Pros: Extremely good measurements but more importantly, equally good sound.... amazing clarity makes other great amps sound dull and veiled.
Cons: Spiky feet can be dangerous!
1) Super Linear theory
In 1975 Mr. Barrie Gilbert created the “Translinear-TL” theory, and published in ISSCC . This marks the birth of Current Mode theory. A translinear (TL) circuit is a circuit that carries out its function using the translinear principle. In TL principle, ideal BJT Collector Current’s log domain (Lc) and Vbe graph is a straight line. See below graph, BJT Collector Current increase from 1pA (1E-12) to 10mA, which is a billion times increase, while log (lc) and Vbe is a good straight line. Current Mode circuit is using this super linear feature to make up a TL loop. This is the basic theory for Current Mode ultra-high precise, ultra-low distortion function. .
2) Ultra-high speed
In Current Mode circuit, the inter-electrode capacitors in between transistors which affect speed and band are working in a very low impedance point (usually several ohms or dozens of ohms, which is hundreds or even thousands of times less than a voltage mode amplifier). With large amplitude current wave, the inter-electrode capacitors complete their charge and discharge cycle very fast, so the efficiency is much higher than that of a voltage mode circuit.
CMA800 Work Principle
Since 1989 in the 87th AES forum, when Dr. D.C Wadsworth first point out current mode circuit in audio can achieve ultra-high speed, ultra-low distortion function, people start ceaseless dedication towards current mode amplification. So far some brands created CAST (Current Audio Signal Transmission) technology, SATRI technology based BPM7110 modular, etc. These are pioneers on developing current mode technology application on audio. But they are mainly “non-feedback” mode, mainly for current mode “transmission” rather than amplification. Questyle is different, as we specially focus on “amplification” instead of “transmission”, so CMA800 provides more outstanding specifications and sound performance.
Above is the CMA800 current mode amplification circuit diagram. The circuit is consist of VCCS (Voltage Control Current Source), A(i) --Current Mode amplification modulator, I/V converter, OPT and Negative Feedback sections. In an electronic circuit, both voltage and current are exist - neither can be missing. CMA800 signal is controlled by current so as to provide high performance, while the input and output are in voltage mode. This is to ensure better compatibility with other audio devices.
From the diagram we can see, input signal will first to to VCCS, voltage mode is converted to current mode here, then goes to Ai for amplification (in current mode), next go to I/V converter to become voltage mode again, and output to OPT stage to drive headphone.
For better performance, in this circuit loop we applied Negative Feedback design. The impedance of the whole Negative Feedback loop is in the hundreds of ohms, meaning amplification is very fast; thus the Negative Feedback process speed can be as much as 100 times faster than that of voltage mode designs. In this way CMA800 is free of problems like TIMD or other which occur in voltage mode designs.
CMA800 Full-Power Bandwidth approaches Closed-loop bandwidth, and the converting speed shows a linear increase as input signal amplitude increases. These features are totally different from that in voltage mode, and together they support CMA800 to achieve ultra-low distortion and ultra-high efficiency.
Current mode amplification internal translinear-TL loop has critical requirement on transistors. A Canadian semiconductor manufacturer, Microsystems International Limited, had attempted to make a current mode amplification IC but failed on workmanship.
In a TL loop consisting of 6 transistors, a mismatch as small as 5uV Vbe will create 0.01% harmonic distortion. Precise matching is critical. Questyle engineers studied over 5 years on this, working together with Dr. Charles from California university who is dedicated on transistor workmanship research. By selecting from lots of customized specific components, and designing specific circuit structures, we finally achieved success.
1KHz@300Ω , THD vs Power
1KHz@300Ω , Po=10mW FFT Spectrum Analysis
Intermodulation Distortion (IMD)
Some engineers use Sine waves to test amplifier performance. They think that as long as the THD+N value is not high, it should therefore be a good amplifier. But in actuality each music signal is made up of thousands of harmonic waves - it’s expected they will interfere each other, so IMD value is a more fitting approach to indicate signal accuracy as opposed to THD+N. “IM (Intermodulation)” means two or more signals that come together which will interfere with one another and make up a new signal. IMD decides amplifier’s purify, density and details. CMA800 IMD spectrum is almost as perfect as AP2 signal source - we hardly see any mixtures at all.
TIMD @300ohm 0dBV FFT
Full frequency low distortion
Ultra-wide bandwidth provides amplifier with great sound, but ultra-low distortion is also required for high frequencies. Otherwise the sound will be terrible. CMA800 distortion graphs at all frequencies show a straight line; the value is ultra-low.
Broadband High-speed Amplification
Only an amplifier with broadband high-speed amplification can properly recreate a burst pulse wave signal, accurately reproducing the necessary speed and clarity. CMA800 frequency response is essentially a straight line, with only a -0.3dB drop at 200KHz.
Frequency Response @300ohm, 0dBV
Specific Power Supply
For an analogue amplifier, power supply plays a very important role. We work together with Plitron Manufacture in Canada Their senior designers specially designed many versions of dedicated transformers, evaluated and selected the best one to fit our needs. In addition, CMA800 adopts schottky rectifiers to reduce noise, as well as 22 instances of 35V 1000uF Nichicon FG capacitors providing clean power.
CMA800R adopts best Hi-end components to ensure best sound. Here “best” means best fit, like OPA627 operational amplification module, DALE military resistor, WIMA customized capacitors, Alps volume potentiometer, Nichicon FG capacitor, etc.
Besides good parameters, good components, a good sound performance is the last step but most critical factor. Before production, CMA800 was tested with various headphones, and was sent to many studio engineers and musicians for critical evaluation. Once they were all satisfied with our sound, the CMA800 detailed specifications were then fixed and we started production.
Mechanical resonance will affect performance for any Hi-end device. CMA800 chassis is made up of pure Aluminum with “mutual bit” structure, 10mm thick. Each section weight is equal, for maximum reduction of resonance. Feet are made of solid aluminum as well
The spiky feet, sitting on some audiophile puck I had laying around:
Powerful enough to happily run two headphones at once:
LEDs are not too bright - a good thing!
Rear panel with XLR and RCA inputs:
Selectable upsampling uses different methods for 44.1kHz and 48kHz signals:
It can also disable upsampling, plus has multiple filter options:
Other DACs I used included the BMC PureDAC:
The Yulong DA8 (pictured here playing DSD):
Source was my Auraliti PK90 music server with NuForce LPS-1 power supply:
I also tried multiple DAC options from Resonessence Labs:
Note - this review originally appeared under the "CMA800R" category. My mistake. Now that I'm posting a review of that model, I had to create the proper entry and move this review over.