Blast from the past - Kenwood KA-80 - diyAudio
Go Back   Home > Forums > Amplifiers > Solid State
Home Forums Rules Articles diyAudio Store Gallery Wiki Blogs Register Donations FAQ Calendar Search Today's Posts Mark Forums Read

Solid State Talk all about solid state amplification.

Please consider donating to help us continue to serve you.

Ads on/off / Custom Title / More PMs / More album space / Advanced printing & mass image saving
Reply
 
Thread Tools Search this Thread
Old 17th May 2016, 08:22 PM   #1
Banned
 
Join Date: Sep 2010
Location: Hungary
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, 324 views)
File Type: jpg kenwood_ka-80_sm-007.jpg (288.6 KB, 314 views)
File Type: jpg kenwood_ka-80_sm-014.jpg (510.7 KB, 307 views)
  Reply With Quote
Old 17th May 2016, 08:40 PM   #2
diyAudio Member
 
JonSnell Electronic's Avatar
 
Join Date: Dec 2012
Location: The Jurassic Coast, England. GB
"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?
__________________
Support for Fender, Marshall and all Valve Equipment; Audio Innovations, Audiorama FU29, Quad and Leak. www.jonsnell.co.uk
  Reply With Quote
Old 17th May 2016, 09:01 PM   #3
diyAudio Member
 
picowallspeaker's Avatar
 
Join Date: Apr 2007
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 !
__________________
Everything under and above heaven is in utter chaos
  Reply With Quote
Old 17th May 2016, 09:23 PM   #4
Banned
 
Join Date: Sep 2010
Location: Hungary
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
Attached Images
File Type: jpg SAM_ 1332.jpg (94.2 KB, 274 views)

Last edited by dragonweed; 17th May 2016 at 09:27 PM.
  Reply With Quote
Old 24th May 2016, 08:27 AM   #5
diyAudio Member
 
Join Date: Nov 2004
Location: Germany
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.
  Reply With Quote
Old 11th May 2017, 10:23 PM   #6
roclite is offline roclite  United Kingdom
diyAudio Member
 
Join Date: Dec 2011
Location: Middlesex
Send a message via Skype™ to roclite
Default TRIO KA-80

Going through a bit of a TRIO phase at the moment and picked up a spares or repair KA-80 off the bay. It was a bit of a pig to fix, usual blown Darlington power pack but someone had tried to repair before and introduced a solder short elsewhere in the circuit. It was so nice to hear the speaker protection relay kick-in when I finally sussed that one out. As mentioned, these amps run hot because of the high bias and the inverted PCB design so I replaced all the low value caps with high temp versions. While I was at it, I replaced the unreliable filament display bulbs with white LED's - this also helps to reduce the internal heating because you get rid of the high wattage voltage dropper resistors and give more power to the amplifier.

I completely agree. This is a very good amplifier, great bass, very detailed with spine-tingling dynamics. Its just a shame that there are probably so few of them running now that not too many people will get the opportunity to experience.
Attached Images
File Type: jpg DSC_0276[1].jpg (692.1 KB, 185 views)
File Type: jpg RIMG0114.jpg (945.8 KB, 115 views)
File Type: jpg RIMG0112.jpg (788.4 KB, 111 views)
  Reply With Quote
Old 14th May 2017, 03:33 PM   #7
roclite is offline roclite  United Kingdom
diyAudio Member
 
