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Neutral and Natural, The Auditor

A Review On: SPL Auditor

SPL Auditor

Rated # 88 in Desktop Amps
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Review Details:
Audio Quality
Posted · 4387 Views · 2 Comments

Pros: Very neutral and accurate sound

Cons: Can't drive anything under 80Ohms that well.







Hi all, Zombie-X here again with another review for the good boys and girls of Head-Fi.org. Up today I have the SPL Auditor, one of SPL's own high end headphone amplifiers that is intended for studio use. I've had this amp for about 6 months and it's quite impressive. Truly it's sound is high end. But you want to more, don't you?


I would love to give a big thanks to both Robert at Aphrodite Cu29, and all the employee's at Front End Audio for supplying the SPL Auditor as well as information and guidance. These guys are really helpful and have a lot of insight into the unit's. I'd also like to give a shout out to SPL for supplying me with more information as well as granting me the use of images in this thread.


During this review I will be comparing to my other amps: Audio-GD ROC, Woo Audio WA3+ (modded), and the Musical Fidelity X-CANV8P. Each amp will be compared quite extensively and will be tested with a wide variety of headphones. The Audio-GD ROC and SPL Auditor will utilize the XLR output from my Violectric DAC V800 while the WA3+ and X-CANV8P will be feed by the RCA Output from the DAC V800. The RCA output will be using Audioquest RCA splitters and the XLR output will be going through the SPL Auditor to the Audio-GD ROC.




SPL, or Sound Performance Lab, is a leading manufacturer of studio gear based out of Germany. All their equipment runs on their famous 120V technology. They manufacture lots a gear ranging from equalizers to headphone amps, even software plug-ins. All of their gear is high end and is for studio use, but does not seem to exhibit the typical studio sound, that being the music is dull, lifeless, and clinical. In fact from the gear I have tried their stuff is pretty natural sounding.






Auditor Headphone Monitoring Amplifier
The Auditor is our interpretation of a high-end headphone amplifier. We strictly followed a purist approach to achieve the highest sound quality.
The basis for such high-end developments is our proved 120 volts reference technology. Core elements are the handmade SPL SUPRA op-amps running on 120 volts – it corresponds to approximately twice that of most modern analog audio semiconductor technologies. Through such 120 volts circuitry and processing we reach performance levels far beyond conventional designs in dynamic range and distortion levels, and such technical specifications exceed all known analog or digital standards. In other words: simply sounds.
Why Headphones?
There is more than meets the eye in working with headphones. For example the modern audio production is often a decentralized process, maybe recording and track laying in one studio and mixing in another and as a result a production often take place in acoustically questionable rooms. In such circumstances, a mix might occur in an acoustically deficient ambiance (for example, in an extremely modal room), and employing headphones then begins to make sense when a successful mix would otherwise turn out to be impossible. Another fact is that many musicians or producers might wish to – or be able to – mix at home (to say nothing of having to). Then the headphone becomes a clear must, enabling an evening or late night session that can only take place thanks to its being unhindered by the local acoustic environment.
Acoustic Magnifier
But also when working with full range monitors in the studio, headphone monitoring is an extremely important alternative to loudspeaker monitoring: analytical monitoring via headphones offers a very high precision to observe details. Headphone monitoring is like working with an acoustic magnifier, excluding external room influences. Working with the magnifier effect of headphones has the advantage of safely hearing clicks or similar defects and helps in fine tuning crossfades or to judge tonal problems in individual tracks.
The End of Ear Fatigue
With headphones, ear fatigue can begin relatively soon, and there are several causes. First, some cans themselves may not be that comfortable to wear...
Moreover, a standard headphone amplifier is often an additional important reason for premature ear fatigue. Almost without exception, present-day headphone amplifiers employ comparatively undemanding IC’s. In the best cases they might work with symmetrical voltages of +/-15 V to +/-18 V, and in less favorable cases, with only a simple supply of 9 or 12 V from cheaper external “wall-wart“ power supplies. But the voltage level acts in circuitry much like the cubic inch capacity to the productive power of a combustion engine: Cubic inch capacity is replaceable with nothing but more cubic inch capacity – and in the productive power of electronics, voltage level functions similarly.
For some years, now, SPL has addressed this issue in all of its mastering product series through its own specifically developed 120 volt technology. Consoles and signal processors of the SPL Mastering Series appear as central elements in installations of today’s most renowned mastering houses (e. g. Bob Ludwig’s Gateway Mastering & DVD in the USA, Simon Heyworth’s Super Audio Mastering in Great Britain or the Galaxy Studios in Belgium). This 120 volt technology is based on discrete operation amplifiers from SPL’s own production, developed and perfected over many years by SPL’s co-founder and chief developer, Wolfgang Neumann. The SUPRA OPs have a signal to noise ratio of 116 dB and offer a 34 dB headroom – that yields an unequalled 150 dB dynamic range.
The musical result is not to be mistaken: Regardless of the monitoring means, regardless of how loud you monitor – the Auditor always remains a distant, impartial factor unaffected when used to capacity and beyond being overloaded. The phase stability is always perfect, its THD next to immeasurable. The SUPRA OPs cannot be stressed in the most stressful circumstances, and for precisely this reason its musical sound is always relaxed and spacious. All frequencies are reproduced in balance, basses are stable and tight, mids are clear and differentiated and highs remain transparent and soft.
Such supreme and heretofore unreachable neutrality in audio reproduction is the direct consequence of our technical approach and basis in 120 volt technology: Possible disturbances from such as noise or distortion are so slight that we even arrive at the boundaries of the best measuring equipment, and what remains is quite simply unaltered musical sound.








