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Bench tests and listening impressions

Chinese amp #1 initially sounded lean, constricted and bass-shy. After a number of hours of use, I either grew more accustomed to the sound, else it smoothed out a bit, but the basic sound didn't change much. The internal USB DAC worked without additional drivers from Windows Vista and Mac OS X computers, but buzzes noticeably with my HP notebook if I use the AC brick. Very likely there's a ground loop issue. Within a few weeks, one of the Beijing 6P14P output tubes failed. These aren't warranted, and the dealer sells replacements only in sets, along with the 5755 input tube. I chose to switch to Russian 6P14P-ER tubes, and all subsequent comments are based on the amp equipped with these tubes.

On the test bench, I looked at it's square wave performance and frequency response. Manufacturer's specs state an amazingly flat response from 30Hz-30kHz, and surprisingly, this is actually true--with sine waves. Square waves on the other hand, showed considerable overshoot and high-frequency oscillation. High-frequency square waves are oddly misshapen as a result. After manually tracing out the schematic and asking lots of questions, I discovered that the circuit board had been laid out incorrectly and hence, the circuit contained errors. Correcting the mistakes required cutting foil traces and jumpering others. That accomplished, the amp measured much, much better, and it sounded better too. There was free space inside the chassis, so I also ended up replacing the tiny stock output transformers with larger ones from Edcor.

Amp #2 had gotten rave reviews, and some folks were comparing it to amps costing thousands of dollars. To my ears, it was very forward and somewhat dry-sounding. Square waves showed massive overshoot and ringing. A tiny tweak to the feedback circuit greatly improved it, and I wondered why the factory hadn't invested the extra 10 cents to do it right in the first place. Sonically, the corrected amp was actually pretty decent from about 70Hz to 32kHz.

Amp #3 seemed promising, with a big, bass-rich sound right out of the box, but with noticeable hiss and hum. Possibly a bit bland compared to my slightly-modded $40 Dayton/Sonic Impact T-Amp but not bad. Radio frequency interference was a real problem though--as soon as I turned the amp on, I lost all HD Radio reception! On the test bench, I noted a fair amount of square wave overshoot + ringing, and the waveform overall looked "dirty", and my oscilloscope had difficulty locking onto it at all. The Dayton actually measures quite a bit better! Tripath's tech notes indicate a couple of small-value capacitors which are used to improve amplifier stability (ie reduce that square wave overshoot) but when I examined underside of this amp's circuit board, I discovered that they had provided space for such parts but had left them empty. I also discovered that major parts like the power supply capacitors were being run at full rated voltage! Ouch. This is an ongoing project, and all I'm retaining of the original amp is the case and power transformer (I suspect the original circuit board has layout errors that I can't correct), and even the transformer got modded to lower the voltage a couple of notches.

The dead-stock Elekit easily measured better than any of the 3 stock Chinese amps. Some square wave overshoot and ringing, but not bad, with bass rolling off a bit sooner, maybe due to the small size of the output transformers. The hardware was a bit of a surprise too: The sheet metal may be humble, but it's solid, and various parts are aligned perfectly: Screw holes are dead-centered, the bottom cover had no significant warps at all, and wonder of wonders, the edges of the front panel are ground smooth. I found that I even kind of like the stock speaker connector because it's extra-large and handles moderately fat wire without a fuss, whereas there's really not enough room to use binding posts here. I've also found that I really like the narrow form factor of the amp and it's 2nd line input.

Despite my various gripes about the Chinese amps, I do enjoy the sheer variety they offer, and I've come to expect that these are "projects" which may require reworking, sometimes extensive reworking. At the same time, I've enjoyed NOT having to fuss with the Elekit at all: It's a very unassuming thing, but there's some depth to it, and individual bits may be inexpensive, but there's a sense that some real thought went into selecting each one of them. To me, there's something very Japanese about humble everyday items of "regular" quality which are really well-done.


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