I have a bag full of MJ11033 TO3 transistors... - diyAudio
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Old 8th April 2012, 05:08 PM   #1
TheMG is offline TheMG  Canada
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Default I have a bag full of MJ11033 TO3 transistors...

More precisely, I have 19 of them.

I was thinking of building a stereo amplifier using 16 of them (8 per channel), in a quasi-complimentary configuration.

However, this would be the first time I've ever attempted to build a fully discrete class AB amplifier.

My goal is to build this using mostly recycled and surplus parts that I have available to me at no cost (thus the MJ11033 transistors).

Are these transistors even worth using for an audio amplifier?

Using 8 transistors, what kind of output power can I expect to achieve into 4 ohms, while remaining well within the transistor's SOA?

Any recommended reads about quasi-complimentary amplifier design?

Thanks.
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Old 8th April 2012, 05:22 PM   #2
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Look at the SOA diagram in the datasheet and determine your rail voltages for your desired load impedance, 1A at 50V is pretty bad, so i'd say max 2x35V rails and a max power of 100 watts per channel into 4 ohms.

However these are PNP devices, so the output stage is gonna be a bit different from the usuall stuff, a bit like my pchannel amp with with your transistors in place of the fets.

Last edited by Tekko; 8th April 2012 at 05:27 PM.
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Old 8th April 2012, 05:46 PM   #3
sreten is offline sreten  United Kingdom
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Hi,

I'd research and build a 2 devices per channel amplifier using a baxandall diode
for output biasing and not particularly shooting for maximum power output.

Then decide if you really need a big amplifier with 6 devices per channel.

Those devices are not cheap, around 10 a pop.

rgds, sreten.
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Old 8th April 2012, 05:51 PM   #4
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Heres an example based on what you describe: http://i.imgur.com/KDUz1.png

Even that is on the very edge of SOA for your devices. Thats 35V supplies and a 4 ohm load.

Datasheet: http://www.onsemi.com/pub_link/Collateral/MJ11028-D.PDF

The SOA is not that much different than SOA for a MJE15033 driver transistor above 30 volts.

Last edited by Tekko; 8th April 2012 at 05:54 PM.
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Old 8th April 2012, 06:09 PM   #5
sreten is offline sreten  United Kingdom
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Quote:
Originally Posted by Tekko View Post
Look at the SOA diagram in the datasheet and determine your rail voltages
for your desired load impedance, 1A at 50V is pretty bad, so i'd say max
2x35V rails and a max power of 100 watts per channel into 4 ohms.
Hi,

No idea what you are talking about, the above is miles off.
SOA curves for a device aren't the same as an amplifiers
SOA curves, each device is rated at 80V at 1amp, x 8.

rgds, sreten.
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Old 8th April 2012, 06:23 PM   #6
sreten is offline sreten  United Kingdom
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Quote:
Originally Posted by sreten View Post

.... using a baxandall diode for output biasing .....

rgds, sreten.
Hi, for darlingtons you should need two series diodes, not one, rgds, sreten.
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Old 8th April 2012, 06:24 PM   #7
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It's a good idea to stay away from the second breakdown part of the SOA curve.

But still 8A into 4 ohms is still only around 250 watts, 8A into 8 ohms however would be over 500 watts.

Heres another sim with current(3A per output device pair) vs supply voltage set to right AT SOA, 3A for 50V:
http://i.imgur.com/6gYiu.png

So i rate this a 200W RMS amp into 4 ohms.

Now keep in mind this is merely an EXAMPLE of how it COULD be done, rather than a actual design.

Last edited by Tekko; 8th April 2012 at 06:42 PM.
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Old 8th April 2012, 06:43 PM   #8
sreten is offline sreten  United Kingdom
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Quote:
Originally Posted by Tekko View Post
It's a good idea to stay away from the second breakdown part of the SOA curve.

But still 8A into 4 ohms is still only around 250 watts, 8A into 8 ohms however would be over 500 watts.

But building an amp that halves its power going from 8 ohms to 4 ohms is just bad design.

The highest i'd go in psu voltage with 8 of these per channel is 40V.
Hi,

Sorry but your clueless, and talking meaningless numbers.

rgds, sreten.
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Old 8th April 2012, 06:50 PM   #9
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Okay fine, i take back everything i've posted in this thread since im obviously 100% wrong.

Please forget i ever posted a reply in this thread.
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Old 12th April 2012, 10:19 PM   #10
TheMG is offline TheMG  Canada
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Quote:
Originally Posted by Tekko View Post
Heres another sim with current(3A per output device pair) vs supply voltage set to right AT SOA, 3A for 50V:
http://i.imgur.com/6gYiu.png
Your image links aren't working. It just says "the image you are requesting does not exist or is no longer available".

Anyways, if I'm not mistaking, the highest stress (maximum instantaneous power dissipation) on the output transistors in a class AB amplifier occurs at about half the peak of the audio waveform. Thus, an amplifier with 50V rail voltage for example, at 4 ohms would have a peak current of 12.5 amps, however since the voltage across the transistors at this point is very small, transistors with an SOA of 50V/12.5A is not required, a smaller SOA will suffice. What I'm trying to find is the math calculations for determining maximum instantaneous power dissipation.

Is this what you were trying to show in the images?

Last edited by TheMG; 12th April 2012 at 10:23 PM.
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