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Old 26th January 2014, 08:48 PM   #1
Sandor is offline Sandor  Italy
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Default Suggestions for a balanced input class A amplifier

I'd like to build a pair of monoblock amplifiers that meet the following requirements:
1) rms power 50 W or more
2) class A
3) balanced input
4) ability to drive speakers that go down to 2 ohms

Any suggestion for a good diy project would be much appreciated.
Thank you.
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Old 26th January 2014, 10:09 PM   #2
tvrgeek is offline tvrgeek  United States
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Head over to the Pass Labs forum where the do insane thing like this with MOSFETS.

The EU energy efficiency police will not like you for this.

Or just go find a used Threshold/Krell/Levinson. Probably a lot cheaper than DIY, but not as much fun.
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Old 26th January 2014, 11:35 PM   #3
sreten is offline sreten  United Kingdom
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Hi,

A pure class amplifier that does any sort of power into 8
ohms is an exercise in futility making it 2 ohms capable.

rgds, sreten.
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Old 27th January 2014, 12:50 AM   #4
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Quote:
Originally Posted by Sandor View Post
1) rms power 50 W or more
2) class A
3) balanced input
4) ability to drive speakers that go down to 2 ohms
Easiest and cheapest to build would be the Pass/Thagard A75.
(2-part building article for download at FirstWatt)
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Old 27th January 2014, 08:38 PM   #5
Sandor is offline Sandor  Italy
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Thank you all for advices.

The A75 is quite interesting but I think it is expensive and challenging.
Do you think a Pass A40 could drive well a pair of Magneplanar MMG? It could be an easier to build and more affordable alternative to A75. Sadly, its input is not balanced...

@sreten
I'm not a native english speaking person, I'm afraid I've not understood the sense of your contribute. May I ask to clarify a bit? Thanks a lot.

Last edited by Sandor; 27th January 2014 at 08:45 PM.
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Old 27th January 2014, 11:05 PM   #6
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With loudspeakers that go well under 4 ohm, it's safer and easier to pick a power amplifier with a MOSFET output stage.

2-ohm capable amplifiers with bipolar output transistors would need a relatively big output stage, or protection add-ons, which makes it more complex.

A 50W class A power amp will require a big case, large heatsinks, and big power supplies anyway.
Full 50W class A in 8 ohm means a minimum idle dissipation of 110W per channel.
220W dissipation for 2 channels is a heat figure that is hard to get rid of with a stereo amp case. Without fans, it would have to be huge.

(the A75 is easy to downsize, btw, and suitable output devices are really affordable)

Easiest for you to do would be to order boards from the diyA store, the BA stuff.
Making PCB's and amp cases are the difficult parts of building an amplifier for most.
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Old 27th January 2014, 11:18 PM   #7
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Sandor,

I built a pair of A75s as a first project, and they worked first try. I had some electronics background, but it is possible. The article is long because it's full of learning. It's a good choice for your 2Ω requirement.
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Old 27th January 2014, 11:19 PM   #8
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Quote:
Originally Posted by Sandor View Post
I'd like to build a pair of monoblock amplifiers that meet the following requirements:
1) rms power 50 W or more
2) class A
3) balanced input
4) ability to drive speakers that go down to 2 ohms

Any suggestion for a good diy project would be much appreciated.
Thank you.
Well here is my DIY project. I've been using these amps for about 10 years now, and never had a problem (but I admit when its 45 deg in Melbourne like it was 2 weeks ago, I don't turn them on).

40 Watts class A, balanced in. I drive a 4 ohm ribbon tweeter with these. Never gone down to 2 ohm, but it wouldn't hurt (though you'd get less power out).

balanced class A fet amplifier

Last edited by hazard500; 27th January 2014 at 11:23 PM.
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Old 28th January 2014, 02:47 AM   #9
Bonsai is offline Bonsai  Taiwan
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Sandor, go with the Pass proposal - order the PCB's from the DIYaudio store. Lots have been built and you can get help/advice. You are also lucky - Modushop are based in Italy and you can get a very BIG 5U case with heatsinks for about 200 Euros. This should get you to 50W class A per channel with heatsink temps at 60C max.

modushop.biz

Hazard500 amp looks good also - fun project!
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