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  1. John_Siau

    Review and Measurements of Benchmark AHB2 Amp

    We have no plans to make other versions of the AHB2. Our biggest challenge is keeping up with the demand for the AHB2.
  2. John_Siau

    Review and Measurements of Benchmark AHB2 Amp

    Hey guys, go easy on @Paradigm5h1f7. It is a common mistake to assume that the AHB2 is class-D. The small size and high efficiency imply a class-D topology. The switch-mode AC/DC power supply adds to the confusion. The AHB2 is a very unusual amplifier and it takes some time to wrap your head...
  3. John_Siau

    Review and Measurements of Benchmark AHB2 Amp

    The above is incorrect. All outputs from the power supply in the AHB2 are DC.
  4. John_Siau

    Review and Measurements of Benchmark AHB2 Amp

    The AHB2 does not have a class-D amplification stage. The outputs of the power supply are DC. The AHB2 has an AC to DC switched mode power supply that provides two sets of regulated supply rails to a high-power linear class-AB amplifier. It also provides a set of DC rails for the class-A...
  5. John_Siau

    Review and Measurements of Benchmark AHB2 Amp

    The above is incorrect. The AHB2 has a class-A/B output section with dual bipolar output rails (class-G). A class-A correction amplifier provides feed-forward correction. The main amplifier and correction amplifier are passively summed at the output.
  6. John_Siau

    Review and Measurements of Benchmark HPA4 Headphone Amp/Pre

    Yes, the passive -10 dB and -20 dB attenuators on the Benchmark DAC1, DAC2, and DAC3 converters lower the output signal and the output noise. If you add passive attenuation, you will need to rotate the volume control clockwise to achieve the same output level. If you can still reach your desired...
  7. John_Siau

    Review and Measurements of Benchmark DAC3

    Interesting that you mention the PMC Speakers. I use a set of MB2S passive monitors in one of our listening rooms. I am driving each MB2S monitor with an AHB2 running in bridged mono. In the attached photo, you will see 4 AHB2 amplifiers, but 2 are reserved for driving other speakers when we...
  8. John_Siau

    Review and Measurements of Benchmark DAC3

    1) All bets are off when you are trying to measure something below the limits of your test equipment. I was never able to replicate Amir's results using the original DAC3 while testing it on one of my APx555b test systems. My conclusion was that the first DAC3 was not defective in any way...
  9. John_Siau

    Review and Measurements of Benchmark DAC3

    Like all Benchmark converters, the balanced outputs on the DAC3 deliver professional +24 dBu output levels while the RCA outputs deliver 2 Vrms (8.2 dBu). This means that the level difference between the XLR outputs and the RCA outputs is approximately 16 dB. In contrast, most consumer-grade XLR...
  10. John_Siau

    Review and Measurements of Benchmark HPA4 Headphone Amp/Pre

    Like all potentiometers, the pot in the DAC3 has some L/R imbalance at low volumes when passing an analog input. We have a L/R trimmer in the DAC3 that trims out the L/R imbalance in the upper half of the volume range, but below that point, the L/R imbalance will begin to increase. With a...
  11. John_Siau

    Review and Measurements of Benchmark HPA4 Headphone Amp/Pre

    This is exactly what the HPA4/LA4 is intended to do. It can provide transparent gain reduction to interface professional DACs to Consumer power amplifiers. It can also provide transparent gain to allow consumer DACs to drive professional power amplifiers such as the Benchmark AHB2 power...
  12. John_Siau

    Review and Measurements of Benchmark HPA4 Headphone Amp/Pre

    Thank you for the suggestions. We will look into this. In the meantime, here is a workaround that will solve your problem: Enable the "DAC on input 1" mode. Connect one of your signal sources to input 1. Connect your second input to any of the remaining 3 inputs. Disable any unused inputs. Use...
  13. John_Siau

    Review and Measurements of Benchmark HPA4 Headphone Amp/Pre

    You can disable unused inputs. This will allow you to scroll through a limited selection of inputs.
  14. John_Siau

    Review and Measurements of Benchmark HPA4 Headphone Amp/Pre

    The THX888 headphone amplifier is not in the line output signal path. There are two identical fully-balanced relay-controlled preamplifiers in the HPA4. One drives the line outputs and the other drives the THX888 headphone amplifier.
  15. John_Siau

    Review and Measurements of Benchmark DAC3

    Not exactly. This configuration gives the lowest output noise, but the signal level is reduced more than the noise, so the SNR is actually reduced. In the -10 dB setting, you will not be able to drive the AHB2 to full output. If you do not need the full output, you could run at the -10 dB...
  16. John_Siau

    Review and Measurements of Benchmark DAC3

    The output noise of the amplifier due to the amplifier itself is 132 dB below 100W. Since 1 W is 20 dB lower than 100W, the amplifier output noise is 112 dB below 1 W. You speakers have a sensitivity of 90 dBSPL at 1 W so if you do the math, (90 dBSPL - 112 dB = -22 dBSPL). This means that the...
  17. John_Siau

    Review and Measurements of Benchmark DAC3

    The relay-controlled attenuators in the LA4 will always optimize the gain between the DAC and the amplifier. Do not engage the 10 dB pads on the DAC. You will get better noise performance with the DAC set to 0 dB. In either case, the system noise will be well below audibility. You should not be...
  18. John_Siau

    Review and Measurements of Benchmark DAC3

    The DAC2/DAC3 output impedance changes from 60 Ohms to 425 Ohms (when engaging the -10 dB pads), but this is not an issue when driving the AHB2. The input impedance of the AHB2 is about 1 megohm when fed with a balanced signal. The input capacitance of the AHB2 is also very low, and you will not...
  19. John_Siau

    Review and Measurements of Benchmark HPA4 Headphone Amp/Pre

    Stan21, Thanks for your kind words and thanks for taking the time to do the math. I always tell our team here "do the math". When I design audio circuits, I calculate the SNR of the the entire circuit using Excel spread sheets. The entire signal chains of the LA4, HPA4, AHB2, DAC1, DAC2, DAC3...
  20. John_Siau

    Review and Measurements of Benchmark HPA4 Headphone Amp/Pre

    I would love to sell you a second amplifier, but it would not be necessary. The only reason to upgrade to dual AHB2 amplifiers would be to increase your peak SPL by 6 dB. If your LA4 is set to -25 dB, your peak levels are 22 dB below the 100W output of the AHB2 in stereo mode (assuming your...
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