Monoprice 3 Budget Monolith Mx100X Amplifiers Bench Test Results

Monoprice is known for their internet savings cables and cords, but for the better part of the last decade Audioholics.com has been consistently impressed by Monoprice's affordable audiophile line of home theater equipment run through their Monolith team of engineers.  Only 5 years ago contributing writer Theo Nicolakis said of Monoprice, "I don’t know about you but I never would have thought about buying reference gear from Monoprice." Then Gene bench tested their Monolith 7 amplifier and said:

If you have any doubts about the Monolith, cast them aside. Build quality is impeccable. The heat sinks on this amp are massive and integrated on the amplifier modules themselves inside the chassis. 

During his listening tests, Theo went on to say, "The Monolith crushed any test I threw at it.  I loved the energy and emotion that the Monolith was capable of putting on display.  Dynamics, control, and stable imaging were easy-to-identify hallmarks of the amp’s capabilities." High praise for what used to be a camera and monitor company. We often agree with their slogan, "Monoprice's rugged design and rigid quality control standards deliver high quality products at fair prices." To that point, Monoprice is one of the few companies who's product measurement often downplay the actual precision and power of the unit compared to what we measure them at.

Monoprice has now aimed its sights at Emotiva's more affordable line of amplifiers called BasX.

Monolith M2100x Inside View

Monoprice's newest lineup of multi-channel home theater power amplifiers include the M2100X, M3100X, and M5100X. The three amplifiers are a Class AB design with a large centralized torodial transformer and filter capacitors varying in size and amount based on the channel configurations. Two low noise fans fire directly at the single heatsink housing the amplifier power transistors which only engage on power up or under extreme load. All the amplifiers in the lineup feature class AB amplification that are stable into 4 ohm loads and would be a way to relieve the power supply in your AV receiver which could handle your font LCRs while the Monoliths power the height and other effects channels in a larger system. The amplifiers feature both unbalanced RCA inputs or unbalanced XLR inputs which should make it easy to connect to any receiver or pre/pro. Monoprice rates the amplifier's power at 90 watts per channel into 8-ohms and 135 watts into 4-ohms (all channels driven) with less than 0.02% THD and a signal to noise ratio greater than ‑100dB (full power). 

Note: Monoprice hired me to beta test this line up of amplifiers as well as to honestly rate them. With their permission, I turned this beta-test into a published this report AFTER my work was completed.

In my recent Youtube Video, I talked about rating amplifier power before the knee of the power vs distortion curve. As you will see in my bench tests, it's refreshing that Monoprice has rated ALL 3 of the amplifiers in this series accordingly.

How Should Amplifier Power Be Rated?

Bench Test Results

As mentioned earlier, the Monolith Mx100X series is linear Class AB design. In this test report, we cover the M2100X, M3100X, and M5100X multi-channel amplifiers. The # of channels is represented by the first digit of the respective model (ie. 2 – 2ch, 3 = 3ch, etc). All testing was conducted on the Audio Precision APx585 HDMI analyzer.

Monolith Mx100X Gain Structure

All models in the Mx100X series tested exhibited similar gain structure of 28dB via unbal (rca) and bal (XLR). The high gain structure makes these amplifiers easy to drive from any AV receiver with preouts. All you need is a clean 1.1Vrms output from your AVR to reach rated power (90 watts/ch, 8 ohm) for each of these models.

Frequency Response

The Mx100X series is a wideband design with ruler flat frequency response from 10Hz to 80kHz( -0.8dB) when tested into 8-ohm and 4-ohm loads. Full power bandwidth and frequency response uniformity was tested with All Channels Driven (ACD) into 8-ohm and with 2CH driven into 4-ohm loads with measurement results in the power tables.

