high pitch frequency on motu 896mk3 hybrid inputs

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Klaus
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Re: high pitch frequency on motu 896mk3 hybrid inputs

Post by Klaus »

Yes, resolved

Klaus
mweiss
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Re: high pitch frequency on motu 896mk3 hybrid inputs

Post by mweiss »

I just took delivery of a 896mk3 Hybrid this week and I'm noticing noise on the inputs, though it's not a steady tone. It sounds like a buzzing sound with high frequency components to it (commutator noise), when something is plugged into the TRS input.

I first noticed this noise on the output bus when Cue Mix software Bus 1 was set to Main 1-2. Then the output would have this buzzing sound. When I first tested the unit in my electronics lab on a spectrum analyzer, I noticed two dominant spikes at 60hz and 180hz that were 80dB below clipping level. I thought that rather poor for a digital interface built in 2012. When I installed it in the studio, I heard that noise as a buzzing with high frequency harmonics on it, not a steady tone as described here, but perhaps a related problem. The output mix problem was mitigated by setting Cue Mix so as not to route Bus 1 to Main 1-2. Then the output got DEAD QUIET. In terms of playback from a computer, the mk3 Hybrid is stellar. Way better than the Original 896, which hissed quite a bit.

Unfortunately, I discovered that the mk3H preamps seem quite a bit noisier than my 896 Original, which died on Thanksgiving and which MOTU will no longer repair. The hiss level on the Original preamps was below the noise level of the mics I was using. However, this new 896mk3 Hybrid shows a noise floor of only -78dB and I can clearly hear the hiss even with the mics connected and only 25dB of gain on the MOTU preamps. The hiss is the same for the channel with a mic connected and the channel with nothing connected. On my Original, the unconnected channel was quieter.

But there's more. With a TRS connection to the input, there's this 'commutator noise' (a buzzing that varies over time and has lots of high frequency hash) that's quite severe. And with a dynamic mic connected, I am also hearing this buzzing noise. Definitely a big step down in preamp s/n ratio, compared with the 896 I used to use for the past 8 years.

When I recorded symphony concerts with the 896 Original, the background was dead quiet. You could max out the playback volume and hear orchestra players' clothing rustling during a clarinet solo. With the mk3H, I'm afraid there is going to be a lot of hiss and that I'll lose that 'black' background noise floor that I enjoyed with the 896 Original. That was one fantastic preamp. Why did MOTU screw it up on the mk3?

ADDENDUM:
I wanted to test the mk3 with RightMark Audio Analyzer, but for some reason, all three versions of RMAA report 'AUDIO ERRORl Cannot add memory buffer to recording queue. DRIVER REPORTED: There is not enough memory available for this task.'
This is the first time I have seen this error. My MOTU 896 Original tests without any errors, but something about the new mk3, or its drivers, does not get along with RMAA and I tried v6.25 and v5.5, both of which crash when attempting to start tests.


ADDENDUM 2:
With the preamp gain at 0dB and a condenser mic connected, I have the hiss, but I also hear a twin tone. The Spectrum Analysis feature in SoundForge shows the two tones to be 3,946Hz and 4,088Hz. The tones vary in amplitude with respect to eachother and with respect to the background noise at a rate of about 1-2 Hz. This is visible on the spectragraph and is audible on the normalized audio.

ADDENDUM 3:
I suspect these tones may be acoustically picked up in the room from various equipment. With the mic unplugged from the cable, the tones are gone. With the mic stuffed into a box, the tones are 27dB down from where they were out in the open.

I also suspect there's a huge non-linearity in the preamp gain pots on the 896 Original. For orchestra recording, I had them at 12 o'clock and a 106dB crescendo would result in 6dB of headroom left from the loudest peak recorded during a concert. "12 o'clock" on the mk3 is more than 25dB of gain. But when I supplied an arbitrary 112dB SPL to the mic (with a sound level meter mic right next to it for measurement), the Mk3 with gain at 0dB, showed levels at -7dB. So it appears that I need only 6dB of preamp gain to equal the 12 o'clock gain setting on the 896 Original. Apparently most of the gain on that unit occurs past 3 o'clock on the gain pots. So I guess there's no serious problem with the mk3, when using condenser mics. That said, the phantom power is hugely noisy. Preamp s/n ratio drops by 8dB when it's switched on, just measuring on an unused input. It's mostly infrasonic random noise, but it's way noisier than with the phantom power switched off.
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lifeforming
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Re: high pitch frequency on motu 896mk3 hybrid inputs

Post by lifeforming »

So is the "Fix" basically to get a replacement unit?
I'm not happy with that solution - power supply should not get this hot and it should not introduce humming or any other signal that degrades the sound quality and introduces random buzzing.

Nevertheless, some impressive engineering troubleshooting on here.

However- what's up MOTU? You in the business of selling janky multi-hundred dollar gear?
Not only unfair but straight up embarassing.
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terrybritton
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Re: high pitch frequency on motu 896mk3 hybrid inputs

Post by terrybritton »

lifeforming wrote:So is the "Fix" basically to get a replacement unit?
I'm not happy with that solution - power supply should not get this hot and it should not introduce humming or any other signal that degrades the sound quality and introduces random buzzing.

Nevertheless, some impressive engineering troubleshooting on here.

However- what's up MOTU? You in the business of selling janky multi-hundred dollar gear?
Not only unfair but straight up embarassing.
They make good stuff as well. Don't blow this incident up into a generalization about their gear.

I have two of their units - an Ultralite mk3 hybrid and an 828 mk3 hybrid which are beautiful things.

Terry
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