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Old 11th February 2017, 08:15 AM   #1
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Default Circuit Suggestions Wanted for 2SC5200 / 2SA1943

Hi,

I've had a browse of the forum where there are similar threads and I'll continue to look through but I'm considering a project to build a new amp from an existing one and have fairly specific parameters so wanted to ask for ideas.

I have an amp. I've been a little indiscreet in the past and named it but I'll try to avoid that this time. It's a cinema amp, of the variety which cost thousands new. I bought it on the cheap, broken and had it fixed / restored, intending to use it as a high quality stereo amp with the option to go active. It doesn't sound good to me. Every time we find an issue and fix it there's another two behind that. I'm now over two years and a thousand pounds in and my engineer (I'm just a beginner) has given up. I don't blame him, he's a saint to have gotten this far. We could keep chasing problems for another two years and end up finding there's not a decent amp in there after all, and it's a bastard to work on.

What the tech is up for is throwing out the circuit boards and using the thing as the basis for a new build. This seems like a good idea to me as it would give us the following most excellent platform:

- 2 x 800VA 60V toroidals
- 10 x 10k uF capacitors (I'm sure we could fit a few more in
- 10 pairs of 2SC5200 / 2SA1943
- Big, heavy, fan cooled internal heatsink
- Very sexy case of 3U(ish) size

I presently have 8 ohm 300W speakers (ATC SCM19) and a belief that headroom is very desirable. I'm a hifi buff and looking for best possible quality but also want at least 300W into 8, ideally with the flexibility to handle trickier loads to cover possible speaker changes in the future (you never know). We'd like to use all 10 pairs (so 5 per channel). First instinct is just to copy a Krell or Bryston design but they might not match the transistors and the first Krell we looked at was too complex / esoteric. I only mention them to give an idea of the sort of thing which would be ideal. Because this isn't just a fun project but rather will end up as my main amp in a quality system and can't be just chucked out I'm trying to avoid the build it and see if it's any good thing and get it right first time.

Any suggestions to throw into the mix would be greatly appreciated

Last edited by MrLem; 11th February 2017 at 08:32 AM.
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Old 11th February 2017, 09:36 AM   #2
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What are the dimensions of the heat sinks?
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Old 11th February 2017, 09:51 AM   #3
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Approximately 400 x 80 x 120. It's comprised of two sections which bolt together in the middle but probably can't be split in practice because combined they're the entire height and width of the case, with a little gap for cable routing etc. It has mountings for two fans at the moment (120mm IIRC).

Last edited by MrLem; 11th February 2017 at 10:04 AM.
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Old 11th February 2017, 10:15 AM   #4
AndrewT is offline AndrewT  Scotland
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Quote:
2 x 800VA 60V toroidals
what transformers have you got?

60Vac single secondary,
or 60Vac centre tapped secondary,
or dual 60Vac secondaries?

You have to supply information that can be understood, if you want some help.
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Old 11th February 2017, 10:20 AM   #5
AndrewT is offline AndrewT  Scotland
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Forget ClassA.
If it needs a fan assisted cooler for it's former duty, there is no way you can copy, or clone a ClassA.

I would not recommend cloning a Bryston either. It's output stage is quite complex and without good debugging capabilities you may never get a clone to run.
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Last edited by AndrewT; 11th February 2017 at 10:39 AM.
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Old 11th February 2017, 10:32 AM   #6
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What is the dimensions of the mounting surfaces of the heatsinks? Is there one mounting surface of two?
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Old 11th February 2017, 11:05 AM   #7
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Quote:
Originally Posted by AndrewT View Post
what transformers have you got?

60Vac single secondary,
or 60Vac centre tapped secondary,
or dual 60Vac secondaries?

You have to supply information that can be understood, if you want some help.
Thanks for taking an interest. I do apologise for any ambiguity. I'm trying not to casually use any terms I'm only 90% confident of and lead people up the garden path without spending a little time reading up but do ask if I miss something and I'll check it out. Also, I don't have the unit here to check right now. I'm in the early stages of kicking ideas around and seeing what sort of options are available before deciding whether to commit to the project.

At present the transformers are wired in parallel and putting out +60 and -60 (AC) so I'm fairly confident that is available from each but I want to physically check and read up a little rather than assume before I talk about centre taps and dual / single. I shall respond on that as soon as I'm able.
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Old 11th February 2017, 11:08 AM   #8
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Quote:
Originally Posted by AndrewT View Post
Forget ClassA.
If it needs a fan assisted cooler for it's former duty, there is no way you can copy, or clone a ClassA.

I would not recommend cloning a Bryston either. It's output stage is quite complex and without good debugging capabilities you may never get a clone to run.
For sure, I'm not looking at class-A. I know I don't have the cooling or the electric bill budget for 300W of that.

Thanks for the Bryston advice, it may save some time ruling it out. I mainly mention those to give an idea of the kind of application / spec I'm chasing. Cloning one is probably a little pie in the sky even if I hadn't already determined the transistors.
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Old 11th February 2017, 11:17 AM   #9
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1/4 C/W per heatsink maybe? 300W class B => 150W dissipation => heatsink T = 38C + ambient. Say ambient 25C, heatsink 63C. Add 20% margin for error, heatsink 75C.
Note: 2SC5200 datasheet max W is derrated about 40% at 75C along with SOA.
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Old 11th February 2017, 11:26 AM   #10
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Originally Posted by jwilhelm View Post
What is the dimensions of the mounting surfaces of the heatsinks? Is there one mounting surface of two?
At the moment the transistors are all mounted on one surface running almost the entire width of the heat sink. There doesn't seem to be a lot of flexibility to change that. The heat sink provides a flat surface top to bottom of the case on the side it connects with nothing in the way.

In fact, I think I can sufficiently redact a picture of it, sec...

Red 2.jpg

I can't give exact dimensions because the unit is in the workshop at the moment and dimensions are not in the service manual but that should give you an idea. It sits about half way back in the amp separating the power and display sections from the rest of the circuitry.

I must stress that boards can be made, metal can be drilled etc and I'm really at the early stages of seeing what circuits are around which might fit the bill.
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