Monoprice Monolith THX 887 Balance Headphone Amp: New Champ?

[JohnYang1997]

Not my experience. The gain switch links to a few resistors that affect input impedance of the amp. You are going to feel the difference.

No.
First of all, there is buffer and volume control before the gain stage.
Secondly, the gain stage is non-inverting amplifier the gain settings will NOT affect input impedance.

 

[JohnYang1997]

Well I digged into datasheet of opa1612. http://www.ti.com/lit/ds/symlink/opa1612.pdf

You should check Fig. 9. Under 600ohm load and gain=11, THD+N measured better than gain=1.

Well. It has nothing to do with input impedance.
Then, fugure 7 clearly showed that at lower frequencies lower gain settings have better performance for the range that human is more sensitive. (lower noise and more feedback for lower distortion)
After that, the distortion rising at higher frequency is due to the common mode distortion at input stage. As we see in the figures the output voltage is fixed, the input voltage is lower with higher gain. This way the distortion gets reduced for higher frequencies. However the last harmonic you will likely to hear is 2nd harmonic of 10khz or 3rd harmonic of 6666hz. The difference will much smaller if you use 20khz BW. And if you use it in real circuits the distortion will always be higher at lower frequencies for higher gain.

 

[JohnYang1997]

I agree on the input load part.

But gain stage is outputting to amp section. Internal input resistance for the amp input is going to be high. Perhaps the 2k ohm load is more meaningful.

Then it's the potentiometer. It's a 5k pot for 789 I'm not sure what's for the 887.
The load is large for the gain stage. But for whatever reason (can be ground layout, can be feedback resistors as load, can be larger output voltage) the measured distortion is higher. Usually 6-10db higher than unity gain.
And the max impedance is relatively small for the output stage as well. Then for the most part you either use small position for listening and max volume when measuring, 50% position is rarely used. So it is not important either.

 

[JohnYang1997]

Resistance before opamp/chip and after it is different.

Small portion can have channel imbalance. In fact 50% position for a log taper (audio taper) is about 10% of the total resistance. I won't say it is not a small portion.

However, that's why I prefer a separate preamp these days.

I meant 50% resistance position. We all know it's about 75% position for log to be 50%. It's not accurate.

BJT suffers from high source impedance like 10k ohm. It's due to the input capacitance of the opamp being non linear. Same thing is true for FETs but they generally behave much better. The better one of BJTs is actually opa1602. And opa1642, opa827 are actually immune to this issue, having no distortion rise with high source impedance.

 

[Inkey31]

BTW, the product listing is already on Monoprice website: https://www.monoprice.com/product?p_id=39359

This is a review and detailed measurements of the brand new Monolith Balanced Headphone Amplifier using THX AAA 887 technology. It was kindly sent to me a couple of days ago prior to its official unveiling on September 10th. It costs US $399.99 and is slated for general release on October 27th.

The Monolith is a direct attack on Massdrop THX AAA 789, aiming to better its performance while keeping feature set essentially the same. You can tell that from the overall design:

View attachment 33025​

As Avid readers of the forum know, there are now three headphone amplifiers that stand alone as the best of the best: the JDS Labs Atom, the Massdrop THX AAA 789 and Benchmark HPA4. The Atom is great but is not balanced (input or output). The massdrop is but availability has been quite poor. The HPA4 is superb but very expensive. So having more options here is good and that is what we have.

The controls are familiar to anyone who has seen the Massdrop THX AAA 789. Three gain settings, jacks for XLR, 1/4 and 3.5 mm headphone jacks. And an input selector. The power button feels better than the one on Massdrop.

The back panel is as you expect:

View attachment 33026​

As is the 24 volt "laptop" style external power supply.

The Massdrop THX AAA 789 set a very high bar. Is it possible that Monoprice can exceed that? The number "887" may give you a clue as to them getting there. Don't go looking for that on THX website. It was an informal designation that was created just for this unit. To wit, official units will not have those numbers on it.

