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Review and Measurements of Benchmark HPA4 Headphone Amp/Pre

laidick

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The unit you tried was just configured for use with Benchmark DACs having older software. When the DAC has current software, you can scroll through all four inputs on the HPA4.
Hi John, thanks for pointing this out.

Did you mean the HPA4 has latest current software instead of DAC3?

It's weird if I need to have a DAC3 in order to scroll through the inputs of HPA4.

So instead of scrolling, can we have direct input selection on any remote codes? Such as Logitech Harmony. I expect this would be a very easy change on the HPA4/LA4 software ?

Thanks
 

John_Siau

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DAC3 has 9.2uV noise at max volume. HPA4/LA4 has 1.9uV noise. HPA4/LA4 doesn't have the lowest noise possible. It's easily achievable to have less than 0.3uV noise.
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This statement needs a little clarification:

The output 20 Hz to 20 kHz output noise of the XLR line outputs on the HPA4/LA4 is 1.9 uV when the volume control is set to 0 dB (unity gain). It is lower at lower volume settings.

It is important to note that the XLR inputs and outputs are capable of passing +28 dBu which is 19.46 Vrms. The ratio between signal (19.46 V) and noise (1.9 uV) is 20*Log(19.46/1.9 uV) = 140.2 dB. A SNR of 140 dB is state of the art performance.

A noise voltage of 1.9 uV on a 19.46 V interface is the equivalent of a 0.39 uV noise voltage on a 4V interface. It is also important to note that the HPA4/LA4 achieves this at unity gain, not just minimum gain (output noise is lower at minimum gain).

The 1.9 uV output noise drops when the volume control is turned down. At minimum volume (-112 dB), the output noise is 1.18 uV. This is equivalent to 0.24 uV on a 4 V interface. At full mute, the output noise is 1 nV and is determined by the thermal noise (Johnson noise) of two 30-Ohm resistors .

At maximum volume (+15 dB boost) the output noise increases to just 5 uV. Given the maximum output level of +28 dBu (19.46 V), the SNR is 20*Log(19.46V/5uV)=131.8 dB. It is not easy to achieve a 15 dB gain boost and still have a 132 dB SNR. Again, this is SOTA performance.

If you measure the 2 Vrms RCA outputs, you will find that the noise is 15.7 dB lower than that of the professional XLR balanced outputs. The noise on the RCA outputs is 0.31 uV at unity gain and 0.18 uV at minimum gain (-112 dB). The SNR is identical on both interfaces, but it is very difficult to find RCA inputs that have a 0.31 uV EIN (equivalent input noise). Low noise systems need to be built with professional-grade balanced interfaces. High-voltage professional balanced interfaces make it much easier to achieve an excellent SNR (at the system level).

Comparing the noise voltage of a 4 Vrms balanced interface to that of a 19.5 Vrms studio-level balanced interface is comparing apples to oranges. You need to do the math to calculate the ratio between the signal and the noise.
 
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John_Siau

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Hi John, thanks for pointing this out.

Did you mean the HPA4 has latest current software instead of DAC3?

It's weird if I need to have a DAC3 in order to scroll through the inputs of HPA4.


Thanks
No, you can normally scroll through all of the inputs. I am guessing that the dealer just had the scrolling disabled on input 1 because the DAC had older software. This is not an issue with newer software.
 

John_Siau

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Well the initial question comes back. How did they achieve 445mW at 300ohm with opa564 which can only run on +-12V?
Great question! Great observation!

The answer is that the +/- supplies track the audio.

This tracking also eliminates common-mode distortion components that would normally be produced by the OPA564. The reason for this is that audio signal is always centered between the two voltage rails in that stage of the amplifier. The common mode voltage is always nearly 0.
 

laidick

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No, you can normally scroll through all of the inputs. I am guessing that the dealer just had the scrolling disabled on input 1 because the DAC had older software. This is not an issue with newer software.
This is good. Thanks!

How about the temperature? What is the temperature normally for this preamp running for 2hrs in room temp of 24degree with air conditioner ?
My observation today the HPA4 was running about 40degree plus (hand feeling) while touching the top of it.
 

John_Siau

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The main sources of heat are the four fully-balanced 256-step attenuators. There are 12 relays on each of the four attenuators. In addition to these 48 relays, there are 6 that control inputs and outputs plus two that provide protection for the THX-888 headphone amplifier. Much of the heat comes from the relay coils, but there are other contributing factors:

Headphone amplifier:

The THX-888 can draw significant power when driving headphones.

