On the Bench
I tested the No 433 and No 436 amplifiers with a Mark Levinson No 326S
preamplifier and Carver Mark IV ribbon speakers.
First, the No 433.
At 20 volts into 8 ohms (50 watts), THD+N was a very low 0.005%. The
second harmonic is larger than the third, which is good. An absence of a 60
Hz peak and its harmonics indicates a clean power supply.
The SMPTE/DIN IMD test yielded 0.01% distortion. The calculations are
complex, and the more important thing is to look at the actual graph and
examine the peaks next to the
fundamental at 2 kHz. Compare this to the fundamental on the No
436, which has fewer of these IMD peaks. Nevertheless, the No
433 is still a top performer. The No 436 just has that
extra little bit of fine detail quality which qualifies it for being one of
the best power amplifiers ever designed.
THD+N vs. Frequency is shown below for a 20 volt output into various
loads. Notice that the distortion stays flat to about 200 Hz, then begins to
rise, depending on the load. Compare this to the No 436.
Power output at clipping (1% THD+N) was 250 watts at 8 ohms, and 465
watts at 4 ohms. This exceeds specification by a wide margin.
Below are graphs from the No 436.
At 5 volts output into 8 ohms, THD+N was less than 0.03%. Again, there is
not much evidence of any 60 Hz peak or its harmonics, so the power supply is
very clean.
At 4 ohms, it was just over 0.03%.
At 20 volts output into 8 ohms, THD+N was reduced to 0.006%. This is
typical of many amplifiers, where distortion goes down when comparing low
output vs. mid-level output.
At 4 ohms, THD+N was the same as at 8 ohms.
For the SMPTE/DIN IMD test at 5 volts and 8 ohms, although the calculated
IMD value is higher than that with the No 433, notice that
there are actually less peaks next to the fundamental. This is what gives
the No 436 that added detail in the sound.
At 4 ohms, IMD increased just a bit.
Here again, compare the actual peaks next to the fundamental, with those
shown for the No 433 at 20 volts and 8 ohms. The No
436 has the better performance.
At 4 ohms, the performance actually improved. Unusual.
Click Here to Go to Part IV.