Class D Audio SDS-254 Amplifier in an Audio Research D-150 chassis
Completed 1-2015, updated meter circuit 7-2017.

The Audio Research D-150 amplifier is a classic.  It was the first high end, high powered tube amp, created when transistor amps were the norm in 1975.  Only 200 were made, and they are highly collectable today, selling for far more than there original price of $2,685.  The amp had a total of 22 tubes (10 of them 6550s) and was spec'ed at 150 watts per channel.  Its also a great piece of industrial design, and the new Audio Research G Series components take inspiration from it and its smaller offspring, the D-79.  Lots of photos and schematics are on the ARCDB.WS website, a must visit for any Audio Research fan.  The Audio Research website (audioresearch.com) also has the schematic and parts list.

Above: Original Audio Research promotional flyer describing the D-150, front side.  


Above: Original Audio Research promotional flyer describing the D-150, rear.  Click to enlarge. 


This Project:

This project started with an ebay find some time ago of a D-150 faceplate, in very good condition.  I then began a search for a suitable chassis as well as the ARC meters, etc. to approximate the exterior look of the D-150.   I was lucky enough to find a D-350 being parted out, and bought all 3 working meters along with the chassis, which although not identical to the D-150, was close and could be made to work with some metalworking.  Finding the correct knobs, fuseholders, red/green panel indicators and rack handles to match the original D-150 also took some time.

At this point I had to decide what amp circuit to build in the chassis.  Although I considered a variety of options, ranging from a smaller tube circuit to various solid state circuits, ultimately I was drawn to the efficiency of a class D amplifier.  The one I settled on was the Class D Audio (classdaudio.com) Model SDS-254, which is a 125W per channel stereo amp circuit.  I designed my own power supply as well as a remote trigger/soft-start circuit for power to the amp.  

The photos and captions below detail the construction of the amp.  You can click on each photo for a larger view.


Above: The faceplate that started this project.  Overall in very good condition as received.


Above: The D-150 had no meter illumination, but all of the similar form-factor amps from ARC that came after the D-150 (including the D-110, D-350, D-79) all had meter illumination, with holes in the faceplate below each meter cutout for the lamps to protrude thru into matching holes in the meters.  I decided to add LED lamps to my faux D-150, and drilled holes in the appropriate spots (behind the meters). This photo is that drilling process underway.


Above: The D-350 chassis being modified to work with the D-150 faceplate.  I cut two square holes for the two bias switches (which actually mount to the faceplate) and I used a Greenlee punch to make holes for the 4 large fuse holders that actually mount on the subpanel behind the faceplate.


Above: A side view of the chassis, which is 10" tall by 17" deep by 17 5/8" wide.  The internal dividers in the lower front were originally used to mount power supply transformers (dual mono) and the cap bank atop them. 


Above: rear view of the chassis, with most of the connectors attached.  Unlike to original D-150, I used an IEC connector for power.  I used Superior BP-30 dual gold binding posts and Connex gold/teflon RCAs.  I also added a convenience outlet which I use to power associated equipment (which is turned on by the remote trigger circuit in this amp).  Plastic hole plugs were later added to cover the unused holes on the chassis, so the back looks relatively neat.  The D-350 label is still visible on the lower right.  Note that the connectors are on a recessed panel, so the amp can be stood on the back side (faceplate up) without damaging the connectors.


Above: The Class D Audio SDS-254 amplifier board before being wired up.  Input circuit is on the right, output and power supply inputs on the left.  The IRS2092 chip (the heart of the digital amp, and the same one used in the current Audio Research DS series of amps) is mounted below the PCB. This amp is two channel, 125 watts into 8 ohms. Although I'm using it unbalanced, it is designed for fully balanced operation.  It typically is set up with variable gain, with the gain pots plugging into the white connectors on the right.  I opted to install fixed resistors to set the gain to 26db, to match other equipment.


Above: The power supply PCB.  This is a Peter Daniels (Audiosector.com) circuit board that works nicely in this application, with dual full wave bridges, one for each rail.  The 8 fast recovery time rectifiers, barely visible on the far side, are MUR1560G, and I used two 12,000/60V 105 degree Panasonic THA caps per rail, for a total of 48,000mfd of capacitance.  There are bleeder resistors across the caps to discharge them for safety reasons.  Below the board....


Above: Below the power supply PCB are a series of polypropylene bypass caps.  Each rail has a 20mfd, 4mfd, 0.33mfd, as well as a Vishay MKP1837 0.1uf cap.


Above: I decided to mount the amp PCB above the power supply PCB.  That would allow very short (under 3") connections between the boards.  Above shows the relative location of the two boards.


Above: Test fitting the sub-assemblies on the bottom chassis plate.  The red PCB is the actual SDS-254 amplifier, the blue PCB below it is the power supply, and the green board to the right is the remote trigger/soft start circuit I designed.  The transformer is an Antek AN-4434 (dual 34V secondaries, 400 VA).


Above: test filling the sub-assemblies in the complete chassis.  Note that I added a mu-metal shield between the AC power IEC and the rest of the chassis.

Above: Closeup of the remote trigger/soft start circuit.  This allows the amp to be powered up by a 12 volt remote trigger signal from the preamp across the room, as well as providing a soft start (thru that large brown 10 ohm resistor above) that is shorted out of the circuit by a relay after a couple seconds.  I designed the board using ExpressPCB, and had them fabricate the board, highly recommended.



Above: Rear of the power meter and power switch (which is a rather uncommon 6 position 60 degree switch to match the front panel markings).  Note that this amp is actually turned on by high current relays on the remote trigger/soft start board, so the switch only sees the low current of the transformer on that board.  It is, however, 120V, so the live connections are covered with heatshrink tubing, and  I added a plexiglass panel, secured by screws to the rear of the switch, to prevent accidentally touching the switch when servicing the amp.  The leads to the meter (also at 120VAC) are insulated with heatshrink tubing as well.  You can also see the terminal strip mounts and wire leads to the meter LEDs in this photo.


Above: Completed amp, looking to the rear of the chassis.   I used all teflon insulated cable for chassis wiring, and teflon coax (the white wires in the center) for the inputs from the RCA jacks.



Meter Circuit update 7-2017:


Above: Completed amp looking toward the rear of the front panel.  There is  a plexiglass cover over the remote trigger-soft start board on the right, as it has 120V on the board.  The two 1mA full scale power meters are driven by a BA6138 logarithmic buffer board, shown on the left above.  I added a pair of reverse connected 1N4007 diodes across each meter for protection, visible just above the meter terminals. 


Above: The completed amp in operation.  I added simple LED circuits to the red "Fuse Out" lamps on the front panel to indicate when the soft-start circuit is operating at turn-on (and also indicate a malfunction of that circuit at any time).  The Class D Audio SDS-254 amp board works great, and sounds equally good.

From an efficiency perspective: the amp draws only 16 watts at idle.