This Harman Kardon Citation 12 amplifier came through the shop for an overhaul recently. The owner bought it on Craigslist, and it was working okay, but the owner knew it was due for an overhaul based on its age and so it came through for service.
Unfortunately, this one had been serviced in the past by a hobbyist who made a number of questionable modifications.
They replaced the 4 power supply capacitors, as well as the capacitors on the PCB. In addition to using the wrong values of capacitors on the circuit board, they placed 1 uF film capacitors across all electrolytic caps – but must have used an unsteady hand as many had melted spots on their cases, which isn’t great.
They added 2.2 uF capacitors across the main filter capacitors, as well. These also had burnt and melted spots on them.
They added 2.2 uF across the original, failing, 220 uF electrolytic input capacitors, too. Also, nearly all under-chassis wiring was replaced with the thick braided cloth-covered wire for some reason.
They also damaged one of the pin connectors, and used a piece of a third pin to make up some of the difference. Fortunately, despite the “upgrades” performed previously, this amplifier was fully repairable.
The previous owner did some major damage to the chassis pan when they replaced the filter capacitors. It’s unclear why.
Fortunately, everything cleans up nicely. The new capacitors are considerably physically smaller. They’re slightly down-sized from the installed values, but are still about 2x the factory size.
The replacement PCB capacitors were starting to bulge, too, so it’s a good thing this came in. All replaced with the correct values on the board.
After reinstalling the board and tightening it down, all of the pins were checked for a good electrical connection and were all good.
Bias adjusted to 10 mV in the center of the range, giving 40 mA of idle current per channel. Bias was re-adjusted after 15 and 30 minutes of idling to ensure proper adjustment after warm-up. The original potentiometers were clean and smooth through their travel range, and so did not need to be replaced for this operation. One channel was nearly cut off, with ~5 mA of idle current; the other channel was adjusted way too high with 110 mA of idle current. This amplifier does not have a discrete DC Offset adjustment, as the offset is set by the gain matching on the differential input pair transistors, but these were well matched and delivered <20 mV DC offset on each channel.
Finally, since this power amplifier was due to be used with the HK Citation 11 pre-amp, it was time to run them together. The pre-amp’s level adjustments were set so that 100% Volume on the pre equals maximum output of 60W on the power amplifier. Time for some measurements!
Frequency response is specified from 5 Hz – 70 kHz, although a flatness specification was not provided so we selected an exacting +/- 0.5 dB deviation specification. The analyzer’s bandwidth only extends to 10 Hz on the low end, but through the available range 10 Hz – 70 kHz, the unit is perfectly flat.
Zooming in to standard audio frequency ranges, 20 Hz – 20 kHz, the amplifier is flat better than +/- 0.06 dB!
Harman Kardon provided data points at 30W and maximum power, both specified <0.2% THD+N. At both 30W and 60W power, actual THD+N performance was below 0.025%, or more than 8x better than the specification. In addition, HK specifies the Citation 12 to 60W per channel into an 8 Ohm load with 1.5V input, but that is a conservative rating as 60W is achieved at 1.2V. There’s about another 50% of headroom left.
All set! Works great, and sounds fantastic!