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Review and Measurements of Neurochrome HP-1 High-Performance Amp

amirm

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#1
This is a review, detailed measurements and comparison of Neurochrome HP-1 high performance headphone amplifier to Massdrop THX AAA 789. It is on kind loan from company designer and owner, Tom Christiansen. I learned about Tom in chasing a review he had done of a passive volume control. I was very impressed with his depth of knowledge and thorough analysis using an Audio Precision analyzer. Tom is an analog designer, having worked for famous and large leaders in the industry (National Semiconductor and Texas Instrument). He is quite active in DIY audio forum, sharing his knowledge of engineering and audio. Needless to say, I was excited to get my hands on his products to test starting with his HP-1 headphone amplifier given its superlative measured specifications.

The HP-1 comes in all forms from bare PCB to fully populated board and finished package. I received the latter:

Neurochrome HP-1 Headphone Amplifier Review.jpg

The retail price for this version is $1,249. Bare board with the knob and RCA jacks is $225. And there are options in between. See: https://www.neurochrome.com/product/hp-1-ultra-high-end-headphone-amp-2/

The package feels hefty and dense due to inclusion of AC power supply inside the unit (unlike the Massdrop THX 789 and its external switching supply).

Naturally there are both balanced and unbalanced inputs. There are three gain levels (+6, +12 and +20 dB). On output side we have 1/4 inch and XLR headphone jack. As the measurements show later, the power does not change by using the XLR connector.

Let's get into measurements and see how HP-1 compares to its stiff competition, the massdrop THX AAA 789. Given the state of the art that these two products represent, I re-ran all the measurements for the Massdrop again. We are talking about some small performance aberrations that seemingly change in the weather might impact them! I kid of course but it is partly true.

All tests are performed with the XLR input. Output is 1/4 single-ended headphone output unless otherwise noted.

Measurements
As usual, we start with our dashboard. Given three gain levels and infinite choices of input and output levels, you get fairly wide variation in performance. I usually play some to get close to the best performance possible and here is that outcome (same output level as my review of Massdrop THX 789:

Neurochrome HP-1 Headphone Amplifier Measurements.png


Looking to the right side of the 1 kHz FFT (frequency) spectrum we see essentially no harmonic distortion! This is in the "high-performance" mode of my Audio Precision APX555 which uses dual ADCs to essentially suppress the noise and distortions of the signal capture (so don't try it with any hobby level sound card).

The reason SINAD is not some 10 dB higher (limit of my analyzer) is the spikes with have to the left which are from the power supply (harmonics of 60 Hz). Fortunately our threshold of hearing is extremely high in low frequencies so no concern whatsoever. I am left wondering though what a statement this device would have made if those weren't there. As it is, the HP-1 takes a fine position in our ranking of headphone amplifiers when it comes to SINAD (signal over power of noise and distortion):

Neurochrome HP-1 Headphone Amplifier SINAD Measurements.png


I am not happy with my signal to noise ratio measurements as test conditions make a big impact on what value you get. Here is a conservative sample that is close to what I used for massdrop THX:
Neurochrome HP-1 Headphone Amplifier snr bar Measurements.png


If I played with input and output levels, I could get above 127 dB. Going with what we have as is, here is how it compares:
Neurochrome HP-1 Headphone Amplifier snr Measurements.png


For the next graph, we have a new measurement that is SMPTE intermodulation test (60 Hz and 7 kHz tones) versus output power level (300 ohm load). IMD tests like this are not bandwidth dependent like THD measurements and hence can be more revealing:
Neurochrome HP-1 Headphone Amplifier Intermodulation vs Power Measurements.png


I have shown the Massdrop THX 789 performance in dashed lines. The 789 is very well behaved and changing the gain only increases the output power and doesn't impact noise or distortion levels. Highest power is achieved by using high-gain mode which eventually clips as see in dashed green color.

In lowest gain mode the HP-1 matches the noise and distortion levels of 789 but produces even more power the latter outputs in highest gain mode. No clipping occurs here so to get max power, you need to use medium or high gain. Once there, you get over twice as much power as Massdrop THX 789. This is done without any increase in distortion or noise over 789.

