PartTimeProjects DX HRII Amplifier (From Carlos)
Carlos is from Brazil and boy does he have a lot of personality! He has built lots of amplifiers and designed many amplifiers as well. This amplifier is a variation on Carlos' DX amplifier, and is called the DX HRII. You can read about Carlos and some of his amplifiers here. I undertook the task of building this DX HRII Class AB amplifier. This is a Class AB amp so should normally run cool. It is about 50 WPC into 8 ohms.
I have posted a .pdf scan of the Schematics and parts list (the link in the document is dead) which has all the components for the design, and the parts that I used are indicated. Note- I did not check off everything (check marks are missing) because this was scanned before I finished. R18 has been changed to 1.48K to decrease the gain and I have not connected a jumper across the "optional treble boost" part because I did not want the treble boost (but the parts are in there anyway).
For the case I started with a very old Sansui 2000 receiver, which I bought at the thrift store for $5.00. It had only one channel working so I did not feel bad taking out all the internal components and putting in the DX circuits. Man, that receiver had a lot of options, and as you can see below a lot of wiring. It had separate bass and treble controls for left/right speakers, selectable MPX filtering for the FM radio, and a number of shielded internal compartments. It used Elna capacitors and TO-3 output transistors. Good technology from long ago.
I used "ABS Plastic Sheet" to cover the openings in the rear panel connectors which were not exposed due to removal of the connectors. This is rough on one side and glossy smooth on the other, which looks very nice on the back of an audio component. I did keep the original fuseholders. New connectors were put on the rear panel. I designed a front panel from Front Panel Express using their software and the existing screw holes. It came out looking quite nice.
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I did not think the silver bezel with gray aluminum panel would look good with a black cover and back panel. But wow, I think it looks great! It kind of looks like an old Goldmund amplifier. The inset LEDs behind the original smoked glass (used for the original tuner) are offset inwardly and give this a very neat 3-D appearance. It looks like a piece of high quality equipment- which it is.
Here is the beginning, where I got the Sansui 2000 receiver and beat it up.
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The power supply uses two bridges mounted at the front on the metal case. One is inside the shielded compartment where the controls used to be. But it is vented from the bottom. There are 2 capacitors per rail, each is rated 15,000 uF at 63V/ 85C and these are mounted using stand offs that actually mount them below the floor of the chassis, as the floor is suspended from the bottom of the case. Kind of neat. The transformer is 2x25 VAC at 500VA. I get relatively noise free 35 VAC with a lot of current on tap.
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With 4 capacitors at 15,000 uF and a 500 VA transformer, this unit will blow a 5A fuse with regularity after a few on-off cycles. Its kind of weird, you can see gradual imperfections on the fuse as it wears out. Switching to a 6A fuse I have not had any problems. Here are the diodes and capacitors-
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The amplifier uses the DX HRII boards from "Nordic" and three "large" TO-3 heat sinks. The heat sinks are connected together via aluminum wedges to even out the heat distribtion which will someday be replaced with copper heat spreading bars. I have set bias to be about 50 mV across the 0.22 ohm emitter resisters and it does get warm after 1 hour of idling.
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This amp sounds fantastic! It plays with authority, goes loud, with depth in the bass and without mushing the treble. It is quiet, no noise at the tweeters when you hold you ear to them. It runs cool but not as cool as you think, big heatsinks are a plus. It is a little tricky and frustrating to build due to the close part spacing but its worth it. The DC offset can be made very low (mine is < 20mV) and the gain modification I made on mine results in gain that is a little low but nothing to worry about. The amp is solid, looks nice and sounds great. It sounds like a 100 Watt amp, not a 50 watt amp.