Custom order High Gain model for the HE6 and low sensitivity headphones:

      The product default had 16DB gain, customers can custom order extra +6 DB to +12DB gain, in total 22DB gain,  had better match with the headphone which is sensitivity low than 95DB.
       Price is USD45.
       If the headphone sensitivity lower than 100DB, choice extra +6DB.
       If the headphone sensitivity lower than 90DB, choice extra +12DB.

Audio-gd products had design and test based on the AP SYS-2722 (The top-drawer professional equipment in world)


Connect with KRELL amplifiers:

          Master 9 is compatible with KRELL amplifiers CAST inputs.

           A cable can be made with one side connecting to Master 1 ACSS outputs, other side to a KRELL amplifier CAST inputs.

           The ACSS output pins:
           Pin 1, GND
           Pin 2, Hot
           Pin 3, Cold
           Pin 4, GND

           KRELL amplifiers have lower gain versus audio-gd Master series amplifiers when using the CAST connection, so customers can custom order the Master 9 to have higher ACSS output gain if expecting to connect to KRELL amplifiers.

Summarize
Fully balanced pure class A design built by discrete analog stages without any OPA or coupling caps in the signal channels.

Non- feedback ACSS analog output stage.

Total applied 12 pcs exact matching 150W transistors ( In total 1800W ) as the output stages and PSU stages for implement large class A output power .

Uses three R-core transformers (In total 135W ) separate power supplies for the digital control components, right and left channels.

More than 55,000uf audio grade NOVER capacitors are used to ensure ample and smooth power delivery.

A total of 9 groups of PSU's for dedicated DC supply.

Uses most Hi-end grade parts and exactly matched transistors, OCC wires.

Four-channel 100 steps digitally controlled relay-based volume control avoids channel imbalance and improves sound quality .

Supper Exponential Volume Characteristic, easy match the volume with very high headphone / speaker without change the gain mode.

20DB gain can drive the low sensitivity headphone very well.

5 groups input: RCA X2, XLR X2, ACSS X1

Preamp output : XLR / RCA / ACSS

Headphone amp output: 4pins XLR X1 / 3pins XLR X2 / 6.3MM X1

　

Modification guide on headphone jack for 4-pin balanced headphone output.
   Click here to read:   modification guide
　

Technology detail

Detail of the Current Conveyor Technology

Impedance matching:
              There are a lot different DACs in world, they had different design and output impedance .

              For avoid the sound quality effect by different DAC's output impedance, in the Master 9, it had design 4 groups expensive Single polarity class A FET cascode buffers in the input . All the input signals through these FET buffers, the output impedance had become very low, even the source had high or low output impedance.

               The FET buffers working on pure class A state, and it is so short signal journey , it can keep the sound quality without coloration ,dry and harsh .
             
Core:
             Master 9 is a true balanced design applying the newest ACSS design which can improve the XLR connection to sound as good as the ACSS connection. They seem complex , but in fact they are the most simple design the world has ever had , their a lot components to process the power supply. The simplest and shortest signal journey brings out the most neutral and least colored sound.

              Master 9 applies ultra-low distortion single stage ACSS amplifier and soldered directly on the motherboard. It is a fully class A preamp / headphone amp and applies class A power supplies.

               The ACSS is a non-feedback technology made with discrete amplifiers. Most people know the global feedback design can offer better specs in test measurements, and non-feedback can't do well in test measurements but can offer better sound for the human's ears. Here is a conflict of the classic circuits. But the ACSS opens a new field, it can offer a least coloration sound which is more neutral with very low distortion and high linearity. So it can retain the dynamics, detail and neutral sound but not sound bright or harsh.

                All signal gain is in current, all transistors work in pure class A mode.
 
                The Master 9 output buffers had applied 4 pair power transistors ( 150W / 15A ) working on pure class A state, the output impedance is only 1 ohms , it can drive most headphones and amps as well .

                 Master 9 has all components of four channels exactly matched before installation.

Strong class A output stages:
             The Master 9 had design 4 groups diamond difference output stages as the balance drive stage, every parts had exact matching before assemble so don't need any human adjustment . 

