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As our burn-in period slowly neared its end, we occasionally played the T20U when we left the house. It also became time to charge the SLAP. Just then only a Chinese manual was available and we have to admit that was a bit difficult to comprehend. Yet luckily the global 6moons staff has a wide array of languages under its belt. Again David Kan came to the rescue:

1. The charger is monitored by a TI UC3906 IC which is a smart SLA battery charger controller that prevents overcharging and deep discharges.
2. During charging, the PSU is cut off. The LED on the front will stay red.
3. When the DC output of the PSU drops below 10.5 volts, it will cut off automatically. The LED on the front will flash red/green.
4. When the charging is complete, the LED remains green. The charger will continue maintaining a low-current charge (80mA as compared to 750mA during normal charge) to avoid discharge.

It follows that the PSU switch should be 'on' at all times and the charger switch 'off' while using the SLAP to power the amp. The PSU switch also powers the LED indicator so the status of the battery is displayed all the time. Now we plugged the SLAP in the wall outlet and left it to charge overnight. The next morning saw the KingRex T20U in its proper place, all cables connected and the SLAP's output leashed to the T20U with the provided umbilical.

Some software used at KingRex during R&D

With the switching adapter, we had already noticed a loose fit of the power plug into the back of the amplifier. The umbilical suffered the same. The connection was made properly but the feel of it was a bit chintzy. With all connections checked, the first CD went on the belt drive - Renaud Garcia Fons' Arcoluz, a live trio recording of the master acoustic bass player. Our first impressions were that the amp really and truly had gained in performance. Gone was the dental-filling-loosening sound. Here was a very decent sounding amplifier. And decent is really the best word we could scare up. This sound had none of the tube-ness as claimed elsewhere, none of the bloom a small valve amp is capable of sending into the room. There was plenty of power though. The amp is rated at 20wpc into 4 ohms with a usable 10wpc when low distortion is key. So with our 16-ohm Duo Omega loudspeakers, there were at least 2.5 usable watts on tap, more than enough for serious damage.

Tonal balance was on the light side since bass extension was limited. Some stridency was evident in the mids especially on violins and brass. Treble was gentle enough to live with, micro dynamics handled well enough too but macro dynamics seemed restrained. These impressions proved consistent over a wide range of music and CDs. With the amplifier running well over 250 hours, the perceived limitations had to have another cause given all the reviews.

From the design of the amp, we could only point at two possible culprits - the casing and the chip itself. We have noticed with other switching amps like this Tripath that an enclosed casing can compress the sound. We experienced that with the Kharma MP150s which opened up when one screw was removed from the bottom plate. Another option is to use wood enclosures as the Red Dragon Leviathan does. However, we must look at the price tag. Those two mentioned class D amps charge way beyond the $299 KingRex commands for today's unit.

So we opened the amp's enclosure. That was easy and also showed how the KingRex metal work is of good quality. Screw holes match nicely and screw heads are smoothly level with the surface. Inside the amp we spotted the PCB with the DAC on the left and the T2020 chip prominent in the middle, snuggled up tightly against its heat sink.

After reconnecting the cables, we played more music. Definitely an improvement, playing the KingRex in the nude. With even less physical mass however, cables now had a ball trying to capsize the amp off the SLAP. We added a little weight with a small wooden cutting board over the amp's top loaded down with a 1 kilogram door stopper.

Still, there was that harshness in the mids. As we have experienced a few times already with computer-based audio, chips are readily capable of messing up the environment within which they work. All manner of RFI/EMF noise is emitted to find its way back into the audio signal somewhere. We did not test the USB DAC of the T20U due to the fact that the only computer we had available at the time has its USB port relatively far away from the USB board. The cable that ran to the KingRex's USB port picked up too much interference and the signal was not usable.

How to tackle RFI/EMF emissions? When our AudioNote DAC had needed repair, we'd handed it over to Peter van Willenswaard of Audio Magic in our hometown of Rotterdam. When we got the DAC back, all chips inside had been covered with a piece of carbon/graphite foam. This black spongy foam blocks various RFI/EMF frequencies. Why not put a little on the T2020 chip to see whether it helped? We cut a small piece of the foam the size of the T2020 chip, being careful to not having the foam touch the pins of the chip. A tiny amount of silicone glue kept the foam in place.

And it did help. The harshness in the mid frequencies was diminished though not completely eradicated yet. Removing the foam 'on the fly' instantly returned the stridency to prove that we were on the right track! While looking for the foam in various drawers, we also came across some ferrite beads that could be clipped on cables. With the SLAP providing clean power, why infest this with RFI/EMI from the chip(s) and diodes in the amp? We thus clipped a small bead on the incoming DC cable just behind the connector on the amp's end. In conjunction with the foam, this eliminated the harshness completely. We now only had to cope with the bass lightness and dynamic hesitation.

With the amp finally free of noise -- which was far more important than a lightness in the bass -- we moved in the SLAP for the switching adapter to asses the worth of the battery power supply. This test only took 2 minutes. The difference between both power supplies wasn't big but immense. The T20U is like the two-headed Janus. Change power supplies, it's the same thing still but with an entirely different perspective on the world. The relaxation and ease evident with the SLAP in place feeds the fishes with the adapter on duty.

After these tests to determine the Duo-worthiness of the KingRex mini, we tried one last thing. We keep plenty of Acoustic System resonators around which work miracles inside amplifiers as well. Behold and AMR are just two brands that offer a resonator upgrade for their amps. A silver Pro fit neatly next to the T2020 chip on an empty piece of PCB. Because the resonator eliminates compression inside the casing, we could close up the KingRex again. Admittedly, this tweak alone costs nearly as much as the entire T20U but the result was well worth it. It introduced another layer of relaxation and instruments and voices gained in air around them.

Next to its somewhat sloppy power cable connection on the amp, we came across a few other niggles. One is the turn-on transient. When the SLAP detects that the T20U is switched on, there's a click a few seconds later as the SLAP triggers into action. On our super-sensitive horns, this was fairly loud and quite annoying. We also could not open the SLAP to peek inside as some of the Philips screws were botched like the one to the left. Granted, users are not likely to open a SLAP right away but reviewers are. And, when it comes times to replace the batteries, all owners need to get inside, eventually.

With all our tweaks applied, we could finally sit back and listen to music for pleasure - which we promptly did. In conclusion for our sidebar to David's upcoming feature review, we have to say that for the asking price, the T20U/SLAP combination at $749 is a bargain for its quality - well built, good sounding and with room for tweaked improvements if you run the T20U/SLAP on extremely revealing, ultra-expensive, super efficient speakers like our Avantgarde Duo Omegas.

Quality of packing: Amp came in good quality box. SLAP was added inside outer box with bubble wrap.
Reusability of packing: Yes for amp, no for SLAP.
Ease of unpacking/repacking: Easy.
Condition of component received: Perfect. However, SLAP had botched screws to prevent getting inside.
Completeness of delivery: Includes manual for amp, cables and adapter.
Quality of owner's manual: The amp's is good, in English and with drawings. The SLAP manual arrived later via e-mail and is good and in English also.
Website comments: Informative enough.
Pricing: Almost bargain.
Human interactions: Prompt and very forthcoming with all the information requested.
Suggestions: Add shielding of the T2020 chip against RFI/EMF. Add ferrite bead to DC umbilical. Avoid resonance of the PCB in the amplifier. Fix the turn-on transient of the SLAP.

KingRex website