The Importance of Low Noise and Distortion
Some experts believe that modern amplifier technology has reached a state of development such that the only real differences are related to subtle frequency response differences caused by the varying interaction of speaker loads with amplifier output impedance. From this viewpoint, once such frequency response differences are eliminated, most modern amps not driven into overload sound basically alike.
However, I believe that non-frequency-response effects can help the listener differentiate among amps. Two such effects are the signal-to-noise ratio and distortion levels.
While you may think that any amp whose quiescent residual noise you cannot hear from your usual listening position is quiet enough, that has not been my subjective experience. This is one reason I'm now a big fan of the Benchmark equipment, including the AHB2 amp. I've been using a pair of them in bridged mono, as well as the Benchmark DAC3 HGC to drive my Harbeth M40.2s for awhile now.
With this combination of electronics, for the first time in my audio life I can pin my ear to each speaker driver and hear absolutely no sound coming from the speakers in a quiet room with the system volume control at any position from minimum to maximum with no source material playing. Most amps produce quiescent noise under such conditions loud enough to be clearly heard at least a foot from a speaker driver, often more like several feet. The Sanders and Behringer electronics driving the Sanders 10C speakers I once owned, combined with the high sensitivity of those speakers, meant I could hear hiss from the speakers immediately upon entering the listening room, a good 10 feet from the electrostatic panels, even with the volume control at minimum. The amps in the Janszen Valentina Actives were noisy enough that I could sometimes hear a trace of hiss from my listening position about 55 inches from the electrostatic drivers, and that was after careful gain structuring of the system to minimize that hiss.
I find that lowering the background noise paints the music on as black a canvas as possible and thus seems to bring out realistic low-level detail in recordings. While I may not actually hear residual amp noise as hiss/hum/rumble/roar, I more clearly hear the environmental ambient noise captured on the recording if the amp noise is lowered and this increases my perception of hearing the music in its recording venue, as well as my perception that the music is being made by real humans producing little noises like forceful breathing in rhythm, sub-vocalizations, lips parting, risers and chairs creaking, etc.
Benchmark's raft of published tests, as well as published test reports, show that the Benchmark equipment also exceeds most other audio equipment in terms of lower distortion of various types. See, for example, Stereophile's review which includes John Atkinson's measurements and comments about the AHB2 amp. This is probably why the Benchmark equipment reaches new highs in my experience in terms of allowing the music to sound cleaner and clearer than ever before, without any hint of brightening. Residual electronic artifacts are wiped away to an unprecedented extent.
The Pesky Volume Control
Once you have the expertise and technology to allow your digital decoding and amplification chain to reach these extremely low levels of noise and distortion, the weak spot in the system may well be the lowly volume control. You can't live with most, but you certainly can't live without any!
The difficulties of designing a method of controlling system SPL/volume without simultaneously adding distortion and/or noise is explained in the Benchmark Application Note, "Relay-Controlled Volume – The Ultimate Solution For Analog Audio." Controlling the volume in a way that did not "spoil" the performance of which the Benchmark DAC3 HGC and AHB2 amp are inherently capable was one of the two raisons d'etre of the newest Benchmark product, the HPA4 Headphone/Line Amplifier. Only through designing a new type of volume control could the low noise and distortion of the AHB2 amp remain uncompromised while still allowing precise control of system SPL. Even the method of controlling the volume used in the DAC3 HGC I've raved about is a bit compromised. To quote just a bit from the Application Note:
In the end, for the HPA4, Benchmark:
Benchmark's literature claims:
Driving Demanding Headphones Like the Audeze LCD4
Ultimately the power/voltage demands of low sensitivity, high-current-demanding headphones like my favorite Audeze LCD-4 forced me to use the very powerful SimAudio Moon Neo 430HA headphone amplifier to drive them, rather than the headphone amp built into the DAC3 HGC. This, in turn, forced me to use another cascaded DAC3 to optimally feed that separate headphone amp a balanced analog audio signal. For details on this saga, see, in particular, my post, "Doubling Down on the Benchmark DAC3: Adding the DAC3 HGC."
Now, as I noted in my very next post in that thread, "Benchmark HPA4 Headphone/Line Amplifier," with the release and delivery of the HPA4, I potentially can eliminate a lot of complexity in the system hook up and sell both the Benchmark DAC3 DX and SimAudio Moon Neo 430HA headphone amplifier.
