The HPA4 or LA4 will improve the measured system SNR in an all-Benchmark system by 2 db at most volume settings. This is counter intuitive. How can adding an active component decrease the noise? Here is how:
The HPA4 or LA4 will allow the DAC2 or DAC3 to operate at full output which is +24 dBu at 0 dBFS. This means the D/A converter is always delivering its full rated SNR. Please note that at this calibration, the DAC2 and DAC3 converters still have an additional 3 dB of headroom available for the accurate reproduction of intersample peaks (but this is another topic).
The clip point of the AHB2 power amplifier is reached at an input level of +22 dBu. This was set 2 dB lower than the typical +24 dBu at 0 dBFS calibration used in most studios so that studio D/A converters could drive the AHB2 2 dB into mild clipping. The down side is that this calibration elevates the D/A converter noise by 2 dB. This 2 dB is fully recovered if the HPA4 or LA4 is inserted between the converter and the power amplifier.
If you were to measure the system SNR, you would find that it improves by 2 dB when the HPA4 or LA4 is inserted between a Benchmark D/A and Benchmark power amplifier. This is only possible because the AHB2 and HPA4/LA4 have a greater SNR than the D/A converter. If you use any other power amplifier, the power amp will limit the system SNR. Likewise if you use any other preamplifier, the preamplifier will limit the system SNR. With the combination of the Benchmark AHB2 and HPA4/LA4, the D/A converter will almost always be the limiting SNR factor in the system (the Benchmark power amplifier and preamplifier have higher SNR performance than most D/A converters. The key point is that the Benchmark preamplifier will allow the full use of the D/A converter's SNR.
Another advantage of the HPA4/LA4 is that it optimizes the interface between any D/A converter and any power amplifier. This optimization will produce the highest possible system SNR, given the limitations of the D/A and power amplifier.
The reason that this optimization is possible is that the preamp section of the HPA4/LA4 has a SNR that exceeds 140 dB.
If you are using an AHB2 with a non-Benchmark D/A converter, the HPA4/LA4 preamplifier may yield an improvement of much more than 2 dB. The reason for this is that other D/A converters are not necessarily matched to the gain structure of the amplifier. The insertion of the preamplifer optimizes the gain staging between the D/A and the AHB2.
The system-level SNR improvements can easily be measured.