Loading cap effects

[jmarkwart]

Hi All,

I measured the compression driver loading cap effects on impedance and unfiltered sonic output for a stock 604E last month and thought it might be of interest to some here:



The first pic above is SPL measured on axis. Data below 200Hz is not accurate due to measurement window size.



The second pic above shows impedance. Big effect here, especially smoothness above 2kHz.

Jeff

[selmerdave]

Thanks Jeff, very interesting. I don't feel quite like a schmuck for having done it for my E's.

Dave

[RonSSS]

Hey Jeff,

How's things?

I can't seem to see the pictures you posted....

Ron

[bfish]

I can see the plots (Thanks Jeff!), but am unclear on the test conditions.

Is this just the HF section direct, sans XO?

Was it driven with a current or voltage source?

Considering loading cap influences should be non-linear in relation to power, what changes would you predict for large signal or even typical listening levels?

<font color="#FFFFFF" size="1">[ April 08, 2008 03:53 PM: Message edited by: bfish ]</font>

[jmarkwart]

Ron,

Things are good...no complaints...how about you?
The pics I posted are .gif format and I believe all of the browsers should display it without problem. I can send them directly to your Email box if you want me to. Let me know.

bfish,

Yes, just the HF section without crossover. The MLS test signal is generated by CLIO and amplified by a Crown D75 which is connected directly to the device under test.
The sinusoidal impedance measurement signal is generated from, and driven by, CLIO directly in Internal mode, which is similar to constant current.

I'm not sure what changes would occur at large signal levels, but it ought to be faily easy to elaluate distortion at increasing output levels with the sinusoidal stimulus of CLIO.

Jeff

[bfish]

Cool! Thanks Jeff!

[bfish]

So if I read that right, the loading cap is rolling response off at ~6dB/oct at 1kHz, while being of minimal (+/-2dB) influence above 1kHz. The horn starts to unload around 1.8kHz, and is done by 1500, then the cone loads up.

The rough impedance in the 1.8-3.5k range is in a bad place, but isn't really reflected in the spl plot.

I've never A/B'd a duplex, but by the plots, it looks like it might be pretty hard to tell the diference in a std deployment, yet the cap offers protection as intended. Is that borne out subjectively?

OT...I'm just imagining the breadth of that midrange canyon if there were no horn...

[jmarkwart]

bfish,

Yes, I think you're got it read correctly.

I was told that Altec used the loading cap to avoid over-excursion of the diaphragm for theatre applications when using 800Hz and 500Hz crossover points. That makes sense when you look at the plots above.

When you look at the phase plot below you get even more data that supports this idea:



The loading cap equipped unit has a much more benign phase response, especially below 1kHz. Frankly, I don't see much need for a cap with a duplex crossed over at 1500Hz or above and a 3rd order filter slope like the late 8 ohm models, and maybe that's why Altec dropped it eventually. Well, that and saving a few cents per unit!

I have found the unloaded small format driver more natural sounding than the loaded ones in home hi-fi settings.

Jeff

[bfish]

Loooks like the capped impedance adheres better to the "nominal" definition by a few ohms.

'Bout all that's left for objective consideration is impulse response. Just guessing the cap loaded version would ring less, though perhaps at the expense of slower rise time.

Hmmm...a light coat of Aquaplas or equiv. on the inner surface of a loading cap....hmmm...

[Old Guy]

Jeff I don't think it is said enough, your efforts are always appreciated-knowledge after all, is power.