Set Up
The user's manual for the HPA4 is online here. Besides detailing the unprecedentedly low noise and distortion of the HPA4, the thorough instructions in the manual will give you a very good idea of how to set up and use the innovative preamp interface it offers. But here are my suggestions for optimally using the HPA4 together with the Benchmark DAC3 HGC DAC and Benchmark AHB2 amp/amps:
Speaker Sound
About all Benchmark's literature claims for the sonics of the HPA4 compared to using the DAC3 straight into their AHB2 amp is: " With the HPA4 inserted, the DAC3's 32-bit digital gain control is replaced by the HPA4's more-resolving fully-analog relay gain control."
So "more resolving" is how Benchmark sums up the benefits. A modest claim.
Does the HPA4 deliver the goods? Does it work as advertised? Is it really "more resolving"?
For years I'd found that eliminating the analog preamp was the surest way to get better, purer sound. I've done many comparisons of a volume-controlled-DAC-to-amp chain with a chain injecting an analog preamp between the DAC and amp. Never before did the addition of an active analog preamp gain me more than it lost. The sonic comparisons weren't even close. Whatever extra dynamics might have been gained by inserting an active preamp were far outweighed by added distortion, noise, lack of clarity, less expanded and more ambiguous space, more ambiguous image placement, brightness/grain/grit in the highs, volume control tracking problems changing the left/right balance, and a significant loss of transparency—recorded information was gobbled up by the added analog stage.
And I've tried my share of passive preamps as well, from several iterations of the old Mod Squad Line Drive (remember that?) to the best balanced nude Electronic Visionary Systems Ultimate Attenuators (the pic is of the RCA variety, but you get the idea). These deliver a certain undoubted purity of sound, but in the end are disappointing in terms of seemingly truncated dynamics, less bass slam, and lack of high frequency air and extension even when used with the recommended short, low capacitance cabling.
But now the rules apparently have changed. Most volume controls must truly be terrible sounding compared to this one. The HPA4 does indeed deliver the goods and is yet "more resolving" than the prior Benchmark chain in my system.
In one fell swoop, adding this line amplifier between my Benchmark DAC3 HGC and pair of bridged mono AHB2 amps creates the following sonic improvements:
Headphone Sound
My reference headphones are the Audeze LCD-4 fitted with an aftermarket "balanced" Moon Audio Silver Dragon cable with a 4-pin XLR connector at the amp end. I also use the NAD Viso HP50 headphones in this system. Up until now, the best sound I'd achieved from either of those headpones was via the SimAudio Moon Neo 430HA headphone amp. The SimAudio was Tyll Herstens' reference headphone amp until his recent retirement from InnerFidelity. To get this sound it was necessary to drive the SimAudio amp via balanced analog audio connections from the Benchmark DAC3. Since the DAC3 does not have two separate balanced output busses, I found it necessary to use a second Benchmark DAC3, the DAC3 DX, dedicated just to driving the headphone amp. For the Audeze phones, it was also necessary to use the balanced output for best sound. For more on this saga, see my "Doubling Down on the Benchmark DAC3" discussion.
All the non-soldered headphone connections are treated with Caig Deoxit Gold G100L, the brush-on variety. See my headphones thread for further discussion of the Audeze/SimAudio and my prior headphone set ups.
The following comments apply both to the Audeze LCD-4 headphones heard via the 4-pin XLR jack and the NAD Viso HP50 headphones heard from the 1/4-inch phone jack. These jacks are both on the front panel of the HPA4, of course. The HPA4 drives both the 4-pin XLR and 1/4-inch phone jack simultaneously if headphones are plugged into both jacks.
Score another home run for the Benchmark HPA4! First, the presence of separate line and headphone output busses on the HPA4, together with the fact that those separate busses are independently volume controllable, means that there is no longer any need to use two DAC3's in my system. The HPA4 potentially replaces both the DAC3 DX and the SimAudio headphone amp.
Second, the HPA4 is more than just a substitute replacement for those two components in terms of sound quality. Heard via the 4-pin XLR jack, my Audeze LCD-4 headphones sound more realistic than ever before, as do the NAD Viso HP50 phones heard via the phone jack. There is yet-better overall tonal balance: the highs seem to have a bit of extra air but no extra brightness, the presence range below that is emphasized a small bit, the bass remains powerful and full, but better defined, the lower mids remain warm but are better defined, and the midrange is clearer and more lifelike than ever.
