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Monoprice Liquid Platinum Headphone Amp Review

amirm

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This is a review and detailed measurements of the Monoprice Liquid Platinum Headphone Amplifier by Alex Cavalli. It is on kind loan from a member. It costs USD $770 and is currently out of stock on Monoprice website. Alex Cavalli used to design, sell and manufacture his own tube amplifiers. In this case, I have read that he has done the electrical design and PCB and has veto power on any part substitution by the contract manufacturer. In other words, even though manufacturing is in China, no used, or off-brand substitution of parts is allowed according to him.

I find the look of Monoprice amplifiers to be bland and uninteresting. The Liquid Platinum is a bit better with the tubes sticking out and vents on top:

Monolith Monoprice Liquid Platinum Headphone Amplifier by Alex Cavalli Audio Review.jpg

This is a hybrid design meaning the output stage is solid state. This means that it is able to deliver very low output impedance, avoiding the issue that many tube amps have with high impedance and resulting change in headphone frequency response. There is the standard 1/4 headphone jack and XLR "balanced" connection.

On the backside, we are happily greeted with a set of XLR input in addition to RCA In/Out:

Monolith Monoprice Liquid Platinum Headphone Amplifier by Alex Cavalli Back Panel Audio Review.jpg

The unit is quite light because of the external high voltage switching power supply and internal switching converters to generate the various voltages it needs. At 36 volt though, you may have a hard time finding aftermarket power supplies.

There is only one gain switch which is just as well as I am assuming most customers won't bother using it with IEMs and such.

The volume control has the right amount of stiffness and feels fine in use.

There is protection circuit built-in with a very long turn on time. I didn't time it but felt like 20 to 30 seconds before the red LED turned white. Got me worried at first, wondering if it was broken.

The unit as tested came with Genalex tubes. Spec says they normally come with Electro Harmonix tubes instead. Owner says he upgraded the tubes and put risers in there.

I loved the icons and somewhat detailed specification on monoprice page:

1552532919276.png


The icons are dead on. Some genius was at work here creating them.


Let's do our own measurements and see how real those specs are.

Measurements
As usual, we start with our dashboard, asking the amp to simply produce the same voltage on its input (unity gain):

Monolith Monoprice Liquid Platinum Headphone Amplifier by Alex Cavalli Audio Measurements.png


Despite using XLR inputs and me playing with grounding, I could not get rid of that pesy 120 Hz mains harmonic (from rectifiers in power supply). That peak is actually limiting the SINAD (signal over noise and distortion) as the first real harmonic distortion is around -95 dB. Going with what we have here, this is where the Liquid Platinum lands:

Best Headphone Amplifiers Distortion SINAD Rating.png


So quite a bit lower than our state of the art headphone amplifiers but for a tube amp, it is decent. It certainly runs circles around the Woo Audio WA7.

Frequency response is dead flat and excellent:

Monolith Monoprice Liquid Platinum Headphone Amplifier by Alex Cavalli Frequency Response Audi...png


Here is the classic signal to noise ratio measurement:

Monolith Monoprice Liquid Platinum Headphone Amplifier by Alex Cavalli Signal to Noise Ratio A...png


I have said that I don't like this measurement because the output is not the same for every device (it is usually the max it can muster). I read some place that the important thing for headphone amplifiers is to not generate noise for sensitive headphones so why not set the ceiling at a low number like 50 millivolts? So I did that and this is what we get:
Monolith Monoprice Liquid Platinum Headphone Amplifier by Alex Cavalli 50mv SNR Audio Measurem...png


I think that power supply spike is limiting what it can do here. I will have to measure this for other amps but I am thinking of setting 60 dB as the minimum required. Give me some feedback on this measurement.

Output impedance is excellent at just 1.0 ohm:
Monolith Monoprice Liquid Platinum Headphone Amplifier by Alex Cavalli Output Impedance Audio ...png


Yes, they advertise 0.07 ohm but that is with no wires or connectors as I am using. So consider 1.0 ohm as good as zero.

