604 LF: EV Electro-Voice CX-1 XO module delay calculation

[gearfreak]

This link may work:
http://mypicsonline.net/archive/arch...CX-1%20EDS.pdf

So the EV CX-1 dual octal plugin module allows you to set an XO point (500/800/ or custom w/resistors) and has the option for LF delay.

The delay modules must be built (BMK 'blank module kit') ... but seem to be just 16pin DIPs, of which I have some - though the pins are not as pleasantly long as the factory modules.

R value for the modules is calculated via 2 formulas which I seek advice on.

First formula (attached) is

'theta s' = 360 x f x td1

"where 'f' is the crossover frequency"
"and td is the desired time delay in milliseconds"

but...
- is f in hz or khz?
- is the value we seek called 'theta s' or are they indicating that theta value is in seconds ?
- td may be td, or may be td(sub)1. Can't tell. Does it make some difference?

For td:
Jeff M spec's the ferrite 604's as having a 2.8125" LF/HF offset
so
td = 2.8125in / 13557 in/s @ 70? )
td = .00020745 s = .20745ms

using:
'theta s' = 119491.2 = 360 x 1600hz x .20745ms
vs
'theta s' = 119.4912 = 360 x 1.6khz x .20745ms

119.4912? Does this seem correct?
If so then they intended to use khz?
We're talking degrees here right, not radians?

thanks!

[GM]

Hmm, 13557"/1000 = 13.557"/ms


2.8125"/13,557" = 0.20746 ms


13557"/1600 Hz = 8.47313"


2.8125"/8.47313" = 0.331931843*360 = 119.49539 deg., so apparently it's ms @ kHz


But this ignores the 'td ......@70 deg' notation, so what's at 70 deg?


GM

[gearfreak]

Thanks as always GM!

I should have clarified - it's a temperature ref (F?), for the speed of sound value used.

Once I get the rest of the math down, I can tinker to see how much difference ~5? F or so makes. (probably not much).
Likewise, what tolerances are acceptable for the final R value used in the module.

So now the second formula (attached) :

- we see 'theta s' as numerator is made negative. This doesn't seem to make sense in my final value - am I interpreting that negative symbol properly or is it something else?

- we see f in the denominator has a ', does this mean 'f prime' or is it just an errant period? (...it doesn't look like their other period).
- if it is 'f prime', then is their intent to indicate a 'derivative'? ...what do I do with that mathematically?

If I presume 'negative theta' and ignore the 'prime' notation:
- and using:

theta s = 119.497?

(just because that's what the spreadsheet comes up with at the moment)

then:

(numerator) = tan(-theta s/4) = tan(-119.497?/4) = tan(-29.8742?) = -0.5744274?

and

(denominator) = 0.00000002 * pi * 1.6khz = 0.000000100531 khz


thus:

R(ohm) = -0.5744274? / 0.000000100531 khz = -5,713,933.01 ohm = -5.713933 Mohm


I presume the (-) indicates resistance, so R = 5.7 Mohm - does it seem right?


thanks!


* having trouble with the attachment. Will try again soon.

[GM]

You're welcome!

Yeah, HB is back to being a super slow, frustrating PITA, so combined with my foray over on Facebook being mostly a frustrating waste of time, for now will be going back to only frequenting DIYaudio once done here.

Regardless, all higher math is literally 'Greek' to me, so only know to input formulas into a Ti36X Pro as I see them and accept what it spits out, so FWIW my math agrees except I get ~5.687 Mohms due to more rounding off, which begs the ? of how accurate does it need to be.

No clue why they would set it up to sum a negative number other than to indicate delay, which seems ridiculous to me assuming of course we're reading it correctly, though I notice that the '-' is noticeably longer than the one in 10^-8, so maybe we are.

You got me curious though, so will discuss this with my friendly local computer/DSP/XO guru next week as he only responds to dire emergencies on the weekends.

GM

[gearfreak]

Thanks GM, I appreciate it.

Yes, also begged is the question, why not just provide a constant, instead of:

0.00000002 * pi

I suppose they wanted to allow the user to use their favorite version of pi out to n decimals.

[gearfreak]

I spoke to a friend knowledgable in mathmatics. Thier opinion was that the negative was a negative (for whatever reason), and that the 'prime' was not - instead a comma following the equation.

So presuming my spreadsheet is crunching properly, and with a revised/improved speed of sound value, we're looking at e.g.

@ 70? F air temp, 2.8125" offset [Altec ferrite 604]
= .207794 milliseconds delay

@ 2.0khz = 6.084 Mohm
@ 1.9khz = 5.982 Mohm
@ 1.8khz = 5.888 Mohm
@ 1.7khz = 5.803 Mohm
@ 1.6khz = 5.725 Mohm
@ 1.5khz = 5.654 Mohm
@ 1.4khz = 5.589 Mohm
@ 1.3khz = 5.530 Mohm
@ 1.2khz = 5.477 Mohm
@ 1.1khz = 5.430 Mohm
@ 1.0khz = 5.387 Mohm

Modifying the temp +/- 5? makes a very small difference.
Modifying the offset +/- .1 inch makes a noticable difference.

Seems an awfully tight spec to come up with, I'm not seeing a wide variety of 5.x Mohm 1/4W resistors out there.
(and the small size of the bmk presumes you are using a single value, no combos)

Maybe we're on the right track, maybe not.

[gearfreak]

*Hmm the board is spry tonight.

So the latest: setting the XO point

R = 15900000 / F

9937.5 Ohm = 15900000 / 1600 hz

So I matched 8 resistors @ 9.97 K ohm ea. , and installed them into R22, R23, R24, R25 on 2 CX-1 modules.

9970 Ohm = 15900000 / 1594.78 hz: ...but the HF seemed a bit low to me.


So For comparison, I took out an Altec 15581A set for 1600hz: ...and I'll be damned, the HF seemed about twice as loud.

Both CX-1 had the same 'issue'.

Careful re-inspection of the mini-datasheet on the inside of the CX1 metal cover reveals:

HF Relative Gain:
0 db ...SW1: 2

So the solution: Dip Switch #1: Position#2 on

Yup. That fixed it.



They don't tell you this info or any settings on the actual datasheet (or at least the 5 page PDF version that I have).

Though they do allude with:

Gain HF: Switch Selectable
HF gain setting (on the block diagram)


If someone happens to know the factory default (non-unity) value, or any other HF gain settings, please consider listing for future generations.

thanks for watching