As with the SimAudio Moon Neo 430HA, there is no lack of current/voltage on tap. Thus the presentation never sounds strained or dynamically restrained even at high SPL with the very-difficult-to-drive Audeze LCD-4 headphones.
The whole presentation sounds more "solid" with the HPA4 than it did with the 430HA, giving the impression that the amp has a more solid grip on the drivers. The presentation gains a more "out front" aspect, with the presentation seeming significantly more in front of my head than it was before, while having equally wide spread. This is especially true of the NAD Viso HP50 headphones, probably because the subjective frequency response of those phones lacks any depression of the presence range and lower highs whatsoever and especially because of the "RoomFeel" response technology built into those phones. (See the excellent Computer Audiophile review of the NAD Viso HP50 for more on the PSB/NAD RoomFeel design.)
Imaging and staging on naturally miked material is rock solid. Simultaneously, the stage presents phase-rich material in studio-mixed productions in a more spread out and ethereal manner, enhancing the feeling of envelopment and diminishing the center-of-head focus so common with headphone listening. Overall, even though the HPA4 lacks any special HRTF processing circuitry, this is the most interesting spatial presentation I've yet heard from headphones using a "normal" headphone amplifier.
Dynamically, everything is kicked up several notches compared to the SimAudio. This amp really lets both the LCD-4s and Viso HP50s shine in terms of presenting sudden shifts in the SPL of the overall presentation, as well as dynamic nuances of any particular instrument from moment to moment. There is overall considerably more punch and thrust to the music compared to the SimAudio reproduction.
No obnoxious hardness, brightness, grunge, or other detectable distortion has been added compared to the SimAudio reproduction. In contrast, the reproduction sounds even clearer and cleaner. That is no mean feat, considering that the SimAudio was already the best I'd heard in these respects.
And that silent background! Music emerges from true blackness. There is absolutely no residual noise at any volume setting. This is true even with the quite sensitive NAD Viso HP50 headphones. The NADs are fully sensitive enough to be easily driven to very satisfying volumes via a smart phone. When both are plugged into the HPA4, I would judge the NAD phones to be 10 to 20 dB louder for any given setting of the HPA4 volume control.
Conclusion
The Benchmark HPA4 is the best sounding line amplifier I've yet heard. It's innovative approach to controlling system volume/SPL results in what for me is a uniquely transparent-to-source experience. It is also the best sounding headphone amp I've heard, both for very-insensitive, hard-to-drive phones and those sensitive enough to easily be used directly from a smart phone. The HPA4 truly stands apart in terms of measured and subjectively low background noise and distortion when driving either loudspeakers or headphones.
While not cheap, I rate its subjective value as extremely high. In my system, this $3,000 component sounds better than the $6,500 total of the Benchmark DAC3 DX and SimAudio Moon Neo 430HA which I used as my previous-best headphone front end, and that's not even considering that the HPA4 also makes the music played through speakers sound more realistic. I more than recouped the price of the new HPA4 through sale of the used DAC3 DX and 430HA which the new HPA4 replaced.
What's not to like about the Benchmark HPA4? The only thing I can think of is the clicking/chattering noise produced by the operation of the relay-controlled volume control. I suppose that this could be a deal-breaker for a few. I cannot guarantee that your reaction and accommodation to this noise will be the same as mine. I hardly notice it anymore and it certainly is no longer distracting to me. If that mechanical noise is necessary to get this level of sound quality, bring it on!
Some experts believe that modern amplifier technology has reached a state of development such that the only real differences are related to subtle frequency response differences caused by the varying interaction of speaker loads with amplifier output impedance. From this viewpoint, once such frequency response differences are eliminated, most modern amps not driven into overload sound basically alike.
However, I believe that non-frequency-response effects can help the listener differentiate among amps. Two such effects are the signal-to-noise ratio and distortion levels.
While you may think that any amp whose quiescent residual noise you cannot hear from your usual listening position is quiet enough, that has not been my subjective experience. This is one reason I'm now a big fan of the Benchmark equipment, including the AHB2 amp. I've been using a pair of them in bridged mono, as well as the Benchmark DAC3 HGC to drive my Harbeth M40.2s for awhile now.