Channel balance is typical of analog volume controls with some variations:
Monolith Monoprice Liquid Platinum Headphone Amplifier by Alex Cavalli Channel Imbalance Audio...png


Strange to see that peak in the middle of the range though.

The all-important test is power versus distortion and noise. Here it is at 300 ohm:

Monolith Monoprice Liquid Platinum Headphone Amplifier by Alex Cavalli Power at 300 ohm Audio ...png


That it is much noisier and has more distortion than our reference Massdrop THX AAA 789 is a given. What was not was ample amount of power which I am happy to see. My threshold for desktop amps is 100 milliwatts and the Liquid Platinum delivers on that.

Switching to the other extreme at 33 ohm where current ability is required we get:

Monolith Monoprice Liquid Platinum Headphone Amplifier by Alex Cavalli Power at 33 ohm Audio M...png


Performance gap widens more but again, we have lots of good power at 1.3 watts. This is just shy of the spec which is likely measured at higher distortion than I use.

At 50 ohm, we get to test balanced power against unbalanced:

Monolith Monoprice Liquid Platinum Headphone Amplifier by Alex Cavalli Power at 50 ohm Audio M...png


I was pleasantly surprised that the noise level was much lower in balanced. I am not sure what is the cause there but I take it! We once again have tons of power at 3.5 watts prior to clipping setting in.

Examining the spectrum of distortion versus frequency we get:

Monolith Monoprice Liquid Platinum Headphone Amplifier by Alex Cavalli THD versus Frequency Au...png


The right side is as expected. The sudden jump at 30 Hz is not and usually indicates lack of reserve power in the power supply. The peaks at low frequencies last longer and are able to drain the power supply capacitors causing severe distortion. I suspect it is the negative part of the waveform here doing this as that is usually the weak point.

Listening Tests
Sticking to my usual protocol, I start testing with the Sennheiser HD-650 using 1/4 jack. The experience was very enjoyable. There was lot of power with excellent resolution and bass response. At max level, I could get my ear lobes to tingle as they vibrated with the bass notes. Life is good this way! :D

Switching to Hifiman HE-400i, once again the Liquid Platinum was able to shake the dust out of the drivers, producing a high level of fidelity that only high power and clean headphone amplifiers can deliver. Both the HE-400i and HD-650 act very differently than when fed with lower power amplifiers. You don't know what you are missing if you have underpowered headphone amplifiers.

Conclusions
Other than some power supply noise that gets through, the Monoprice Liquid Platinum seems well engineered. Performance is the best of any tube headphone amplifier I have tested (I think). We have tons of power which is the #1 think that impacts subjective performance of a headphone amplifier. On this front, it is at the top of the class competing with our reference headphone amplifiers.

Not so good new is why. Why stick a tube in there? It did nothing at all to fidelity. It is there simply to add cost, complexity and performance. The $99 JDS Labs Atom headphone amplifier produces similar power ratings with literally no distortion or noise. You can step up to Massdrop THX AAA 789 if you want balanced inputs and outputs. Yes, that is hard to get but apparently so is Liquid Platinum.

Overall, if you are determined to put a tube amp on your desk, the Monoprice Monolith Liquid Platinum by Alex Cavalli is the best choice out there. It seemingly does little harm to audio signal unlike the classical tube amplifiers sold to audiophiles.

------------
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Any chance you have 36V linear power supply able to power it? So we could see if the noise is indeed from the switching power supply?
The reason I am asking is because @ head-fi.org forums Mr. Cavalli urges folks not to upgrade to LPS for this unit as this does not make any audible difference.
 
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amirm

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Any chance you have 36V linear power supply able to power it? So we could see if the noise is indeed from the switching power supply?
The reason I am asking is because @ head-fi.org forums Mr. Cavalli urges folks not to upgrade to LPS for this unit as this does not make any audible difference.
My Agilent lab supply only goes up to 31 volt. Seeing how the Liquid Platinum has internal dc to dc converters anyway, I gave it a try and it works just fine. Here the results:

Monolith Monoprice Liquid Platinum Headphone Amplifier by Alex Cavalli With Linear Power Suppl...png


The power supply spikes are completely knocked out! We are now distortion limited by 2nd harmonic. It is a bit higher now for some reason so the gain in SINAD is about 2 to 3 dB.

