This review page is supported in part by the sponsor whose ad is displayed above
Reviewer: David Kan
Digital Source: Philips DVP-9000S, Apple PowerBook G4, Acer TravelMate 3010 notebook
Preamp: Audio Zone Pre T-1, Dared SL-2000A
Speakers: Loth-X BS-1, Klipsch Synergy F2, Dynaudio Facette
Cables: Luscombe LBR-35 interconnect, Clear Audio Silver Line interconnect, OCOS speaker cables by Dynaudio
Power Cords: stock power cords, Unity Audio Link [on loan]
Power Line Conditioning: Monster Power HTS-1000 MkII
Room Size: 15' x 13.5' x 8' diagonal set up
Review component retail: $245 for T20U; $190 for PSU; $360 for Pre-Amp
Part I: KingRex T20U
U for ubiquitous. U for unprecedented. U for unbelievable. U for USB. The KingRex T20U is basically the same as the Tripath-based T20 integrated amp but with an onboard USB interface and DAC. USB has plugged into our so-called modern living and taken over. Audio devices with USB interface are becoming commonplace. We've seen tube amps and outboard converters with USB inputs. But combining a USB DAC with a Tripath amp is definitely unprecedented. It makes so much sense that one should ask, what took you. The fact that the T20U is priced at $245, a mere 25 bucks more than the T20, is absolutely unbelievable. Opening up the black boxes to quickly compare the two further hammered home one simple mathematical fact: there's no way I would take the trouble to build my own USB DAC even if you paid me 25 smackers.
Before we actually look at the DAC portion of the T20U, let's see what's wrong with computer sound cards from an audiophile's perspective. Why would we need the U in the T20? If one wanted to play music from a computer through a mainstream audio system, simply connecting the audio-out of the computer to an input of the audio system will give sound. Alas, the sound card or audio device installed in the computer is not - um, dedicated enough. In the audiophile's cookbook, there's no multi-tasking. Duo core do more? No, thank you. For audiophiles, less is more! A typical sound card these days (or the AC97 chip mounted to the motherboard) is hardly just stereo. Whether you like it, need it or ever use it, it comes with 5.1 or 7.1 capability for gaming and movies, a synthesizer and MIDI interface for music making, analog-to-digital conversion for recording and digital-to-analog conversion for playback. For audiophiles, anything but the latter are unnecessary evils and unsolicited sources of interference. To shut off these back doors to block unwanted noise and distortion, an outboard digital-to-analog converter that receives digital signals via USB seems a welcome proposition.
Instead of creating another standalone DAC that takes up extra space and one more AC power outlet, KingRex took a shortcut and build the DAC inside the T20 Tripath amp - shortcut in the sense that it saves you a pair of interconnects (could you buy a decent pair with $25?). Shortcut in the sense that the signal path from the DAC's output stage to the Tripath chip is as short as five centimeters. Fitting snugly inside the same compact chassis of 180 x 138 x 45mm, the amp and DAC come to you all in one piece - plus a free USB cable and the standard AC/DC adapter.
Sitting side by side with their bonnets removed, one can easily spot that in addition to the DAC circuit, the original Tripath amp section has been modified. The two input coupling capacitors have been changed from electrolytics to metalized polypropylene films for better frequency response. There are also four high-current inductors for demodulating the switching waveform into the audio signal. The high-current type ensures more headroom for the Tripath chip to perform with maximum output capabilities.
|What kind of DAC does $25 buy you? The heart of the T20U DAC is the Burr-Brown PCM2702E, a familiar choice for USB-ready amps including Dared's MP-5 and its many OEM reincarnations. Let's face it, the PCM2702E has very conservative specs. It's merely a 32/44.1/48kHz ready 16-bit stereo DAC with USB interface, on-chip clock generator, 8 times oversampling digital interpolation filter and analog low-pass output filter. Knowing that the AC97 chip supports 96kHz/20-bit stereo resolution, these seven-years old specs might give techno freaks cold feet. Not me. One of my reference CD players, the Restek Radiant, is also a 44.1kHz/16-bit affair and more than 14 years old. .