After getting this amp from Front End Audio, I noticed the size of the box itself. It's quite big actually! The amp is housed in a specially designed box, and you have to open it up a certain way. In my anxiousness I just used a razor blade and sliced the box open. The amp itself is quite big and despite it's size it's not that heavy at all. The finish of the amp is quite nice and the matte finish looking striking to say the least. The Alps potentiometer is buttery smooth and feels "right" when you turn it. All the jacks in the back are of high quality and use Neutrik parts, notably the XLR in puts and outputs. The power switch on the back  is also nice and had a good click to it when you flip the unit on.


When turning the unit on, have the volume all the way down and make sure no headphones are plugged in. When powering down the unit be sure to lower the volume and disconnect the headphones before power down. Not doing so will result in no damage to the headphones or gear, but the amp will discharge residual voltages which will  make the headphones "beep" a few times or until all the voltage has been discharged. Like I said this will not harm your gear at all but it's a nice precaution in doing so. I always do this.


The XLR outputs in the back are for a direct output, so a line level signal. They are independent from the volume control and are quite useful if you plan on daisy-chaining a couple amp or other equipment together. You can also use RCA cables on this amp but they will require the use of an adapter. Using the RCA cables with give you have the voltage of a normal XLR cable so the volume will have to e turned up more. I have tried it both ways and using RCA input will hinder this amp as it's designed for taking a balanced signal. I recommend a balanced DAC or other source if you plan on suing this amp.


You can also initiate a "Unity Gain" mode by turning the volume to 0. This will give you a non-attenuated signal, but you need another way of adjusting the volume earlier in the chain. Unity Gain mode will give you an unaltered signal and as such sounds a tad more full and slightly slower sound. It's a nice feature but is dependent on something else to adjust the volume before hand.



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This amp in a few words is neutral,  transparent, and faithful. There is nothing added or subtracted from the sound at all. No treble spikes or dips in the treble. The typical studio sound, as described earlier, is cold and analytical. Well you know what? If doesn't have any of those issues. It maybe ultra transparent and very neutral, but despite those it is quite natural sounding. Also there is a stigma that German gear tends to sound bright and thin and clearly this amp does not exhibit these issues at all. The amp will only sound colored if the source or DAC is colored. If your DAC is clinical, the amp will sound clinical. It really shows the flaws in anything higher in the chain. You can even hear if your other gear is not up to snuff. Quite a nice feat and in my opinion the amp should be the last thing in the chain to color the sound.