Join Date: Dec 2011
Location: Middlesex
Send a message via Skype™ to roclite
Aargh! Just when I thought I had the TRIO tamed, it put up a fight. Listening tests revealed the left channel was dropping out to less than half volume setting with annoying regularity. Any number of things could bring it back - tapping the chassis, PCB or a sharp turn on the volume control - classic dry-joint or dirty control/contact issue. I cleaned out the volume pot, no improvement then turned my attention to the speaker protection relay, thinking probably dirty or burned contacts. Its easy to check on the KA-80 because the speaker relay contacts are after the headphone socket and inserting the headphone jack automatically disconnects the speakers - neat! So the next time the channel went quiet, I checked the headphone output and sure enough it was good, balanced stereo. I removed the speaker relay, refurbed the contacts which were dirty, replaced and re-tested. Hurrah, perfect stereo....for about 10 minutes! Running out of options, I had to resort to the carpet-bombing re-solder of all the component leads on the left channel. One of them was obviously dry because now the TRIO is working perfectly again, and sounding much better for its signal path spring clean. I have ordered a replacement relay just in case but it already sounds really stunning, definitely a keeper
Attached Images
File Type: jpg RIMG0159.jpg (737.5 KB, 65 views)
  Reply With Quote
Old 14th May 2017, 06:29 PM   #8
Banned
 
Join Date: Sep 2010
Location: Hungary
Great idea about replacing the light bulbs with white LED-s! Now the power transformer gets definitely less warm after a few hours of use. If you have the patience and the necessary parts, it is definitely worth to do the fast rectifier and supply cap replacement mod mentioned in the starting post.. The sound gets even better, cleaner than before.
I used MUR 420 ultrafast diodes, and 10.000 uF/50V Kendeil filter caps in the main power supply, and Elna Silmic II-s in the rest of the circuitry (actually replaced all electrolitycs with those). I admit, the sound is a bit too "analytical" for some tastes, but I have yet to find an other amp with this kind of detail and speed....
I am using it mainly as a testing tool for loudspeaker design, and for that purpose it is just the perfect choice.

Last edited by dragonweed; 14th May 2017 at 06:32 PM.
  Reply With Quote
Old 15th May 2017, 01:28 AM   #9
diyAudio Member
 
Join Date: Jul 2004
Location: Edmonton, AB Canada
STK0060 II family internal schematic and pic of one I took apart.
I have datasheets and app notes for them. Usually their bonding wires open-circuit (tap it with a plastic pen) or the transistors short.

For the output relay, I use a piece of paper to scrub off oxide on the contacts.
Attached Images
File Type: jpg STK0060II module.JPG (17.1 KB, 66 views)
Attached Files
File Type: pdf STK0060_sch.pdf (108.9 KB, 20 views)

Last edited by prairiemystic; 15th May 2017 at 01:34 AM.
  Reply With Quote
Old 15th May 2017, 06:56 AM   #10
Banned
 
Join Date: Sep 2010
Location: Hungary
Quote:
Originally Posted by prairiemystic View Post
STK0060 II family internal schematic and pic of one I took apart.
I have datasheets and app notes for them. Usually their bonding wires open-circuit (tap it with a plastic pen) or the transistors short.

For the output relay, I use a piece of paper to scrub off oxide on the contacts.

Since you've one already dissected, is there any chance that you could measure the resistance values? That way it could be rebuilt on a similar size PCB.....
  Reply With Quote

Reply


Hide this!Advertise here!
Thread Tools Search this Thread
Search this Thread:

Advanced Search

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off
Trackbacks are Off
Pingbacks are Off
Refbacks are Off


Similar Threads
Thread Thread Starter Forum Replies Last Post
heybrook speakers , a blast from the past needs help busman3856 Everything Else 0 29th October 2015 10:09 PM
TV Valves blast from the past.. M Gregg Tubes / Valves 0 19th February 2014 05:55 PM
A blast from the past .. PAPA1 a.wayne Pass Labs 4 30th April 2010 08:06 PM
Totally cool, blast from the past... amt Everything Else 5 26th March 2010 03:34 AM
pioneer sa 9800. blast from the past burbeck Solid State 5 15th January 2008 02:03 PM


New To Site? Need Help?

All times are GMT. The time now is 02:41 PM.


vBulletin Optimisation provided by vB Optimise (Pro) - vBulletin Mods & Addons Copyright © 2017 DragonByte Technologies Ltd.
Copyright 1999-2017 diyAudio

Content Relevant URLs by vBSEO 3.3.2
Wiki