I have owned my venerable Phase
Linear D-500 amplifier since 1978. It was the last amplifier that Bob Carver designed
before he left the Phase Linear Corporation to start his new business.
Over the years, I have done several maintenance items on this amplifier, mostly related to
re-capping. The majority of problems were due to capacitors drying out on the board that
controls the amplifier's functions. I've replaced these over the years.
Just recently, the transistor that controls the latch circuit which powers the main relay
for switchings mains power to the large power transformer had developed an intermittent
breakdown where the amp would occasionally turn itself on with no one pressing the front
panel button. Since I didn't have an MPSA42 on hand, I replaced with a 2N3440, with just
as good as new results.
Integrating this amp into my racks a number of years ago brought about a problem: hum. The
amp threw a lot of magnetic energy into everything made of metal that was nearby.
Especially with direct conduction from the D-500 chassis to the rails of the rack.
I decided to redo my rack configuration this month in an attempt to fix the problem by
isolating low level systems from power systems. While I was at it, I did a little bit of
needed maintenance on several pieces of gear.
While inside the D-500, it occurred to me what an awful job of engineering went on as far
as wiring and grounding topology was concerned. These guys wired the amp in the most
wasteful, noise-attracting way possible!
I found several wiring no-nos and gradually worked on fixing them. Before I get to
details, I should point out that this amp always showed a bit of instability in the form
of ultrasonic oscillation, when driven to clipping, since it was bought new. (I love to
put all my amps into dummy loads and put a scope and signal generator on them to measure
the real power output.)
Getting back to details now, I ended up cutting out about 60% of the wiring lengths where
B+/- voltages were running around the chassis. They had the DC, AC and low level audio
wiring bundled together. No wonder I would hear a buzz when a light dimmer was turned part
way on in the house. Every wire went into one harness.
Even more unbelievable was the fact that the ground for the driver board and audio inputs
was taken off the negative terminal of the left and right speaker outputs! No wonder I was
measuring crosstalk garbage in the undriven channel when the driven (other) channel was
driven to clipping.
I proceeded to correct these deficiencies, observing the "star topology" of
grounding methods. I moved the driver board ground to the power supply ground, at the
junction of the two big capacitors. I unbundled all the leads carrying AC, DC and audio
and shortened the DC power leads by directly routing them to the capacitors from the
output stage. I shortened the ground leads in the same manner.
The next thing I did was add a second 25 amp bridge rectifier to the power supply. I ran
separate parallel wiring from that to the Low/Hi Z relay and parallel wiring to the +/-
At an earlier time, I had replaced the original electrolytic capacitors with much larger
45,000uF capacitors. Recent research made me aware that the ESR of electrolytics is not so
good above 1000hz, so I added 4.4uf of poly capacitors across each power supply rail, to
improve the ESR at higher frequencies.
I shortened the audio wiring from the rear apron, routing it directly to the front panel,
instead of letting it run parallel to 110 vac wiring in a bundle. To cure a slight left
channel hum, I ended up placing/gluing a strip of copper clad PCB on the left edge of the
driver PCB, right over the 5" trace of unshielded audio input, grounding that strip
to audio ground. That quieted the left channel hum.
Over the course of these modifications, wiring was shortened, leads carrying diverse
signal types were segregated, and grounding was corrected so that all systems share a star
The result? The crosstalk between left and right channels is gone. The amp doesn't
oscillate when driven to clipping anymore. The power supply is stiffer and doesn't sag
Subjective impression? The noise is gone. Music emerges from a blanket of silence. There
seems to be a greater clarity and detail and the soundstage is wide and deep. Transient
response is crisp. The amp sounds better today than it did when it was new.
In fact, now that I isolated the amp from the rest of the equipment by insulating it from
the rack rails electrically, the pervasive hum that plagued me is gone to inaudible on my
I'm sure there are other things I could further do to make subtle measureable
improvements, but the changes made so far have resulted in pretty radical improvements.
I'm quite surprised that Phase Linear would design such marvelous circuit topology, but
completely screw up the wiring topology. But having corrected that, the true sheen of this
amplifier is realized.