The unit I received for testing is the limited production run. I am told it is representative of the retail products as manufacturing ramps up.

Let's get into the measurements as that is what you are all here for.

Headphone Amplifier Measurements
I must say I was not prepared for what unfolded in front of my eyes:
View attachment 33028

This is stunning level of performance. Distortion levels are at or below -140 dB! SINAD is therefore determined by noise levels of the Monolith but also my analyzer. As it is, it matches the Benchmark HPA4 in that department:

View attachment 33029

That is 2 dB better than Massdrop THX 789. From here on, I was crossing fingers that it would maintain this lead throughout the rest of the measurements.

Signal to noise ratio at 4 volts input and output was exemplary at 123 dB:
View attachment 33030

Dialing the output way down using the volume control to assess noise level for very sensitive IEMs we get better than 92 dB:

View attachment 33031

Again, we have a 2 dB advantage over Massdrop.

Let's look at intermodulation distortion relative to level across all three gain settings:

View attachment 33032

A bit of a jungle in there despite me zooming in. Summarizing, the Monolith THX 887 maintains a few dB lead in noise and distortion department (solid lines).

We can see that more clearly with THD+N versus power using 300 ohm:
View attachment 33033

Neck and neck on power but slightly lower noise. Similar story exists at 33 ohm load:

View attachment 33034

Switching to a balanced 50 ohm load we get:
View attachment 33035

Here the performance gap is a bit larger. And with nearly 4 watts of power on tap, you should be able to drive just about any headphone.

Frequency response is as flat as you can get:

View attachment 33036

Channel balance shows some variations:
View attachment 33037

Given the very low gain setting, I don't think in practice it will be an issue. The Benchmark HPA4 is the king of hill here if you want perfect channel match.

Output impedance is comfortably low:
View attachment 33038

My setup has close to a 1 ohm impedance of its own so the actual impedance is probably close to 0.4 ohm.

Headphone Listening Tests
I tested the Monolith THX Balanced THX Amp using my Sennheiser HD-650 using both 1/4 inch jack and "balanced" XLR. I had no need to go beyond middle gain setting. No matter how much I turned up the volume, there was zero hint of any kind of distortion. Fidelity was exceptional. With XLR connection and highest gain, you get scary loud so there is plenty of headroom here for higher impedance headphones.

I then switched to Drop Mrspeakers Ether CX headphones. Despite the very low impedance of these headphones (around 25 ohm), there was no issues whatsoever driving them. Dynamics were excellent and nothing between your music and the headphones.

I play the same tracks for headphone testing. Having heard them hundreds of times, I am sick of them so usually stop pretty quickly. Not this time. The fidelity was so good that I kept the amp running for the rest of the evening.

There is not one subjective flaw I can identify with this amplifier.

Conclusions
I did not think it would be possible to build a mid-priced headphone amplifier and beat the Massdrop THX AAA 789. It is clear that the Monolith team managed to go against the tide and produce even a better headphone amplifier. Both subjectively and objectively, the "Monolith by Monoprice THX AAA Balanced Headphone Amplifier featuring THX AAA 887 Technology" (yes that is the official name) beats the Massdrop offering. Mind you, there is not much room left to squeeze one up there but they did. It is like beating the world record holder in Olympics.

Needless to say, it is my absolute pleasure to recommend the Monolith THX AAA Balanced headphone amplifier. It delighted me to see a company dedicated to absolute excellence and full transparency to our sources and content. This is a type of product you buy once and you are done. It made me happy just testing it!

------------
As always, questions, comments, recommendations, etc. are welcome.

Thinking about needing to buy more test gear to handle the broader set of products we are testing. So please support us by donating using: https://www.audiosciencereview.com/forum/index.php?threads/how-to-support-audio-science-review.8150/

Oh no. You have an exceptional product. Don't get buyer's remorse that way. The monolith is for people who have not yet managed to get a Massdrop one.

