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Get The Noise Out Of Your Cables!

Wednesday, October 8, 2014

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Benchmark AHB2
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Bench Measurement and Commentary
Benchmark AHB2 Power Amplifier

  The new Benchmark AHB2 is the result of considerable research and patents on its resultant circuitry. The intent was to make it an ultra low distortion design. I would have to say they succeeded as it is the lowest noise and distortion power amp I have measured and I have measured a LOT of power amps in my career.
  Measurements were made at the 2V sensitivity and in stereo mode unless noted. I did spot check amounts of distortion at the lower sensitivities and they were little different than at the 2V sensitivity.


The S/N ratio was about 131.6 dB. Pretty impressive, and this is the lowest output noise level I have seen in any power amplifier I have measured!

  Frequency response as a function of loading, not surprisingly, is mostly independent in the audio range with a typical effect of more roll-off with lower loading in the ultrasonic range. Overall HF bandwidth is quite high with a 3 dB down point of 200 kHz or more.  This is shown plotted in Figure 1.

Figure 1 - Frequency Response: Open, 4 and 8 Ohm Loads

  What is a good measure of success in lowered distortion is a plot of THD+N of a 1 kHz and SMPTE IM distortion as a function of power output for 4 & 8 Ohm loading.  This is shown in Figure 2. The THD+N is pretty much noise dominated up to clipping whereas the IM distortion does show above the noise above 10W of output.

Figure 2A - IM Distortion 4/8 ohm Loading


  Clipping with the design is very abrupt and my usual attempts to put extra points in the clipping curve to get the highest power before distortion rises was an exercise in futility. When connected in bridge mode, power was close to 400W into an 8 ohm load.  1 kHz THD+N and SMPTE IM distortion are plotted in Figure 2B.

Figure 2B - 1 kHz  THD+N/SMPTE IM Distortion



  In Figure 3 THD+N is plotted vs. frequency and power.  Here, a 4 ohm load is used and distortion is down in the noise at all power levels up to 2-3 kHz where is start to show as distortion.  The amount of rise with frequency is very low and stays below 0.01% at 20 kHz.  Amazing!

Figure 3 - THD+N Re: Frequency/Power
Red=2W, Magenta=20W, Blue=60W, Cyan=120W, Green=180W


  Damping factor vs. frequency is plotted in Figure 4 and is quite typical of solid stage amplifiers being high at low frequencies and falling above a few hundred Hertz. Damping factor in mono mode, while not measured, would be expected to be about half that each channel in stereo mode as the outputs of the two channels are in series with the load in mono mode. 

Figure 4  - Damping Factor Vs. Frequency


  Another benchmark – pun intended – measurement is a spectrum of 10W 1 kHz THD+N arranged logarithmically along the horizontal axis to show line frequency harmonics as well as the signal harmonics.  I have not seen one of these with such low numbers.  I couldn’t increase the sensitivity any more to show the noise floor.  But look at that third harmonics component at 0.0001%!  That is a low distortion number for sure.  A bit of 5th harmonics also shows with the even harmonics down in the noise.

Figure 5 - THD+N Re:Frequency/Signal Harmonics


  Of interest is how low the noise level is in this amplifier. I measured noise in three bandwidths, wideband (WB), 10Hz – 20 kHz (10-22K), and with A weighting (A wtd) for all three input sensitivities. The inputs were terminated in 300 Ohm per phase.  Results in uV are shown in Figure 6 for both channels. Taking the number of uV of noise and relating them to the reference power output voltage for 100W/8 Ohm, the A wtd. noise for the 2 V sensitivity is a little shy of the claimed 130 dB S/N ratio at 128.1 dB. But at the 4 V sensitivity, the S/N is 130.4 dB and at the 9.8 V sensitivity, the S/N ratio was about 131.6 dB. Pretty impressive and this is the lowest output noise level I have seen in any power amplifier I have measured! (Editor's note: Benchmark measured 3 dB better when input was terminated with 60 Ohm impedance input.)


Figure 6 - A-Weighted Noise (in uV) Re: Wideband, 10 Hz- 22 kHz

 A few final observations: AC line draw at idle was 20W with a power factor of 0.46 and 0.37A. Input impedance was an easy to drive 47K at 1 kHz.


—Bascom H. King


  Bascom King is owner and chief technician for BHK Labs in Santa Barbara, Ca. and a regular contributor to the Everything Audio Network.



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