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Old 5th May 2006, 07:45 PM   #1
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Default DC drive of twin coil speaker

just thinking out loud realy, and sorry if this should be in speakers but it is to do with zero crossing amps and did a search and nothing...

but here goes...

after reading the Nchanel thread, and several others with zero crossing problems, and problems encountered with associated niose and distorsion, i had a mad thought...

if a upper and lower halvs of the amp output drive tansistors are seperated (each half outputs individualy, not joined to a single output line) and driven WITH dc bias of a volt or so...

then take a twin coil speaker and wire one half drive posotive, and the other drive line wired negative, effectivly giving push pull speaker...

since only one output is driving when signal is applied, but when not the small dc bias's cancell out but also help return the speaker to centre efectivly incrasing suspension stifness...

ignore this bit if you want, its speaker stuff,
if that is so, and the coils could take more dc current, could the suspension be done away with altogether??

thats it... just some thoughts, didnt know if this has been tried before, or if it would work.

cheers for listening, steve.. ..
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Old 5th May 2006, 08:04 PM   #2
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Interesting idea, and driving a speaker this way would work for certain, although I think you would need a small pickup coil somewhere on the speaker to provide a signal for the negative feedback, which would control crossover non-linearities. This layout isn't so different from a push-pull valve amplifier, where the transformer primary windings would be in place of the two voice coils.

As for the standing current through both coils making you able to do away with the speaker suspension, no, I can't see it would work simply because the effects of both coils cancel out leaving no centering force.

However, with a bit of additional servo amplification and associated control circuitry, I dare say this could be done electronically, although equally well with a single voice coil speaker. This idea in itself is interesting, and I wonder if such designs exist to maximise speaker compliance.
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Old 5th May 2006, 08:22 PM   #3
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Wouldn't the DC through the voice coil overheat the speaker? Correct me if I'm wrong, but doesn't the movement of the speaker and air through the voice coil help to cool the speaker under signal conditions? I thought this is why DC on a speaker was not a good thing (other than the coil being driven out of the gap, which your dual coil/reverse polarizing solves).
Also, I thought that the suspension centers the coil in the gap. Could you do that magnetically?
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Old 5th May 2006, 08:31 PM   #4
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Default negative feed back???

assuming the output can be split efectivly, could not the negative feedback be taken before the split? i know some of the chip amps run with transistors for current gain tda2030 has one on the data sheet. i know this would then not compensate for the output devices, but hey its a thought..

also could neg FB be aplied individualy and to pos and neg sides??

also, could the output from each side be level shifted, resistor or other means, then diode back, to form the neg fb?

cheers for listening, steve.. ..
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Old 5th May 2006, 08:38 PM   #5
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hey nobody, yes dc on a coil causes heat, but ive run well past sell by amps with 1.8v dc ofset. a normal small watt speaker would over heat but if the theory works, and proves to be sonicaly better im sure a twin coil could be wound to hold it. and its not that much dc anyway so some of the twin coils out there would probly work as is.

but im not recomending you try it with something expensive, i may experimen with one ive got with a blown surround.

as for centering, iron nail floating in a coil from O level physics class says yes, dont know how eveftive it would be..

see you soon, steve.. ..
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Old 6th May 2006, 04:51 AM   #6
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Another thought:

The idle DC will cause an amount of "preheating". With the coil idle too, the cooling should be moderate, but not very effective. OK so far, has been stated already. With increasing cone movement both the cooling of the coil will get better as the thermal energy dissipated will get higher.

An interesting test would be how much DC-preheating could be applied to keep the voice coil at (quite) similar temperature (and thus maintain a then higher VC-DC-resistance) for both idle state and max. acoustical output. Thermal compression effects could probably be vastly reduced by finding some appropriate "thermal bias"...


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Old 6th May 2006, 10:21 AM   #7
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http://patft.uspto.gov/netacgi/nph-P.../4,360,707&RS=

PN/4,360,707

http://patft.uspto.gov/netacgi/nph-P...S=PN/5,748,753

PN/5,748,753
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Old 6th May 2006, 11:42 AM   #8
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Default interesting mr bond!!!!!!!

the first patent seems to need two amps, efetivly bridge conection witch has posibilitys, but doesnt include the problem of zero crossing in a single amp.

the second seems to be push pull subwoofers with a solid conection between the coil/cone, co axial conection again posibilitys but no single amp zero crossing.

worth looking into further as i only scan read them.

i only started typing to help me think, now im thinking much more and need much more help, AHHHHHHHHHHHHHHHH!!! lol..

cheers for the thought tho, steve.. ..
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Old 6th May 2006, 02:06 PM   #9
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"then take a twin coil speaker and wire one half drive posotive, and the other drive line wired negative, effectivly giving push pull speaker..."

The first patent (CTS) is exactly that. Because each coil is seperate, you only need one polarity power supply to provide push-pull cone motion.

"since only one output is driving when signal is applied, but when not the small dc bias's cancell out but also help return the speaker to centre efectivly incrasing suspension stifness...

ignore this bit if you want, its speaker stuff,
if that is so, and the coils could take more dc current, could the suspension be done away with altogether??"

The CTS patent mentions this, and the speaker only requires a centering device for concentricity, not restoring force.

The amplifier is class D, from a single supply, does not require four outputs, but behaves like a bridge.

The second patent (Carver) is the same, but with a linear output stage fed by a switching regulator.

http://patft.uspto.gov/netacgi/nph-P...y=PN%2F4130725

The third patent (Tenna, also known as Pyramid) is also the same, a straight linear output stage, and was sold under the trade-name of 'Mindblower'.

http://patft.uspto.gov/netacgi/nph-P...S=PN/7,039,213

The fourth patent (Adire XBL^2) is for a modified magnetic gap for the dual coil driver. By allowing the same number of turns in the gap at all times, this would improve linearity at small signal levels, and allow for more maximum excursion.
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Old 6th May 2006, 09:05 PM   #10
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told you i only scanread them, lol

sold you say? mmmmmmmmm and how did they sound? what was the distortion figures like?

the cts pattent needs two amps tho??

dont know much of class D amps seen some stuff on H bridge topology, can the out put be split??
what is the zero crossing like??
assumihg it uses tranistor of some form for output how can the waveform cross zero?
is bias aplied at all? if so, is each side over biased like my thoughts at the begining?

cheers for listening to my rantings, steve.. ..
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