HPA4 - Bench Test
"I measured the Benchmark HPA4's performance with my Audio Precision SYS2722 system (see the January 2008 "As We See It"), repeating some tests with the magazine's sample of Audio Precision's current top-line APx555. As a line preamplifier, the HPA4 performed almost identically to Benchmark's superb-measuring LA4, which Kal Rubinson reviewed in January 2020, so I will refer you to that review. This report therefore focuses on the HPA4's behavior as a headphone amplifier."
"The gain for the balanced inputs to both the headphone outputs with the volume control set to "+15" was exactly 15dB. Reducing the control to "0.0" resulted in a gain of 0dB, ie, the output voltage was the same as the input voltage."
"The HPA4 preamplifier preserved absolute polarity (ie, was noninverting) with both balanced and unbalanced inputs and outputs."
"The HPA4's output impedance from both headphone outputs was extremely low, at 0.6 ohm at 20Hz and 1kHz, rising slightly to 0.83 ohm at 20kHz. (Both values include the series resistance of the adaptor cable and 2m of interconnect.)"
"The headphone amplifier's frequency response was flat in the audio band and down by just 1 dB at 200 kHz. With the very low output impedance, the response into 300 ohms was identical to that into the high 100k ohms load, and there wasn't any audio band rolloff with impedances as low as 10 ohms."
"Both the response and the superb channel matching were identical at lower settings of the control and from the unbalanced headphone output."
"Channel separation was superb, at >130dB R–L and >138dB L–R below 2kHz, decreasing to 110dB in both directions at 20kHz."
"From balanced inputs to balanced headphone output, the Benchmark had extremely low noise, with the power supply–related spuriae in its output close to –130dB, even with the volume control set to +15dB. With the volume control set to unity gain, the noise floor was 12dB lower!"
"We define clipping as the output voltage where the THD+N reaches 1%. However, as this graph shows, the Benchmark's protection circuit muted the output when the voltage reached 11.27V, when the THD+N was just above 0.0002%. Reducing the load to 300 ohms gave an almost identical picture, as did repeating the test with the unbalanced headphone output."
"The THD+N percentage was extremely low throughout the audio band into both 100k ohms and 300 ohms."
"I concluded my report on Benchmark's LA4 preamplifier by saying it was the widest-bandwidth, widest-dynamic-range, lowest-noise, lowest-distortion preamplifier I had encountered at that time. To those virtues, the HPA4 adds equally superb headphone outputs."
- John Atkinson, Stereophile
