RDA-7.1 Measurements and Analysis
The RDA-7.1delivered a whopping 188wpc into 8 ohms at < 1% distortion (onset of clipping) with an equivalent Signal to Noise Ratio (SNR) of 114dB (unweighted) which translates down to 91dB @ 1 watt. The RDA-7.1 delivered 320wpc at clipping with similar SNR figures. These are excellent measurements demonstrating the amplifier delivered more than speced power and more importantly had an extremely low noise floor which is critical when listening at low power levels to preserve detail ensuring all of the musical nuances may be heard.
At the manufacturers rated power of 150wpc into 8 ohms, the RDA-7.1 exhibited impressively low distortion (30.657+52.55dBv)dBv = 83.2dBv or 100*log^-1(-83.2/20) = .007%
At the manufacturers rated power of 300wpc into 4 ohms, the RDA-7.1 exhibited similarly low results (30.656+49.508)dBv = 80.16dBv or 100*log^-1(-80.16/20) = .01%.
Gain Product Bandwidth (GPB) of this amplifier is excellent. Even at near full power levels, we see a -3dB point above 100kHz.
The RDA-7.1's output impedance is well under 100mohms throughout the entire audio bandwidth. This requires the right amount of negative feedback and usage of multiple power transistors to achieve this feat. The benefits of 8 BJT's per channel are clearly evident here. The reason we emphasis so much importance on minimizing amplifier output impedance is because the lower the impedance, the less chance of system frequency response variation when driving reactive loads such as speakers and/or many exotic cables.
An amplifier that exhibits low output impedance will also exhibit excellent damping factor. The RDA-7.1 maintained an impressive damping factor of over 125 throughout the entire audio bandwidth. These type of measurements are uncharacteristic of tube amplifiers which is where they usually fall apart when attempting to drive low impedance, inefficient speaker loads. This design is a far departure of that design characteristic.
Although the faceplate of the RDA-7.1 has the name Balanced Audio Technologies (a company known for its tube amplifiers), the bulk of the real circuit design was done by ATI Amplifier Technologies. Note that the published spec of this amp claims a damping factor of a meager 40, implying a much higher output impedance. I suspect this is a case where marketing is under-exaggerated to gain audiophile appeal for tube lovers that believe esoteric amps sound more "chocolately." Conversely, we were relieved in the performance seen here. While tube amps may have their place for esoteric two-channel audio, they don't cut the mustard for dynamic home theater environments. Rest assured, this amp has no such issues.
Update (11/13/05): According to BAT, the RDA-7.1 is their design, but the manufacturing was done by ATI.
At full power we see the RDA-7.1 maintains its composure into an 8 ohm load.
The true test is how
well an amp will hold up into low impedance loads. Ideally and amplifier should
act like an ideal voltage source meaning power will double as load impedance
halves since output voltage will remain constant. In order to do this, the
amplifier output impedance must maintain a low enough level to not be the
limiting factor. It takes more than just negative feedback to keep a low output impedance, especially when driving low impedance loads at high power. You need a good stable power supply, lots out output devices and heat sink area.
As you can see, the output impedance is very similar to the 8 ohm full power test!
Based on the principle of voltage divider, we should see about 1/2 the damping factor from our 8 ohm case.
No surprises there! This amp delivered the goods.
There is no substitute for good old fashion amplifier design with plenty of power devices to maintain a low output impedance at high power levels, an ample power supply to keep the source resistance low and a well laid out circuit topology to keep the noise floor down in the mud.