With this combination of electronics, for the first time in my audio life I can pin my ear to each speaker driver and hear absolutely no sound coming from the speakers in a quiet room with the system volume control at any position from minimum to maximum with no source material playing. Most amps produce quiescent noise under such conditions loud enough to be clearly heard at least a foot from a speaker driver, often more like several feet. The Sanders and Behringer electronics driving the Sanders 10C speakers I once owned, combined with the high sensitivity of those speakers, meant I could hear hiss from the speakers immediately upon entering the listening room, a good 10 feet from the electrostatic panels, even with the volume control at minimum. The amps in the Janszen Valentina Actives were noisy enough that I could sometimes hear a trace of hiss from my listening position about 55 inches from the electrostatic drivers, and that was after careful gain structuring of the system to minimize that hiss.
I find that lowering the background noise paints the music on as black a canvas as possible and thus seems to bring out realistic low-level detail in recordings. While I may not actually hear residual amp noise as hiss/hum/rumble/roar, I more clearly hear the environmental ambient noise captured on the recording if the amp noise is lowered and this increases my perception of hearing the music in its recording venue, as well as my perception that the music is being made by real humans producing little noises like forceful breathing in rhythm, sub-vocalizations, lips parting, risers and chairs creaking, etc.
Benchmark's raft of published tests, as well as published test reports, show that the Benchmark equipment also exceeds most other audio equipment in terms of lower distortion of various types. See, for example, Stereophile's review which includes John Atkinson's measurements and comments about the AHB2 amp. This is probably why the Benchmark equipment reaches new highs in my experience in terms of allowing the music to sound cleaner and clearer than ever before, without any hint of brightening. Residual electronic artifacts are wiped away to an unprecedented extent.
The Pesky Volume Control
Once you have the expertise and technology to allow your digital decoding and amplification chain to reach these extremely low levels of noise and distortion, the weak spot in the system may well be the lowly volume control. You can't live with most, but you certainly can't live without any!
The difficulties of designing a method of controlling system SPL/volume without simultaneously adding distortion and/or noise is explained in the Benchmark Application Note, "Relay-Controlled Volume – The Ultimate Solution For Analog Audio." Controlling the volume in a way that did not "spoil" the performance of which the Benchmark DAC3 HGC and AHB2 amp are inherently capable was one of the two raisons d'etre of the newest Benchmark product, the HPA4 Headphone/Line Amplifier. Only through designing a new type of volume control could the low noise and distortion of the AHB2 amp remain uncompromised while still allowing precise control of system SPL. Even the method of controlling the volume used in the DAC3 HGC I've raved about is a bit compromised. To quote just a bit from the Application Note:
Digital DSP multipliers can achieve near-perfect transparency if enough of the output bits are preserved and delivered to a D/A converter with sufficient resolution. Any significant volume reduction would place difficult demands on the SNR performance of the D/A converter. In our analog-to-analog application, an A/D converter would need to be added at the front end. Any significant volume boost would place difficult demands on the SNR performance of the A/D converter. In any practical system, the converters are the performance limitation. This is why we provide analog attenuators at the output of all Benchmark DAC1, DAC2, and DAC3 converters. The attenuators match the nominal output level of the DAC to the downstream device (usually a power amplifier). This matching minimizes the use of the digital volume control. In our analog-to-analog application, the cascaded A/D-D/A would add excessive THD and noise.
In the end, for the HPA4, Benchmark:
. . . settled on a very low impedance 16-step gain stage that feeds a very low impedance 256-step attenuator. Each channel uses 12 DPDT gold-contact relays and 64 high-precision resistors. All stages are fully buffered and fully balanced. The balanced design provides a 3 dB SNR improvement while increasing the immunity to interfering noise sources. The buffers provide high impedance inputs and low impedance outputs. This means that the Benchmark gain control places very little load on upstream devices while providing ample drive for downstream devices. The buffering also allows the use of very low resistance values and this reduces the thermal noise produced by the attenuator.
Benchmark's literature claims:
The HPA4 is the perfect complement to the Benchmark AHB2. It may be the only preamplifier or line amplifier that exceeds the signal to noise ratio of the ultra-quiet AHB2. This means that the HPA4 will extract the full performance of the AHB2. In contrast, other preamplifiers limit the system noise performance because they cannot match the SNR performance of the AHB2. The HPA4 is the only line amplifier/preamplifier that we recommend inserting between a DAC and the AHB2. The HPA4 can be inserted between a Benchmark DAC3 and AHB2 without degrading the system noise performance. With the HPA4 inserted, the DAC3's 32-bit digital gain control is replaced by the HPA4's more-resolving fully-analog relay gain control.
