frequency."

Right, at 8 Hz, but it's a lot easier to add an outboard filter than to dig around in the amp.

[richie00boy]

You cannot add a 3rd-order passive filter to that amp because the amp input impedance is quite low at 10k. Even 2nd-order would be a struggle. And you would need a strong source.

The filter will be very droopy and affect the passband.

You need to build an active filter, which will give far superior performance in all respects, and can be very small. I think I might have a pair of 3rd-order active filter PCBs lying around at home, or could find a layout

[Tweeker]

Speaking of which, can I just use the 10K amp impedance as R2 and dispense with the extra resistor?

If you could you could do it on the first order filter also.

This second order filter is just a cascade of two first order filters. Use the equations from t-linespeakers.org or Art Ludwig.

To use the java tool, you need to add in the amp impedance, it lies in parallel with R2.

The Java tool aggrees with Richie, effect on passband and high insertion loss.

[Tweeker]

On the bright side, they are cheap and easy to try.

[noah katz]

Tweeker,

"If you could you could do it on the first order filter also."

I don't understand.

Looking at the schematic for 2nd-order HP at t-linespeakers.org, the amp impedance can be merged with R2, but there's nothing directly in parallel with R1 to merge it with.

Richie00boy,

"The filter will be very droopy and affect the passband."

Yikes, if R1 = R2/10, doesn't that automatically make the insertion loss 20 dB?

But why can't R2 be, say, R2/4?

"I think I might have a pair of 3rd-order active filter PCBs lying around at home"

Sounds interesting. Does it include a PS, run off a wall wart, or ?
I just need one BTW.

You can email me details at noah.katz at lmco.com

Or is there something I can just buy ready to go?

Thanks