Yaqin MS 300B Giantkiller for Around a Grand.

 

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I’ve upgraded many Chinese amps now and I enjoy working on them. Most are fairly easy to work on, and the circuitry can be improved to offer dramatic improvements in sound quality. There are two key questions:

  1. Safety
    Many of these amps are designed to run on 220v, and not our Euro standard mains of 230v +/- 10%, which means that UK mains can be as high as 253v and still be within spec. In many cases the mains transformer on the Chinese amp will run coolly at 240 or so volts however safe or not that might be, you are straining the valves’ heaters. For example an extra 10% on the heater voltage puts the heater supply at 6.93v, above the recommended and desirable limit for valves’ heaters.

    So the solutions are to replace the mains transformer, use a bucking transformer, or a Variac. Personally I prefer to replace the mains transformer with a UK made one. The cost is comparable to the cost of a bucking transformer or Variac and there are no operating disadvantages. Bucking transformers may hum, and Variacs may affect sound quality, although folk report good results with both.

  2. The Quality of the Output Transformers
    This was always put always put forward as the strong reason not to bother with Chinese amplifiers. But I’ve upgraded Yaqins, Music Angels, and Bowei clones and the results have been from spectacular to excellent and on these amps the improvements were not limited by the output transformers.

So on to the Yaqin MS300B. This amp can be turned into a giantkiller at the cost of a few hundred pounds. As you buy the amp for around £600 from Hong Kong, that means you can achieve a fabulous level of sound quality for around £1,000. And I mean sound quality that will live with many more expensive 300b amplifiers.

For reasons best known to themselves Chinese amplifier designers produce decent circuitry, but then cripple the sound quality with large amounts of feedback. Coupling capacitors and cathode bypass capacitors also leave much to be desired. Resistors are of decent quality so can be happily left alone.

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The photo above compares the replacement upgrade Elna Silmic cathode bypass capacitors  with the originals. The originals are also branded ‘Elna’ but I’m a bit suspicious of them as they have the writing in a different direction to the usual Elna style. Not to say Elna didn’t produce a special run for Yaqin, but other Chinese amps I’ve upgraded have had capacitors labelled ‘Rubaycon’ an obvious counterfeit of the well-known Japanese brand Rubycon!

The coupling caps also didn’t match on the two channels, one having a 470nf value and the other channel having 220f value.

The Yaqins use a local feedback circuit in the driver stage and additionally global feedback. the global feedback is switchable to become less when the input labelled ‘direct’ is used. My modifications are to remove the local feedback network completely and to make the global feedback switchable. We can either use the headphone switch if an owner doesn’t use the headphone output or a small switch can be installed on the rear panel. Using this switch it is very easy to hear the difference of feedback and no feedback. I don’t know of anyone who uses their amp with feedback once this mod has been done.

Another mod I do is to build in a slow start delay circuit as 274B rectifiers can start up too quickly for the 300b’s comfort. Using such a delay circuit with a slow ramp up and 75s delay means that you can use any rectifier tube. Another way to give the amp a slow starts to use an indirectly heated rectifier such as the GZ34 or GZ37, but the latter in particular is getting pricey and other rectifiers may provide better sound quality. My personal favourite for sound quality is the Chatham 5R4GY. This is a military spec rectifier that some claim is the best ever made. The glass and base certainly demonstrate the rugged military origins of this tube and it just seems to provide more clarity and dynamics than other tubes including some ridiculously expensive KR Tubes rectifiers we tried.

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Slow start module installed in space by smoothing capacitors in photo above.

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The Mundorf ZN coupling capacitors and Elna Silmic coupling capacitors are clearly visible in the photo above. The one remaining blue capacitor at the top of the photo is in the feedback loop and can be switched out of circuit after the modifications.

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Under the bonnet before the mods!

So what is sound quality like? After the mods the amp has a sound quality to compete with the best. It compares very well with my customised 300b stereo power amplifier, producing a that glorious single-ended midband but with real bass authority, speed, dynamics and slam. Remember though, power output is only around 8 or 9 watts so you need some fairly efficient speakers to get the best out of these amps. My own custom built speakers (93db sensitivity) or modified Goodmans Magisters (94db sensitivity) perform beautifully with this amp.

 

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