Tube Power Amp Reviews

PS Audio BHK Signature 300 monoblock power amplifier Measurements

Sidebar 3: Measurements

I began to perform a full suite of measurements on the PS Audio BHK Signature 300 (serial no. NPWA-A1-5G0062), using my Audio Precision SYS2722 system (see www.ap.com and the January 2008 "As We See It"). First, however, I preconditioned the amplifier by running it at one-third the specified power, 100W, into 8 ohms for 60 minutes. At the end of that time, its top panel was warm and its heatsinks were hot, at 122.1°F (50.1°C). The percentage of THD+noise at this power level was 0.0145% with the amplifier cold, 0.0164% with it fully warmed up.

The BHK 300's voltage gain at 1kHz into 8 ohms was a little higher than usual, at 30.5dB with both balanced and unbalanced input signals. Both inputs preserved absolute polarity (ie, were non-inverting), and the single-ended input impedance was 47k ohms at low and middle frequencies, decreasing slightly and inconsequentially to 39k ohms at the top of the audioband. The balanced input impedance was exactly twice these values, as expected. The output impedance (including the series resistance of 10' of speaker cable) was on the high side for a solid-state amplifier, at 0.16 ohm at all audio frequencies.

After that point, having been powered up for two hours, the BHK 300 became unstable. Both with signal present and with the input shorted to ground, there was a tone at 270kHz present at the output at a level of 3V. The front-panel light remained illuminated. (With a fault condition, it is supposed to flash.) I put the amplifier into standby with its front-panel button, then turned it off with the rear-panel switch. When I turned it on again and let it go through its turn-on routine, it worked fine for about a minute, operating with a 1kHz tone at 1W into 8 ohms with very low distortion, after which the ultrasonic noise reappeared. I turned the amplifier off again, after which it wouldn't turn on at all.

I redid all of the testing with serial no. 5G0063. This sample performed without any problems and matched no. 5G0062 very closely, at least with the tests I'd performed up to the point where I had the problem.

The gray trace in fig.1 shows the BHK 300's small-signal frequency response into our standard simulated loudspeaker. The variations in response were just ±0.15dB, and into resistive loads the amplifier had a wide bandwidth, with –3dB frequencies of 195kHz into 8 ohms (blue trace), 175kHz into 4 ohms (magenta), and 100kHz into 2 ohms (red). With this wide bandwidth, it came as no surprise that the BHK 300's reproduction of a 10kHz squarewave (fig.2) featured very short risetimes, though the slightest hint of overshoot is visible.

Fig.1 PS Audio BHK Signature 300, balanced frequency response at 2.83V into: simulated loudspeaker load (gray), 8 ohms (blue), 4 ohms (magenta), 2 ohms (red) (0.5dB/vertical div.).

Fig.2 PS Audio BHK Signature 300, small-signal 10kHz squarewave into 8 ohms.

The unweighted, wideband signal/noise ratio (ref.1W into 8 ohms and measured with the input shorted to ground) was good, measuring 82.2dB. (When it was working correctly, S/N 5G0062 was about 5dB noisier than 5G0063.) This ratio improved to 86.7dB when the measurement was restricted to the audioband, and 90.2dB when A-weighted. Spectral analysis of the low-frequency noise floor while the BHK 300 drove a 1kHz tone at 1W into 8 ohms revealed a regular series of AC-related spuriae (fig.3), though these were all very low in level.

Fig.3 PS Audio BHK Signature 300, spectrum of 1kHz sinewave, DC–1kHz, at 1W into 8 ohms (linear frequency scale).

The PS Audio comfortably exceeded its specified power outputs of 300W into 8 ohms and 600W into 4 ohms (both equivalent to 24.8dBW), clipping at 370W into 8 ohms (25.7dBW, fig.4) and 620W into 4 ohms (24.9dBW, fig.5). The BHK 300 clipped at 950W into 2 ohms, but was less comfortable at lower powers into this low an impedance. This can be seen in fig.6, which plots the percentage of THD+noise into 8, 4, and 2 ohms at a level, 8.975V (equivalent to 10W into 8 ohms), at which I could be sure I was looking at actual distortion rather than noise. The THD was very low at low and middle frequencies into 8 ohms (blue trace) and 4 ohms (magenta), but rose into 2 ohms (red) and in the top two audio octaves.

Fig.4 PS Audio BHK Signature 300, distortion (%) vs 1kHz continuous output power into 8 ohms.

Fig.5 PS Audio BHK Signature 300, distortion (%) vs 1kHz continuous output power into 4 ohms.

Fig.6 PS Audio BHK Signature 300, THD+N (%) vs frequency at 8.975V into: 8 ohms (blue), 4 ohms (magenta), 2 ohms (red).

The distortion signature was predominantly the relatively benign third harmonic in nature (fig.7), which was low in level even at high powers, lying at –80dB (0.01%) into 8 ohms (fig.8). Reducing the load to 4 ohms at this output level (fig.9) brought up the fifth and seventh harmonics a little (to –83 and –99dB, respectively), but the third harmonic didn't increase. Despite the decreasing linearity at high frequencies seen in fig.6, the BHK Signature 300 did very well when asked to handle an equal mix of 19 and 20kHz tones at a level a few dB below visible clipping on the oscilloscope screen. Even into 4 ohms (fig.10), the 1kHz difference product lay at –107dB (0.0005%), and the higher-order products at 18 and 21kHz were still almost 80dB down from the peak signal level.

Fig.7 PS Audio BHK Signature 300, 1kHz waveform at 60W into 4 ohms, 0.025% THD+N (blue); distortion and noise waveform with fundamental notched out (red, not to scale).

Fig.8 PS Audio BHK Signature 300, spectrum of 50Hz sinewave, DC–1kHz, at 100W into 8 ohms (linear frequency scale).

Fig.9 PS Audio BHK Signature 300, spectrum of 50Hz sinewave, DC–1kHz, at 200W into 4 ohms (linear frequency scale).

Fig.10 PS Audio BHK Signature 300, HF intermodulation spectrum, DC–24kHz, 19+20kHz at 200W peak into 4 ohms (linear frequency scale).

Amplifiers do tend to break when I set them up on the test bench—it's the audio equivalent of taking a car out on the skid pad to find out at what lateral g-force it rolls over—so I give PS Audio's BHK Signature 300 the benefit of the doubt. Its measured performance indicates that it is a well-designed, well-engineered powerhouse of an amplifier, though it will perform at its best with speakers having an impedance of 4 ohms and above.—John Atkinson

COMMENTS

Hum problem and deep bass performance

Submitted by K.Reid on February 5, 2016 - 5:32pm

Mike, great write up. You mentioned the hum problem, but it is not clear how you eliminated it. Please clarify. Also, you express that the deepest bass was not spectacular in comparison to your 6 figure price tag reference monoblocks. In your opinion, what choices did Bascom King make in the design of his amp that led you to conclude that the extreme deep bass performance was less than world class say in comparison to Soulution, dartzeel and Constellation.

BHK-300 or Parasound JC1

Submitted by Audiophileman on October 4, 2017 - 5:57pm

Hi Michael I just finished reading the review of the BHK-300. I have the Goldenear Triton Ones. I am looking to buy this amp, or the Parasound JC1 monoblocks. You reviewed both amps, I know the JC1, was back in 2003. I highly value your opinion. What amp would you buy to pair up with the Triton Ones today? Thanks for the help.