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A diy amp for looow impedance
A diy amp for looow impedance
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Old 16th November 2001, 02:49 PM   #11
Bill F. is offline Bill F.  United States
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Ok, now for some synthesis:

It sounds like the recipe for a 10W amp at this spec would look like this: Take one half-ton toroid, two bushels of chips, and a substation, mix thoroughly, solder to taste, and serve with asbestos room treatment.

Maybe I'll shoot for just a couple watts pushing half an ohm--two amps and a volt at the output terminals. (Not much juice, but my speaker array models to be 104+ dB/W.) Could this work with plain vanilla MOSFETs? Is this moving in the direction of reasonability (especially considering from whence I came)? Should serve as a good disproof-of-concept, anyway.


I like your ideas of discrete drives for subsections of the array. How about one driver stage feeding 8 small complimentary pairs each driving 2-4 ohms? If I insert a continuously variable phase control in front of each output, I could dial in an electronically focused (virtually curved) array or maybe a polar response something like a point source...

...see how it is? just as I begin to find the path, I'm diving back into the weeds.

So much for synthesis.
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Old 17th November 2001, 12:42 AM   #12
Kilowatt is offline Kilowatt  United States
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Don't you want to go more than 10W? Sure that would be loud, but those 30 drivers must be able to handle some serious power. If I were you, I would put the drivers in series-parallel so they total about 2 or 4 ohms per channel stereo, and hook them up to an amp that can put out as much power as the drivers can handle in case you ever want to make LOTS of noise! Like others have said, there are many problems with building an amp for low such impedance, it would be much easier to do it for 2, 4, or even 8 ohms. There's nothing saying you have to put all those speakers in parallel, series-parallel should work much better. With all those drivers, you could achieve about any impedance you want.

Good Luck!

[Edited by Kilowatt on 11-16-2001 at 07:47 PM]
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Old 17th November 2001, 02:57 PM   #13
Bill F. is offline Bill F.  United States
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Yes, my arrays could easily handle 300W+ per channel. At that power, my smallish listening room would be serenaded with peak SPLs of nearly 130dB--and I'd be crawling down the hall in the opposite direction, blood trickling from my ears, asking myself if I was having fun yet.

I do like to turn it up till the sound becomes compelling, but quality is the goal.

That's some of the attraction of arrays. I'm planning on 32 full-range drivers per side (about 3.7" cone diameters, 90 dB/W 15W max pwr.) Working together, they roughly equate to a 20" driver of 105 dB/W while maintaining the mechanical characteristics of small drivers.

So, as I enjoy powerful tunes at peak SPLs near 110 dB, my amp is putting out 4 watts per channel and each driver is dissapating 1/8 of a watt. Cone excursions stay very small and IM distortion is a thing of the past. (Beware: IM distortion crops up way before X-max.)

Truth is, 10W is all kinds of headroom, as far as I'm concerned.
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Old 21st November 2001, 08:54 AM   #14
maxkillahz is offline maxkillahz
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What kind of small room can fit 30 drivers/ch? I think you would have a hard time getting a good speaker placement. What kind of crossover are you using?
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Old 21st November 2001, 04:51 PM   #15
Alex M is offline Alex M  Europe
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Does anyone here remember the discussion on the BASS List a
while back where Tom Danley and Nathan Sargeant were
speculating how to power the Servodrive? This was a clever
and original way to overcome distortion in sub-bass drivers
by driving the cone from a rotary electric motor with a
servo control to reduce the distortion. Only thing was, the
motor that was the most suitable in every other respect for
this application turned out to have a DC resistance of under
an ohm. I think Tom designed an amp with an exotic
transformer-coupled output, but I forget whether anyone
actually built the amp or they found another motor.

Details of the Servodrive for those interested are to be
found at http://www.servodrive.com/servo .

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Old 21st November 2001, 08:13 PM   #16
tschrama is offline tschrama  Netherlands
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Dear Sir.....

I do believe it's a great idea... like the Startrek phrase..
... to (??) .. go where none have gone before.. well or something like that... anyways....

