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Author Topic: Amplifier Hiss Manny Knowles "What are these things and WHY are they BLUE???" Posts: 4247 From: Bloomington, IN, USA Registered: Feb 2002 posted 11-03-2010 07:21 PM                     I'm hearing a hiss coming from my screen channels with nothing playing. Am I just being picky or is there something we can do to get rid of it?  |  IP: Logged Richard May Phenomenal Film Handler Posts: 1057 From: Floral Park, NY USA Registered: Aug 2004 posted 11-03-2010 07:27 PM                     Does it go away when you turn the processor fader down?  |  IP: Logged Steve Guttag We forgot the crackers Gromit!!! Posts: 12814 From: Annapolis, MD Registered: Dec 1999 posted 11-03-2010 08:06 PM                     That is generally due to the gain structure not being set up well. Now, some degree of hiss once everything is tuned is normal. One thing I liked about Panastereo over most all others is that it was 10dB quieter than ANY other processor on the market...you didn't get hiss out of it. I suspect I will be out there soon enough though to go through it a bit and see how the installers did. Steve  |  IP: Logged Manny Knowles "What are these things and WHY are they BLUE???" Posts: 4247 From: Bloomington, IN, USA Registered: Feb 2002 posted 11-03-2010 08:50 PM                     quote: Steve Guttag That is generally due to the gain structure not being set up well. Oh my. Well, don't look at ME! LOL We're looking forward to your visit.  |  IP: Logged Eric Robinson Jedi Master Film Handler Posts: 538 From: Santa Rosa, CA Registered: Jan 2005 posted 11-03-2010 09:58 PM                     To minimize hiss, I adjust the preamp output levels to about 80% of max which gives 20% headroom if any adjustment were required. Then I adjust the amp gain to get the proper level. It sounds like your amp gain may be very high and your preamp output low. This is just a guess though which is based on having hissy amps. I know at our chain, the techs who B chained before me set all the amps to full gain. I think they did it to prevent people from damaging the sound system if they happened to tamper with the amp gain knobs.  |  IP: Logged Manny Knowles "What are these things and WHY are they BLUE???" Posts: 4247 From: Bloomington, IN, USA Registered: Feb 2002 posted 11-03-2010 10:00 PM                     Makes sense. Everything is digital prior to the amps. Hence, I suspected it was "amp hiss." I'll have the tech lower the amps until the hiss is gone and then boost the pre-amp gain and see what happens. I suspect that'll do it. Dolby states having the amps full-open as the preferred state. Interesting.  |  IP: Logged Steve Guttag We forgot the crackers Gromit!!! Posts: 12814 From: Annapolis, MD Registered: Dec 1999 posted 11-03-2010 10:39 PM                     No no no...Where does Dolby state that. Oddly enough...a procedure was given in the spec for setting the gain structure. If they amp gains are substantially the way up...they they were not set correctly. QSC will have one set the amps all of the way up...but that only applies if the dataport cables are used with the 9dB pad on the dataport inputs. Did they tune it with a D2 analyzer? -Steve  |  IP: Logged Tony Bandiera Jr Film God Posts: 3067 From: Moreland Idaho Registered: Apr 2004 posted 11-03-2010 10:51 PM                     Manny, what processor, crossover/monitor and amps are involved? Which of the screen channel(s) are hissing? Did this come on suddenly or has it always been there? How loud is it? I had a few CP750's that were bad and hissed badly.  |  IP: Logged Manny Knowles "What are these things and WHY are they BLUE???" Posts: 4247 From: Bloomington, IN, USA Registered: Feb 2002 posted 11-03-2010 11:04 PM                     quote: Steve Guttag Where does Dolby state that Here's what it says on page 4-3 of the CP650 Installation Manual: "Set all the gain controls on all power amplifiers to a known repeatable setting. The preferred setting for most amplifier gain controls is maximum. If a different setting is required in order to optimize the noise performance of the system, the controls should be locked in position or marked clearly." (Emphasis is theirs.) Their preferred way didn't quite work out, so now I'll ask the tech to go with Plan B. I sent an email already so he's aware -- coz he's planning to leave tomorrow.  |  IP: Logged Mark Gulbrandsen Resident Trollmaster Posts: 16657 From: Music City Registered: Jun 99 posted 11-03-2010 11:04 PM                     If memory serves me correctly Dolby wants to see 750 mv RMS at the output with volume set to 7. Adjust your amps gain accordingly then depending of course on theor actual gain. If I am wrong then perhaps Sam could chime in on this... Mark  |  IP: Logged Tony Bandiera Jr Film God Posts: 3067 From: Moreland Idaho Registered: Apr 2004 posted 11-03-2010 11:19 PM                     Manny, I still would like answers to my questions, because at UC Irvine I have my CP-650 in HIB operating with the amps wide open and don't have a hiss problem. (MxA Series amps stage and sub, USA 900 for surrounds). Did this come on suddenly? Or was it always there?  |  IP: Logged Manny Knowles "What are these things and WHY are they BLUE???" Posts: 4247 From: Bloomington, IN, USA Registered: Feb 2002 posted 11-03-2010 11:35 PM                     Tony I only noticed it this evening but it's a construction zone so it may have been masked by all the noise. Or maybe I just never noticed it before because I wasn't looking for it. Today our grand drape was being installed and I was having a conversation with the rigger so I was standing right on the stage. The tech came in to talk to me and waited in the 2nd row for me to finish my convo with the rigger. I looked over to him and asked "are you hearing this hissing?" He couldn't hear it from there. So a little later I was still on stage with the rigger and my boss came into the theatre and -- standing in the 2nd row -- asked "is that amp hiss?" I went down to where he was and, sure enough, I could still hear it. It really is THAT loud. But by then our tech was gone for the day. That's why I came on here to ask the question. I needed to know if I was being unrealistic in my expectations. Dolby DMA8 Plus --> CP650 --> QSC DCP300 --> QSC Amps (various) --> QSC 4-way loudspeakers  |  IP: Logged Tony Bandiera Jr Film God Posts: 3067 From: Moreland Idaho Registered: Apr 2004 posted 11-04-2010 12:25 AM                     Ah,ha. the signal chain you described brings the next questions: Does changing modes on the DCP-300 make the hiss go away? Adjusting the fader on the DCP-300, same question? Turning off the CP-650, any effect? Fader on CP-650, any effect? Since you essentially have two processors in the chain you need to isolate which one affects the hiss. Odds are though that it is a gain settings issue as Steve correctly surmised.  |  IP: Logged Steve Guttag We forgot the crackers Gromit!!! Posts: 12814 From: Annapolis, MD Registered: Dec 1999 posted 11-04-2010 06:49 AM                     Please also note, the signal is in the digital domain all of the way until the output of the DCP300...the CP650 has the Cat 778 card so the output is also digital. There is only one D/A stage. There were instructions on how the set the gain structure...they don't appear to have been followed.  |  IP: Logged Manny Knowles "What are these things and WHY are they BLUE???" Posts: 4247 From: Bloomington, IN, USA Registered: Feb 2002 posted 11-04-2010 03:14 PM                     Problem appears to be associated with DMA8 Plus. Goes away when other formats are selected on CP650, or when the DMA8 unit is disconnected. DMA8 is connected to external input (Format 11) Analog?  |  IP: Logged

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