PartTimeProjects "Krill" Amplifier
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Here is the "Krill" amplifier published by Steve Dunlap at Dunlap Audio. At 100 WPC, this amplifier stands apart from most 25-50 WPC DIY designs, and it incorporates Steve's special and controversial "Krill" biasing of which there is continued debate. As I was looking for an amplifier with more than 50WPC, this amplifier became of interest since Steve has boards available for purchase, there is a full and detailed parts list, and I was looking for something a little bit different.
I bought the parts and boards; and put the design into an old Harmon Kardon AV receiver that was not working. The receiver is very big and so are the Krill boards- they just fit with a little overlap.
Starting Out
Here is the starting point, a Harmon Kardon AV receiver. I took it apart and closed off the unused connections. I added a AC inlet, fuses and high quality RCA input connectors, but I used the existing speaker binding post connections, set up for biwiring. If I did this again, I'd remove the speaker binding posts and put in better ones that can accept spade connections. For some reason I wanted to keep the originals in there, but they cannot take a spade lug (banana's plug right into the back though). |
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The back looks pretty good and the AC wiring can go down the side of the amp for reduced interference.
The Boards, Bias, Offset and PH104.
After years of jamming components into small spaces, it was fun to work with these roomy boards. The parts all fit and it was easy to put together. The problem I found was that the DC offset and bias were hard to adjust. I tried and tried but could not solve these issues. At the suggestion of others, I cut the board traces to separate the input stage from the output stage, so that input offset could be set independent of the output stage. Once the input stage offset was about 20mV the thought is that you reconnect the input stage to the output stage and tweak it again. Sounds like a good idea but I could not get it to work. Also there was a split of opinion on how much to heat sink the driver transistors, so I tried putting small heatsinks on them. Note the many heatsinks in the photos. In the end, I failed. So, PH104 to the rescue! With on-line diyer "PH104" doing all the real work and Steve helping in the background, the driver transistors got mounted to the heatsinks, a few resistors got added to help with DC offset and the amp worked. So if you are going to build the Krill, make sure you check with Steve and the web link (above) to get the latest information on how this is supposed to be put together.
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Notice that there are a lot of wires going to the driver transistors, many of which ended up mounted to the main heatsink. There are 2 additional resistors added to the leads of the driver transistors to stablize bias.
Front Panel
The front panel of the Harmon Kardon AVR-45 is not quite flat. A curved ridge at the bottom protrudes sligthly further than the flat faceplate, so I cut the curved ridge out with a dremel and standard cutting disc. If the dremel speed is too fast the plastic will melt onto the disc preventing cutting; I found the speed of 2.5-3 worked best, no melted plastic botching up the disc. Having a flat plastic front panel surface allows the mounting of a new flat metal panel over the original faceplate to hide the faceplate. I liked the look of the DX Amplifier (one of my prior amps) so much I kind of copied its look- silver aluminum on black with a display window that uses the original receiver's smoked plastic/glass display. This time instead of two indicator lights, I put in dual seven segment LED display, wired with a separate regulated power supply to say "On." when the amp is turned on. As usual I designed the front panel and sent away to Front Panel Express for the panel. I created a small perfboard variable voltage regulator circuit running of the unused 12V transformer windings to power the dual LED display. It reduces voltage to around 6.5 VDC to drive the LEDs on the front panel display with 220 ohm resistors in line on each segment.
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During testing, the DC offset was at about 100 mV when the amp is first turned on. After about 30 minutes its down to about 40mV and 50 minutes to about 25mV. After an hour it is down to less than 20mV and stays there. I had the amplifier on an entire day and the last offset readings were <10 mV. The bias is set to about 30mV across each emitter resistor. This gets the heat sinks up to about 41 degrees C (21 C above ambient) where they sit unless you really crank the volume. There is a small thump when you turn the amp on. The amp will blow a 5 amp fuse so I am using a 7.5 amp line fuse, which so far has not given me any problems.
The chassis from the HK AVR-45 is made up of segmented steel and is very floppy without the cover. You have to be really carefull when picking up one side because the other side will flex and you may bend the boards. Luckily, adding the faceplate and cover (which attach to the sides, front and back) solves this probelm; making the case very solid, I could get any flex with all the parts installed. Also, to mount the heatsinks and transformer, there are long brackets and aluminum tubes inside the amp which add some structural rigidity.
Sound
Well finally, a 100WPC diy amp! Its great to have a little extra power when you need it. And this amp is not at all bass-shy, it has no problem putting out some deep base. The amp is smooth and mellow, the highs are not as detailed or crisp as the DX or gainclones so even with speakers that have sensitive tweeters the Krill does not sound shrill. I'd characterize the sound as more like a relaxed tube sound from the midrange up, and powerful solid state below the midrange. If you are the kind of person who thinks digital audio is bright, edgy and fatiguing, you should think about this amplifier.Looking at the amp on an oscilloscope, the square wave was fine, nice and stable. Sine, Triangle and Square waves all got bigger with no obvious distortion, and clipping was soft with no oscillation. It easily puts out 100 WPC into 8 ohms, and more into 4.
A nice amp that is the result of some nice people.