Attenuators and audio path:

The attenuators are a very low impedance design and this means that it takes some power to drive them. The internal signal path is fully balanced and it normally operates at about +24 dBu. The low impedances and high signal levels are what make the high SNR possible. Low-noise audio circuits burn power. There is no way around the laws of physics.
 

nemesisrobot

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The main sources of heat are the four fully-balanced 256-step attenuators. There are 12 relays on each of the four attenuators. In addition to these 48 relays, there are 6 that control inputs and outputs plus two that provide protection for the THX-888 headphone amplifier. Much of the heat comes from the relay coils, but there are other contributing factors:

Headphone amplifier:

The THX-888 can draw significant power when driving headphones.

Attenuators and audio path:

The attenuators are a very low impedance design and this means that it takes some power to drive them. The internal signal path is fully balanced and it normally operates at about +24 dBu. The low impedances and high signal levels are what make the high SNR possible. Low-noise audio circuits burn power. There is no way around the laws of physics.
Hi John, how much does the heat affect the lifespan of the amp? Other than saving electricity, should I always be turning it off whenever not in use instead of leaving it on all day?
 

John_Siau

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Hi John, how much does the heat affect the lifespan of the amp? Other than saving electricity, should I always be turning it off whenever not in use instead of leaving it on all day?
The main issue with the lifespan of solid state electronics is the lifespan of the electrolytic capacitors. Electrolytics tend to dry out over time and loose capacitance. We use long-life high temperature 105 C electrolytic capacitors to maintain a long lifespan. These components are designed for power supplies (which tend to run much warmer than our products). Obviously, lifespan can be increased by turning the product off when not in use. There is no warm-up time (the sound does not change as the product warms up) - this can be verified with measurements. If you were to leave the unit on 24-7, you can still expect a lifetime of at least 15 years from the electrolytic capacitors (probably much more). You will find that the power dissipation is much lower when the unit is muted. The reason for this is that all of the relay coils are off in this state.
 
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I have the hpa4 and love it. I do notice when booting it that a line goes across the display then disappears.

 
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Sal1950

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I have the hpa4 and love it. I do notice when booting it that a line goes across the display then disappears.
Just curious, are we supposed to be able to see that line in your video?
I see only the light gray of the backlight first coming on, then the Benchmark factory wallpaper?
 
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Yeah, just slow down the motion on youtube. I was just wondering if anyone else or the benchmark rep has seen it and if its normal behavior.

Before the wallpaper comes on there is a fast line that shoots across the screen. Its not uniform or anything so it catches my eye. Its on the right side and about 1/4 of the size of the screen and its vertical.
 
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Sal1950

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Yeah, just slow down the motion on youtube. I was just wondering if anyone else or the benchmark rep has seen it and if its normal behavior.

Before the wallpaper comes on there is a fast line that shoots across the screen. Its not uniform or anything so it catches my eye. Its on the right side and about 1/4 of the size of the screen and its vertical.
OK, at .25 speed I see it. Just looks like the loading of the background image scan to me. I doubt it's of any consequence.
 

anmpr1

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I have the hpa4 and love it. I do notice when booting it that a line goes across the display then disappears.

Reminds me of when I was a kid. How the toob tv would turn in to a little dot in the middle of the screen once you switched it off. It would last maybe a minute, getting smaller and smaller. I'd watch that, fascinated just to see how small it'd get before it became invisible. That little dot was much more interesting than any actual programs, which even back then I understood to be idiotic. At least it was that way until Joseph Stefano created The Outer Limits. Then I actually wanted to watch. Interestingly, Joe used some TV tube tricks in the opening credits of the show. Rolling the image, blurring it, etc. Rod Serling's thing was pretty cool, but I never saw it until later in life. It must have come on too late, or the folks were watching something else. We only had one TV, and the old man called the programming shots.
 

Sal1950

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Reminds me of when I was a kid. How the toob tv would turn in to a little dot in the middle of the screen once you switched it off. It would last maybe a minute, getting smaller and smaller.
I remember that little dot, LOL And also the credits on Outer Limits and LMAO at my mom as she got up to try and adjust the V & H controls to fix the picture.
Wow home video's come a long way since then !!!
 

RichB

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Yeah, just slow down the motion on youtube. I was just wondering if anyone else or the benchmark rep has seen it and if its normal behavior.

Before the wallpaper comes on there is a fast line that shoots across the screen. Its not uniform or anything so it catches my eye. Its on the right side and about 1/4 of the size of the screen and its vertical.
I tested this on my LA4. The is a right-bar that shows of < 1 second by only when powered on by trigger. It does not appear when using the power button. As other have posted, it is not something to worry about.

- Rich
 

amirm

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I remember that little dot, LOL And also the credits on Outer Limits and LMAO at my mom as she got up to try and adjust the V & H controls to fix the picture.
I remember logic in some TVs to make the dot move so that it would not burn that hole in the CRT Phosphor!
 

LuckyLuke575

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It has higher baseline noise. However, it has so much less distortion that when you sum the two in those sweeps, it is better than 789.
Given the price differential, it seems to me that this test just reinforces just how good the 789 is for the money.
 
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