Hi pass question

[woody]

I have a speaker I want to put on an open baffel and run almost full range. The 6ohm speaker has a QTS of .8 and a fs of 42 hz but
of cource the impedience rises at fs. Just using a hipass cap would there be a way to figure the optimum capacitor value to get sort of flat responce down to about 60hz. I know using impedience compensation at
fs would make this a lot easier but this calls for some realy large caps.

The reason I am thinking about this is an amp I am building the ZENLIGHT
calls for an output cap. I am sort of thinking I might get by here with
a cap in the 100uf to 200uf range if I just want it to go down to 60hz.

[DcibeL]

Unfortunately I think you are out of luck here. Because of the impedance peak at Fs, you will need some very big and expensive parts to create a flat impedance at 42Hz for the cap to roll the response off smoothly.

This is also the reason why the "high level" outputs on some subwoofer plate amps are a terrible idea. They generally just have a 100uF cap in series to act as a high pass filter for the speakers. Unfortunately whomever thought up this idea didn't realize that a speaker is not a resistor, so the resulting response is quite terrible.

What I would recommend is a simple active high pass filter before the amp. If you intend to use the amp always with the same source, then you will know input impedance and can get away with a passive line level filter.

[tsmith1315]

With no impedance compensation, use the actual impedance of the speaker at 60Hz as a starting point rather than the 6 ohm nominal figure.

It would probably be easier/less expensive to filter to the amp's input instead.


edit: sorry for the simultaneous posting, DcibeL