Audio Research D-100B

I have this vintage D-100B in my collection. Over the years, I never had the time to work on her as I was too busy designing loudspeakers. Since I can only write about power amplifiers until I fully recover from my heart surgery, it is an opportune time to visit this amplifier.

Apparently, the D-100B was introduced sometime in 1979. The D-100B was Audio Research Corporation (ARC) early introduction in solid-state designs. Her rated power was 100W into 8Ω. During those early years, 100W is rare coming from a power amplifier.

There are a few unusual features in the D-100B that I will touch on. The most prominent are two sealed modules on the pcb (Fig 2). These are the blue boxes in the schematic (Fig 1) below.

Fig 1 – Audio Research D-100B Amplifier Schematic

Fig 2 – Audio Research D-100B Modules

It is not uncommon for manufacturers to resort to hiding their designs to prevent competitors from copying their work (Fig 2). Potting and modules are just some of the ways. It is safe to assume that the first blue box on the left (Fig 1) contains the input circuit while the second box is likely the VAS. After that comes the drivers arranged as a Buffer Diamond instead of a more conventional emitter follower.

Fig 3 – Audio Research D-100B Power Transistors

The output transistors (Fig 3) are four pairs of TO-3. Two are Toshibas but the other are from Motorola. I suppose there are no complementary by the same manufacture at that time hence this mismatch.

Fig 4 – Audio Research D-100B PSU

Another unusual feature in the D-100B is the amount of filter capacitors. There are a total of 120,000uF 60V capacitors servicing two channels (Fig 4). That’s massive.

Fig 5 – Audio Research D-100B Regulated Supply

Lastly, they even have a tracking regulator for the input module in the voltage amp. The unregulated voltage is at +- 75Vdc. After regulation, the voltage is +- 55Vdc. This translates to a headroom of 20V. Frankly, this is a bit of an over-kill.

Summary

Restoring this amplifier is not a problem but I’m not motivated enough for that. I would rather do away with the original pcb and design a new amplifier from scratch. This is more challenging because a new circuit has to be designed, followed by drawing out the pcb and finally getting the pcb manufactured. After that comes bench testing.

So, what I’ll end up is a new amplifier using the D-100B chassis, heatsinks and possibly the power transformer.