The high gain mode in HP-1 doesn't produce more power so it is there for convenience of dealing with lower input levels (either in source hardware or music being played).

Switching to our traditional THD+N versus power at 300 ohm we get our usual power ratings:

Neurochrome HP-1 Headphone Amplifier Power at 300 Ohm Measurements.png


The HP-1 produces nearly 0.4 watts at this very high load impedance which is impressive. As noted in the IMD test, this is 2.4 times more power than Massdrop THX 789 musters.

Running the same test with 33 ohm load we get:

Neurochrome HP-1 Headphone Amplifier Power at 33 Ohm Measurements.png


The HP-1 delivers twice as much power as THX 789.

The THX 789 gets its revenge though in balanced mode where it produces much more power and HP-1 does not:
Neurochrome HP-1 Headphone Amplifier Power at 50 Ohm Measurements.png


Output impedance is comfortably low on HP-1 at 0.8 ohm:
Neurochrome HP-1 Headphone Amplifier Output Impedance Measurements.png


The volume pot is analog so we get some channel imbalance:

Neurochrome HP-1 Headphone Amplifier channel imbalance Measurements.png


If this is an issue for you, get a DAC with digital volume control and change the level there. A quick test of changing levels in my analyzer instead of the volume control on HP-1 showed same performance. I plan to do a more exhaustive study of volume control impact on performance in the future.

The Massdrop THX 789 uses "forward error correct." This is in contrast to negative feedback commonly used. The advantage of forward error correction is supposed to be in higher frequencies where negative feedback can be weak (due to need to reduce gain to avoid oscillation). So I thought I run a frequency sweep of THD versus frequency at same output levels:

Neurochrome HP-1 Headphone Amplifier THD vs frequency Measurements.png


Interestingly, one channel in HP-1 matches the THX 789 in high frequencies. The other (in red) does nose up though.

In reverse, negative feedback works the best for low frequencies and we see the HP-1 pull ahead there.

Note that these are astonishingly low noise and distortion levels so we are nitpicking. And other factors may be impacting the measurements of error this low.

Listening Tests
As always, I started my listening tests with my Sennheiser HD-650. Ah, you have not heard clean bass until you power this headphone with this class of amplification. It is authoritative and at high volumes, it has no trouble making your ear lobes resonate with the music! :eek: Here I started with medium gain but then switched to high-gain for inconvenience for my "audiophile" recordings that have lower levels.

Switching to Hifiman HE-400i of course brings more power due to their lower impedance. You lose some of the bass due to design of these headphones but you get that airy sound that is distortion free until the headphone drivers start to cry (and you do too with ear piercing SPLs).

I recently bought an AKG K92 closed back headphones so I thought I test with those too. Man, I hit play and it scared the hell out of me! These headphones seem more efficient. While not in the same league as the above two headphones, I like them for better isolation. I checked for level level channel balance and it was fine.

I can't emphasize enough how revealing these amplifiers are. As I noted, it obsoleted a lot of "audiophile" tracks I had collected with my speaker system. So many of them have distortions that become so obvious to hear now. For this reason, I hope more recording and mastering engineers invest in this class of headphone amplifiers to better hear (and hopefully fix) the amount of distortion that can exist in their work product.

Conclusions
Boy, are headphone users spoiled with these superlative headphone amplifiers. We are talking objective performance that strains measurement gear. The HP-1 delivers that with superb engineering and design using classical negative feedback method. Using standard headphones (i.e. not balanced), the HP-1 produces more power and as such as a lot more headroom than Massdrop THX 789. Any distortion you hear is the recording or your headphones. It is extremely liberating to have such transparent and powerful amplification in the chain. If you have less efficient, high impedance headphones with unbalanced connections, the HP-1 is the perfect companion.

The down side is the cost. Fully packaged, we are talking four times higher cost than Massdrop THX 789 which in my opinion, is priced too low for what it does. Tom has given up his day job to build this product in very low volumes so can't compete on price. You can reduce the impact here by either building the unit yourself or getting the assembled board and putting it in your own case. You will have a pride of ownership and uniqueness which users of massdrop THX 789 won't have.

