Outrageous Speaker/Amplifier Idea

[smoking-amp]

I was trying to think of some way to get rid of some nasty light bulbs used in another thread as an inductor substitute for a SE amplifier when this scandalous idea occured to me. The inductor problem for SE is quite similar to the speaker cabinet issue for speakers. Could they be combined in such a way as to null out both problems. Yes!

Now imagine a speaker driver that is truly sealed. An airtight driver in a small hermetically sealed backing. This backing would be quite small so as to act as a sealed air spring. To operate this speaker in Class A mode, we would apply a DC current to it of 1/2 the maximum current for the loudest output. This driver would be set up in such a way that the 1/2 max current pushes the speaker coil and diaphragm backwards to the center of its linear range within the magnet, while also pressurising the speaker backing to 1/2 the max pressure exerted by the loadest sound. (conversely, one would simply pre-pressurize the backing until the 1/2 Max DC current centered the speaker coil, probably less than a PSI, in any case it is no more pressure than 1/2 what a normal speaker encounters at its loudest output. )

Then AC would be applied (summed with the DC) to the driver to move the driver back and forth for normal operation. Now since the AC varies from -1/2 Max DC to +1/2 Max DC, the net effect is of 0 to +1 x DC Max current input, or simply, single ended (SE) speaker operation.

Now, how to combine this with a SE amplifier lacking its inductor or OT inductance. We start out with a non-gapped OT (output xfmr) powered by a SE circuit. This would normally saturate the non-gapped xfmr due to 1/2 Max Class A primary bias current. But we will apply 1/2 Max DC current thru the secondary (transformed magnitude) to balance out the primary side DC. So no air gap is needed. This 1/2 Max DC current bias is just what the SE speaker wants too, so we run the 1/2 Max DC current thru both the OT secondary and the SE speaker coil. Both are happy. AC signal just rides right thru the OT and sums with the DC 1/2 Max bias to drive the SE speaker.

This does not require a CCS by the way. No voltage compliance is wanted, we want a low Z DC source in series, which would just be a fractional volt power supply. Adjusted so that the secondary winding resistance and the SE speaker coil resistance produced, or limited, the 1/2 Max DC current. The sealed air speaker backing is providing the energy storage/inductance for the SE system. Hardly any power would be used by the DC bias supply, maybe say 1/2 Volt at 2 amps.

Essentially no backing volume is needed for the speaker (ie, a cabinetless speaker) here since it must be small enough to pressurize to 1/2 Max sound pressure with just 1/2 the Max motion of the speaker diaphragm. The SE amplifier gets to use the much cheaper and better performing P-P OT. Win-Win! Revolutionary.

[Wavebourn]

Do you mean to connect the speaker in diagonal of a bridge formed by an amp with DC offset and a compensating voltage source?

Or to inflate a speaker box by an air displacing such a way it's diaphragm?

Please clarify.

[smoking-amp]

By the diaphragm air displacement pressurizing the box (or the "backing" since the box size has shrunk to almost nothing here) 1/2 Max motion of the diaphragm would increase the backing pressure from 1/2 Max to full Max. or in the forward direction from 1/2 Max pressure to 0 (relative) pressure. This could also be made to work for SE SS amplifiers as well (just no intermediate OT involved then). When the SE speaker is disconnected from its amplifier, it would move (diaphragm) to its forward Max position, thus neatralizing backing pressure with atmospheric.

This could be a minor manufacturing variation from current speaker drivers. Normally the speaker diaphragm compliance is centered in the center of the magnet. For an SE speaker, the compliance would center at the forward Max position. (assuming it has little or no springiness of its own)
The backing could be built right into the speaker assembly, no need for any cabinet. A pressure relief valve push button could allow neutralizing the backing pressure when disconnected or un-powered so that the compliance would move to the forward position and entrap the correct amount of air. (this is really just a calibration feature to allow for and compensate slow leakage problems)

[Wavebourn]

Do you mean something like electrostatic inflated concentric bubble?

I like the idea. Can be made of a metal mesh ball (anode) and a made of conductive rubber ball around (grounded cathode)

[sreten]

Hi,

Where does one start to explain why this will not fly ?
Its not complicated, its never been commercially done
AFAIK, there are very good reasons for that, find them.

/Sreten.

[smoking-amp]

"Do you mean something like electrostatic inflated concentric bubble?"

No. This is very much a conventional speaker driver setup. Although there is no reason it could not be extended to an electrostatic design, planar or spherical if that's do-able.

[Wavebourn]

A conventional speaker usually has such a motor that will leak an air through a magnet gap.

[boywonder]

Dialing in the speaker cabinet would require dialing in both pressure and volume, since you'll want a certain bias "pressure" for 1/2 the excursion but you'll also need the correct air spring rate for proper max excursion based on the applied voltage. A water manometer connected to the cabinet would probably provide enough pressure while keeping everything dry. It would be easy enough to vary the pressure by changing the height of the water. 30" of H2O is roughly 1 psi. I would imagine that you would need way less than 1 psi, especially for larger drivers.

Of course, keeping the pressure in the cabinet for any length of time will be one challenge. (wait, don't turn the amp on yet-I need to pump up the speakers...)

Using a old 13" car tire for an enclosure may fit the bill here....and it would be non-resonant with non parallel walls. Or maybe a 15" woofer in a 15" car tire. Some sheet metal screws and RTV right into the bead. Pretty low WAF, though.

[smoking-amp]

I think a DIYer could even modify an existing speaker to do this. Just start with a poly... coned speaker or something that is reasonably air tight with a non-permeable surround. Mount a solid metalic cone of the same dimensions as the speaker diaphragm to the front of the speaker with just enough space to the diaphragm to accomodate the backing volume required (essentially it just goes at twice the max forward speaker extension). Sound would then radiate from the rear of the speaker driver. Enough DC current to move the diaphragm to the max backward extension would then be applied, and the metal cone sealed tight to entrap the correct volume of air inside the front space.

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"Of course, keeping the pressure in the cabinet for any length of time will be one challenge. (wait, don't turn the amp on yet-I need to pump up the speakers...)"

Well, that would be the advantage of having the speaker surround centered at the forward position rather than the center travel position. When un-powered, the speaker would move naturally to the zero pressure differential position. A tiny leak could be configured so that the speaker would always tend to entrap the correct volume of air during inactive periods. The leak would have to be small enough to prevent any significant air loss during operation, since the diaphragm would sit on average at 1/2 Max pressure position during operation.

[kenpeter]

Pressurize the speaker cabinet? Why not just evacuate it.
Put some old vacuum tubes inside, and shake it till they break.