This amp has an output impedance of 9Ohms, and as such requires a headphone of 72Ohms or more. This is usually the case but not this time it seems. I have tried numerous IEM's and portable, low impedance headphones and they did perform nicely. One thing I have noticed is that the impedance mismatch causes the sound to become fuller sounding, but it's not bad in anyway. In fact I like how this amp sounds with my Shure SRH-440 (with SRH-840 ear pads). It gives the Shure more bass impact without sacraficing control. Typically the sound of a mismatch between amp impedance and output impedance makes the sound muddy but I ahve not heard this with this amp as of yet.


The amp itself is geared toward 600Ohm, which according to SPL, is the industry standard. As such the amp can output 1.7W into a 600Ohm load and can swing 120V. That's a huge voltage swing and this amp can drive the 600Ohm beyer dynamics quite well. The 600Ohm Premium headphones are typically quite hard to amp as they needed a much higher than normal voltage to get loud and retain control. I'm glad to say this amp drives them all quite well, no that's an understatement. It drives them effortlessly and perfectly. The DT990 needing a bit more power than the DT880 in order to retain control over the bass, and this amp delivers. The bass remains tight and punchy without sounding muddy or bloated. In fact this amp may be the best $1,000 choice for the 600Ohm beyerdynamic models.


The K501 sound exceptional from this amp. As many know, the K501 is quite hard to drive well as it is lower sensitivity and higher impedance. Safe to say this amp gets them loud and then some. The volume is only at a bit over 1/4 of the way up and the headphones are quite loud. It's not gain, but pure voltage that gets them there so fast. On my other amps the volume has to be almost half of the total volume the amps could do. The SPL Auditor really delivers the bass on this headphone as well. When properly amped the K501 has quite a bit of bass, and it's deep, tight, and controlled. The whole sound opens up and the soundstage expands further.


The K400 is even harder to amp than the K501 and this amp does not disappoint at all. It drives the K400 effortlessly and without fuss. The K400 requires a bit more power than it's K501 sibling, but this amp handles it no problem. You immediately hear the bass is tight and punchy, the mids are detailed, and the treble is extended and grain free. The soundstage opens more, same as with the K501. When the K400 is underamped it can sound anemic in the bass but when most bass heavy of the current AKG audiophile headphone line.



The treble on this amp is very well extended and never bright or strident. Completely grain free and smooth. The treble itself is quite clear and highly detailed. Out of my other amps this amp amp certainly has the most refined treble an is the most detailed treble. Extension is quite high and although you can't quite hear that high, you can tell it's extended. 



The midrange is this amps strong point no doubt. It's easily better than my other amps in this regard, and you don't even loose or sacrifice anything. The midrange itself is highly detailed and resolving. I can hear stuff in the mids that I have yet to  hear before. Examples would be coughing in the background of tracks, light breathing, or someone else talking in the background. Guitar's are more, what's the word, distinct. You can clearly hear how complex some riffs are and even light tapping of the fret board is easily heard.



Bass, bass, and bass. The amp has some of the deepest and most controlled bass I have yet to hear. It reaches really deep and grabs you by the throat. Remember this amp is neutral, but the bass is so pure and can sound visceral. The texture and ability of resolved complex bass passages of guitars is quite astounding to me. If your music is heavy on the bass, this amp will deliver it in gobs, but if light then it will give you light bass.



The soundstage is really good on this amp, yet again it has the biggest I have ever heard. Quite easily the most open of my headphone amps, though my ROC does come close. It's pretty three dimensional and has excellent depth, height, and width. Everything snaps into place when listening to music and sounds coherent and distinguishable.






