Kenwood KA4004 Amplifier

General:

Around 1970 Kenwood-Trio company came with its KA4004 quality amplifier on the audio-market.

 

Special futures:

It was one of the first full complementary OTL-OCL output staged amps.
It has great Sanken power transistors.
20 Watts per channel was the power of this amp. Tone is defeatable.

 

Schematic:

Download here the service manual of the KA4004 in zip format.

 

inside the amp: top view: upgrading the amp.

inside the amp: bottom view: upgrading the amp.
 

Modifications:

Electrolytic caps may be replaced and some transistors can be replaced,
as  most of the caps are leaky after many years of service and some of the
transistors in this amp are aging and becoming noisy.
The drivers, Q7 to Q10, can be replaced by the well known BD139/BD140 of
Motorola, STMicroelectronics or any other equivalent.
In the power amps section replace the Q1 to Q4 for BC550C, and replace Q5 and Q6 for BC560C or equivalent.
This replacement eliminates DC at the speaker output and lowers the noise figure of the entire amp,
you can get the noise figure below the original specs of Kenwood if you use these suggested modern transistors.
Leave final power transistors and driver transistors as is, of course only if possible.
The final power transistors are TO-66 cased, they are not available anymore,
these 2SC1445 TO-66 transistors of Sanken were of exceptional quality,
the only replacement is plastic transistors like 2SB595/2SD525 or equivalent,
you have to change the mounting on the heatsink somewhat if you use plastic transistors,
and you will find that the sound quality will not be the same anymore.
All other low-power transistors in the pre-amp section replace by BC550 and BC560 C-type or equivalent.
The output control/security section suffers often of bad transistors and leaky capacitors:
All 100 uF 10V caps should be replaced by at least 100 uF 16V types.
In the RIAA amp there may also be some noisy transistors that can be replaced by similar types,
in any case replace all electrolytic caps in the power rail of the RIAA amp and check caps in the signal path.
I added also some extra caps as you can see on the photos to get the power rail of the entire amp as quiet as possible.
Check idling current and DC offset in the power amps.

 

Sound:

The sound is great after a restoration of the amp.
This amp used to be a competitor of the lower powered Luxman amps like the SQ700 etc.

 

© Hans Hilberink - PE1MMK ® 2003 - Last update: 25-08-2010.