 

Monolith M2100X Frequency Response @ Full Power (4 ohm, 2CH driven)

Monolith M5100X Frequency Response @ Full Power (8-ohm, 5CH driven)

FFT Distortion Analysis

ALL Monolith Mx100X amplifiers exhibited some excessive HF harmonic distortion in varying degrees depending on model#. The M2100X power supply hum was minor as can be seen in the graph below. 2nd order harmonic distortion was at -86dB below the fundamental when driven at 1 watt, 8 ohms. This is good performance for an amplifier of this price class. The unbal inputs exhibited minor (inaudible) 60Hz power supply hum while this was not seen using the XLR inputs.

Monolith M2100X 1 kHz FFT @ 2.83Vrms (1 watt, 8 ohms)
Top graph - Unbal, Bottom graph - XLR

Some power supply hum was seen on the M3100X via the unbal and XLR inputs but the levels are low enough to likely NOT be audible in most systems. The FFT harmonic spectra isn't quite as clean as the M2100X model.

 

Monolith M3100X 1 kHz FFT @ 2.83Vrms (1 watt, 8 ohms)

Monolith M5100X 1 kHz FFT @ 2.83Vrms (1 watt, 8 ohms) – Unbalanced

Monolith M5100X 1 kHz FFT @ 2.83Vrms (1 watt, 8 ohms)  - Balanced

The M5100X performed similarly to the M3100X with respect to power supply hum and residual harmonic distortion. However, the hum and distortion got slightly worse using the XLR inputs.

Signal to Noise Ratio (SNR)

Monolith M2100x SNR @ 1 Watt (a-wt)

SNR was measured for each of the Mx100X amplifiers with 110mVrms input and 2.83Vrms output (1 watt, 8-ohm) and was found to be acceptable between 80-86dB (a-wt). SNR was consistently 2-3 dB better on the unbalanced vs balanced input connections between models which is how we would recommend using them in most installations.

 

Monolith M5100X SNR @ 1 watt (a-wt)
Bal (left), UnBal (right) images

This translates to 100-106 dB SNR at rated power depending on which model and connection type you are using. Monoprice rates these amplifiers at 100dB SNR (A-wt) at full power which matches what I saw on the test bench.

Channel to Channel Crosstalk

Monolith M5100X (1CH Driven) CH-CH Crosstalk @ Half Power

Monolith M5100X (1CH, Undriven) CH-CH Crosstalk @ Half Power

Channel to channel crosstalk was acceptable at -60dB @ 20kHz for all of the Mx100 models for the one-channel driven tests. CH-CH crosstalk performance degraded about 10dB with all channels driven except the undriven channel of the M3100X, and the 5100X. These are acceptable results.

Power Measurements

Using our Audio Precision APx585 8-channel HDMI analyzer, we conducted a full barrage of multi-channel amplifier tests on NAD M23 per our Amplifier Measurement Protocol. We tested power using three methods all of which were taken at < 0.1% THD + N:

• Continuous Full Power Bandwidth (CFP-BW) from 20Hz to 20kHz into 8 and 4-ohm loads (up to two-channels) (5 sec sweep time, typical)

• 1kHz Power Sweep vs. Distortion (1kHz PSweep) - popularized by the print magazines, this is an instantaneous power vs. distortion test at 1kHz. The sweep time is 1 second duration, 31 points.

• Dynamic PWR - 1kHz CEA-2006 Burst Method testing. This is a dynamic power measurement adopted from the car industry similar to IHF method only a bit more difficult for an amplifier and more representative of real musical content.

All tests were conducted on a 120Vrms, 20A dedicated outlet. Line voltage was constantly monitored to ensure it never drops more than 2Vrms from nominal, which in our case was 120Vrms.  

For more info on amplifier measurements, see:  The All Channels Driven (ACD) Test

Monolith M2100X 1kHz PSweep (2CH, 8-ohms)

When driving 8-ohms, the M2100X delivered 103 watts/ch x 2 at 0.1% THD+N and 118 watts/ch x 2 at 1% THD+N. The knee of the amplifier is about 90 watts. SINAD on the right channel peaked at 86dB while the right channel was 5dB less at 81dB. This is good performance and shows Monoprice rated power before the knee which is NOT typical for amplifier manufacturers selling budget amps like these.

