Taken from item page:
c421 is our fine new headphone amplifier, housed in a sleek aluminum enclosure. Audio is inherently analog, so we spent 18 months designing a 100% analog amp around features that matter: straightforward sound and functionality.
The audio chain passes only through ultra precise 0.1% thin film resistors. Thin film parts increase cost by two orders of magnitude over consumer grade 1% thick film resistors. Why spend more? Because thick film resistors can be heard. Thin film resistors improve channel balance and reduce random noise by up to 8x. The result is transparency--enjoy your music like you're on stage.
c421 is packed with dual gain, bass boost, low battery detection, and synchronous voltage boosted +/-7V rails with low ripple, all implemented on a 4-layer PCB with dual internal ground planes for maximum signal integrity.
We created a new active-true ground topology for c421. Three opamp channels and two successive reference circuits in the power stage enable fail-safe charging while still providing the load isolation benefits of an active ground circuit. Great sound, enough said.
Too many headphone amplifiers impose functionality limitations here--not c421. The power circuit uses constant voltage boosting so that your music stays strong until the battery is empty. c421 gives low battery indication during the last 45 minutes of run-time.
- Want to listen while charging c421? You can!
- Want to charge from your laptop, motorcycle, or AC outlet? Great!
- Only have a few hours? The battery reaches 80% capacity in about two hours.
- Don't want to unplug the cord? No problem, c421 charges intelligently.
- Dual Gain, 3.5x or 7.0x (+10.9dB or +16.9dB)
- Bass Boost, +8dB peak
- Alps Dual Analog Potentiometer
- Low Battery Indication
- 0.1% Thin Film Resistors
- 1200mAh, 3.7V Li-Ion Battery
- USB Powered Smart Charging
- Synchronized +/-7V rails, NO power degredation!
- 3.5mm Input and Output Jacks
- 4-Layer Printed Circuit Board
- Aluminum Enclosure
- Run-Time: 11+ hours
- Charge Time: 2 hours to 80%, <5 hours to 100%
- Frequency Response: +0.01, -0.41 dB (20Hz-20kHz)
- SNR: Exceeded test equipment*, > 110 dB (20-20kHz)
- THD: Exceeded test equipment*, < 0.0041%
- Crosstalk: Exceeded test equipment*, < -57dB
- Output Impedance: 0.1 ohms
- Enclosure Size: 97.5 x 14.0 x 62.5mm
- Weight: 4 oz
*Test Configuration: RMAA 6.2.3 Loopback on c421 at low gain w/AD8620 via external engineering sample 24-bit, 192kHz Asynchronous, bit-perfect USB ADC+DAC
c421's dual gain switch and active-true ground topology enable it to drive headphones of all types and impedances. No customization is necessary.
Two options are available for the primary stage opamp: AD8620 or AD8066. Our #1 choice for c421 is the AD8620, which gives smooth, refined output and excellent imaging. A/B tests deemed the AD8620 more favorable than the OPA2227 and AD8397! The AD8066 is slightly less transparent, with similar highs but greater emphasis on bass. AD8066 is better suited to driving especially high impedance headphones (300+ ohms).
All opamps are soldered to the 4-layer PCB for maximum IC performance."