Blast from the past - Kenwood KA-80 - diyAudio
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Old 17th May 2016, 09:22 PM   #1
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Default Blast from the past - Kenwood KA-80

Well, I was always wondering about this one since I heard it some 20 years ago.
Then last week by chance found a perfect condition specimen on the local flea market for peanuts, so I decided to give it a chance.
Doing the compulsory homework on a 30 years old amp -replaced all electrolytics in the power stage with Silmic II-s, and Kendeils in the power supply + ultrafast rectifiers- I was totally blown away from what came through on this old piece of hardware....
Having owned, built and listened around 100 different amps (some really good ones too) in the last 30 years, let me tell you, this is a very different animal as far as speed and transparency concerned.
Maybe I am just getting old and losing my marbles, but let me tell you, these guys at Kenwood knew something by then that very few manufacturers are able to repeat today.
The sheer clarity, transparence, and attack of this little amp is breathtaking.
I am hearing information in the upper octaves that sometimes just plain scare me, because they come through with such a realistic way, I often confuse them with environmental noises. Never experienced that with ANY other amp before.
On the attached pictures you can see, that they did their homework on output stage design.... Split feedback for the VAS and current amp stage, critically applied phase correction and speed-up components at all place.
Even that old Darlington power pack does its job in an excellent way, this thing reproduces 40 kHz square waves perfectly, without overshot and ringing.
After all, I am curious about some expert opinions from you, any comments advices, criticism concerning the design?
Attached Images
File Type: jpg kenwood_ka-80_sm-006.jpg (239.4 KB, 112 views)
File Type: jpg kenwood_ka-80_sm-007.jpg (288.6 KB, 109 views)
File Type: jpg kenwood_ka-80_sm-014.jpg (510.7 KB, 107 views)
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Old 17th May 2016, 09:40 PM   #2
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"this thing reproduces 40 kHz square waves perfectly, without overshot and ringing".
A square wave has all harmonics of the frequency produced and they are present on the top leading edge of your wave form and presents itself as ringing. I suspect you are hearing harmonics, which sound satisfying to the human ear. How fast is your oscilloscope?
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Old 17th May 2016, 10:01 PM   #3
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Quote:
Originally Posted by dragonweed View Post
Even that old Darlington power pack does its job in an excellent way
STK 070-III, IIRC

You forgot to say: two transfomers, one for each rail ( for keeping the height of the case low ) ; The two caps after the rectifier, I'm still keeping them in another amplifier and they're still fine.

Was the 4558 based tone stage defeatable ?

Yeah, great amp !
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Old 17th May 2016, 10:23 PM   #4
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The scope is a Leader 1041 dual channel 40MHz scope. Well, the load is an 8 Ohm power resistor, the amplitude is 10v p-p. With a speaker it could be different, but I am not risking feeding my speaker with such a signal.....
The power pack is a STK 0060 II 8A Darlington array, the tone stage is defeatable, (replaced with LM4562 just in case) the input RCA-s feed the power stage through the selector and the defeat switch, with nothing else in the signal path
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Last edited by dragonweed; 17th May 2016 at 10:27 PM.
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Old 24th May 2016, 09:27 AM   #5
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Do I see a common-mode bootstrap in the input stage there? Cool.

Unfortunately these amps enjoy a reputation of being notoriously unreliable, mostly due to running hot. Apparently they often had to be run at reduced bias to avoid blowing up (again).

I am still quite fond of the tuner going with this one, the KT-80. The 10/16 coupling caps at the output are quiter probably dead at this point though, these should be either bridged or replaced by bipolar / low-leakage types. Very nice-sounding piece of kit after that. Never been that happy with my KT-1100, always sounded thin to me... maybe it needs a recap. I'd just need to do something about the stupid lamp driver circuit, and the wiper contacts in the tuning cap could use some cleaning action again.
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