             The output stages had  four pair 150W transistors ,they also working under pure class A state , the balance output mode had 4X times power than the same state single-ended mode .The pure class A design had larger  drive ability than the specs, it drive the headphone not worst than a double power class AB design , not to refer, the pure class A is the best sound design  in worth. The balance drive mode can kill the noise and distortion in the end of the headphone, much improve the sound stage, background and the transparency ,ect.

Power supply:    
            Three special transformer are used to supply power separately for the audio and control circuit.

            Master 9 uses 8 groups of high-quality PSU .

            The class A parallel connection PSU has very high input impedance to avoid pulse through the PSU from affecting the preamp and low output impedance with very fast speed and high linear, so it is a very clean power supply.

            Master 9 applies seperate linear PSUs for the display and control parts, avoid the disturbances through power line from affecting the amplifier.

Input/Output:
           Master 9 can input not only XLR balanced signal or RCA unbalanced signal, but also the top-grade ACSS current signal. It can also output RCA signal, balanced XRL signal, ACSS current signal to the power amplifier. Using RCA input, Master 9 can convert to balanced ACSS signal and output real balance signal from XLR and ACSS.

          The ACSS input / output can compatible with KRELL CAST products and SATRI technology gears . The XLR / RCA can compatible with any audio devices in world.

          Master 9 can output RCA, XLR and ACSS at the same time in preamp output model , in headphone amp model, Master 9 can output XLR ,single end for headphone.

          The input channels of Master 9 adopt separated shielding method. When switching, the idle signal lines linked to the equipment and the ground wire are completely disconnected. The idle input ports of the equipment are shielded. In that case, the sound quality won't be affected even with more connected equipment.
　

Volume control:
            The volume control is also different from conventional technology. Conventional volume control technology is the volume control implement in the analog amp input stage through a volume pot to reduce the signal . The volume pots quality effect the sound quality direct , the low quality volume pot cause lost the detail and channels imbalance cause the soundstage distortion . Even the high performance volume pot still cause the detail lose. Whatever quality volume pots, must cause the S/N worst .

             But in the Master 9 , we applied the I/V conversion volume control , the volume control just a variable passive I/V conversion place at the ACSS amp output , where the output is the current signal, not the voltage signal. The volume control is I/V conversion, and the volume depends on the current (I) conversion to the voltage (V). (Like R-2R D/A chips output passive I/V conversion) It can keep the signal frequency band flat and not lost any detail. It can't degrade on sound quality in every volume level . After the volume , there are four groups diamond non-feedback buffer output stages offer the very low output impedance .

             The volume control quality is much important in the real balance gear . It must guarantee the four channels signal control exact to achieve the performance of the real balance gear . If the cool and hot of the balance signal can't guarantee exact, the balance output will has large distortion cause the sound quality and performance even much worst than a single end gear waste the balance gear design and the cost.

             In Master 9 is applied a four channels digital control relays based volume control. .Master 9 has total four channels through changing the DALE resistors to control the volume for avoid channels imbalanced ,achieve the best performance and sound quality of the gear.

             Master 9 have 100 steps volume control, control range achieve 90DB (usually the steps volume ports, potentiometer volume port, relays base volume control all have around 70DB range ), very smooth control range on first 50 steps , cooperate with 20DB high gain, so can very easy adjust the volume for both high / low sensitivity headphone and speakers. While change the volume, the different relays break or close, it may had slightly switch sound on output and disappear while stop change the volume.
              Supper Exponential volume characteristic                   The Volume steps comparison to normal volume knob       
                                                                                          
Shield: 
         Master 9 has complete separation of left, right amplifier channels, power supply, and transformer.

          Master 9 uses heavyweight all-aluminum alloy chassis to prevent the eddy current of transformers from interfering on circuit board.

Components:
         Uses all audiophile grade components, including DALE resistors, WIMA caps, custom order NOVER (UK brand) caps, all audiophile grade input & output sockets (Neutrik ) and gold IEC socket. Master 9 all parts of four channels had exact match before install.