In this blog post I test the Meixing Mingda MC368-B902 KT90 Single Ended Triode Integrated Amplifier. I purchased this amplifier about seven years ago and it has served me well. The amplifier uses the stock KT90 tubes but I've had to replace the rectifier tube with a Electro Harmonix 5U4GBEH.
I've also replaced all of the internal coupling capacitors with Mundorf EVO oil. This provided a noticeable step up in sound quality.
Below is the published specification from Meixing.
Output Power: 18W × 2
Output Impedance: 4Ω, 8Ω
Input voltage: 330mv
Input Impedance: 100K
The degree of distortion: ≤0.8%
Power consumption: 160W
Input options: 3 groups
Output options: 2 groups
Frequency response: 18Hz ~ 50KHz ± 1dB
Vacuum tubes: KT90 (special tube) x2,
5U4G(274B) x1 (rectifier), 12AU7 x2
Amp Dimensions: L33 × W31 × H23 (cm)
Carton size: L51 × W45 × H36 (cm)
Net weight: 16Kg
Gross Weight: 20Kg
Power Supply Voltage: AC 100V - 240V / 50Hz-60Hz
For the measurements I decided to test the amplifier's distortion at these output voltages.
I test the following distortion types.
- Intermodulation (IMD)
- Gedlee (Gm)
From a frequency response perspective we see that there is a +2dB rise from 100Hz to 10kHz. We see H3 at 0.19% for the 10kHz region.
Testing at the 0.25V we see H3 rise out of the noise floor sitting at 0.05% for the 10kHz region.
The 1.00V test shows H3 remaining ultra low at only 0.02% for the 10kHz region.
For the 4.00V test we basically see H3 unchanged at 0.02% for the 10kHz region. However H2 has increased slightly to 0.22% from 0.07% that we saw with the 1.00V test.
Using a multitone test signal ranging from 100Hz to 20kHz in 12 band per octave spacing, I focus in on the 1kHz to 50kHz region only.
We see IMD at -67dB for the 10kHz region. We do see the noise floor coming up in the midrange since this is such a low test voltage. My speakers produced 66dB at 1m for the signal level. My speakers are 95dB sensitivity. This represents a low listening level.
We see IMD at -76dB for the 10kHz region.
The 1.00V test see IMD at -67dB for the 10kHz region.
For the 4.00V output, we see IMD at -50dB across the spectrum.
Gedlee Distortion (Gm)
I tested Gm as an overlay between the four test voltages. For the 10kHz region we see the amplifier do well for the 0.06V and 0.25V increasing noticeably for the higher voltages. We see the inverse happening in the midrange and bass where distortion is lower at the higher test voltages.
From the test data we can see that the MC368 amplifier does really well in the lower output voltages when looking at the upper treble (10kHz region). The 0.25V test voltage provides the best treble quality in terms of IMD and Gm. This equates to a listening level of 80dB with a stereo pair of speakers assuming the speaker sensitivity is 95dB (1w) which represents a pretty average listening level.