In this review, I’m taking the DROP + xDuoo TA-84 DAC/AMP for a whirl. The unit costs $299 USD and can be bought from DROP.
Disclaimer: DROP and xDuoo are not related to Headfonia. They sent me the TA-84 to be featured on Headfonia.com.
DROP + xDuoo
Drop (formerly Massdrop) is a California-based e-commerce platform that caters to many hobby enthusiasts, including audiophiles. Its most well-known offering is the collaboration with Sennheiser, which birthed the venerable HD6XX, a more budget-oriented version of the HD650. I’ve personally bought from them many times, and occasionally, they offer some deep-cut choices.
xDuoo Tech. Co., Ltd is a Guangdong-based audio equipment manufacturer. They first appeared publicly around 2011 and have made their own personal audio electronics as well as served as a high-quality OEM for other brands. Generally, xDuoo has stood out with interesting tube-based designs and all-in-ones with swappable DAC cards.
DROP + xDuoo TA-84
The DROP + xDuoo TA-84 is an unusual beast – it’s an OTL tube amp with a built-in DAC. Like most OTL amplifiers out there, it only has single-ended inputs and outputs because differential tube electronics without transformers are crazy difficult to design and manufacture. In the input section, the TA-84 uses a pair of ECC82 (European 12AU7 equivalent), and a pair of EL84 pentodes drive the output.
Both are quite affordable and abundant tubes with many compatible equivalents. Of course, when tube rolling, do consult the respective datasheets to double-check electrical compatibility and pinout.
Like with all OTL amplifiers, two factors have to be kept in mind. Tubes like driving high impedance loads and without a transformer isolating them from the headphones, they increase in power delivery as the load impedance goes up. Therefore, the higher your headphones’ impedance, the happier the amp will be.
OTL output capabilities can be increased with larger power tubes and by using many parallel tubes. Since the EL84 isn’t exactly crazy-powerful, the power delivery on TA-84 is quite meager.
Separate from the amplifier section, we get a DAC board that is based around the ESS9018K2M chip. It gets the digital signal from a USB-C output and then sends it to the potentiometer, and then to the tube input stage. As far as I can tell, the output stage of the DAC section uses op-amps.
The potentiometer is labeled “Alps RH2702”, and the case resembles the crowd-favorite RK27 “Blue Velvet”. The kicker is that there’s no info about “RH2702” on the Alps’ official website, and many speculate that it’s actually a copy. What’s more interesting is what’s inside! As it turns out, the RH2702 uses discrete resistors, so it’s a stepped attenuator instead of a traditional potentiometer.
It also has 4 pins per wiper instead of the regular three. The fourth pin is labelled “loudness” in the datasheet and apparently engages an RC-network to boost the bass when the volume is under around 11 o’clock.
When using the volume control on the TA-84, one can feel the volume steps, and indeed, around the middle, the sound changes. In theory, applying loudness EQ when listening at lower volumes can be beneficial, but the EQ must be applied depending on the actual SPL that’s heard by the listener.
A potentiometer cannot know that, so the effect depends on how sensitive the headphones are. An interesting solution by xDuoo, but frankly, I would have preferred a regular RK27 instead.
Features
- DAC chip – ES9018K2M
- Amplifier type – tube, capacitor coupled (OTL) output
- Tubes – 2xECC82, 2xEL84
- Inputs: USB-C, RCA for line-in
- Outputs: 6.35mm TRS, RCA for line-out
- PCM data rate: up to 32bit 384kHz
- DSD data rate: up to DSD256
- SNR: 102dB amp, 113dB DAC
- THD+N: ≤1% amp, 0.002% DAC
- Gain: +18dB, +20dB
- Output power: 44 mW (at 100 ohms), 83 mW (at 300 ohms), 93 mW (at 600 ohms)
- Output impedance: ≤0.3Ω via 4.4mm; ≤0.2Ω via 3.5mm
- Power draw: around 50W
- Dimensions: 10.6×6.4×4.6” (26.8×16.3×11.6cm)
- Weight: 3kg
Design, Build & Haptics
Similar to other DROP collabs, the xDuoo TA-84 has understated black looks. Of course, the Pavane-made tubes still have their glassy silver looks, with small orange embers visible when the filaments are on. If this is your first tube amplifier, always adjust your expectations of tube glow; most photos exaggerate it by using long exposure. I’m also happy that xDuoo didn’t decide to use LEDs to make the tubes brighter.