Monolith M2100X SINAD (2CH, 8-ohms)

Monolith M3100X 1kHz PSweep (3CH, 8-ohms)

When driving 8-ohms, the M3100X delivered 100 watts/ch x 3 at 0.1% THD+N and 110 watts/ch x 3 at 1% THD+N. The knee of the amplifier is about 90 watts for Left/Right channels. The center channel appears to have significantly higher distortion when driven > 50 watts.  

Monolith M3100X SINAD (3CH, 8-ohms)

SINAD on the right channel of the M3100X peaked at 84dB at 50 watts while the right channel was 5dB less at 81dB. The center channel exhibited 15dB worse SINAD than the right channel at 75 watts. This is mostly academic as the center channel amp is still below 0.05% THD+N at 90 watts.

Monolith M5100X 1kHz PSweep (5CH, 8-ohms)

When driving 8-ohms, the M5100X delivered 101 watts/ch x 5 at 0.1% THD+N and 109 watts/ch x 5 at 1% THD+N. The knee of the amplifier is about 90 watts for all channels. SINAD on CH5 peaked at 82dB while the CH3 was about 10dB less.

Monolith M5100X SINAD (2CH, 8-ohms)

Monolith M2100X CEA 2006 Dynamic Power – (2CH, 4-ohms)

The CEA-2006 burst tests simulate musical program material to illustrate dynamic capabilities of the amplifier.  The Monolith M2100X produced 153 watts/ch into 8-ohms and 249 watts/ch into 4-ohms with both channels driven at 1% THD+N.  

Monolith M5100X CEA 2006 Dynamic Power – (5CH, 8-ohms)

The M3100X, and 5100X exhibited similar 150 watts/ch ACD dynamic power into 8-ohms at 1% THD+N, while the M7100 produced slightly less at 140 watt/ch. Into 4-ohms, the M3100X, and 5100X produces 250 watts/ch with 2CH driven at 1% THD+N.

Measured Power Test Results for Mx100X Series

     Monolith M2100X Power Measurement Table

      Monolith M3100X Power Measurement Table

     Monolith M5100X Power Measurement Table

Summary

The Monolith Mx100X series of amplifiers appear to be very high-value designs with generous power supply capabilities for each model. The XLR connections are superficial to meet a feature check box but in most cases, we’d recommend using the unbalanced connections with a quality double-braided double-foil shielded interconnect like the Belden 1694A from Bluejeanscable.com.

Power Ratings Considerations

The Monolith Mx100X series can be considered 100 watt/ch amplifiers based on how they tested. However, a more conservative approach based on rating power before the knee would win over Truth-In-Power consumers. We happy that Monoprice decided to rate these amps conservatively which is a rarity for products of this price class and target buyer as previously stated.

The Mx100X series could be rated as 90 watt/ch amplifiers ACD at 8 ohms and 150 watts/ch (2CH, 4-ohm) for M3100 and higher models and 130 watts/ch (2CH, 4-ohm) for the M2100. The larger power supplies of the higher channel count amplifiers (M3100X, 5100) have the advantage of increased dynamic headroom for one or two channels driven compared to the M2100X model. This is a good selling feature for anyone deciding between models with the idea of expanding their system at a later date but still wishing to take advantage of extra power on reserve now. However, the best noise and distortion performance was achieved by the M2100X model so if those aspects are important to you when building a 2CH system, that should be your amp of choice.

Conclusion

The measurement results are far from perfect on this series of Monolith amplifiers. While I wish for better CH-CH consistency for noise and distortion, it’s hard to fault a series of budget amplifiers giving you honest All Channels Rated Power for only $349 3CH and $415 5CH models. These amps would serve well for height channel applications and the $295 M2100X 2CH amp should do excellently when building a budget HiFi system. Monoprice stands behind this product with a 3 year replacement warranty, and offers a 45 day money back guarantee as well!

Deal Alert: At the time of publishing, Monoprice is offering a special discount on the M5100X model for only $414!

Unless otherwise indicated, this is a preview article for the featured product. A formal review may or may not follow in the future.