Passively-assisted sealed alignments

[bentoronto]

An exciting, even breathless thread. As if somebody has finally discovered the secret to achieving perpetual motion.

But shouldn't somebody help us understand why this kind of mod has been totally discounted in the entire history of low output impedance audio amps driving low-freq cone drivers? (And that's even before my stomach settles down after all those thoughts of putting disgusting electrolytic capacitors in the speaker signal path.)

Brian deserves thanks for his careful presentation and measurements... at least as far as they reveal some limited if important part of the story.

B.

 

[00940]

Some more theory : Subwoofer Tuning with Capacitor And Resistor

 

[MarcelvdG]

An exciting, even breathless thread. As if somebody has finally discovered the secret to achieving perpetual motion.

But shouldn't somebody help us understand why this kind of mod has been totally discounted in the entire history of low output impedance audio amps driving low-freq cone drivers? (And that's even before my stomach settles down after all those thoughts of putting disgusting electrolytic capacitors in the speaker signal path.)

There are plenty of other interesting inventions that are rarely used. I don't know why, but maybe a sociologist or a historian of science could explain that, or even an ethologist specialized in herd behaviour of social animals.

 

[Brian Steele]

I decided to "try out" this type of alignment on the sealed subwoofer that I use in my car audio system. While the gain is not as great as my first example, there is still some improvement, in this case the response being flattened all the way down to 20 Hz. I can accomplish this already though with the EQ have have in the car's audio system and the subwoofer amp has plenty of power to spare, so sinking the extra $25 into the required series capacitance may not be worth it.

For a lark, I looked at the THD of the raw system from 200 Hz down. Less than 1% THD from 120 Hz down to 30 Hz, at ~93dB average during the passband. Not bad, eh? It's actually the midbass drivers that let this system down - they can't keep up. Not really their fault - door panels make horrible enclosures.

 

[grec]

Interesting.
How added capacitance will affect woofers with 0.3-0.35Qts?

 

[Sangram]

You'll need a very small box. The overall system Q should be in the 1.1-1.5 range for best results.

The 'trick' also works with some added series resistance in lieu of capacitance, specially when the woofer is a low impedance type.

 

[grec]

 

[AllenB]

I'm curious. Doesn't it affect somewhat "damping factor"

Yes, damping is changed. The entire resonance has changed. It is now third order. This kind of tuning is in this way akin to a vented box adding its two orders.

 

[bolserst]

Surprisingly 28 hz boost comes free of increased distortion. This is a really interesting tweak thanks brian

Here are the HD curves. Unfortunately REW does not provide a means for directly comparing HD measurements.

@Brian Steele,
It is unusual to see reduced woofer distortion where LF response is boosted. Do you happen to remember the settings you used for the REW distortion plots? If you happen to still have the files saved, you might try using the check box for “Use harmonic frequency as ref”. This provides a more meaningful comparison of distortion when the response is not flat, or changes between test configurations.

Copies of the Temme paper referenced by the REW manual and B&K Appnote can be downloaded here:
http://www.pearl-hifi.com/06_Lit_Ar...pers/How_To_Graph_Distortion_Measurements.pdf
https://www.bksv.com/media/doc/BO0385.pdf

 

[bolserst]

It is a nice trick to obtain some extension from small closed boxes….Harmonic distortion should not change, assuming the system is equalized to the same frequency response.

I can recall in the late 80’s early 90’s when passive subwoofer-satellite systems were popular, the capacitor used for HP filtering the satellite speakers could be sized to take advantage of the trick to yield the required LF extension to cross with the subwoofern with a smaller satellite enclosure.

…somebody help us understand why this kind of mod has been totally discounted in the entire history of low output impedance audio amps driving low-freq cone drivers?

Besides the application in subwoofer-satellite systems, at least one major manufacturer used the technique in some bookshelf and full-range loudspeakers. If I recall correctly, the C20 used a 600uF capacitor and the C40 a 450uF. KEF described the technique as “…3rd order LF loading for increased LF extension and power handling”

 

[Boden]

Maybe I am mistaken here, but I vaguely recall Joe d' Appolito doing a capacitor loaded subwoofer for one the Orca systems he designed in the late 80ties early 90ties, using 10 or 12" Focal woofers.

 

[bentoronto]

There are plenty of other interesting inventions that are rarely used. I don't know why, but maybe a sociologist or a historian of science could explain that, or even an ethologist specialized in herd behaviour of social animals.

Sometimes people who are very scientific in physical matters are not comparably sophisticated in other sciences and might only attribute "unscientific" behaviour to herd or deranged judgment. An example is the reluctance of automobile makers to change things because the large "installed base" of mechanics wouldn't know how to fix new stuff. Maybe that's not how cars should be designed but it isn't deranged logic either.