Personally and it is of course subjective, I like us as a community to support designs and efforts such as what Tom is doing. Just as you donate money to support ASR Forum, I think it makes sense to spend extra money to make sure we have options in the marketplace. This is especially so given Tom's willingness to help the community by providing knowledge and kits.

Needless to say, on performance side, the Neurochrome HP-1 gets my highest recommendation for headphone amplification.

-----
As always, any questions, concerns, comments, corrections, etc. are welcome.

If you like this review, please consider donating using Patreon (https://www.patreon.com/audiosciencereview), or upgrading your membership here though Paypal (https://audiosciencereview.com/foru...eview-and-measurements.2164/page-3#post-59054). It will enable me to pay for gear that is not offered for loan.
 
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wadec22

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#2
really neat product! very cool to see such expertise outside of the typical big players.

totally respect the small business and craftmanship - but supporting at that price is an awful lot to ask IMHO. Not only can you get the Massdrop unit for 4x less, but you are also competing with units like the RME ADI DAC 2 - a really nice DAC & headphone amp - for less money.

there is a premium to be paid for US made and supporting small business - but most people don't have the disposable income to justify that large a price premium gap.
 

Veri

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#3
"So I thought I run a frequency sweep of THD versus frequency at same output levels"

@amirm in the image there you put "Monoprice" instead of Massdrop THX 789 :)

thanks for the great, in-depth review!
 

restorer-john

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#4
This product's performance is at the point where your THD+N becomes just N.

As such, it may be time for you to finally consider a residual noise test in uV with shorted inputs. Does the AP short the inputs when doing its SINAD test at some point?
 

amirm

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#5
"So I thought I run a frequency sweep of THD versus frequency at same output levels"

@amirm in the image there you put "Monoprice" instead of Massdrop THX 789 :)
Thanks. Corrected.
 

tomchr

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#6
Thank you very much for the review and your recommendation. I really appreciate it.

One note, though: You should have no trouble driving the HP-1 to clipping even at the low gain setting. You'll need about 4 V RMS to do it, but it'll get there. Many pro sources have 4 V out, so that kind of voltage is not out of reach. That said, I generally have my DAC volume control at around -40 to -30 dB with a 4 V output and the HP-1 set to the lowest gain with its volume control cranked. That's enough volume for both my Sennheiser HD-650 and Focal Elex (via Massdrop).

Interesting with the left channel showing higher THD. I've seen that in my measurements as well. I need to double-check the layout, but I'm pretty sure the left channel is the one furthest from the power supply, so the little bit of supply impedance there is across the supply planes is likely what you're seeing creeping in. That's life at precision circuit design. Parasitics matter. :)

I am totally stoked to see the FFT of the 1 kHz tone (yes, I'm really that much of a geek!) Would you mind running the FFT separately? I'd like to see 1 kHz @ 400 mW into 300 Ω with 1M point FFT, 8 averages. Pretty please. :)

I agree with your observations regarding SINAD and SNR/DNR. In a low-noise amp, they both become a measurement of how high output amplitude your amp can produce. I measure SNR just below clipping.

As for the price: Yeah... The HP-1 is expensive. That's the unfortunate part of low-volume production in a small business. Also, in the HP-1, I focused entirely on perfomance without regards to price. My next headphone amp will definitely need to be designed with cost at the forefront of my mind.

Thanks again for the review. I'm looking forward to sending the Modulus-286 Kit to you.

Tom
 
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#8
Thank you for the review Amir! This is a nice looking amp, very industrial and serious looking. Prior to the Massdrop THX amp this price tag would have been fine for this level of performance but it seems that the economy of scale is the enemy here. But it's exciting times! So many heavy hitters lately... even more on the way with the Monoprice THX amp/dac and March Audio's headphone amp offering. *Oh and the JDS Labs Atom is a serious performer too!
 
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#9
Thank you very much for the review and your recommendation. I really appreciate it.

