Power output with 1kHz test signal
- 8-ohm load at 1% THD: 99.2W
- 8-ohm load at 10% THD: 118.5W
- 4-ohm load at 1% THD: 47.0W
- 4-ohm load at 10% THD: 83.0W
General
The Raysonic M100 is a beautiful-looking medium-power
push-pull tube power amplifier utilizing four pairs of 5881 output tubes. Said to operate
in class A, that didnt turn out to be quite true, as the full-power AC-line draw was
326W and 257W at idle. True class-A operation would have a constant power draw as a
function of power output up to clipping. In reality, the M100 is a class-AB design with
very high idling current.
Starting to measure this unit, I quickly discovered that it
was a design with too much negative feedback. The unit would go into subsonic oscillation
with an open-circuit load. This would preclude using this amplifier with speakers that
have what is called "LC tuning" or speakers that have a built-in bass amplifier.
Both of these speaker types have a series capacitor as part of either its bass tuning or
as part of the internal crossover to cut in the midrange above the active woofer. This
would unload the amp at DC, thus causing low-frequency instability. Using this amp for
these kinds of speakers would result in the amp using up its power in subsonic
oscillations. To get the open-circuit frequency response, I had to place a suitable
inductor across the amplifier output to give it a low-resistance DC path to ground yet be
a high impedance to audio frequencies. This worked to a sufficiently low frequency to get
the bulk of the curve where the lowering inductive reactance of the inductor as frequency
went down started to load down the amp -- below 40Hz.
Chart 1 shows the frequency response of the amp with
varying loads. The output impedance, as judged by the closeness of spacing between the
curves of open-circuit, 8-ohm, and 4-ohm loading, is unusually low for a tube power
amplifier and another clue to what would appear to be quite a bit more than the usual
amount of negative feedback. Note also the peaking in the ultrasonic frequency range with
any normal load. Surprising, the ultrasonic response is not peaking with the open-circuit
load.
Chart 2 illustrates how total harmonic distortion plus
noise vs. power varies for 1kHz and SMPTE IM test signals and amplifier output load.
Distortion performance is best for a "matched load" or load value the same as
the output terminal value. Power is reduced and distortion increases for the "half
loading" of a 4-ohm load on the 8-ohm output.
Total harmonic distortion plus noise as a function of
frequency at several different power levels is plotted in Chart 3. Amount of rise in
distortion at high frequencies is quite pronounced but not unusual for many tube power
amps. Very, very few power amplifiers of any type have a constant amount of distortion
over the audio band at various output powers. Measurement at the rated 100W proved
problematical, as the M100 really couldnt produce the power above about 5kHz without
excessive distortion.
Damping factor vs. frequency is shown in Chart 4. Here, we
can see the unusually high damping factor over much of the audio range -- again a clue to
the high amount of overall negative feedback used in the design.
A spectrum of the harmonic distortion and noise residue of
a 10W 1kHz test signal is plotted in Chart 5. The principal signal harmonics are second
and third with the remaining harmonics about 20dB or more below the level of the second
and third harmonics. Amount of AC-line harmonics are reasonably low but with some
intermodulation of the AC-line harmonics with the signal harmonics near the nulled-out
1kHz test signal.