The front panel is almost all-black, with the DROP logo in the upper left corner and a status LED that lights up when the device is powered on. In my experience the placement of the power LED is less-than-optimum as in many cases it will be eclipsed by the volume knob. The 6.35mm headphone jack feels nice, and the TA-84 is heavy enough so you can plug your cans in without holding the thing down.
Most of the chassis is made out of anodized bent sheet steel with enough thickness never to have that cheap sound when tapped on. The front panel is milled black aluminum, and both the back and the bottom panels seem to be aluminum as well. Taking the TA-84 apart is quite easy, with most bolts having the same Allen head.
The circuit is simple enough that you can figure out which are the coupling capacitors, but sadly, there’s very little space to go wild with coke-can-sized esoterics. Something like a Bottlehead Crack is way easier to mod to your heart’s (and wallet’s) content.
Versatility
Like many more affordable options, the TA-84 doesn’t have a power button on the front, and you have to reach behind it to power it on and off. Take care not to accidentally press your hand on the hot tubes that can reach over +70C. They won’t immediately burn you like hotter-running large power tubes, but you can hurt yourself by pulling your hand back instinctively.
The back also houses switches for gain toggling and switching between the DAC and AUX RCA inputs. DROP specifies the gain of the TA-84 as 16dB, but my own measurements show that the voltage gain is around +18dB on low and +20dB on high gain. With a Sennheiser HD6XX, this meant that I was around midnight on the volume control for most of my listening.
The knobfeel is peculiar, with very high dampening between the volume steps.
After around 20 minutes of operation, the TA-84 settles at the thermal equilibrium of around +20 °C over the ambient. In my room, that meant the case was at toasty +45 °C, with the hottest part being the power tubes at +70 °C. Because the amp section runs in class-A, driving more difficult loads will not impact the temperatures.
The xDuoo TA-84 is shipped with Classic-series Psvane tubes. A matched pair costs $40 for input tubes and around $50 for power tubes. Unfortunately, my ECC82 pair is quite microphonic, so tapping on the amp or the desk where it sits produces an audible “ping” sound on the outputs. I swapped out the stock ECC82s for fancier RCA 6072s, and the microphonics went away.
Sound Signature and Technicalities
To test out the DROP + xDuoo TA-84, I used my Sennheiser HD6XX and ZMF Auteur Classic headphones. Lower impedance planars and dynamics would immediately show the limits of power delivery as the louder bass sections would become audibly distorted. All impressions are with the stock Psvane Classic-series tubes.
As always, I’ll start with some practicalities. The TA-84 is rated to be used with headphones with 100-600 ohm impedance. With OTLs, you can sometimes bend the rules a bit and go lower if the headphones are super-sensitive, but here, the limits were quite pronounced, and it was easy to current clip the amp stage as lower impedances shrink the available power.
Of course, most IEMs are out of the question due to the +18dB minimum gain and considerable output impedance.
First, I hooked up my Topping Centaurus DAC to the AUX inputs of the TA-84 to see how good the built-in DAC is. The improvement over the built-in ESS DAC was evident, but it also made me realize that it’s no slouch. It surely didn’t make me feel that I was missing much of the music if I chose to use the TA-84 as a proper standalone.
I also felt that the built-in DAC was a perfect match to the warmer tube section as it would give it some extra bite that was missing from the Centaurus matchup.
I also tried the TA-84 as a DAC/preamp for my FiiO SP3 active desktop speakers. The signal for the line-out is taken after the tube input stage. Testing it was easy – I tapped on the stock ECC82 tube, and the ping came right through my speakers. As for the sound, there was a bit of tube warmth, but generally, the sound was rather neutral, with great sound staging.
My only complaint is the high gain, which meant that I had to keep the volume on my speakers very low.
The review continues on the second page with more on sound. Click here or use the jumps below.