Driving Demanding Headphones Like the Audeze LCD4
Ultimately the power/voltage demands of low sensitivity, high-current-demanding headphones like my favorite Audeze LCD-4 forced me to use the very powerful SimAudio Moon Neo 430HA headphone amplifier to drive them, rather than the headphone amp built into the DAC3 HGC. This, in turn, forced me to use another cascaded DAC3 to optimally feed that separate headphone amp a balanced analog audio signal. For details on this saga, see, in particular, my post, "Doubling Down on the Benchmark DAC3: Adding the DAC3 HGC."
Now, as I noted in my very next post in that thread, "Benchmark HPA4 Headphone/Line Amplifier," with the release and delivery of the HPA4, I potentially can eliminate a lot of complexity in the system hook up and sell both the Benchmark DAC3 DX and SimAudio Moon Neo 430HA headphone amplifier.
Set Up
The user's manual for the HPA4 is online here. Besides detailing the unprecedentedly low noise and distortion of the HPA4, the thorough instructions in the manual will give you a very good idea of how to set up and use the innovative preamp interface it offers. But here are my suggestions for optimally using the HPA4 together with the Benchmark DAC3 HGC DAC and Benchmark AHB2 amp/amps:
- If you are using the -10 dB or -20dB XLR output pads of the DAC3, your first set up step is to remove the cover of the DAC3 and move the jumpers so as to disable those output pads. You want the DAC3 set to have no XLR output attenuation, the 0 dB setting for the internal jumpers. You should use a grounding wrist strap while performing this work inside the DAC3 case. Ground your wrist to the back panel silver metal of the DAC3 chassis. When finished, put the cover of the DAC3 back on.
- Use the HPA4's balanced XLR analog Input 1 for connecting the DAC3 HGC to the HPA4 and the HPA4's balanced XLR left and right analog outputs to connect the HPA4 to the balanced XLR inputs of the AHB2 amp/amps.
- Use Benchmark Studio&Stage StarQuad XLR Cables to make these connections. Really. Accept no substitutes. These use the specific brand of starquad cable (Canare 4E6S) found by Benchmark to have the greatest ability to reject magnetic interference (non-starquad cables have no ability to reject magnetic field interference, but different brands of starquad cables vary in their resistance to magnetic field interference) and the best Neutrik XX XLR connectors and strain relief, together with silver solder. These sound better than any other XLR cables I've used, are extremely flexible and thus will not put strain on the connection points or conduct mechanical vibration between components, and are also relatively inexpensive at about $72/meter pair.
- As long as you connect the DAC3 HGC to the HPA4's Input 1, a single Benchmark remote control will seamlessly control both boxes. To fully enable this compatibility, also go to the Remote settings screen on the HPA4 and select "BENCHMARK DAC ON XLR1."
- Bypass the hybrid analog/digital volume control of the DAC3 HGC. You do this by enabling Home Theater (HT) mode on each the DAC3's inputs you are using, in my case, the two coaxial digital inputs, D3 and D4, and the USB input. You do that by selecting each of the inputs you use and then pressing and holding the mute button on the DAC3's front panel or remote for at least three seconds. The HT light on the front panel of the DAC3 will illuminate and the motorized volume control will automatically advance to about 2:00 or higher. Select the next input you use on the DAC3 and do this again. Do this for each of the DAC3 inputs you use. You have now bypassed the DAC3 volume control since the DAC3 output is now set to its calibrated maximum level for each of the inputs you use and will stay there regardless of the position of the HPA4 volume control.
- The easiest way to initially set up the HPA4 is through the touch screen on the front panel. After that, you can access most settings from the remote control if you like.
- Set the HPA4 screen brightness. The screen has 15 brightness levels from full off 0 to full on 15. Set the timer for dimming the screen in a selectable number of seconds. I like 10 or 20 seconds. Set the dimmed screen brightness for a low level, preferably off for a guarantee of best sound. If you frequently do your serious listening in a semi-darkened room as I do, I would also suggest experimenting with setting the undimmed screen brightness at some level below the maximum of 15. I currently find 10 to be the best level for my application in terms of both being clearly readable in all lighting conditions and not overly distracting in a semi-darkened room when the screen pops on because I'm adjusting the volume. Or, if you don't want or need to see the screen during volume changes, you can change the settings so the screen remains DIM while you adjust the volume. This setting is found on page 2 of the Display settings (HPA4 Manual, page 17).