Yet it's still putting a lot of newer players to shame. The best part of the KingRex design is the use of a 12MHz crystal resonator as the clock source for the PCM2702E. It works with the on-chip crystal oscillator to generate a stable low-jitter clock for internal PLL and DAC operation from the USB interface audio data, freeing them from the computer's inferior anti-audio clock. As we said, the PCM2702E is dedicated to stereo and we like it that way. In terms of measurable typical performance, the PCM2702E renders a set of numbers that satisfies the Redbook requirement: 100dB dynamic range, 105dB S/N ratio (both A-weighted) and 103dB channel separation. By the way, the PCM2702E is USB 1.0 compliant. The DAC chip has a large internal storage buffer to store up audio packets and you won't need the higher speed USB 2.0 so even less up-to-date computers can plug 'n' play.
Leaving the DAC chip via the low-pass filters, the analog two-channel signals are handed over to the Burr-Brown OPA2134 operational amplifier which acts as output stage preamp and low-pass filter, taking out the residual digital garbage and turning the audio signal into the analogue stream. To make sure the audio signals are clean without a DC stage, a pair of Texas TL072 low-noise dual JFET op amps are assigned to the DC servo circuit. Every DIYer knows how changing the op amp will change the sonic characteristics. KingRex adeptly left that option open by making it a plug-in feature on the circuit board. In addition to the USB interface, the T20U has one set of line inputs. Both the input sockets and toggle switch are located on the rear panel, keeping the front panel as clean as the T20's. Input selection is electronically executed by a relay.
Last time, KingRex kindly sent me two T20s and two PSUs for bi-amping per my request. This time, they sent me two T20Us and two updated PSUs without asking and added two extra Burr-Brown OPA2604 op amps to roll with the factory-fitted OPA2314. I soon realized that I could not bi-amp with USB. PC or Mac, I could select but one audio output device. (The USB hub would still connect to one randomly). Still, I could enjoy the convenience of comparing the OPA2314 and OPA2604 by setting up two amps with different op amps. KingRex chief engineer James Lee told me to switch on the T20U before booting up the computer to insure that the USB device would be recognized. I followed his instruction but thought he might be too precautionary.
Both PC and Mac detected the T20U without problem even if I booted up the computer first. On my PowerBook, when two amps were connected via two USB ports, both were detected as "Burr-Brown Japan PCM2707 USB". On my Acer laptop, they were recognized as "USB speaker" and "USB speaker (2)". The PowerBook had no problem switching between the two. Even with music playing continuously, switching back and forth required merely a click. The Acer was more temperamental. Sometimes it switched after the music stopped or it wouldn't switch at all unless rebooted. Using a USB switch connected to one USB port was slightly better at times. The USB switch is not a USB hub but a push-button selector. The USB hub didn't help. It just connected me to one USB interface. By the way, the operation system on the PowerBook is Mac OS 10.3.9. On the Acer, it's Windows XP Professional. Would Vista recognize the USB better? Perhaps.
James also advised me to disable the computer sound card or audio device to avoid interference. That was possible only with my PC. Apple only selects one audio output device at a time. There's no option for enabling or disabling other devices. Does that mean the remainder is automatically disabled? I'm clueless. After the USB audio output is selected, volume and L/R channel balance can still be adjusted. That probably indicates that the volume and balance settings are carried out in the digital domain. It is therefore wise to set the volume to maximum to optimize the 16-bit word length.
So, this was the setup: Mac PowerBook, two USB cables, T20U with OPA2134 and T20U with OPA2604 both running on PSU, each driving a pair of identical Loth-X BS-1. I conducted a blind comparison by labeling the chips on the back of the amps and then shuffling them. During the audition, they were identified as chip A and chip B. Their identity was only revealed at the end. Since I always have my Loth-X stacked in D'Appolito arrays, I just left them like that and hooked up chip A to the top pair, chip B to the bottom pair for direct comparison. I swapped the speaker connections at random to make sure placement (the top pair was higher and inverted) would not play tricks on my ears. I picked five CDs, selected a few tracks from each, ripped them to MP3 and WAV, saved them to the hard disk of the PowerBook and compared the different file formats with the original CDs played on the PowerBook super drive. One CD in particular was also converted to FLAC and EAC for more extensive comparison with the original CD played from the PowerBook as well as from the Philips DVP-9000S SACD/CD player for the sake of USB DAC vs. Philips DSD upsampling. The Philips was only connected to the line input of one of the T20U since that didn't involve the OPA2134 or OPA2604. If that's not too complicated, let's continue, shall we?