Specifications
Measured Stock Performance
While I have individual information on each channel, the information was close enough to simply average the results for
reporting. This also helps to minimize information overload.
1. POWER OUTPUT AND DISTORTION: As measured into 8 ohms from the 8 ohm tap, with the Biaset voltage set for 1.56 vdc
in each channel under stabilized quiescent conditions. The power output figures given are maximum power output levels at the
onset of clipping, while the THD numbers are produced at a power level that is 1 db down from rated power (or about 27 watts) --
this to conform to the distortion specifications as provided by Dynaco.
Single Channel -- @ 20 Hz = 39.8W RMS. @ 1 db down, THD = 1.0%
@ 1 kHz = 45.6W RMS @ 1 db down, THD = .18%
@ 20 kHz = 43.5W RMS @ 1 db down, THD = 2.3%
Both Channels -- @ 20 Hz = 31.6W RMS @ 1 db down, THD = 1.9%
Operating @ 1 kHz = 37.5W RMS @ 1 db down, THD = .32%
@ 20 kHz = 34.1W RMS @ 1 db down, THD = 3.3%
2. IM DISTORTION: .45% at 35 watts equivalent power output. .74% with both channels operating.
3. FREQUENCY RESPONSE: (ref 1 kHz)
@ 10 Hz = -.3 db
@ 1 kHz = 0 db
@ 20 kHz = -.25 db
@ 30 kHz = -.5 db
@ 36 kHz = -1.0 db
@ 40 kHz = -1.2 db
4. HUM AND NOISE: 92 db average below rated power
5. STABILITY: No value of capacitance only loading would cause the amplifiers to become unstable.
These figures were all produced with the unit operating directly from the AC line in my home, which was providing its typical
122.0 vac to the unit during the tests. The power output figures were rerun with the unit operating from 117 vac (the published
operating voltage for the unit), which reduced all power output figures by about 2.5 watts. The reduction in line voltage did not
have any significant effect on the distortion levels produced.
Within the results then, they meet the majority of the specifications as presented back in the day, which of course were based on
single channel performance figures -- this because no standard existed for performance presentation at the time. The notable
exception is for the 20 kHz THD figure, and frequency response. To that point, I've never measured an ST-70 that was only .5 db
down at 40 kHz, with the performance my unit shows being somewhat better than typical in my experience. I can only assume
that the specification given then is somewhat optimistic. I have found it is common for ST-70s to show a HF response that is
between 1 and 1.5 db down at 40 kHz, relative to a 1 kHz reference. What is important, is that the frequency response within the
20 Hz to 20 kHz audio bandwidth is flat within .25 db, which is very good, particularly so for equipment of that time -- and
especially so for the level of HF stability achieved.
As for the 20 kHz THD figure, the distortion level produced is typical of that achieved AFTER installation of the HF stability
networks. These networks reduce the open loop gain at 20 kHz, which then increases the distortion produced at 20 kHz due to
the effective reduction of the feedback factor. But they are necessary to produce a practical feedback amplifier that is stable into
varying loads. With these networks lifted in each channel, 20 kHz distortion comes in at an average of .48% at 1 db down from
35 watts RMS power output.
I have always felt that this practice was deceptive relative to the way the specification is given, but have come to the conclusion
that it was accepted practice back in the day none the less. These networks came late to the amplifier design party, after a
number of early feedback amplifier designs were branded as tweeter killers (Heath W-3M for example). The networks were
then added to aid in stability performance, but they caused the HF distortion performance to deteriorate from the specifications
of previous models published without such networks. Therefore, for lack of any better explanation, I can only assume that many
manufacturers continued to publish distortion specifications based on that of the raw amplifier circuit (that is, without the
finished HF stability circuits added) -- likely under the guise of preventing confusion, and certainly of course, to not to have their
product look worse than that of other manufacturers who were following the same practice.
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