Specifications

In classic Dynaco fashion, the Z-565 transformers (at left) used in the SCA-

35 are excellent performers. The best examples are capable of delivering the

full 20-20 kHz bandwidth at full rated power with very low distortion,

although on the low end, the figure is often closer to 22 Hz. They also

provide an optimized Ultra-Linear output stage configuration that gives the

SCA-35 a clear advantage in damping and regulation of power delivered to a

speaker load, versus that of an equivalent pentode output stage. The output

transformers always determine the ultimate capability of a given design, and

in the SCA-35, the transformers set a very high benchmark indeed.

Now the specifications for this amplifier would have you believe that the full capability of these

output transformers are fully utilized, and they are – but only under very limited conditions.

Power output is stated as “35 watts continuous” – but offers no clue as to what frequency range

or distortion level this power is achieved at. In fact, when those elements are included in the

power ratings, the information is then provided only on an “each channel” basis. On the bench,

the ambiguity of the “each channel” designation is shown for what it really is: each channel at a

time. Measured as such, the SCA-35 is generally capable of meeting its performance standards.

That may have been fine for early stereo that often threw nearly all the sound to one channel for

dramatic effect, but with the more balanced presentation of today’s stereo sources, full perfor-

mance is required in both channels at all times. The fact is, when a stock SCA-35 is tested under

conditions of both channels driven, the specifications suffer significantly such that it becomes

about a 27-28 watt RMS (total) amplifier, at greatly elevated distortion (typically 3-5 times that

of single channel performance). To be very clear here, that means that when both channels are

driven, the power output of each channel drops to about 80% of single channel performance, and

at that reduced power, distortion is 3-5 times higher than what is produced at the higher power

output of single channel operation. In an amplifier where every watt counts with today’s dynamic

program sources, this is a significant drop in performance in deed. The bottom line is, under real-

istic stereo conditions where both channels are operating, the low distortion capabilities of the

output transformers in the SCA-35 are not used effectively, because the operating point of the

output stages themselves have shifted significantly to one of higher distortion under conditions

of elevated power output.

The typical cause of this lies in the capability of the power supply, versus the total needs of the

amplifier. Specifically, the SCA-35 uses an economical solid-state power supply, and amplifiers

that operate in class AB1. This means that as the amplifiers produce more power, the current

drawn by them increases significantly. Against the increased demand from the amplifiers, the

output of the power supply drops. Of concern here, is that the B+ “sag” created by this increased

current draw is much greater when both channels are driven, versus that of a single channel. This

happens in virtually all stereo amplifiers, but a closer look at what is really happening here will

give the first clue as to how greater performance can be had.

Consider what happens when a single channel is measured on the test bench for power output

and distortion. The specifications for “each channel” performance are typically met – but not just

because of the merits of the channel being measured. The idle channel is also playing a signifi-

cant roll in helping to achieving the driven channel’s results. But how does that happen?