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: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: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: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.As usual, we start with our dashboard, asking the amp to simply produce the same voltage on its input (unity gain):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: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:Here is the classic signal to noise ratio measurement: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: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: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: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: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: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: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: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.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!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.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 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.------------As always, questions, comments, recommendations, etc. are welcome.I will make this simple: you can never have enough money. So please consider donating using: https://www.patreon.com/audiosciencereview ), or https://audiosciencereview.com/foru...eview-and-measurements.2164/page-3#post-59054 ).