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Old 20th November 2005, 04:33 PM   #1
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Join Date: May 2003
Default Would this be worth the effort?

Buying the Forte 1A kind of killed my Aleph budget, but I have plenty of parts available to make an amp of some other kind. I've been thinking about it a lot lately (instead of doing homework assignments, projects, etc.)... I've always loved the X-SOZ and the Zen-lite... and I would love to have both combined into one. This will turn into my Christmas break project.

Attached is a first-draft schematic. I have an old 30-0-30 transformer that is big and heavy (probably 10 pounds or more). I don't know the VA rating, but I think it'll be sufficient. I figure after it gets loaded by four FETs, it's recitified voltage will settle in at around 80VDC, which is what I included in the circuit. I could drop it some more with a CRC filter (which I'll probably do).

I don't have the spare money for huge heatsinks, so this won't be a 20W monster SOZ, it'll be more like a 2W version. Would these heatsinks be suitable if I put one FET per sink? I figure with one 150W light bulb, the idle current will be around 700mA per FET, which is low, so each will then dissipate 50-60W. At the moment, I am leaning toward a single rail supply over a double. Any reason why I shouldn't do this? I am also unsure of the value to use for the input pot. I really want to include it because I would like it to be an integrated solution. This was a big problem with the Forte; I had to build a separate passive preamp (oxymoron?) so I didn't deafen myself. I am currently thinking about IRF230 for the FETs, it I can find them.

I have a pretty good general idea of where I'd like to go with this, but I really wonder... would it be worth it? I'll only get about 2W, which is totally fine. I usually listen at about 50-60dB through my 102dB 1W/m speakers, so I am only using milliwatts anyway. But, will this be a good, clean 2W, or will it be loaded with distortion? Thanks for any pointers.
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Old 21st November 2005, 06:02 PM   #2
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Looks to me like the circuit will work, but I wonder about the
80V supply - it seems like way more than you will want.
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Old 21st November 2005, 08:03 PM   #3
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Thanks for the reply.

I was thinking the same thing about the supply voltage... Perhaps if I cut it in half to about 40V? I only lack a few parts, so I suppose I can collect the remaining ones and give it a try (after finals and projects, though...).
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Old 22nd November 2005, 12:46 AM   #4
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Will the distortion go up significantly for such a low bias current? Would it be too much distortion to take?
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Old 22nd November 2005, 06:22 PM   #5
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The distortion is very dependent on the current, less so on
the voltage, so you want to look for a balance there. When
I do development work, I often just use a variable supply
sao that I can dial it in most easily. In your case, an existing
fixed ~40V supply and a Variac ($39 cheap) would assist you.
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