Pushing SINGLE Pair of MJL4281A/4302A to the max limits at lower impedance - diyAudio
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Old 17th May 2011, 01:16 AM   #1
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Cool Pushing SINGLE Pair of MJL4281A/4302A to the max limits at lower impedance

If using really large MJL4281A/MJL4302A but only a single pair, what kind of power can you expect. Not worried about years of reliability, just what max you can get away with on a single pair on an average range of cheap heatsinking.

How much power @ 4 ohms and what rails

How much power @ 2 ohms and what rails

and @ 1 ohm?

*I'm planning out a small 2 channel amp for some people with 2 channels, 1 pair per channel on a 12.6/0/12.6V transformer to give me approx 18V-0-18V rails. Low rails, but high current.

I want the ohms impedance to be VERY flexible because many average people like to parallel a bunch of speakers on their amps, not knowing any better. many times getting less than 4 or 2 ohms per channel. I'm planning 1 ohm stable, so he can wire whatever combo of speakers he likes. Would one pair of MJL4281/4302 handle these rails at 1 ohm?
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Last edited by EWorkshop1708; 17th May 2011 at 01:40 AM.
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Old 17th May 2011, 01:39 AM   #2
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2R and 1R , no no no. 8R only , maybe 80W @ 50V rails. OEM's get away with 4R w/ one pair by undersizing the power supply , collapsing the rails before SOA is exceeded.

2 pair NJW0281/0302 @ mouser are the same price , fit in the same space and
will do 120/200W 8/4R w/ 58V rails (all day).

OS
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Old 17th May 2011, 01:44 AM   #3
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Quote:
Originally Posted by ostripper View Post
2R and 1R , no no no. 8R only , maybe 80W @ 50V rails.
OK so lower rails at lower ohms. 8 ohms only, HAHAHAHA I've seen TO264 devices on 75V rails at 8 ohm.

Not concerned with 8 ohms at all. Anything can push 8 ohms.

I can push 4 ohms easy with a pair of TO-220 devices at the right voltage.
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Old 17th May 2011, 03:02 AM   #4
tinitus is offline tinitus  Europe
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into such low impedance it will run like a classA amp, and BIG heatsinks needed

can't do it with 'average cheap heatsink', using your own words
even if classAB, you will have to build like it were a classA

but built like classA, you can do it with about 25-30V rails
1ohm load might be possible with very effective forced cooling
amp design needs to be very stable and reliable
2ohm is possible, but 1ohm is really pushing the limit
playing loud on a really hot day, and it may be gone
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Old 17th May 2011, 04:04 AM   #5
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The MJL4281A and its compliment can deliver a continuous 10A safely with a worst case of 10V across it. I see no reason that it would not drive 1 ohm speakers since music is far less demanding that steady state DC.
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Old 17th May 2011, 08:33 AM   #6
sajti is offline sajti  Hungary
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I know diy amplifier with single pair MJL21194/93 with +/-50V rails, and push about 200W/4ohms every day since 2 years. Fan cooling with lot of noise, but it's no problem in club application

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Old 17th May 2011, 10:22 AM   #7
lineup is offline lineup  Sweden
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MJL4281A/4302A
Collector current
-continuous: 15A
- peak: 30A
So I see no problem with supply only +/-18 VDC
and 1 Ohm load regarding the transistors.

But in a high current low voltage amplifier there are other difficulties.
This includes rail inductances.
It is just not like a 'normal' amplifier for 4/8 Ohm.
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Old 17th May 2011, 02:03 PM   #8
effebi is offline effebi  Italy
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If you want low voltage / high current nothing beats the 15003/15004.
Just make sure you get the original ONsemi from a reliable source.
It's only a matter of 6 more holes on the H/S but there is nothing like that for power handling.
Of course the design of the amplfier/PCB/power supply/heatsink/wiring(!) must be adequate if you do not want to cheat on output power.
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Old 17th May 2011, 07:53 PM   #9
CBS240 is offline CBS240  United States
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Quote:
Originally Posted by lineup View Post
MJL4281A/4302A
Collector current
-continuous: 15A
- peak: 30A
So I see no problem with supply only +/-18 VDC
and 1 Ohm load regarding the transistors.
Hmmm. Looking at figure 1 & 2 in the datasheet with Vce=5V the current gain plummets after a few amps, as does the GBWP. Since lesser Vceo is not a problem here I presume, perhaps a higher conductance device would be more appropriate?

I've always wondered how much low Z sub-woofer power could be made with a pair of these super slow things. They have substantial SOA.
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Old 17th May 2011, 07:57 PM   #10
lineup is offline lineup  Sweden
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Yes. I should have considered the curves you refer to.
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