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Emotiva CMX2 AC Line Filter With DC Offset Eliminator Review and Measurements


Major Contributor
Forum Donor
Jan 15, 2020
Emotiva CMX2 AC Line Filter with DC Offset Eliminator (Review and Measurements)

There has yet to be an AC line conditioner that shows any sort of measurable benefit. One area that hasn't been looked at is the value of DC blocker which can contribute to mechanical transformer hum. While it's not rocket science to build your own DC blocker, it is potentially fatal working with high volage / high current AC power -- which I guess makes it just rocket science.

Up for review today is the Emotiva CMX2 AC Line Filter. It sells for $82 including shipping. These were out of stock as soon as the pandemic started but just started coming back in stock.

The manufacturer doesn't make a lot of exaggerated claims. They just talk about reducing DC on the AC power line.

There is no UL listing or other regulatory information on the unit itself although the box does brag about being manufactured in an ISO9001 compliant facility. The unit is nicely machined with no sharp edges (apparently an issue with the original batch) and the included power cable is one of the nicest I've seen. It's a beefy 14 AWG cable that is surprisingly supple (and does have UL listing). The unit also includes a simple receptacle tester to confirm that it is correctly wired to the wall (like those $10 GFCI outlet testers).

Raw AC Outlet Measurement
I used a 20,000 count RMS multimeter. After confirming accurate AC voltage measurement, I switched to DC voltage and recorded the result. An RMS multimeter is needed to remove the AC component from the measurement. The actual value oscillates a lot, but I think it's fair to say that the DC measured straight was consistently higher, generally in the high 20s to low 30s in millivolt compared to the low 10s to low 20s with the CMX2. Whether this is just random variation in electrical noise/RMS calculation, or an actual measurement is hard to say. I don't have more sophisticated instrumentation for measuring AC power.


Through CMX2

I cannot find a great source on the minimum amount of DC that causes transformer hum, but I've seen phrases as "as little as 100 mV" being tossed around by audiophiles, so it would seem that my home electrical power is below the threshold needed to cause hum by an order of magnitude.

Test #1: Kenwood L-08M: Mechanical Hum
My restored Kenwood L-08M, which measures quite well and still has the claim-to-fame of a damping factor of 20,000 (with Sigma Drive applying negative feedback that accounts for the speaker cable) has a mechanical hum that's quiet but annoying for an otherwise incredible amplifier.

A UMIK-1 was placed next to the L-08m and RTA was run straight and with the CMX2.

Straight (LEFT) vs Emotiva CMX2 (RIGHT): There is no meaningful difference.


Darn. The hum isn't caused by DC offset on my AC line. It's something else.

Test #2: Raphaelite CS-30 Mk II: Tube Hum
My 300B SET, which sounds great despite measuring as far from transparent as we have seen so far, also generates 60Hz hum. This is presumably due to the AC heaters of the 300B as well as the tube based rectifier. Will the CMX2 improve this?


There is no meaningful difference although the measurements are slightly better with the CMX2.

Test #3: Bose 901 Series VI ver 2 Active Equalizer (Electrical Analysis)
The Bose 901 Series VI ver 2 active equalizer has the lowest noise and distortion off all my Bose 901 equalizers, but it has more AC mains noise than I'd like. For these tests, I just ran the equalizer directly to the E1DA Cosmos ADC with no input signal.

The E1DA introduces its own DC offset, and that should be ignored.

The CMX2 didn't make a meaningful difference but the measurements were slightly better with the CMX2
. You can see this on the spikes to the left and right of the 60 Hz peak. They're a bit lower with the CMX2.


Test #4: Topping D90 MQA (Electrical Analysis)
Last, I tried the Topping D90 MQA as a reference. This has a highly filtered power supply and world-class "120 dB SINAD". I reduced the averages for these tests to avoid the D90 running into power saving mode with no input. Again...

The E1DA introduces its own DC offset, and that should be ignored.

There is no meaningful difference, but the CMX2 measurement is again slightly better. The 120 Hz and 180 Hz spikes are lower with the Emotiva CMX2 in use.


I bought the CMX2 because I had some mechanical hum on some of my vintage gear. This is audible to me and is why I went down this route. The CMX2 didn't correct this. This isn't a negative reflection on the product but it means that I made an error in diagnosing the mechanical hum as DC offset. From that perspective, the CMX2 was a great way to test for DC-induced transformer hum without having to DIY anything. It would also be pretty hard to build something as nice as the CMX2 on your own unless you had a machine shop.

If you have mechanical transformer hum, I think trying a DC blocker such as the Emotiva CMX2 is a good and safe way to assess this. If it doesn't work, you're only out the cost of return shipping. Even using a multimeter probe on your AC mains can be dangerous if your multimeter is poorly constructed/defective.

What was interesting was that while none of my measurements suggest that anything audibly different, the performance measured through the CMX2 was consistently equal or slightly better than without. In the world of power filtering snake oil, that's actually nice to see that there is actually something measurable rather than detrimental.

Credit also goes to Emotiva for making no outlandish claims about this product.


Major Contributor
Forum Donor
Jan 15, 2020
My Arcam SR250 does have a minor mechanical transformer hum with my ear to the vents. The CMX2 does successfully eliminate this mechanical buzz!

Measurements to follow. The buzz is below the noise floor of my UMIK-1, so I can hear it with my ears -- but not with the mic.
Last edited:


Dec 28, 2019
So the reality is , there is a lot of people that incorrectly think small amounts of DC offset are the cause of hum in their equipment. when the reality is, It is simply something else.
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