Our hearing sensitivity in low frequencies is atrocious so he is probably right about audible effect. Still, for so much money, owners deserve a better a power supply than the one provided.
 
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#7
On topic of tubes, it would be an interesting exercise to measure if the parameters change as the tubes warm up. I don't know how long you had it on before testing. It is a common wisdom that the tube amp performs best if it is warmed up for 20 min minimum. I am not sure if such test would work well with hybrid amp like this because I would expect the effect to be more pronounced in all-tube amp but running a series of tests after 1 min, 5 min, 10 min, 20 min, 40 min, 2h to see if there are any measurable differences would be super interesting.
 

amirm

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What significant alterations would occur if using the original tubes (or any other tubes)?
I did some testing on another tube amp and difference was very small from what I recall. For this one, I don't have alternate tubes to play with. @SIY has more direct knowledge here and can tell us more I am sure.
 

Veri

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#10
Pretty good tube amp :) I would dread the hum this thing produces with sensitive headphones though.
 

amirm

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On topic of tubes, it would be an interesting exercise to measure if the parameters change as the tubes warm up.
I also tested this in one of the recent tube amp reviews and was surprised how quickly the output stabilized.
 

amirm

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From what I've read from here and here, there are some serious problems with this amp. Wonder if those claims are legit.
There is no there there. Half the time they are talking about different versions of this amp from Alex. And this is definitely not my experience:

1552549472582.png


I just tried it a few times with my linear supply and no problem turning it on and off. I can't even imagine how a mechanical on/off switch would go bad this way. Why would it care what the input source is???

Anyway, if I can open it we can look for ourselves.
 
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#13
Pretty good tube amp :) I would dread the hum this thing produces with sensitive headphones though.
Not sure what hum you are dreading. Are Focal Utopias sensitive enough? I have this amp and use with Utopias over balanced output and I don't hear any hum.
 

Veri

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Not sure what hum you are dreading. Are Focal Utopias sensitive enough? I have this amp and use with Utopias over balanced output and I don't hear any hum.
Not sure how sensitive the Utopias are but since you run them balanced you get that power so quickly noise floor probably isn't a problem. I wonder if you have sufficient volume control though, balanced? I read this liquid platinum goes loud very quickly.
 

xingcoo

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#15
There is no there there. Half the time they are talking about different versions of this amp from Alex. And this is definitely not my experience:

View attachment 23586

I just tried it a few times with my linear supply and no problem turning it on and off. I can't even imagine how a mechanical on/off switch would go bad this way. Why would it care what the input source is???

Anyway, if I can open it we can look for ourselves.
That would be great! Thx amirm
 
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#16
Yes, indeed, it gets loud quickly and there is no gain switch. Folks are inventing ways to lower the level of the input signal from DACs via passive attenuators for it.
Utopias are the the most detailed and sensitive headphones I have heard. Maybe rivaled only by Meze Empyrian (from the short time I was able to hear them at Denver's Audio Fest last year)
 

vitalii427

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#17
Not sure what hum you are dreading. Are Focal Utopias sensitive enough? I have this amp and use with Utopias over balanced output and I don't hear any hum.
There’s no hum on balanced headphone output. Noise floor on balanced out is almost equal to reference THX 789.
 

Veri

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Yes, indeed, it gets loud quickly and there is no gain switch. Folks are inventing ways to lower the level of the input signal from DACs via passive attenuators for it.
Ah yes I see. A gain switch would have come in handy :)..
 

anmpr1

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#19
Why stick a tube in there? It did nothing at all to fidelity.

I remember watching an interview with Bob Carver. I think he was answering questions at an audio show, or something. Anyhow, if I remember correctly, he admitted that he really can't reliably tell any sonic differences between his tube amp and solid state designs, but with the lights low, he added that it's nice to watch the glow. I have both tube and SS amps, and can verify Bob's subjective conclusions, 100%. LOL
 

graz_lag

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#20
Indeed, especially on Xmas Eve ...

b400tubes1.jpg
 
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