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The faintest hum from Michi X3 through speakers

@Mart68 I bought it used from tmraudio. It’s past their 30-day no-questions-asked return date, but still within their 90-day faulty-product return date.
 
I was thinking about buying a rotel ra1592mkii new, but saw this used michi for the same price, so I thought “who not?” Now I know why not…
 
@Audio Science Pal Sure. But this thing is soooo heavy. FWIW, I did try on two different circuits with my apartment, including one supposedly dedicated circuit for air conditioning. It’s all the same.
 
... recently noticed a faint hum or buzz coming through the speakers. One speaker hums a little louder than the other -- I can just hear the louder one from like 2 feet away, while the other I can only hear with my ears right on the speaker. It seems to be the amp itself, as I unplugged everything connected to the amp (besides speakers and power) and it still hums (that rules out ground loops), and if I swap the speaker cables on the back of the amp, the humming swaps speakers (that rules out speaker cables and speakers).

The fact that your swapped left and right does seem to indicate an internal problem.
 
Thanks again to everyone who chimed in. I should probably call tmraudio and try to return it. I’m not 100% sure that I will though…I just have too much else going on rn to deal with this. I guess I need to learn the lesson “buy cheap, buy twice” for once…
 
@Kal Rubinson just want to add one more piece to this puzzle. The Michi x3 has an “auto mute” feature which apparently opens a relay when no signal is detected for a little while. Thid is a relay because one can hear a little click when it opens. One can disable this feature but it’s enabled by default.

So I’ve tried it both ways and the hum is constant no matter what. So how can there even be a hum through the speakers if this mute function is doing what it supposedly does?? Seems odd to me.
 
@Kal Rubinson just want to add one more piece to this puzzle. The Michi x3 has an “auto mute” feature which apparently opens a relay when no signal is detected for a little while. Thid is a relay because one can hear a little click when it opens. One can disable this feature but it’s enabled by default.

So I’ve tried it both ways and the hum is constant no matter what. So how can there even be a hum through the speakers if this mute function is doing what it supposedly does?? Seems odd to me.
Well, it depends on how the mute function is implemented in the amp. It can be at the input, it can be at the output or it can almost anywhere in the signal path. Wherever it is (and you can probably find it in the schematic), the hum/noise culprit is probably after it so that it is unaffected by the muting.
 
@Kal Rubinson Thanks! I have not been able to find a schematic for the Michi x3 online anywhere. The user manual says this about auto mute:

When enabled and the unit stops receiving an audio signal for 30 seconds, the speaker outputs will be muted. When an audio signal is detected the unit will un-mute the speaker outputs and restore the audio. This setting can reduce noise when there is no active audio source

The “speaker outputs muted” and “reduce noise” bits made me think that this relay must be pretty late in the signal path …but I suppose that’s all speculation without seeing the schematic.
 
For completeness, here is a recording of the speaker hum: https://www.dropbox.com/scl/fi/p064...ey=y7t6k75oithowt9ryf3jxroz9&st=yx6zv2t7&dl=0

I measure it at 35db using NIOSH SLM app on my iPhone with the microphone within 1 inch from the tweeter/midrange.

Tmraudio said that since it’s within the 90 day window for dealing with defective equipment, I could send it back for them to look at it, but it’s not clear what the outcome would be Maybe they would find that it is operating within spec? Maybe they could fix it? Not clear. Shipping cost is minimal.
 
For completeness, here is a recording of the speaker hum: https://www.dropbox.com/scl/fi/p064...ey=y7t6k75oithowt9ryf3jxroz9&st=yx6zv2t7&dl=0

I measure it at 35db using NIOSH SLM app on my iPhone with the microphone within 1 inch from the tweeter/midrange.

Tmraudio said that since it’s within the 90 day window for dealing with defective equipment, I could send it back for them to look at it, but it’s not clear what the outcome would be Maybe they would find that it is operating within spec? Maybe they could fix it? Not clear. Shipping cost is minimal.

At two inches that will be 29dB. At 4, 23dB, at 9 17dB, and at 18, 11dB. You can't hear it when you are at listening position, and that is in a quiet room. Even with the quietest music playing it will be masked even close in.


That is not a fault IMO. Certainly not one that will be fixed under warranty on second hand gear.

If you don't want to live with it, and you have a return option, I think that is your only choice.
 
@antcollinet That is my concern. That I’ll go through all the hassle of sending it back only to be told it’s not faulty according to them. Add to that the small possibility of something getting broken in the round trip shipping.

The seller did have a “no questions asked” 30-day return window, but that has long passed. I actually didn’t notice the problem until after that deadline…I guess I should have done more checking right away.

Btw, using the same spl app, I measure the ambient noise level of my listening space to be 28-30db (under ideal conditions). So as you say, the noise is totally masked at normal listening positions, even when nothing is playing, and this is confirmed subjectively.
 
@antcollinet it turns out that it’s hard to get precise measurements of db and distance for very short distances. So 35db at one inch was a bit rough and ready, since I don’t really know how far away the “point source” is. I did the following. I measured 38db right on top of the cone and then moved 3in back to get it to drop to 32db. So in theory I have 32db at 6inches to a point source. So that’s still under 10db at 7-8ft, which is my listening position.

Am I overthinking this? For sure…
 
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