Simple LPF for using full-range speakers with a guitar amp?

[jaapie]

I'm working on fitting my superfly build into an old Philco tube radio. I can only fit about a 4" speaker in the case, so I got my hands on some old 8 ohm stereo speakers that will fit (I'm going to include an external speaker jack so it can be used with a guitar cab and actually sound decent).

I'm not really trying to make this speaker sound great, or even good, but I'd like to make some sort of filter to get rid of all the extra HF over 6k or 7k that would be rolled off with a guitar speaker...maybe roll off some bass as well to compensate for the terrible LF response of such a little speaker? I've done some googling with limited success; evidently a Zobel network would be useful for this purpose, but I'm not really sure where to start. Since this speaker is never going to really sound good, I'd like to use something simpler than a full-blown speaker simulator; I just want to tame some of the high-end harshness. Can anyone point me in the right direction?

[Seljer]

Wire a inductor in series with the speaker. Something roughly in the 200uH to 1mH range I think, depends on the impedance of the speaker and how much cutoff you want. This is just a couple of hundred turns of magnet wire with an air core, something you can easily make (there are formulas and calculators all over the internet). If you want a sharper cutoff add a capacitor to ground after the inductor.

[jaapie]

Could I achieve similar results with a Bipolar cap (or two electrolytics back to back) across the speaker leads?

As far as determining the cutoff frequency, would it be as simple as plugging the DCR (or Z?) of the speaker and the reactance of the inductor/capacitor into the 1/(2*pi*r*c) formula?

[PRR]

The original speaker from an AM radio is probably fine.

It will be more efficient than "hi-fi" speakers.

On AM you do NOT want anything over 5KHz, so they didn't give it to you.

The classic guitar speaker _IS_ the classic Large Radio speaker. The roots have been lost with modern magnets, glues, etc, but the air-paddle side is very much the same.

The other trick is to block the front of the speaker with a small hole. For a 4-inch, a 1" hole will low-pass at ~~1KHz, and also chuff if the speaker makes heavy bass. 2" would be a good trial hole. The holed baffle can be cereal-box cardboard, so you can try many sizes for free.

[jaapie]

Unfortunately, the cone of the original speaker was torn and it didn't work. I'll definitely try the baffle/hole trick, though. Thanks!