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Review and Measurements of Massdrop THX AAA 789 Amp
- Thread starter amirm
- Start date
First of all, the scratchiness in the volume pot is quite common issue of 789( not really common to majority but there are a number of reports of this issue).
Now on to the hum. Firstly, even though the connection is balanced there still can be hum. One of the benefits of balanced is that there's no ground sharing between the two devices. BUT the gnd at the input of XLR in 789 is electrically connected to the internal ground. This connection makes clean AP measurements but is not optimal in real world use. However it can be beneficial in devices with high leakage current.
Then there's leakage current of both transformers(su8 has one, leakage is relatively small) and SMPS(789 uses it, pretty large leakage). If su-8 is perfectly earthed, the ground in XLR is going introduce return path to earth from 789 which can gets rid of the leakage current. However if the earth is not clean or polluted even the introduced earth can cause hum.
Also SU-8 needs to be connected to a 3pin power jack to have earth connection. If you use a 2pin at the moment, then try using 3pin. If you are using 3pin I may let you try to connect to a not earthed power strip (THIS IS NOT CONSIDERED VERY SAFE) to see if there's any difference.
IF the problem would be earth, wouldnt i have the hum issue all the time? Also, i noticed that if i touched my Su-8 while this is happening, the hum would go away. Would switching to RCA make any difference here? And, yes i am using a 2 pin power, as my wall outlet is only 2 pin in this room (not optimal at all) But its what i got atm.
Alright. That's the issue. If you fix that the hum will go away.
I guess i will be switching back to my old Dac, it would be a cheaper than an electrician. not gonna risk ruining my headphones.
If the non-grounded plug is indeed the problem, why not try a 3 prong power adapter and ground to the outlet screw?
I switched to RCA and if it happends again i will throw on an extention cord from a grounded outlet somewhere.. This issue is just very wierd, never experienced something like this
From my experience, this can't be more common. It's almost too common that I would even think the polluted earth in the 3pin caused the issue. Turned out it's just the lack of earth pin. We all learn something everyday~
I moved it from my socket, and put it directly into the wall outlet aswell.. Since the dac shared the "ground" with my pc, and monitors without it actually being grounded to the houses ground. But i still think that the random hum is wierd, in my mind i should have had this issue all the time and not every other day, and here and there.
It is a switch for the auto-off function, which turns the amp off if no signals are detected after 2 hours.
Yup, that is what I do
ZeDestructor
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My setup with an SDAC balanced for some reason doesn't trigger the auto-on properly, so I have mine set in bypass mode. It's probably related to the fact that my SDAC never turns off, since it's always powered on via standby power being fed to all my USB ports at all times.
Possibly, though when I leave my SU-8 DAC powered on, the auto-off on my 887 still functions properly.
This is immensely useful. I woke up this morning to find the white light on, but no way to switch the unit off. The on/off switch did not work. I contacted MD, who told me to take a hike in the park because I was out of warranty by a week. I flipped back the switch to the by-pass mode, per your recommendation, and all is dandy! Thank you. Thanks a ton!
ZeDestructor
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Probably specific to the SDAC.. Gonna give it another go today/night and see what happens - of all the DACs out there, one would expect that the SDAB-B would be the one to work perfectly with the 789.
focus_rarely
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I picked up a used 789 and trying to drive my HE560v2s, which should be plenty of power. However, I had it connected from SMSL Su-8 and I had to turn it to gain 3 and max volume on everything just to hear a normal volume. Then I tried just using my PC headphone out and I have to use gain 2 and 60% volume to hear normal volume. Is this normal?
The su-8 is probably faulty. But check volume on the dac using the remote to make sure it's at max.
Then the mid gain on 789 is unity gain the low gain is actually attenuation. So isn't far off.
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