Sunday, December 4, 2011

Upgrading the Yaqin MC-10T tube amplifier

In my more budget system, the stock Yaqin MC-10T has proved itself to be a great performer. Honestly, I wasn't expecting much from this low-priced Chinese tube amplifier, and was shocked by the level of performance I was getting. But plugging the Yaqin into my main system, and I started to notice some minor problems.

The first thing, of course, is that 50Ws of tube power just isn't enough oomph to drive Magnepans. Sure, it's okay for low listening levels or baroque music, but once something like Pink Floyd is played at decent playback levels, the Yaqin starts to stumble - the overall sound gets compressed and the bass becomes really sloppy. My eventual plan is to bi-amp, using the mighty Threshold to drive the bass panel, while allowing (with a suitable crossover) the tube amplifier to drive the tweeter. In theory this combination should be a winner.

The second thing is the coupling capacitor quality. There is a bit of opaqueness and lack of detail that could be coming from the low-rent stock units. I've often been impressed by Chinese amplifier metalwork, but less so with the passive parts quality. Perhaps some improvement could be found here.

The third thing to consider is the tube quality. I may be in the minority here - the early 1990s Chinese KT88 fiasco of yore still lingers in my mind - but I've liked recent Shuguang tubes. They have a smoothness and musicality that betters some of the older Russian stuff, all while being fairly robust. But for all their strengths, I've generally found the old American and European 1950s-1960s stuff to be much better than anything new.

For the purposes of this article, I'll skip on the bi-amping and will instead concentrate on the coupling capacitor upgrades and tube rolling.

The most difficult part of the changing capacitors in the Yaqin amplifier is actually getting to them. Removing the top plate, you can see the circuit boards and transformers. The capacitors (both power and coupling) reside under the PCB. Gaining access requires removing a kajillion screws holding the standoffs to the bottom plate.

You will also have to snip out a few zip-strips and unplug a few wires to safely push the PCB on its side. Here you can see the larger .22uF/250V and the smaller .1uf/400V blue caps. Since I had some real Wima MKP caps on hand that fit perfectly, I went this route. I'll note that the Wimas - though good- are not exactly world class, but trust me, anything much larger is going to be impossible to fit in there. Space is at a premium and with the tight-fitting metal case, there is no room on top either.

After some soldering frustrations, I had everything back in place and ready to start rolling some tubes. But I first did a few hours of listening to determine the differences (if any) with the new caps. The change was fairly minimal with a slightly cleaner top-end and perhaps a touch more detail. Honestly, I would only go this route if you are so inclined and (very) handy with a soldering iron.

Next up was changing the stock Shuguang 12AT7 tubes. In theory this change should have the greatest effect on the sound. Since I don't have a large stash of this tube, I had to go shopping. Trolling hither and thither, I scored some 1950s-era Tungsol 12AT7s via Ebay, and bought a pair of Mullard CV4024 from Upscale Audio. With the Tungsol as the input tube, the Mullard used as drivers, and the stock Shuguang EL34s, I sat down and gave the Yaqin a listen.

This was a major improvement - listening to the 1975 Fleetwood Mac self-titled album brought further depth and instrument clarity.  For some reason, there seemed to be more apparent power too.  Treble detail increased dramatically, showing just how important the driver tubes are, even in a negative feedback amplifier.

I've often railed against the cost of vintage tubes - but in my experience, they've been the best. With a budget amplifier, it really doesn't make sense to spend a fortune on the 'good stuff', but I was curious to see how well the Yaqin would perform with something world-class. Enter in a set of real Mullard XF2s. Though these tubes already have many hours on them, they still test as new, and with the conservative operating points of the MC-10T, they should last for a very, very long time.

At first I thought the output tube change was minimal, but as the Mullards settled in to place, the sound just got better and better.  For example, listening to Steely Dan - The Nightfly, the female backing vocals took on an amazing separation.  Instead of a homogeneous blob, I could now hear the individual voices.  Instrument attack and dynamics also improved, easily besting the Shuguangs.  It isn't that the Chinese tubes were bad, it's just that the vintage Mullards were that much better, casting a bigger and deeper soundstage with a more realistic, less hi-fi presentation.

In the end, my appreciation for the Yaqin amplifier grew. I've owned many tube amps - most of the Dynaco line-up, Harmon-Kardons, Scotts, Heaths, Fishers, and my own DIY designs, and the Yaqin is right up there with some of the best ones.  Stock, out of the box, it's a screaming deal. With the right tubes and a coupling capacitor upgrade, they truly are great amplifiers

Next up is bi-amping. I initially intend to try a simple in-line capacitor to roll off the bass for the Yaqin. In theory this should give the amplifier for headroom since it won't have to handle the low frequencies. Another option is to use an active crossover, which can also be used to replace the passive components that are mounted within the speaker. This is ultimately the best route to go. Stay tuned!

Main System:
preamp: Threshold FET-10/HL
phono preamp: Audio Sector Phono Stage
amplifier: Threshold S/500
analog: VPI HW19 MkIII - Rega RB300 with Incognito wiring - modified Denon DL-103R
speakers: Magnepan 1.6/QR with Sound Anchor Stands
speaker cable: Kimber 4PR/8PR Bi-wire with banana jacks
Interconnects: Cardas Cross and Cardas Quadlink 5C