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Old 16th July 2009, 06:39 PM   #7971
john65b is offline john65b  United States
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Whoops, forgot about those zeners
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Old 17th July 2009, 01:53 PM   #7972
alibear is online now alibear  United Kingdom
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Hi John,Thanks for the reply.
I'll give a quick answer as it's going off the Krell thread. All three amplifiers were built using good quality components. After a lot of listening this is what I think ( it's all subjective of course, input source and loudspeakers will also affect)
The F5 was my least favorite, it sounded " thin"and not very engauging I don't think I could listen to it for a long period of time.The Hiraga and JLH were both smoother and more musical, much easier to listen to, but I prefered the JLH, bass was firmer,midrange was open and detailed, female vocals outstanding. The wife liked it best as well. ( need I say more ? )
Alan


Hi Stuart,
can you please explain how I select the Zener voltage and resistance. I will be using +and - 22volt rails, I do not understand about getting the operating conditions for the long tailed pair correct. Also can you give more details of the CCS you mentioned. Sorry to appear so dim, but I am trying to learn.
Thanks for your help
Alan
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Old 17th July 2009, 08:16 PM   #7973
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Default well...

...the CCS is too much to put in a single post, searching this thread will get you the results you need, but if a simple turnkey solution is your goal it's probably not the right answer anyway.

In a nutshell, the long tailed pairs are constant current driven, and the current source they run from can be made voltage independent, at which point, within the limits of the transistors involved the operation of the front end becomes voltage independent.

I'll find a schematic and let you know about the resistors and zeners shortly, first I have to run to the post office.

Stuart
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Old 19th July 2009, 04:00 PM   #7974
alibear is online now alibear  United Kingdom
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Thanks Stuart
Any help is much appreciated.
Alan
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Old 19th July 2009, 06:12 PM   #7975
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Default numbers...

The zeners probably need to be about 1/2 - 2/3 the rail voltage, so say 14v, the zener 'bias' current in the original amp was about 6ma, so the resistor in series needs to be ((22V-14V)/0.006A) or 1333 ohms. Keeping the same LTP current (3 ma) the emitter resistor in the long tailed pair will be (14V/0.003A) or ~4700 ohms.

Don't fixate too much on the exact values here, if your rails end up being +/-24v no big deal, none of the values above would cause anything to break.

HTH

Stuart
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Old 26th July 2009, 03:57 PM   #7976
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Default Krell clone on audiogon

Hello to all,
I bought the KSA-50 clone that recently sold on audiogon and I have some questions about it. The output transistors run hot, 165F degrees on the right channel and 185 degrees on the left. Also, one of the other transistors (has a small heat sink attached to it) is running at 200F. Are these temps acceptable / safe? Love the sound of this amp, just want to keep it running for a long time.
BTW, I'm a lifelong DIYer and have built many amps and speaker systems over the years. I was able to buy this amp for around the cost of the parts ($350) and couldn't resist.
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Old 26th July 2009, 04:21 PM   #7977
AndrewT is offline AndrewT  Scotland
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165degF = 74degC is about highest temperature for plastic packaged transistors.
To3 may be able to run a little hotter.

I think 185degF is too hot.
200degF is far too hot. Any idea what Tj is at that case temperature?
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Old 26th July 2009, 10:12 PM   #7978
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Default Hot Krell clone

Thanks AndrewT,
I have no idea about Tj but I would guess it's at least 50degF higher. I'm going to check/adjust bias as the left channel is obviously running higher/hotter. Got to listen to it for a couple of hours last night but I put a fan on it (yuck) to play safe.
Here's a link to a member's amp that is using the same boards as mine:
http://img60.imageshack.us/i/dscn12304et.jpg/
I would like to know which schematic to use and the suggested bias for this design.
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Old 27th July 2009, 01:51 PM   #7979
alibear is online now alibear  United Kingdom
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Stuart, thanks for your reply. I now understand about the zeners / resistors, can I assume that the rest of the circuit ( Jans )needs no alterations for the lower supply rails.
much appreciate your help.

Does anyone know if PCB's are still available.

Alan.
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Old 27th July 2009, 02:42 PM   #7980
johnm is offline johnm  United Kingdom
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Hi Alan.

Regarding Jans PCBs I have a pair you are welcome to free of charge if you want? One of many projects that never got started alas

Cheers,

- John
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