One note, though: You should have no trouble driving the HP-1 to clipping even at the low gain setting. You'll need about 4 V RMS to do it, but it'll get there. Many pro sources have 4 V out, so that kind of voltage is not out of reach. That said, I generally have my DAC volume control at around -40 to -30 dB with a 4 V output and the HP-1 set to the lowest gain with its volume control cranked. That's enough volume for both my Sennheiser HD-650 and Focal Elex (via Massdrop).

Interesting with the left channel showing higher THD. I've seen that in my measurements as well. I need to double-check the layout, but I'm pretty sure the left channel is the one furthest from the power supply, so the little bit of supply impedance there is across the supply planes is likely what you're seeing creeping in. That's life at precision circuit design. Parasitics matter. :)

I am totally stoked to see the FFT of the 1 kHz tone (yes, I'm really that much of a geek!) Would you mind running the FFT separately? I'd like to see 1 kHz @ 400 mW into 300 Ω with 1M point FFT, 8 averages. Pretty please. :)

I agree with your observations regarding SINAD and SNR/DNR. In a low-noise amp, they both become a measurement of how high output amplitude your amp can produce. I measure SNR just below clipping.

As for the price: Yeah... The HP-1 is expensive. That's the unfortunate part of low-volume production in a small business. Also, in the HP-1, I focused entirely on perfomance without regards to price. My next headphone amp will definitely need to be designed with cost at the forefront of my mind.

Thanks again for the review. I'm looking forward to sending the Modulus-286 Kit to you.

Tom
A bit off topic, but if you don't mind me asking: What's the dimensions of the HP-1?
 

rajapruk

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#10
I agree with your observations regarding SINAD and SNR/DNR. In a low-noise amp, they both become a measurement of how high output amplitude your amp can produce. I measure SNR just below clipping.
@amirm Why do you not measure SNR in this way, just below clipping? To me that is the SNR I want to know, the highest achievable one for a device. For example Massdrop specifies 136dB SNR for the THX 789 and you state 123dB.
 

restorer-john

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#11
I got to say I like the design of that amplifier! Very cool front plate with easy to understand icons. No more, no less, just perfect.
Subjectivity is funny isn't it, I find it simply horrible looking in every regard, especially the front panel.

The excellent performance of the unit and the retail price being asked, justifies a whole other level of aesthetic design, fit and finish.
 
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#12
Subjectivity is funny isn't it, I find it simply horrible looking in every regard, especially the front panel.

The excellent performance of the unit and the retail price being asked, justifies a whole other level of aesthetic design.
It's all business! It's similar to my JDS Labs o2 front plate. I wouldn't say it's pretty but it's well labeled and gets the job done. Of course you can always get the kit and build your own case :)
 

pkane

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#14
It's all business! It's similar to my JDS Labs o2 front plate. I wouldn't say it's pretty but it's well labeled and gets the job done. Of course you can always get the kit and build your own case :)
I generally dislike purely iconic user interfaces. On this amp, I see five icons on the front, but I’m only sure about what two of them do — and both are images of the headphones :)

But amazing, SOTA performance — congrats to the designer!
 

derp1n

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#15
While the measurements may be exemplary, it's hard to take a product of this level seriously with such a small volume knob.



;) Sorry Tom. It's a great device, I'd love to own one some day.
 

jackenhack

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#16
Subjectivity is funny isn't it, I find it simply horrible looking in every regard, especially the front panel.

The excellent performance of the unit and the retail price being asked, justifies a whole other level of aesthetic design, fit and finish.
Yeah, taste is a funny thing. But have a look at the amplifiers product page. There's a reason for the price when you look at the components used. The performance is stellar.
 

restorer-john

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#17
I generally dislike purely iconic user interfaces. On this amp, I see five icons on the front, but I’m only sure about what two of them do
Armstrong got icons right in the 1960s:

armstrong.JPG


Across the top: volume, balance, bass, treble
Across the bottom: headphones, power, loudness, low filter, high filter, scratchfilter, mono, tapes, phono and tuner.
 
