Review and Measurements of Benchmark HPA4 Headphone Amp/Pre

[John_Siau]

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.
.

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.

 

[John_Siau]

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]

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]

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]

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]

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]

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.

 

[ShiZo]

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

 

[Sal1950]

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?

 

[ShiZo]

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.

 

[Sal1950]

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.

 

[nemesisrobot]

I own the HPA4 and see the same thing. As @Sal1950 said, it’s probably just the hardware clearing the screen buffer before displaying the Benchmark logo.

 

[anmpr1]

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]

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]

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]

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]

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.

 

[Tommygun]

There is a lot of technical jargon on this thread regarding the HPA4/LA4 and certainly the measurements are very very impressive but the question is does it sound any better than just a DAC3 HGC and AHB2 ?

I have owned the DAC3 HGC and AHB2 along with B&W 804D3 speakers for a few years now and to be honest I have not been very happy with the performance to the extent that I was actually thinking of selling my Benchmark kit for something else. I use a music streamer as my source.

The sound is very clean and the detail is good but there is just something missing that to me makes the sound boring and not engaging. Some of my friends even described it as too clinical. I used to sit there and play a track and then midway through it jump to another and another because I was never happy with the sound I was getting.

I always knew the AHB2 was a very good amp and my suspicion fell on the digital volume control in the DAC3. Good as it is I always felt that it was a compromise in the DAC3.

After reading a few reviews I decided to take a chance with the LA4 ( I don't use headphones) to see if it will make any difference.

From the moment I heard the first few bars of the first track I knew this was something very special. That first day with the LA4 I sat there and listened to music continuously for 5 solid hours without skipping a single track. It was as if I had completely replaced my system with something much much better. I was gobsmacked and never in a million years expected to hear such a massive improvement. It is now day 5 and I'm still listening for a couple of hours each night whereas previously I would maybe listen to the system twice a month and that was for the sake of not letting it sit there collecting dust. Standard 16 bit CD music now sounds like Hi-Res 24 bit music and 24 bit is now just completely amazing.

I'm not very good at describing what I hear but here goes

The soundstage was very good to start with but has now become vast, instruments have plenty of space and timbre and I'm hearing detail and micro detail that I never knew existed. Vocals sound better, more natural with no harshness at all even on poorly recorded tracks that sounded a bit harsh without the LA4 in place.
The bass is tighter and much more rhythmic and easier to follow. When I used to turn up the volume control on the DAC3 everything used to get louder even the quiet bits but with the LA4 the quiet bits stay quiet and the loud bits come in louder so the dynamics are much much better. Most important of all the music is a joy to listen to and I'm completely lost in it. My system sounds so engaging and enjoyable,

I now realise that rather than replacing my system it was crying out for the LA4 all along and what a difference that one component has made. There is no going back to the DAC3 and AHB2 combo now.

So anybody out there wondering what difference an LA4/HPA4 will make to their system just go and buy it cos you have nothing to lose and everything to gain.


I will hopefully replace the DAC3HGC with a DAC4 without the volume pot whenever that comes out and I wish Benchmark would make a Streamer with user replaceable integral SSD storage with the same footprint as the rest of the range.

One thing for sure my LA4 and AHB2 will be with me for years to come.

Does anyone know how to upgrade the firmware on Benchmark equipment as there is no mention of that in the manual ?

I hope you all enjoyed my little review.

 

[PaulD]

I would recommend doing a double blind test to see if there is actually any audible difference...

 

[tobes]

Does anyone know how to upgrade the firmware on Benchmark equipment as there is no mention of that in the manual ?

Check the Benchmark web site - there is a firmware (chip) upgrade available for the DAC3: https://benchmarkmedia.com/products...hgc-and-dac3-l-version-2-0-released-9-18-2018

The upgrade provides a HPA4/LA4 compatibility mode which deletes the DIM/mute functions and also the 'arrow' input select buttons from the DAC3. The HPA4/LA4 then provides Dim/mute and the remote input select arrows are used for preamp inputs. The direct select input buttons on the Benchmark remote are used to select DAC input.

This avoids getting DAC/pre in conflicting states or having a flashing unused digital input selected on the DAC when you've used the arrow keys to select a another analog input on the pre etc.
I've ordered the update chip, but there is a cheap fix. Just place a bit of tape over the remote receiver on the DAC3. I rarely change digital input on the DAC3 and switch it on via a HPA4 trigger, so this made remote functionality of the HPA4 far better for me.