Headphone Amplification of Cayin N8
From Digital to Analogue
After a lot of comparison and consideration, Cayin adopted two units of AKM AK4497EQ operates in mono mode as our N8 digital platform. This is the new generation premium 32-bit DAC chipset from AKM with class leading digital decoding capability. The AK4497EQ has four output channels, when operate in normal stereo mode, this will be configured as two channels of analogue output (designated as AOUTL1 and AOUTL2) with P and N phases to support a full differential circuit. Alternatively we can use the AK4497EQ in Mono mode, the alternative phase output will no longer available but the DR and S/N will boost to 131dB (DR and S/N) at mono mode.
To make sure the premium DAC chipsets will operate in optimum condition, we have employed two extremely low-noise high-precision Femtosecond Crystal Oscillator with RMS jitter below 0.1ps@10Hz-1MHz. High precision low jitter digital transmission is the base of high quality digital playback. It is how we deliver a natural, non-digital and musical playback to subsequent audio circuit.
As in regular DAC design, the output from DAC chipset will go through a Low Pass Filter (LPF). We decided to use MUSES02 as LPF after numerous listening tests and comparison with other alternatives. We also added a Buffer stage after the LPF for the first time in our DAP, this will boost the output current and handling capability. According to our listening test, this will enhance the density and reproduce a very natural soundstage. The feedback we have collected when we demonstrated N8 prototype at various shows and exhibition has endorsed this observation.
For the 4.4mm balanced phone out, the output from the LPF+Buffer will go through a 2-stage amplification: the Line Driver + Headphone Amplifier. We decided to stick with OPA1622 in the final output stage after we have tested a lot of alternative. While we can achieve fully differential amplification with 2 pieces of OPA1622, we have decided to double that into 4 pieced of OPA1622 and configure them in parallel amplification, this will increase the output current capability and lower the output impedance. In practical, these improvements will translate into better control, ability to sustain a heavier loading, and drive most portable headphones with ease and offers lots of headroom.
For the 3.5mm single-ended phone out, the first thing we need to do is to convert the differential signal into single-ended, and then we’ll go through timbre selection circuit which involves the following three steps:
- Alternative first stage amplification (KORG Nutube 6P1 as Vacuum tube amplification)
- Timbre selection
- Volume matching
We shall explain the innovativeness and engineering involved in this section later. The output from the Timber selection circuit will go through the same Line Driver + Headphone Amplifier circuit but in this case, we only used two pieces of OPA1622 in parallel in the final output stage.
Dual Output Mode
As mentioned previously, Cayin N8 comes with dual output mode: STD (Standard Mode) and HIGH2 (High Impedance High Output Mode) through our unique power management strategy. At STD mode, N8 will deliver 230mW and 480mW per channel (at 32 Ohm loading) to 3.5mm single ended and 4.4mm balanced phone out respectively. This should provide enough output to handle most, if not all, earphones satisfactory in the market already.
The HIGH2 mode is achieved by altering the operation voltage of the Line Driver and Phone Amplifier modules, this will increase the output level (in Vrms) significantly. The following table compares the two output modes at different loading:
As the figure indicated, the rated output of 4.4 phone out increased by roughly 56% at 32ohm loading, but as the loading impedance increased, the rated output will increase by 167% and 150% respectively at 150Ω and 300Ω loading, that’s why we designed this as High Impedance High output mode.
We want to clarify that the HIGH2 mode are not meant for bringing full size high impedance headphones such as Sennheiser HD800 family into portable applications. These headphones require lots of current to control the diagrams movement accurately (or the magnetic mechanism in case of planars) and the battery in portable DAPs is hardly capable of doing that satisfactory. To the best of our knowledge, there are high impedance IEMs ( oBravo Ra C Series, RHA CL1, Kennerton Algiz, ..) and earbugs (VE Zen V2, …) that certainly can take advantage of the HIGH2 mode. On the other hand, the extra power will not only allow the N8 to drive earphones with difficult load, the improve headroom will benefit a lot of earphones when playback complicated music or extremely dynamic passage.
Battery and Duration
N8 is powered by a 7000mAH custom Lithnium battery, this is the biggest battery in portable DAP according to published specification as of today. Unfortunately even with this huge battery installed, we still can’t achieve the benchmark 10 hours continuous playback with our standard multi-format duration test playlist, but we are very close to that. At standby condition (no playback, screen off, no wireless connection) the N8 can last more than 56 hours. The good news is N8 supports QC2.0 quick charge technology, so you can recover from 20% to 80% in around 2 hours.
The following table provides basic battery duration information for your reference:
For the record, our battery duration test was conducted with 32ohm loading, display off, all wireless off, mid-gain, volume at 35. Theoretically we can develop a super power saving mode to extend the battery duration by lower the operation voltage of selected Op-Amps, but this will certainly affect the sound quality of the N8 audio circuit and we have decided to maintain our “Audio is priority” principle.
Specification (Headphone related)
