Galactron MK2240

Galactron MK2240

Near its clipping point, MK2240 yielded 45Wrms per channel into an 8-ohm load, a figure that rose at 76Wrms when the load was 4 ohms. With a rate of 1.7 in power increase, the amplifier appears to dispose of significant drive headroom with a low impedance loudspeaker. Damping factor was calculated to approximately 11, a value which leaves some room for deviation from the ideally flat frequency response, depending on the magnitude of the loudspeaker impedance, but, probably, these will be not significant.
Static distortion measurements (in reference to the 1/3 of maximum power) were rather typical for the kind of output stage, with total harmonic distortion and noise at 0.68% for 8-ohm loads and 0.85% for 4-ohm loads. Same applies to intermodulation distortion (SMPTE), with 0.57% and 0.72% respectively. Harmonic distortion measurements (0.61% and 0.82% for 8/4-ohm loads) is an indication of good performance in noise since there is no significant difference between these figures and the THD+N. Signal to noise ratio (with a reference level corresponding to 1Wrms /8-Ohm) measured to -80.6dBr(A), which could be considered as a very good result.
Frequency response with resistive loads found to be practically flat up to 60kHz with a slight reduction of 0.5dBr (in full accordance with the specs given by the company). Its main feature is a very slight slope that starts quite early (in 20kHz deviation is near 0.3dBr). Differences between the two channels were very small, near 0.1dB. Response measurement using complex loads revealed a slight deviation with a peak around 0.2dBr at low frequencies and a deep of about 0.5dBr near 3kHz. This was expected, given the low damping factor figure already mentioned.
Output signal spectrum with a 1kHz signal input comprised a number of harmonics, strongest of which was the one of the second-order (near -50dBr). There were also some intermodulation products (observed between the harmonics) mainly from the power supply. These are at very low levels around and below -100dBr. Power supply noise is also visible (at 50/100Ηz) with some components around -80dBr.

Frequency response for both channels. Reference level: 1/3 of maximum power in 8-Ohm load, (green/red curve), and in complex (simulated loudspeaker) load (orange/magenta).

Spectrum analysis with 1kHz signal input. Reference level: 1/3 of maximum power in 8-Ohm load.

Using a 4-ohm load, the spectrum of the harmonics was not significantly altered although one can see that the components are somewhat at a higher level and that the odd harmonics are slightly dominant.
Intermodulation spectrum (with a 19/20kHz 1:1 amplitude ratio signal) comprised the anticipated components (including those relating to noise) at low enough levels, around and below the -60dBr both with an 8-ohm and…

Spectrum analysis with 1kHz signal input. Reference level: 1/3 of maximum power in 4-Ohm load.

IMD spectrum analysis. Two-tone, 19/20kHz, 1:1 level ratio signal. Reference level: 1/3 of maximum power in 8-Ohm load

…with a 4-ohm load. In the later case, values were somewhat higher. Pay attention that the horizontal scale here in both IMD graphs is linear and not logarithmic, therefore the “feeling” is a little bit different from the normal harmonic distortion or frequency response graphs.
Noise spectrum with shorted inputs included primarily hum/power supply noise components at 50/100Hz as well as their intermodulation products. All these are at very low levels (less than -80dBr, with a reference level corresponding to this 1Wrms/8W). The amplifier was, also, very quiet in the high-frequency range, where there are few findings that stand out, a sign that the control circuits are well isolated from the main circuit.

IMD spectrum analysis. Two-tone, 19/20kHz, 1:1 level ratio signal. Reference level: 1/3 of maximum power in 4-Ohm load.

Noise spectrum. Reference level: 1Wrms in 8-Ohm load, input shorted.

Harmonic distortion and noise (THD+N) as a function of signal frequency fell in the region of 0.2-0.3% for the low-frequency part of the spectrum. Above 1kHz showed a mild upwards trend until around 1% where it remained constant until the upper limit of measurement (20kHz) was reached. With a 4-ohm load, the behavior was about the same with distortion values being slightly higher towards the low-frequency region.
MK 2240's power stage proved capable of a rather smooth distortion behavior in relation to the power output. Relevant graphs show a slight upwards trend until the point where the overload begins. This point is about 44Wrms for 8-ohm loads and 54Wrms for 4-ohm loads.

THD+N as a function of frequency at 1/3 of maximum power in 8-Ohm load (green curve) and 4-Ohm load (red).

THD+N as a function of the output power. 1kHz input signal in 8-Ohm (green curve) and 4-Ohm (red) load.

Finally, the MK 2240 proved to be a quite fast amplifier with a rise time of 1.12uS (which corresponds to a conventional range slightly larger than 300kHz) and a slew rate of 38V/uS. The corresponding squarewave waveform includes only minimal overshoot and distortion.

1kHz squarewave response. Maximum output voltage in 8-Ohm load. Horizontal axis: 5uS/Div, vertical axis: 10V/Div.

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