last update : april 1999
After some hesitation an serious doubts, I decided to do my Harman Kardon 6550 a favour with some tweaking. Briefly : this is a fine amp with High Current design, fast ring emmitters, low feedback as most HK amps have.
Still when I started I found that design and philosophics didn't match with the real live produkt. The power supply current (HK claims 40 ampere max) has to pass a long way all over the main circuit board. The difference in the Left and Right sections are are quite different in length and construction. One channel uses several(!) bridges with a very lousy thin wires. YUK!!!
MOD1 : I bypassed the + and - power suply lines with 1.5mm^2 massive copper wire, the output impedance as seen by the power amp is reduced.
The speaker output wires were not satisfactory (to me) either. The output signal travlled from back to front and backside again passing an speaker selector switch. The quality of the wire was not to bad(thin). I used ca. 20 cm Monster 2.5 mm^2 to connect the power output to the new goldplated output spade connectors. (ALLTOUGH I prefer and use bare wire).
I also bypassed the power supply capacitors (13.000uF//63V) with an standerd quality 10uF MKT /100V. I still want to do the same as close as possible to the power transitors.
After this mod job I was very pleased with the result. The music was more spacious and less attached to the speakers.The music was also more relaxed, not laidback. but easy to listen, A bit faster and tighter too.
Some people have experienced with the following : "After bypassing the supply capacitors with MKP 15uF. The amplifier (ROTEL 990BX) got a irongrip on the speakers and I could play much louder. It sounded great."