Look at the good side to it:

1] very high efficientcy: less power needed :-)

2] using more drivers, every driver needs less excursion so will be more lineair, in you case : very lineair :-)

3] current amplifier can be very very simple mosfet buffer, since 2 volt RMS will be just fine... .... :-)

4] such a amp will have a distortion of less then 0.1% wich will be just fine since normal BASS drivers have distortion of 1-10% (!!!) .. your total system linearity will be very good indeed

4] using low voltage MOSFET's will keep input capacity low, but this doesn't matter since it's a BASS-amp, high Hz are not a issue ... :-)

5] damping factor (not that important) .. parralel MOSFET have very low on-resistance.. no issue here

6] low voltage capacitor for the powersupply are cheap.. maybe 5 Volt Cap's will do just fine .. you may be able to get your self some whole Farads of capacitance :-) .. you do need power rectifiers... hmmmmmmm

1] using a very simple 2 stage amplifier give feedback if you want to

2] use as many MOSFET as you have speaker ($$$...:-( )

3] use BIG cable, like car-battery cables, parralel

4] use heavy filament trannys, 5 or 6 of them will deliver 18 Amperes ... that not impossible to get relitive cheap.. or build yourself 10 regulated voltages supplies of 1 ampere.. also not a problem at 5-6 volt ..better yet

5] use ver very cheap voltage supplies from second-hand 10-15 hear old computer supplies... YES

6] you can try you concept first with just 4 or 6 speakers..

welllll....... just do it!!! and be the bravest DIY-man on this forum!!!.. and tell us about the performance...


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Old 25th November 2001, 06:37 AM   #17
petermac is offline petermac
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Default A different way to go

I saw a project like this years ago - the idea was to limit distortion due to the Doppler effect in full range speakers.
Apparently it sounded superb.

Anyway, it seems to me like you have three choices:

1. Buy Krell or McIntosh monoblocs; they would have no problem driving the load, but you probably can't afford them.

2. Build you own; if you don't mind waiting 'til 2004 to listen to your creation - the learning curve on anything but the most mundane power amps is time-consuming and, again, very expensive.

3. Why not amplify each speaker individually? Use a chip like National's LM3876 (about $7cdn at Digi-Key) and make lots of small amps. Huge advantage - as soon as you make the first two, you'll have something to listen to.

Cheers, Peter
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Old 26th November 2001, 12:01 AM   #18
blmn is offline blmn  Brazil
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Petermac gave you a good tip, but, there are some problems in this arrange, specially to put all of these chips working at same way. But I think it will be easier than deal with the very low impedances needed to amplifier the current and to connect your array. I think, considering 2 or 3 volts RMS at output and 6 or 7 amperes, you won't need very much mosfets. Your cables, however, will need to be very huge.

By the way, in my opinion, this kind of question and thoughts are the best think in this Forum.

If you decide to go ahead with your idea, let us to know about its development.

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Old 26th November 2001, 12:13 AM   #19
P.Lacombe is offline P.Lacombe  France
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I have built some years ago a dc-100 Hz 25 W rms amplifier for special purpose, with loading impedance of 1 ohm. It is difficult to maintain low level distortion at such low impedance, and the global efficiency of the amplifier is poor.

Another difficulty is to design dc power supply of low voltage, but high intensity, which requires big condensers and inductors. Efficiency is poor too, because of the voltage drop in the rectifiers.

In my opinion, series-parallel wiring of your drivers, with a resultant impedance of 4 ohms (8 parallel of 4 serial) is the most reasonable solution.

In another way, you can experiment with a cheap auto IC amplifier for *each* driver, paralleling inputs of this ICs, and a general stabilized power supply of 13.8 Vdc, 10 A, which is available at low cost for amateur-radio transmitters. Furthermore, you can modify level and/or frequency response of each driver, as necessary. Total power output would be 64 W RMS or so !

Regards, P.Lacombe
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Old 29th November 2001, 01:10 PM   #20
PassFan is offline PassFan  United States
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Exclamation Square Wave

Be careful with your drivers. I don't know what the rms wattage of your drivers are, but watch for clipping in your amplifier. Clipping will burn the coils out of your drivers. At the output impedances your discussing, there will be a lot of demand on your amp.
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