Chinese firm Questyle Audio Engineering has been working on this particular design for over 6 years, going through a total of 22 different revisions before achieving what they believe is a ground breaking, reference class headphone amplifier. We'll talk about this particular implementation shortly, but first a bit of background on current drive amplification in general.
I'm going to paraphrase Tyll Hertsens because I think he has a knack for explaining complex things in simple terms. In a current drive amplifier (aka transconductance amplifier), the input voltage causes the amplifier to deliver a proportional output current. The output stage of the amplifier will do anything it needs to in order to deliver a current signal that matches that of the audio input, feeding it straight to the voice coils of the connected headphones. It's the current in the voice coils that actually drives the diaphragms, so this type of amplifier creates a very linear relationship between the incoming audio signal and the physical response of the driver. A byproduct of current-mode amplification? The traditional damping factor, which we all know and love, no longer applies. Headphone impedance essentially becomes irrelevant. Or at least that's one theory. Others insist that current drive amplification would only work properly with a headphone that has a completely flat impedance curve. But that doesn't really matter in this case, as the CMA800 actually has a voltage output, despite using current mode operation internally. Confused? I sure was. Still am to some degree, though I can't argue with the results I'm hearing - regardless of the path taken to achieve them.
Let's take a closer look at the CMA800 ($1499). The CMA stands for "Current Mode Amplification", and Questyle describes their implementation as a four part process - an input buffer, a current transmitter, a trans-impedance amplifier, and finally an output buffer. These stages collectively form what Questyle calls a TransLinear Loop Circuit. The key benefit of such a design - it avoids transient intermodulation distortion (TIMD) which according to Questyle causes unpleasant, metallic sounding highs (which is historically a complaint leveled at solid state gear, often from lovers of tube amps). For the longest time I was under the impression that Questyle used a true current-mode output, much like the Bakoon HPA-21. Though my contact there did a fine job at explaining things, I somehow misinterpreted what he was saying, or perhaps I had a preconceived notion of what to expect. After much discussion, their engineers broke it down for me with some in-depth information. English is not their first language but they do a much better job than I would do if I tried speaking their language! What follows is directly from Questyle, with a few minor corrections for readability. I don't normally like to post manufacturer info directly but in this case it makes sense to do so. Some of the charts got a little fuzzy during formatting, sorry about that.
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.