The SPL trumps the ROC in almost every way. The ROC itself is a warmer sounding solid state amp that is toted as being a "wire with gain" amp by Audio-GD. For the most part they are right, but the amp is slightly warm with a slight emphasis on the mids and bass. The ROC falls short in terms of transparency and micro detail. With the Auditor, small details like light footsteps and faint breathing can be slightly masked. The amps itself is still quite good but is not as detailed and resolving as the Auditor. When comparing the two it's as if you are looking through a slightly tinted window with the ROC, but with the Auditor it is like the window is open. The bass on the ROC is deep and tight with a slight bump to it and a little "roundness" in it.


Another thing is how the ROC handles various loads. The ROC is optimized for 100-300Ohm loads from it's power output chart, but it does a better job with low impedance headphones such as IEM's. The ROC has better control of these headphones due to it's lower output impedance. The SPL Auditor has a 9Ohm output impedance and as such has a lower damping factor which makes it more suitable for higher impedance headphones of 80Ohms or more.



These amps sound totally different. The SPL Auditor is dead neutral and transparent while the Woo Audio WA3+ is really warm and generally has a more lush and smeared sound. Smeared may be a bad word to use but what I mean is that the sound more or less blends together, aka smear. The WA3+ is very musical and warm, but it loses detail and extension on both ends due to the tubes. The tubes hinder the treble a bit and cause it to roll off. Even when using the neutral 7236 tube type the sound can still sound warm. It's just the nature of the amp. The amp also adds more bass to the mix and is a typical tube bass that sounds more rounded, aka full. The soundstage is still quite good but the ability to pin point stuff in the stage can be difficult as tubes blend the sound a bit. Besides that the soundstage is rather large.


The WA3+ is OTL by nature and that means it has a high output impedance. The OTL design will give you a higher voltage swing and the higher output impedance will make a good damping factor for high impedance headphones. The WA3+ does not do all that good with low impedance gear as the output impedance in the ballpark of over 60Ohms, and the amp can't manage much current output. The sound on low impedance headphones will sound mushy and bloated. In this case the SPL Auditor is a better choise, but still not ideal.



The X-CANV8P is one of the most versatile amps I have heard. It has an output impedance of less than 2Ohms, which means it can drive most headphone with a very linear sound. The sound of this amp is warm and musical, no doubt it's from the tubes inside. The amp is a hybrid design, in which the sound passes through the tubes and then goes through a solid state output buffer to the headphones. You get the tube sound and power of solid state. The amps also suffers from the tube smear as described above in the WA3+ description.


The amp also has a high damping factor and is good for low impedance gear. It fairs well with higher impedance headphone like the HD600 but falls flat with the 600Ohm DT880 and DT990. It sounds good with them but doesn't have enough voltage. With low impedance headphones this amp really shines. It can power all my IEM's really well and give the Shure SE215-K a boost in bass control and definition. The T1 also sounds nice on this amp but it doesn't sound that dynamic and more flat.


The soundstage is good but not great. When using the stock power supply it's only good at best. When using the Little Pinkie V3i PSU from Rock Grotto the soundstage becomes much bigger and precise. Still smaller than the SPL Auditors giant soundstage but still rather good.




The SPL Auditor is darned good amp for the price of $999. It has one short coming and that's the high output impedance and ability to properly drive low impedance headphones effectively. They don't sound bad, but it's not ideal to have that high of an output impedance for low impedance headphones. If you are in the market for a solid state amp that can drive your 600Ohm beyers effectively and without fuss, then this is the amp for you. It drives them all incredibly well, even perfectly I would say. It's easy for me to recommend this amp to anyone that has a 300Ohm-600Ohm headphone, especially if the 600Ohm headphone in question are DT770, DT880, or DT990.





Is the output impedance 9 or 90ohms? I thought it was 9ohms, which is still kind of high. I am concerned as I have some 25ohm AH-D7000 on their way!
Yes, it is 9 Ohms, and it will probably not suit Low impedance phones such as your ATH...
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