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#19
This is a review, detailed measurements and comparison of Neurochrome HP-1 high performance headphone amplifier to Massdrop THX AAA 789. It is on kind loan from company designer and owner, Tom Christiansen. I learned about Tom in chasing a review he had done of a passive volume control. I was very impressed with his depth of knowledge and thorough analysis using an Audio Precision analyzer. Tom is an analog designer, having worked for famous and large leaders in the industry (National Semiconductor and Texas Instrument). He is quite active in DIY audio forum, sharing his knowledge of engineering and audio. Needless to say, I was excited to get my hands on his products to test starting with his HP-1 headphone amplifier given its superlative measured specifications.

The HP-1 comes in all forms from bare PCB to fully populated board and finished package. I received the latter:


The retail price for this version is $1,249. Bare board with the knob and RCA jacks is $225. And there are options in between. See: https://www.neurochrome.com/product/hp-1-ultra-high-end-headphone-amp-2/

The package feels hefty and dense due to inclusion of AC power supply inside the unit (unlike the Massdrop THX 789 and its external switching supply).

Naturally there are both balanced and unbalanced inputs. There are three gain levels (+6, +12 and +20 dB). On output side we have 1/4 inch and XLR headphone jack. As the measurements show later, the power does not change by using the XLR connector.

Let's get into measurements and see how HP-1 compares to its stiff competition, the massdrop THX AAA 789. Given the state of the art that these two products represent, I re-ran all the measurements for the Massdrop again. We are talking about some small performance aberrations that seemingly change in the weather might impact them! I kid of course but it is partly true.

All tests are performed with the XLR input. Output is 1/4 single-ended headphone output unless otherwise noted.

Measurements
As usual, we start with our dashboard. Given three gain levels and infinite choices of input and output levels, you get fairly wide variation in performance. I usually play some to get close to the best performance possible and here is that outcome (same output level as my review of Massdrop THX 789:

View attachment 17769

Looking to the right side of the 1 kHz FFT (frequency) spectrum we see essentially no harmonic distortion! This is in the "high-performance" mode of my Audio Precision APX555 which uses dual ADCs to essentially suppress the noise and distortions of the signal capture (so don't try it with any hobby level sound card).

The reason SINAD is not some 10 dB higher (limit of my analyzer) is the spikes with have to the left which are from the power supply (harmonics of 60 Hz). Fortunately our threshold of hearing is extremely high in low frequencies so no concern whatsoever. I am left wondering though what a statement this device would have made if those weren't there. As it is, the HP-1 takes a fine position in our ranking of headphone amplifiers when it comes to SINAD (signal over power of noise and distortion):

View attachment 17770

I am not happy with my signal to noise ratio measurements as test conditions make a big impact on what value you get. Here is a conservative sample that is close to what I used for massdrop THX:
View attachment 17771

If I played with input and output levels, I could get above 127 dB. Going with what we have as is, here is how it compares:
View attachment 17772

For the next graph, we have a new measurement that is SMPTE intermodulation test (60 Hz and 7 kHz tones) versus output power level (300 ohm load). IMD tests like this are not bandwidth dependent like THD measurements and hence can be more revealing:
View attachment 17773

I have shown the Massdrop THX 789 performance in dashed lines. The 789 is very well behaved and changing the gain only increases the output power and doesn't impact noise or distortion levels. Highest power is achieved by using high-gain mode which eventually clips as see in dashed green color.

In lowest gain mode the HP-1 matches the noise and distortion levels of 789 but produces even more power the latter outputs in highest gain mode. No clipping occurs here so to get max power, you need to use medium or high gain. Once there, you get over twice as much power as Massdrop THX 789. This is done without any increase in distortion or noise over 789.

The high gain mode in HP-1 doesn't produce more power so it is there for convenience of dealing with lower input levels (either in source hardware or music being played).

Switching to our traditional THD+N versus power at 300 ohm we get our usual power ratings:

View attachment 17774

The HP-1 produces nearly 0.4 watts at this very high load impedance which is impressive. As noted in the IMD test, this is 2.4 times more power than Massdrop THX 789 musters.

Running the same test with 33 ohm load we get:

View attachment 17776

The HP-1 delivers twice as much power as THX 789.