- Two volume control bars are visible on the screen: headphones on the left and speakers on the right. The On button of the remote control will toggle among the three settings of how the volume control operates: both simultaneously, headphones only, or speakers only. When listening to speakers, for guaranteed best sound I lower the headphone volume to its off position, mute the headphone output, and then select speakers-only volume control. That way, the amp is not trying to drive the headphones at all when listening to the speakers. For headphone listening, I fully lower and then mute the speaker volume control, and then select headphones-only volume control.
- The dB display of the volume is too small to read from my listening position, but the bar graphs of the volume control are quite precise and easily interpretable from my listening seat. Don't be surprised if the volume control is usually running in the top quarter of its bar-graph range for serious listening. It does that for both my Harbeth M40.2 speakers and Audeze LCD4 headphones. Even with my much-more-sensitive NAD Viso HP50 headphones, the normal listening range is in the top 40% of the volume range. With the Harbeths, I usually find that no music can be heard at volume settings of -85 dB or lower and that serious listening levels are at -22 dB or higher. The volume control will go up to +15 dB.
- Even though I only use balanced XLR Input 1 of the HPA4, I can still select the three inputs of the DAC3 HGC I use by selecting the appropriate input USB, D3, or D4 with the remote control buttons. These buttons control only the DAC3 input selections, not the HPA4 input selection.
- Input names for both the HPA4 and DAC3 can be renamed. Thus, on the HPA4, I have renamed the DAC3's USB input as "G2 USB," the D3 input "G2 COAX," and the D4 input "OPPO." I renamed the HPA4's Input 1 as "DAC" since that input is connected to the Benchmark DAC3 HGC.
- For best headphone sound, use the four-pin XLR connector on the front panel of the HPA4, rather than the 1/4" TRS jack on the HPA4 or ("worse" yet) either of the TRS jacks on the DAC3. The four-pin XLR connector has no chance of producing measurable distortion and the HPA4 produces far more output current/voltage/power than the DAC3.
- Expect to hear quite-audible-from-the-listening-seat rapid clicking/chattering when adjusting the volume up or down with either the remote or front-panel volume controls. That's the sound of the HPA4 relays. This sound/noise is a bit alarming/distracting at first, but I rapidly got quite used to it. It's louder if you are adjusting both the headphone and speaker volume simultaneously, but that would usually be rare. If your SPL is already moderate or higher, the sound of the music may well cover up the sound of the relays. And if you are wearing closed-back headphones, you won't hear it at all. Just to be clear, this noise is purely mechanical and comes from the HPA4 chassis. There is no electronic clicking noise audible at all through speakers or headphones when the volume is adjusted.
- Give it a solid foundation. As a fully analog piece of equipment with a lot of moving parts, it may be quite susceptible to structure- and air-borne vibration. I avoid using stiff power or interconnect cords to prevent conducting vibrations through these paths. I also have mine sited just above the hardwood floor. It sits atop a combination of two Bright Star Audio IsoRock non-resonant constrained layer platforms, with the larger bottom platform supported from the floor via three Bright Star IsoNode extra large hemispherical soft polymer feet. The upper IsoRock platform has additional damping in the form of added felt to the bottom of it which rests atop the lower Little Rock platform. The top of the HPA4 is damped by the placement of a copy of The Absolute Sound magazine atop it. The DAC3 HGC sits atop the magazine and anther copy of The Absolute Sound magazine damps the top cover of the DAC3 HGC.
- Tweak the connections. Except for the internal relays which are constantly cleaned by motion, all non-soldered connections—fuses, jacks, plugs—are treated with Caig Deoxit Gold G100L, the brush-on liquid contact enhancer. All non-used XLR and RCA jacks are covered with Cardas caps, except for one. That one non-used RCA jack has the RCA jack version of an EVS Ground Enhancer attached.
Speaker Sound
About all Benchmark's literature claims for the sonics of the HPA4 compared to using the DAC3 straight into their AHB2 amp is: " With the HPA4 inserted, the DAC3's 32-bit digital gain control is replaced by the HPA4's more-resolving fully-analog relay gain control."
