SLOB project

[Ray P]

Thanks Steve, I had assumed it was something to do with those other 'ms parameters. Rms doesn't seem to appear on a lot of loudspeaker datasheets though, is there a way to derive it?

Cheers

[Cressy Snr]

Well here is the formula for calculating Qms, which includes the Rms term but don’t ask me how to rearrange the bloody thing for Rms. My algebra doesn’t go that far.

[Nick]

Just swap over Qms on the lhs with Rms on the rhs.

Rms = sqrt(Mms/Cms)/Qms

[Ray P]

Thanks Steve and Nick.

Steve, if it's any consolation, its so long since I've needed to do much algebra that I would have had to work at rearranging that too...

Ray

[Ray P]

So it looks as though Mark isn't going to be a fan of Nelson Pass's choice of bass unit in his SLOB design;

[simon]

Steve, Ray,

If it's any help the magic key is that as long as you multiply both sides of an equation by the same thing you can multiply by anything you like. So, to make the equation read as Rms =, first multiply both sides by Rms. So the equation becomes:

Rms x Qms = Rms x 1/Rms x sqrt(Mms/Cms)

Handily the two Rms terms on the right hand side of the equation cancel (Rms÷Rms=1)

So the equation becomes:

Rms x Qms = sqrt(Mms/Cms)

The last step is to "remove" the Qms from the left hand side, so we divide both sides by Qms (or multiply both sides by 1/Qms, which is exactly the same thing). So we get:

Rms x Qms / Qms = 1/Qms x sqrt (Mms/Cms)

The Qms terms cancel again (anything divided by itself is always 1) leaving the equation Nick gave.

[Ray P]

Thanks Simon, that's helped to dislodge some dormant grey matter...

[IslandPink]

Good work on the maths, everyone !
I have tended to use Rms = 2.Pi.Fs.Mms/ Qms to check drivers for which it's not stated - but I see now I could use Cms for a slightly simpler calc.
See how it goes, Ray - 2.9 is not that high for modern bass drivers, it'll help at bit if you have a low-impedance SS amp driving it.

[Ray P]

Good work on the maths, everyone !
I have tended to use Rms = 2.Pi.Fs.Mms/ Qms to check drivers for which it's not stated - but I see now I could use Cms for a slightly simpler calc.
See how it goes, Ray - 2.9 is not that high for modern bass drivers, it'll help at bit if you have a low-impedance SS amp driving it.

Thanks Mark. The calculation was mainly out of interest as I've already decided to stick, at least initially, with the Eminence bass drivers specified by Nelson Pass because they represent a low risk option giving known good results, especially as the Lii Audio full-ranger is already a deviation from the original. I plan to use a chip amp to power the bass driver.

[Cressy Snr]

Thanks Simon/Nick.
That helped.

[Ray P]

The tracking information tells me that the Lii Audio drive units are in Birmingham and should be with me on Monday or Tuesday.

[Ray P]

They're already in Bristol!

[Ray P]

Anyway, after some further research I found a project on AudioCircle where a guy is doing a very similar build except that he's using a Cube FC8 for the full range driver, but exactly the same Eminence bass units as I plan to use. Fortunately, he posted a schematic of the Pass LXmini active crossover with values specified by Nelson for the Eminence and Cube drivers. I reasoned that those settings won't be too far away from what I'll need so I plugged the values into Micro-Cap, which gave the following results;



I think that gives a pretty good baseline.

[IslandPink]

Exciting times !

[Ray P]

Ed, I tried to reply to your PM about the Pass active crossover but it seems you've disabled PM receipt?

Ray