This thread examines a way of sticking a large unwholesome electrolytic capacitor in series with a low impedance driver with some benefits and some losses. For a small car speaker playing in a car maybe the benefits outweigh the losses.

The decision riddle is that it is easy to make certain audio measurements and these tend to get more attention than other measurements that golden-ears may talk about. That skews the decision improperly.

Here's a simpler way to get "free" low bass: take a handful of window-pane putty* and stick it to your dust cap... cautiously.

Perhaps there should a sticky thread listing such approaches for which special applications. Perhaps KEF or d'Apolito had special uses in mind. (Thanks for great background info, Bolserst.)

B.
*OK, there's a high-class kind of putty used for sealing HVAC ducts. Won't leave a greasy mess on your dust cap. DAMHIK.

 

[Boden]

Yes Ben: it is called "trading efficiency for bandwidth". Well known since the late Middle Ages.

 

[bolserst]

I feel so old…
Looking through my archives of audio files, I see I had references to 2 different AES papers and a patent on the technique. Perhaps the patent is why we didn't see more companies using it…

Low-Frequency Range Extension of Loudspeakers
Author: von Recklinghausen, Daniel R.
JAES Volume 33 Issue 6 pp. 440-446; June 1985
“A passive electric network connected between the driving amplifier and a closed-box direct-radiator loudspeaker can extend the low-frequency response of the loudspeaker system to frequencies below the diaphragm mass-enclosure compliance resonance without requiring additional voltage output capability of the amplifier. Design details for such a closed-box system are shown”
AES E-Library >> Low-Frequency Range Extension of Loudspeakers

Closed-Box Loudspeaker with a Series Capacitor
Author: Thiele, Neville
JAES Volume 58 Issue 7/8 pp. 577-582; July 2010
“Adding a series capacitor to a closed-box loudspeaker modestly extends the low-frequency response, enabling the use of a smaller enclosure. In addition, the capacitor offers substantial protection against excessive voice-coil excursions from subsonic input signals. The capacitor converts a second-order system to third order. A design procedure for the composite system in closed form is provided, allowing the resulting transfer function to be optimized for group delay, frequency response, and cone excursion.”
AES E-Library >> Closed-Box Loudspeaker with a Series Capacitor

US Patent US4383134A:
US4383134A - Loudspeaker systems
- Google Patents

Also, it looks like Infinity used the technique on a few of their reference loudspeakers.
http://www.infinity-classics.de/technik/manuals/RS_II_technical_sheet.pdf
http://www.infinity-classics.de/technik/manuals/RS_III_technical_sheet.pdf
There may be others…

 

[Boden]

500 uF in series is also found in the Elektor PL301(1985), a semi KEF 105. We could make the list of loudspeakers a bit longer if we allow not "just" a single capacitor, but include RCL contouring networks: the IMF Professional Monitor and the AR9. So by and large, it looks asif these passive "contouring/response shaping" techniques have been known and commercially applied since the seventies.

 

[bentoronto]

Grab a good quality woofer driver. Push sharply on the dust cap or thereabouts. Then, take an alligator-clip lead and short the terminals together and push again.

If you can't tell the difference between the two conditions, you are ready to use the capacitor or series resistance "improvement" method with that driver.

B.

 

[AllenB]

Are you talking of damping Ben? Another way of looking at it is that it can be consolidated down to an electrical impedance interaction.

Boden, fourth order An underdamped LC to produce a vented fourth order type response from closed.

 

[bentoronto]

You're knowledge is way ahead of mine. But sounds right.

However, the fallacy some might hold is thinking that by effecting a better FR by cleverly manipulating the signal to the driver you have also captured other improvements. When in fact, you've lost ground in other criteria. And that bad thinking also shows up for trick "alignments", again, that improve FR or low end, but cost ya.

All I'm tying to illustrate about a damped cone, albeit by feel, is that nobody wants loosey circulating ball steering boxes in their car after they've driven with precise rack and pinion steering.

B.

 

[cowanaudio]

Boden, fourth order An underdamped LC to produce a vented fourth order type response from closed.

Do you have any references? A quick Google and AES library search came up with no results.

 

[afa]

The Canton "DC" range of loudspeakers also used the series cap as a form of bass boost, even in vented boxes.

Canton Vento 809 DC loudspeaker Measurements | Stereophile.com

I think they also utilised peaking 2nd order high pass filters for bass boost in some of their other models in the "DC" series as well.

cheers, Arthur