Film-Tech Forum ARCHIVE: audio buzzing: Doremi IMB HDMI

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Posted by Eric Robinson (Member # 2966) on 09-18-2011, 05:18 PM:

This has been my month for ground loop problems. This one has me stumped though. Here is my scenario. I have a BluRay player which is connected to my Barco Series II DP2K projector with integrated Doremi media block (IMB) via an HDMI cable from the BluRay out to the IMB in. The BluRay analog Left Right audio outputs are connected to a CP500 non-sync 1 input. When the HDMI cable is unplugged, there is no hum on non-sync 1 when connected to BluRay analog audio outputs. The moment that the HDMI cable makes contact with the IMB, a hum is present. So far I have reduced the hum by:
1) Connecting the IMB gound plane to the CP500 frame with a very thick wire (10 gauge)
2) Connected the CP500 output grounds to the frame ground of the 500.
Nothing else I do seems to reduce it any further. I also tried an experiment in which I reversed the tip and ground of the non-sync cable on one side (made hum worse.)
3) I also tried connecting to non-sync without the ground wire by clipping the ground/shield wire midway through the cable, I still got sound but there was still hum.
To top it off, I have this same problem on every projection system. The BluRay player has been substituted by several different devices. Thoughts?

Posted by Bajsic Bojan (Member # 4928) on 09-18-2011, 05:31 PM:

Try putting a transformer isolator (we use a fairly inexpensive ART T8) between the bluray player and the cp500. Usually does away with most of our grounding problems.

Posted by Carsten Kurz (Member # 5396) on 09-18-2011, 07:14 PM:

Yep. Two-Channel passive DI-Box will most likely solve the issue.

- Carsten

Posted by Phil Ranucci (Member # 3820) on 09-19-2011, 02:51 AM:

Grounding is a pain. The best idea is one path to ground.
How is the Barco powered? Does the pedestal have a circuit breaker panel for distribution, or did they run conduit directly to the machine? Did they run a ground back to the breaker panel? Does the series 2 have a separate 110VAC power input for the electronics? If so, have you tried powering the blu-ray from the same source? Does your audio rack get it's power from the same panel as the Barco?
A direct box will probably clear it up. But it's interesting that you have this problem on every screen. Sounds like an issue with the electrical system in the building. Have you measured with a VOM AC to ground and between the CP 500 chassis and the blu-ray chassis?

Posted by Eric Robinson (Member # 2966) on 09-19-2011, 01:02 PM:

quote: Phil Ranucci
How is the Barco powered?

Single Phase 220VAC - 2 hots / one ground

quote: Phil Ranucci
Does the pedestal have a circuit breaker panel for distribution?

No direct line to breaker.

quote: Phil Ranucci
Did they run a ground back to the breaker panel?

Yes.

quote: Phil Ranucci
Does the series 2 have a separate 110VAC power input for the electronics?

Yes. I have used this one for the BluRay and I have used the soundrack outlet with no difference.
The 120V circuit in the Barco base is a separate circuit from the Barco 220V circuit. FYI

quote: Phil Ranucci
Does your audio rack get it's power from the same panel as the Barco?

Yes

quote: Phil Ranucci
Sounds like an issue with the electrical system in the building.

Yes possibly. Note also that when the house lights are dimmed and raised, you can hear the humm frequency change. If the house lights are just shut off completely, there is just a lower steady hum.

quote: Phil Ranucci
Have you measured with a VOM AC to ground and between the CP 500 chassis and the blu-ray chassis?

Yes 25mV AC.

Posted by Phil Ranucci (Member # 3820) on 09-19-2011, 02:34 PM:

possibly running a ground wire between the blu-ray chassis and the CP500 would help. The dimmer noise might be a clue,also. Any chance the neutral and ground are tied together and your ground isn't really grounded?
I think that 25mv is a lot of potential, especially when dealing with a -10 RCA signal

Posted by Gordon McLeod (Member # 33) on 09-19-2011, 04:26 PM:

The ns input of the cp500 is unbalanced so you will need a pair of transformers to isolate it from the audio system you can get transformer direct boxes from most music rental places avoid the active ones or buy 2 txmr from jensen that are 10k:10k

Posted by Eric Robinson (Member # 2966) on 09-19-2011, 04:53 PM:

It looks like the Jensen JT-11P-1 would be a good choice. I'll give it a try.

The unit from Art looks nice too, I may try one of those at another theatre.

Thanks for the tips :-)