Updated note: this is a historical article. Today I would no longer recommend buying this amplifier with the specific intention of modifying it, because the starting point is very poor and, to obtain a serious result, practically everything that really matters has to be replaced: the circuit, the output transformers and part of the power supply. The final result of this modification sounded good and the measurements were valid, but with some compromises. With the experience gained over the years, I now generally consider it more sensible to design a new amplifier from scratch rather than trying to save a cheap unit that was badly conceived from the beginning. The article remains useful, however, to show how much transformers, circuit design and proper engineering affect the real performance of a tube amplifier.
This Music Angel amplifier belongs to the category of modifiable Chinese amplifiers. Unlike other kit amplifiers, it is probably not worth buying one specifically in order to modify it. However, if you find one used at a good price, or if you already bought one and put it aside because you did not like it, it can become an interesting basis for a conscious DIY experiment.
Original condition
(Note: this first part of the article is very old, so the measurements were taken with rather poor instruments.) I began by checking the performance of the original unit and, as I expected, it was very poor. Without going through too many figures, it is enough to say that the original output transformers were already at -3dB at 50Hz. The upper bandwidth seemed very extended, apparently up to 60/70khz, but only apparently. In reality, from about 8/9khz upwards, these transformers showed a resonance with a very marked phase rotation and strong resulting distortion. The damping factor was below 2. The spectrum analyses and square waves shown below highlight the problem very clearly.
Spectrum at 1 Watt
Spectrum just before clipping
1khz square wave
10khz square wave
I then completely dismantled the unit and also found several rusty spots inside.
These are the original transformers: the power transformer and one of the output transformers.
The Music Angel power transformer tends to sag under load. In fact, although the amplifier uses EL34 tubes, in its original form it runs them with a very limited bias current. For this reason I decided to use 6V6GT tubes, which are better suited to the transformer available in this unit. Using larger tubes would not have made much sense, because it would not have been possible to drive them properly to power. The output transformers are my SE5K6-UNI, which, without their covers, have the same dimensions as the original transformers. By reusing the two “L” brackets, they can be mounted back onto the chassis using the original covers.
To build a simple circuit, also using tubes that are often ignored by snobbish audiophiles, I chose PC88 or EC88 tubes as drivers.
The 6V6 tubes operate as pentodes, with negative feedback that is absolutely necessary to obtain a correct and controlled sound. Since there was no room to fit a filter choke, I used a solid-state capacitance multiplier. As always, the premium schematic is available for purchase.
Let us look at the build made by “C.R.”.
Measurements:
Power: 3.5Watt RMS per channel before clipping
THD @ 1 Watt: 0.98%
Damping factor DF: 20
Bandwidth: 10Hz – 42khz -1db
Spectrum analysis
Bandwidth on resistive load
Bandwidth on reactive load
Square waves at 100hz, 1khz and 10khz
1khz triangular wave
How does it sound?
I bought this Mini X3 at a very low price so that I could have it modified. I confirm that, in its original form, the sound quality of this amplifier is anything but good. The construction is fairly simple and I will leave Stefano to describe the circuit details. I will only say that, once switched on, the listening test left me speechless: I had never heard such an inexpensive amplifier sound so good. I even like it more than much more complex and expensive units. 3.5W of sonic pleasure. Excellent circuit and excellent output transformers, as always. Stefano has outdone himself once again. “R.C.”
Below is the white build made by “S.Z.”.