Hello,

Thank you for all you due for the community, I really appreciate the time you put in.

Can you advise on the difference between this model and the Benchmark model?

From my understanding, the Benchmark model has better channel matching?

It has no difference in sound signature correct?

Thank you
John

 

[JohnYang1997]

Hello,

Thank you for all you due for the community, I really appreciate the time you put in.

Can you advise on the difference between this model and the Benchmark model?

From my understanding, the Benchmark model has better channel matching?

It has no difference in sound signature correct?

Thank you
John

Hpa4 has stepped attenuator volume control which 887 doesn't have.
Hpa4 has flying supply for the opa564 which lowers the distortion even further. 887 does not live to the performance of thx888. 10db+ more distortion with 16ohm load. Also hpa4 has higher power output for higher impedance load thanks to the flying supply as well.
In short, hpa4 is a higher quality and more exotic product than 887. But for most people the performance don't justify the price difference.

Then there's smsl sp200. From performance perspective sp200 is much closer to hpa4. But again the volume control on it sucked and low gain setting is +6db and XLR input gives another +6db so not ideal for sensitive headphones/earphones. High gain is whooping +18db which is completely not usable. If you have low sensitivity headphones, sp200 is a good choice.

In terms of audibility for normal use. I recommend atom if you can live with RCA input and plastic case.

887 is a solid performer which is miles ahead 99% competitors regardless. I would choose 887 if I were you.

 

[mackat]

I recently acquired one of these 887 headphone amps and have been enjoying it immensely. I have noticed something peculiar, however, when my LCD-3’s are plugged into the balanced jack and my HD650’s are plugged into the single ended jack at the same time. Upon turning the amplifier on or off, there is an unpleasantly loud noise in both headphones that sounds like an electrical discharge of some sort. I wrote to Monoprice about this but have yet to receive a response, it being a weekend. Has anyone also encountered this issue?

 

[Inkey31]

Hpa4 has stepped attenuator volume control which 887 doesn't have.
Hpa4 has flying supply for the opa564 which lowers the distortion even further. 887 does not live to the performance of thx888. 10db+ more distortion with 16ohm load. Also hpa4 has higher power output for higher impedance load thanks to the flying supply as well.
In short, hpa4 is a higher quality and more exotic product than 887. But for most people the performance don't justify the price difference.

Then there's smsl sp200. From performance perspective sp200 is much closer to hpa4. But again the volume control on it sucked and low gain setting is +6db and XLR input gives another +6db so not ideal for sensitive headphones/earphones. High gain is whooping +18db which is completely not usable. If you have low sensitivity headphones, sp200 is a good choice.

In terms of audibility for normal use. I recommend atom if you can live with RCA input and plastic case.

887 is a solid performer which is miles ahead 99% competitors regardless. I would choose 887 if I were you.

I currently own the 789, it has some odd things about it. Well one thing really, I get static in the volume pot with sensitive headphones sometimes. I noticed it does it when I have other headphone amps on. I currently have a PS audio filter that does help remove dirty power.

I would love to see Amirm do a review of some power cables and how they effect the tests he does.

However, I am wondering if I should pickup the 887 because its an overall better amp.

 

[Inkey31]

View attachment 47071

I cant find a D90 anywhere, lucky you.

 

[JohnYang1997]

I currently own the 789, it has some odd things about it. Well one thing really, I get static in the volume pot with sensitive headphones sometimes. I noticed it does it when I have other headphone amps on. I currently have a PS audio filter that does help remove dirty power.

I would love to see Amirm do a review of some power cables and how they effect the tests he does.

However, I am wondering if I should pickup the 887 because its an overall better amp.

If you are not talking about the scratchy pot, it is hard to tell what really caused it.

I don't think 887 is that different of an amp to solve the problem.

Could you explain a bit more about the "static" in the volume pot?

 

[Inkey31]

If you are not talking about the scratchy pot, it is hard to tell what really caused it.

