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Old 19th May 2012, 05:08 PM   #1
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Default Another Extrema 100watt class A power amplifier

Two days ago I finished my ExtremA poweramplifier. This is a 100watt pure class A fully balanced power amplifier by Sander Sassen and Bruno Putzeys. The project started 1,5 years ago when I bought the pcb's, ring-emitters and (relatively hard to find) crd's from some-one who could not find the time to start the project. When I look back the hardest part was to find a case that is able to handle the heat generated by 100watt class A.

Finally I found a case made by an Italian company (modushop.biz / hifi2000). They also made the custom frontplate. Case is full anodized aluminium with large heatsinks which are bearly able to handle the heat (heatsinks up to 65-70 degree celcius). Then the next presented itself; the bottom plate (3mm alu) was bending under the wait of the huge 650va shielded transformers. So I constructed a frame from 2mm alu squares. Now the bottom bends less then 1mm. Problem solved.

I know another builder had some trouble while buidling the amplifier. I am not a novice builder but also defenetely not an expert. Buidling this amp is very do-able. The partlist supplied by Sander is very straight forward. Mechanical construction is the hardest part. And off course, I had some help from my brother who is an electronics designer/engineer, especially during final testing. Just one problem I need to look into; when a input signal with high volume is send to the amp and then the amp is turned on, the amp sometimes latches up. When the source is muted and then the amp is turned on, the problem does not seem to occur. Hmmmm......maybe the value of the powerresitor on the softstart pcb is a bit too high. And maybe I will replace the emitter resistors with higher values. This way a bit of output power is sacrifised but the amp will remain a bit cooler.

Sound? I always have to laugh about comments like "a lot of air", "very black" etc. First impression: "wire with gain", just as the designers intended.

Below some pictures of the project.
Attached Images
File Type: jpg extrema printen klaar.jpg (154.8 KB, 767 views)
File Type: jpg trafo met print.jpg (27.7 KB, 730 views)
File Type: jpg extrema2.jpg (75.4 KB, 723 views)
File Type: jpg extrema3.jpg (87.3 KB, 708 views)
File Type: jpg extrema4.jpg (112.3 KB, 709 views)
File Type: jpg extrema test softstart.jpg (52.9 KB, 447 views)
File Type: jpg extrema en buffalo.jpg (99.9 KB, 419 views)
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Last edited by scanspeakman; 19th May 2012 at 05:25 PM.
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Old 19th May 2012, 05:49 PM   #2
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WOW, very nice work
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Old 19th May 2012, 07:04 PM   #3
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Quote:
Originally Posted by scanspeakman View Post
So I constructed a frame from 2mm alu squares. Now the bottom bends less then 1mm.
Nice work.

Preamp ?
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Old 19th May 2012, 07:13 PM   #4
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Originally Posted by jacco vermeulen View Post
Nice work.

Preamp ?
Hi Jacco,

It is not exactly a preamp; it is a twistedpear audio buffalo32s dac with volume control. So...sort of preamp. It's the only source I have with balanced out so I connected it directly to the ExtremA. Works perfect!

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Old 19th May 2012, 08:53 PM   #5
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65-70C is a bit on the high side. maybe consider a little lower bias setting.
or bigger sinks
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Old 19th May 2012, 10:10 PM   #6
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You do not know what the bias level is now, nor the rail voltages.
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Old 19th May 2012, 10:17 PM   #7
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but i do know that 65-70C sinks is a little to high.
and that it is easyer and cheaper to lower the bias then lower the rail voltage
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Old 19th May 2012, 10:39 PM   #8
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Bias is approx 2.5a of I am correct. By increasing the value of the emitter resistors you are able to adjust (lower) the bias.

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Old 19th May 2012, 10:53 PM   #9
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What an absolute monster...

I love it!!!

I'm often bitching about modern 7x150Wrms amps weighing a lot less than my 2x100Wrms amp.

I think you can beat my 18kg
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Old 19th May 2012, 11:01 PM   #10
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Quote:
Originally Posted by scanspeakman View Post
Bias is approx 2.5a of I am correct. By increasing the value of the emitter resistors you are able to adjust (lower) the bias.

Scanspeakman
there isn't a pot for bias adjustment?
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