Quite nice??? its claiming "here the 2nd harmonic is under -120dB (0.0001%)". Thats 10x less distortion than the Hypex Ncore @2W ....himmm...
The 4QD-TEC amp has a split power supply and no speaker series cap, so should be used only with a $100 speaker protection circuit, or with disposable speakers. I view the speaker protection circuits everybody quotes on e-bay with the 1 cubic inch disconnect relays as snake oil.
The advantage of the AX6 was the $5 speaker protection circuit. One part, a series speaker capacitor, tends to fail safe (no sound) instead of fail stupid (toasted speaker coils or torn suspension). I view the whole split supply .0000x% distortion quest as a pilgrimage in search of the unicorn. The lowest distortion speakers I've ever owned are 20 db down on 2nd harmonic distortion 52-14khz @ 1 w 1 m. Amp distortions below .1% are probably totally adequate.
Blowing up those $600 each new $300 each lucky break price speakers with a $100 amp is not on my agenda. Fortunately all my bright flash + odor amp mistakes have been made with a tube amp with output transformers, or the dynakit ST120 with an output cap in series with the speaker. I'm restricting the PV1.3k $55 split supply amp to $5 car radio speakers until I design a protection circuit that will break 22 amps @ 170 volts. The one it was shipped with melts the PWB trace to the crowbar triac instead of tripping the circuit breaker.
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with the output dc blocking caps, i can not see how this amp can blow up speakers...the split rail amps has more potential to damage speakers when the output trannies fail as shorts...