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Looking at a sealed RS270 three way similar to Tony Gee's "BllackBox" with a passive Linkwitz transform

Thinking about a larger box than the 24 Liters he used, but can't find any calculator or formula for the uF capacitor

I would need. Anyone have the formula or link for a solution?

Thanks.

Thinking about a larger box than the 24 Liters he used, but can't find any calculator or formula for the uF capacitor

I would need. Anyone have the formula or link for a solution?

Thanks.

Don, Donno, or "Hey you" all work for me, But never "Mr Johnson"

Tagged:

## Comments

http://www.humblehomemadehifi.com/download/Humble%20Homemade%20Hifi_Black%20Box.pdf

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Sheesh, I already had the PDF from Tony's writeup, and somehow missed the docs on calculation <doofus> - but in any case, y'alls tips on PCD/WBCD should help empirically.

Stock RS270-8 in that box gave a 53 Hz F3. Not good enough!

Scrounged up about 600uF and tried it again. 43 Hz F3 - just about what I was looking for, and

the ZMA plot shows protection for extreme bass. The build is on.

I expected the impedance peak to shift, lower, but no real change. No matter.

Stock (no cap) ZMA

Here is the ZMA with 600uF inline.

Just what I wanted! - stock (no cap) vs added 600uF inline.

Between 125 and 175Hz I swagged the average (max error maybe .25 dB), then manually found the -3dB point.

The Unibox model predicts a .5dB peak at 96Hz for the stock box model, and you can see a little bit of a rise in the

black (stock) FR. OTOH, the box Qt is .86, which is a bit low for the Linkwitz transform, so you see a little droop in the red (600uF) FR.

Comparing the original Unibox model shows it predicts a 53Hz F3 for the stock box, so measurements agree exactly.

I have NO frickin' idea why Unibox F3 and Fb are different between the params and max SPL screens. Wierd.

@ani_101 - I respect your knowledge and analytics. Any feedback?

Yes the unibox figures seem to be switched. Didn't mean to be curt, but i was looking at the images on the phone and couldn't figure out shift in F3. I haven't figured out the low frequency measurements and simulations and seems more trial and error than measurement hence genuinely interested in know how you measured and what the inference were. Also i have very less experience with sealed boxes and none with caps in series before the woofer - i just go with the unibox simulation and may verify the system FB with DATS.

Also, I had to brush up on reading ZMA curves before tying the below question - so anybody reading this, please correct and educate...

Looking at the impedance images - ZMA, the system impedance look similar at the peak, so the system FB is the same, but you do have rising impedance at the lower frequency. I don't know what are the implications for it - would it serve to protect the woofer with lower frequency - rising resistance for lower frequency so lower power delivered? Usually there is rising impedance towards low frequency and this is due to voice coil inductance.

The third image is more interesting as you do see a flatter curve (red) vs the black which is assume is no cap. And the red curve is a bit more extended and given the flatter curve and lower extension, the F3 is indeed lowered by 10hz. What i don't know is how did you measure with omnimic, as usually i can measure up to 400-350 hz and nothing below. Is this purely near-field?

Another question is, Is there a SPL penalty for using the coil? The red curve is lower than the black curve, so looks like there is, but then the black curve would have been flattened with bsc and crossover, so maybe not much in real world after crossover, but you do gain more low end extension.

I am currently working on a box for a sealed project - this looks like a good experiment to try. Another question - what made you try 600uF?

From the PDF:

The basic formula for working out the rough value of the capacitor is: C=(K x Qts) / (Re x fs) . The value of Qts should include all series resistances as mentioned earlier. The value o f the constant K depends on the voice -coil inductance . For woofers with a small voice -coil inductance K = 316.000 and for woofers with very large voice- coil inductance , like many sub-woofer driver s, K = 100.000. For standard types like the Dayton R S270S -8 choose K = 265.000. In this situation the formula results i n a value of about C = 580uF to 600uF. By fine- tuning the final value of the capacitor one also match the loudspeaker with the room-acoustics. If the capacitor i s chosen a little on the small side, say about 10-20% smaller, this will give the loudspeaker a more lean sounding character.I measured using Omnimic about 1/2 inch from the cone, near the RS270 phase plug.

I chose 600uF because that was most of the big NPEs I had on hand, and that's what Tony Gee used.

As far as I understand, there is not a SPL penalty per se, but you do have to pay attention to excursion issues

for the desired listening SPL since you are extended the bass response.

The ZMA rise at low frequencies is super neat, since it will assist the natural protection of a sealed box concerning Xmax.

Tony's original build used a 24 liter box (with a lot of wasted space for a crossover), but he really didn't document the F3 of the original or the transformed system. It may have been that he specifically wanted to start with a box Qt around 1.1 for the mod. I didn't need a box that small, and wanted to get more low end out the the wonderful RS270, and had the larger test box in my shop, so there. It's a workable size, and I can get the extension I want, so I'll probably build the production version that size.

You know how Unibox and WinISD simulations go - I initially used 100 watts, but was dismayed to find that the woofer exceeded Xmax about 55Hz. Then I realized that the passband showed something like 108 dB SPL, which I was never going to use. Reduced power to 50 watts still gives me 105 dB passband, with no Xmax issues, so I should be "golden" at 95dB max even with the added bass extension.

Really terrific mod, I'm looking forward to my first sealed box bass.

Did I answer all your questions?

Once you finish your build - or maybe you already have. Could you listen with the cap and without and give your impression of what the low end sounds like with the rest of the system / XO remaining the same?

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