So "more resolving" is how Benchmark sums up the benefits. A modest claim.
Does the HPA4 deliver the goods? Does it work as advertised? Is it really "more resolving"?
For years I'd found that eliminating the analog preamp was the surest way to get better, purer sound. I've done many comparisons of a volume-controlled-DAC-to-amp chain with a chain injecting an analog preamp between the DAC and amp. Never before did the addition of an active analog preamp gain me more than it lost. The sonic comparisons weren't even close. Whatever extra dynamics might have been gained by inserting an active preamp were far outweighed by added distortion, noise, lack of clarity, less expanded and more ambiguous space, more ambiguous image placement, brightness/grain/grit in the highs, volume control tracking problems changing the left/right balance, and a significant loss of transparency—recorded information was gobbled up by the added analog stage.
And I've tried my share of passive preamps as well, from several iterations of the old Mod Squad Line Drive (remember that?) to the best balanced nude Electronic Visionary Systems Ultimate Attenuators (the pic is of the RCA variety, but you get the idea). These deliver a certain undoubted purity of sound, but in the end are disappointing in terms of seemingly truncated dynamics, less bass slam, and lack of high frequency air and extension even when used with the recommended short, low capacitance cabling.
But now the rules apparently have changed. Most volume controls must truly be terrible sounding compared to this one. The HPA4 does indeed deliver the goods and is yet "more resolving" than the prior Benchmark chain in my system.
In one fell swoop, adding this line amplifier between my Benchmark DAC3 HGC and pair of bridged mono AHB2 amps creates the following sonic improvements:
- seemingly expands dynamic range, both in terms of the whole dynamic envelope of soft to loud at any given point in the music, and in terms of microdynamics from moment to moment from a given instrument, voice, or groups of instruments or voices
- adds no additional noise—those blackest backgrounds stay that way—and adds no electronic glaze/hash/grit/brittleness etc—in this respect it acts like a passive preamp; unlike many other analog volume controls I've experienced, there also are no problems with volume tracking between the two channels at any setting of the control
- quite significantly improves high frequency resolution and differentiation; the highs are much more filigreed, distinct, and—where the material lends itself to naturalness (as in recordings of acoustic music)—more naturally balanced, more analog, with less tizz
- bass is more differentiated with no loss of extension or impact; sliding electric and acoustic bass tones are more clearly sliding up and down in frequency
- adds yet more clarity, cleanliness, and natural low-level detail to the sound without a trace of added brightness
- better differentiates the recording quality of different recordings, still without adding spurious brightness
- enhances front-to-back stage depth both in terms of specificity of placement of individual images on that front-to-back stage and in terms of expanding the apparent depth of the stage from front to back
- adds a yet more analog/relaxed feel to the music playback—there remains nothing at all "digital" about the reproduction in the bad sense. As to this criteria, I recently added comments about the relative sound of the USB and coaxial output of the Auralic Aries G2 device as heard through my system chain which now includes the Benchmark HPA4. See those comments for more on this sonic aspect.
Headphone Sound
My reference headphones are the Audeze LCD-4 fitted with an aftermarket "balanced" Moon Audio Silver Dragon cable with a 4-pin XLR connector at the amp end. I also use the NAD Viso HP50 headphones in this system. Up until now, the best sound I'd achieved from either of those headpones was via the SimAudio Moon Neo 430HA headphone amp. The SimAudio was Tyll Herstens' reference headphone amp until his recent retirement from InnerFidelity. To get this sound it was necessary to drive the SimAudio amp via balanced analog audio connections from the Benchmark DAC3. Since the DAC3 does not have two separate balanced output busses, I found it necessary to use a second Benchmark DAC3, the DAC3 DX, dedicated just to driving the headphone amp. For the Audeze phones, it was also necessary to use the balanced output for best sound. For more on this saga, see my "Doubling Down on the Benchmark DAC3" discussion.
All the non-soldered headphone connections are treated with Caig Deoxit Gold G100L, the brush-on variety. See my headphones thread for further discussion of the Audeze/SimAudio and my prior headphone set ups.
The following comments apply both to the Audeze LCD-4 headphones heard via the 4-pin XLR jack and the NAD Viso HP50 headphones heard from the 1/4-inch phone jack. These jacks are both on the front panel of the HPA4, of course. The HPA4 drives both the 4-pin XLR and 1/4-inch phone jack simultaneously if headphones are plugged into both jacks.