I don't think 887 is that different of an amp to solve the problem.

Could you explain a bit more about the "static" in the volume pot?

Sorry, yea when you raise or lower the volume you hear static. It happens anytime you turn the knob, sometimes its loud and others its quite. I mean this really only happens when I am listening to low impedance/sensitive headphones. I tried changing DAC's and cables , I had the same problem.

It seems to go away when I turn off everything, like my other headphone amps. I think I might need to upgrade the power cable, it might be getting interference from the other devices the PS Audio filter has plugged in. I have my digital and analog devices plugged into different zones, like I should so IDK.

However this is not a problem at all with any of my other headphone amps I own. Also come to think of It, I had the other 2 headphone amps Drop.com sells. The Liquid X and Liquid Tube Amp, they both had that problem. I ended up returning both of them amps, they were not good.

Maybe its the power brick? All three amps use the same brick.

 

[JohnYang1997]

Sorry, yea when you raise or lower the volume you hear static. It happens anytime you turn the knob, sometimes its loud and others its quite. I mean this really only happens when I am listening to low impedance/sensitive headphones. I tried changing DAC's and cables , I had the same problem.

It seems to go away when I turn off everything, like my other headphone amps. I think I might need to upgrade the power cable, it might be getting interference from the other devices the PS Audio filter has plugged in. I have my digital and analog devices plugged into different zones, like I should so IDK.

However this is not a problem at all with any of my other headphone amps I own. Also come to think of It, I had the other 2 headphone amps Drop.com sells. The Liquid X and Liquid Tube Amp, they both had that problem. I ended up returning both of them amps, they were not good.

Maybe its the power brick? All three amps use the same brick.

Is it when you touch the knob or you have to turn it? This will be a very important clue.

 

[Inkey31]

Is it when you touch the knob or you have to turn it? This will be a very important clue.

Sorry for the delayed response, yes when you turn the knob.

 

[JohnYang1997]

Sorry for the delayed response, yes when you turn the knob.

hmmm, do you have to turn it or not?

 

[Inkey31]

hmmm, do you have to turn it or not?

Yes you have to turn the knob in order for the static to be present.

 

[Inkey31]

Thank you very much for your help. I will have to pull the amp out and inspect it.

 

[JohnYang1997]

Yes you have to turn the knob in order for the static to be present.

So it's simply the scratchy pot.
887 seems to use the same pot according to photos. Some people reported 887 has better pot but I can't confirm this.

I suggest to live with it since it only happens when you turn the volume.

 

[Inkey31]

It’s a grounding issue related to the pot. Pot is an antenna if not ground properly. This is common issue.

Easy fix 1: see if your pot has a washer beneath knob. If you have one, remove it and try sanding the case around washer to make good contact. You can also see below and have a wire pressed beneath it.

Easy fix 2: strip a wire and connect the metal part of the pot to a ground point. You can use xlr jack or headphone jack’s ground pin. If you don’t want to solder, just tie it there tightly.

attachment is my diy project. You can see what i mean. The other end you need to find a place easy for you to work with.

You would think they would have grounded this already, or maybe its lose?

 

[Inkey31]

So it's simply the scratchy pot.
887 seems to use the same pot according to photos. Some people reported 887 has better pot but I can't confirm this.

I suggest to live with it since it only happens when you turn the volume.

My only concern is losing warranty if I pop the sucker open. Then again massdrops warranty isn't the best from what I have heard. That's why I would point people towards monoprice. They are a larger company who will take back the product with little question.

 

[Marutks]

Has anyone also encountered this issue?

Yes, it happens with my headphones as well. Did monoprice say anything about this?

https://drop.com/buy/drop-thx-aaa-789-linear-amplifier/talk/2268898

Yes that could be problematic. We didn't design with that in mind. In your case I recommend either tolerate the pop, or keep the amp always on, or unplug one of your headphones before turning off.