Christian Spaeth Posted July 20, 2011 Report Share Posted July 20, 2011 I am using an SKP 2000 plug on transmitter by Sennheiser for sending a CMIT's signal via a Sennheiser EK 100 G3 diversity receiver to a 788T. Now on some locations I hear this very high frequency noise on that channel, a bit like a feedback sound, a tone (not continouos) at about 12-15 kHz I guess, sometimes stronger, sometimes weaker. It's usually only really audible on quiet passages, and of course it won't be too hard for post to eq it out, but I am clueless as to where it comes from, so I'd like to see if someone else has had this problem and maybe knows what to do about it. I have tried everything I could think of, changed frequencies, switched pilot tone on and off, changed RF power from high to standard to low and back but couldn't get rid of it. It's only on some locations, other days I don't hear it at all. And it's not a tinnitus, as it's only on that one channel. I'll appreciate any of your thoughts on this issue. Quote Link to comment Share on other sites More sharing options...
chris_bollard Posted July 20, 2011 Report Share Posted July 20, 2011 Hi Christian Might be power induced RF, from 788 or other attached devices. Do you have keyboard plugged into the 788? Ive had issues there related I think to the USB cable. Haven't had a chance to workshop it.... Chris Quote Link to comment Share on other sites More sharing options...
Christian Spaeth Posted July 21, 2011 Author Report Share Posted July 21, 2011 thanks Chris, but no, no keyboard attached. Still trying to figure it out... can RF be disturbed by metal? I'm starting to think maybe it's because my cart is all aluminum, and a lot of it... Quote Link to comment Share on other sites More sharing options...
jason porter Posted July 21, 2011 Report Share Posted July 21, 2011 RF is affected by metal, but most carts are metal. Have you tried plugging in a headset directly to the receiver and listening while taking it away from the other gear? Are you running the rx from a separate power source? Could it just be poor reception? Quote Link to comment Share on other sites More sharing options...
Tom Visser Posted July 21, 2011 Report Share Posted July 21, 2011 My guess would be leakage from a switch mode power supply into the ground plane. Try using an isolation transformer between the EK100G3 and the 788T's input. http://www.jensen-tr....com/pb2xx.html Set the dip switches to float chassis ground to pin 1 and shell. The other experiment to try is to use a different phantom power source and disable phantom on the SKP, maybe the 48V is slightly out of spec for the CMIT or is noisy. Denecke and Ambient both make nice phantom supply boxes. I'm not suggesting you add this weight to the boom, but just to determine if the SKP phantom supply is the source of the problem. If it is, then next is to determine if it is that specific unit or a design incompatibility between the SKP and the CMIT in general. Quote Link to comment Share on other sites More sharing options...
Christian Spaeth Posted July 22, 2011 Author Report Share Posted July 22, 2011 My guess would be leakage from a switch mode power supply into the ground plane. Try using an isolation transformer between the EK100G3 and the 788T's input. http://www.jensen-tr....com/pb2xx.html Set the dip switches to float chassis ground to pin 1 and shell. The other experiment to try is to use a different phantom power source and disable phantom on the SKP, maybe the 48V is slightly out of spec for the CMIT or is noisy. Denecke and Ambient both make nice phantom supply boxes. I'm not suggesting you add this weight to the boom, but just to determine if the SKP phantom supply is the source of the problem. If it is, then next is to determine if it is that specific unit or a design incompatibility between the SKP and the CMIT in general. thanks Tom, a mixer I know uses the exact same combination as me and doesn't have that problem. His SKP2000 is in a different frequency range, though. Mine is C, his is E band I think. I will look into the first experiment you mentioned, thanks again for that. Quote Link to comment Share on other sites More sharing options...
Tom Visser Posted July 22, 2011 Report Share Posted July 22, 2011 thanks Tom, a mixer I know uses the exact same combination as me and doesn't have that problem. His SKP2000 is in a different frequency range, though. Mine is C, his is E band I think. I will look into the first experiment you mentioned, thanks again for that. Good luck with that. The problem with ground path issues, is that there is no discernible scientific way to determine why, when, or with what combination of equipment you'll have a problem, even if you know someone else with the same setup. Those transformers are good to have on hand anyways, some day you'll work with a camera that is tethered to you that is running off of facility AC power and have some sort of DC offset, or other electrical problem that floating the audio outputs will solve. Maybe your working with a house feed from a FOH mixer and can't trust that he isn't going to zap your line inputs with phantom power, etc... Quote Link to comment Share on other sites More sharing options...
Christian Spaeth Posted August 9, 2011 Author Report Share Posted August 9, 2011 The shoot is over, I couldn't entirely solve this problem, so I decided to dig this one up again to get some more opinions on this: I have had the impression that when my bag rested on my cart (which is almost entirely made of aluminum) I had these problems with a high feedback-like tone/noise on some of the wireless Rxs. Being quite a noob when RF theory is concerned, I would like to ask those of you who know more about this whether it is at all possible that a metal obstacle like that can notably influence/disturb RF reception? Quote Link to comment Share on other sites More sharing options...
LarryF Posted August 9, 2011 Report Share Posted August 9, 2011 The metal cart, by itself, would not cause the whine. If you have another piece of gear which is grounded to the cart, sitting on it, touching it, etc., that has a switching power supply or is defective and oscillating it might cause the problem. You need to do a process of elimination, which is to turn everything off but the one transmitter and one receiver and see if you have the noise. If you still have the noise, try moving to a different location in case there is a local noise source. If it noisy in a new location, the units themselves may be defective. If the system is quiet when just the receiver and transmitter are on, then try turning on other items one at a time and see what happens. At some point the noise should come back and then you can try tracking it down. Best, Larry F Lectro The shoot is over, I couldn't entirely solve this problem, so I decided to dig this one up again to get some more opinions on this: I have had the impression that when my bag rested on my cart (which is almost entirely made of aluminum) I had these problems with a high feedback-like tone/noise on some of the wireless Rxs. Being quite a noob when RF theory is concerned, I would like to ask those of you who know more about this whether it is at all possible that a metal obstacle like that can notably influence/disturb RF reception? Quote Link to comment Share on other sites More sharing options...
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