Score another home run for the Benchmark HPA4! First, the presence of separate line and headphone output busses on the HPA4, together with the fact that those separate busses are independently volume controllable, means that there is no longer any need to use two DAC3's in my system. The HPA4 potentially replaces both the DAC3 DX and the SimAudio headphone amp.
Second, the HPA4 is more than just a substitute replacement for those two components in terms of sound quality. Heard via the 4-pin XLR jack, my Audeze LCD-4 headphones sound more realistic than ever before, as do the NAD Viso HP50 phones heard via the phone jack. There is yet-better overall tonal balance: the highs seem to have a bit of extra air but no extra brightness, the presence range below that is emphasized a small bit, the bass remains powerful and full, but better defined, the lower mids remain warm but are better defined, and the midrange is clearer and more lifelike than ever.
As with the SimAudio Moon Neo 430HA, there is no lack of current/voltage on tap. Thus the presentation never sounds strained or dynamically restrained even at high SPL with the very-difficult-to-drive Audeze LCD-4 headphones.
The whole presentation sounds more "solid" with the HPA4 than it did with the 430HA, giving the impression that the amp has a more solid grip on the drivers. The presentation gains a more "out front" aspect, with the presentation seeming significantly more in front of my head than it was before, while having equally wide spread. This is especially true of the NAD Viso HP50 headphones, probably because the subjective frequency response of those phones lacks any depression of the presence range and lower highs whatsoever and especially because of the "RoomFeel" response technology built into those phones. (See the excellent Computer Audiophile review of the NAD Viso HP50 for more on the PSB/NAD RoomFeel design.)
Imaging and staging on naturally miked material is rock solid. Simultaneously, the stage presents phase-rich material in studio-mixed productions in a more spread out and ethereal manner, enhancing the feeling of envelopment and diminishing the center-of-head focus so common with headphone listening. Overall, even though the HPA4 lacks any special HRTF processing circuitry, this is the most interesting spatial presentation I've yet heard from headphones using a "normal" headphone amplifier.
Dynamically, everything is kicked up several notches compared to the SimAudio. This amp really lets both the LCD-4s and Viso HP50s shine in terms of presenting sudden shifts in the SPL of the overall presentation, as well as dynamic nuances of any particular instrument from moment to moment. There is overall considerably more punch and thrust to the music compared to the SimAudio reproduction.
No obnoxious hardness, brightness, grunge, or other detectable distortion has been added compared to the SimAudio reproduction. In contrast, the reproduction sounds even clearer and cleaner. That is no mean feat, considering that the SimAudio was already the best I'd heard in these respects.
And that silent background! Music emerges from true blackness. There is absolutely no residual noise at any volume setting. This is true even with the quite sensitive NAD Viso HP50 headphones. The NADs are fully sensitive enough to be easily driven to very satisfying volumes via a smart phone. When both are plugged into the HPA4, I would judge the NAD phones to be 10 to 20 dB louder for any given setting of the HPA4 volume control.
Conclusion
The Benchmark HPA4 is the best sounding line amplifier I've yet heard. It's innovative approach to controlling system volume/SPL results in what for me is a uniquely transparent-to-source experience. It is also the best sounding headphone amp I've heard, both for very-insensitive, hard-to-drive phones and those sensitive enough to easily be used directly from a smart phone. The HPA4 truly stands apart in terms of measured and subjectively low background noise and distortion when driving either loudspeakers or headphones.
While not cheap, I rate its subjective value as extremely high. In my system, this $3,000 component sounds better than the $6,500 total of the Benchmark DAC3 DX and SimAudio Moon Neo 430HA which I used as my previous-best headphone front end, and that's not even considering that the HPA4 also makes the music played through speakers sound more realistic. I more than recouped the price of the new HPA4 through sale of the used DAC3 DX and 430HA which the new HPA4 replaced.
What's not to like about the Benchmark HPA4? The only thing I can think of is the clicking/chattering noise produced by the operation of the relay-controlled volume control. I suppose that this could be a deal-breaker for a few. I cannot guarantee that your reaction and accommodation to this noise will be the same as mine. I hardly notice it anymore and it certainly is no longer distracting to me. If that mechanical noise is necessary to get this level of sound quality, bring it on!
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