The THX 789 gets its revenge though in balanced mode where it produces much more power and HP-1 does not:
View attachment 17777

Output impedance is comfortably low on HP-1 at 0.8 ohm:
View attachment 17778

The volume pot is analog so we get some channel imbalance:

View attachment 17779

If this is an issue for you, get a DAC with digital volume control and change the level there. A quick test of changing levels in my analyzer instead of the volume control on HP-1 showed same performance. I plan to do a more exhaustive study of volume control impact on performance in the future.

The Massdrop THX 789 uses "forward error correct." This is on contrast to negative feedback commonly used. The advantage of forward error correction is supposed to be in higher frequencies where negative feedback can be weak (due to need to reduce gain to avoid oscillation). So I thought I run a frequency sweep of THD versus frequency at same output levels:

View attachment 17785

Interestingly, one channel in HP-1 matches the THX 789 in high frequencies. The other (in red) does nose up though.

In reverse, negative feedback works the best for low frequencies and we see the HP-1 pull ahead there.

Note that these are astonishingly low noise and distortion levels so we are nitpicking. And other factors may be impacting the measurements of error this low.

Listening Tests
As always, I started my listening tests with my Sennheiser HD-650. Ah, you have not heard clean bass until you power this headphone with this class of amplification. It is authoritative and at high volumes, it has no trouble making your ear lobes resonate with the music! :eek: Here I started with medium gain but then switched to high-gain for inconvenience for my "audiophile" recordings that have lower levels.

Switching to Hifiman HE-400i of course brings more power due to their lower impedance. You lose some of the bass due to design of these headphones but you get that airy sound that is distortion free until the headphone drivers start to cry (and you do too with ear piercing SPLs).

I recently bought an AKG K92 closed back headphones so I thought I test with those too. Man, I hit play and it scared the hell out of me! These headphones seem more efficient. While not in the same league as the above two headphones, I like them for better isolation. I checked for level level channel balance and it was fine.

I can't emphasize enough how revealing these amplifiers are. As I noted, it obsoleted a lot of "audiophile" tracks I had collected with my speaker system. So many of them have distortions that become so obvious to hear now. For this reason, I hope more recording and mastering engineers invest in this class of headphone amplifiers to better hear (and hopefully fix) the amount of distortion that can exist in their work product.

Conclusions
Boy, are headphone users spoiled with these superlative headphone amplifiers. We are talking objective performance that strains measurement gear. The HP-1 delivers that with superb engineering and design using classical negative feedback method. Using standard headphones (i.e. not balanced), the HP-1 produces more power and as such as a lot more headroom than Massdrop THX 789. Any distortion you hear is the recording or your headphones. It is extremely liberating to have such transparent and powerful amplification in the chain. If you have less efficient, high impedance headphones with unbalanced connections, the HP-1 is the perfect companion.

The down side is the cost. Fully packaged, we are talking four times higher cost than Massdrop THX 789 which in my opinion, is priced too low for what it does. Tom has given up his day job to build this product in very low volumes so can't compete on price. You can reduce the impact here by either building the unit yourself or getting the assembled board and putting it in your own case. You will have a pride of ownership and uniqueness which users of massdrop THX 789 won't have.

Personally and it is of course subjective, I like us as a community to support designs and efforts such as what Tom is doing. Just as you donate money to support ASR Forum, I think it makes sense to spend extra money to make sure we have options in the marketplace. This is especially so given Tom's willingness to help the community by providing knowledge and kits.

Needless to say, on performance side, the Neurochrome HP-1 gets my highest recommendation for headphone amplification.

-----
As always, any questions, concerns, comments, corrections, etc. are welcome.

If you like this review, please consider donating using Patreon (https://www.patreon.com/audiosciencereview), or upgrading your membership here though Paypal (https://audiosciencereview.com/foru...eview-and-measurements.2164/page-3#post-59054). It will enable me to pay for gear that is not offered for loan.
I've heard many claims that this is the cleanest solid state ever, nice to see some hard objective data from another source to back that!
 
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