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Thread: Weird Noise Thru Amp

  1. #1
    Registered User Mandobart's Avatar
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    Default Weird Noise Thru Amp

    Playing my Carvin AG300 (bought new about a year ago) outdoors at a campground wedding this past weekend. I'm playing both my piezo pu equipped 12 string ovation and my piezo pu equipped 10 string mandocello plugged into the channel 1 1/4" jack (set for Hi Z) thru my tuner pedal, so I can mute during the service and when switching instruments. I have an SM 57 plugged in to channel 2 for the singer performing with me. The pastor is using an SM 58 plugged in to channel 3. The amp is powered by my 200 watt inverter. I set everything up earlier that day with the amp and passive aux cabinet out in front of the mics and instrument (no monitors). Everything sounded fine for the sound check. Service starts and everything sounds great for the prelude music (just me playing the mandocello although both mics are on but not being used). For the wedding party entrance I'm playing 12 string, singer is singing, sound is great. For the majority of the service the pastor is speaking, guitar muted, singer's mic on but not being used. Sound is great. We did a couple of songs for the whole assembly to sing together (I'm playing 12 string, singer leading the assembly). All with no problems. Right at the end of the ~1 hour service, with instrument muted and just the pastor speaking, a loud hiss/static noise sound comes thru the system. I shut the amp off. Pastor finished and I started playing the recessional music on the mandocello. I turned the amp back on, everything sounds fine (just instrument, no vocals but both mics are on) for about two minutes then the loud hiss/static noise returns so I shut everything back down.

    I've used this same setup several times before with just an instrument in channel 1 and SM 58 in channel 2 for 3 hour gigs with no issues. I plan to do some investigating this before my next gig, but do any of you have a similar experience or insight on most likely cause? This was definitely not feedback, sounded more like RF noise. There is no cell coverage or AC power in this campground. There is a two lane paved road running close to the campground (about 100 yards straight line) with light traffic, could a CB transmission from the road cause this? Thanks!

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    Registered User almeriastrings's Avatar
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    Default Re: Weird Noise Thru Amp

    It certainly could be RF interference from a CB transmitter or similar, e.g. walkie talkie or taxi transmitter, though these are a lot less common than they used to be as the majority of users have switched to cell phones by now.

    This used to be a really common problem.

    Other possibilities include a PSU failure coming on, possibly as a result of the type of supply.
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    Although you say there is no cell phone coverage in the area, is it possible that someone standing near the amp has a cell phone turned on? That is the most common source of RF-type noise I hear. The phone transmits even when it is not connected.

    Ed

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    Market Man Barry Wilson's Avatar
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    Default Re: Weird Noise Thru Amp

    Is that a tube amp? maybe a tube starting to go
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    Registered User Mandobart's Avatar
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    Default Re: Weird Noise Thru Amp

    No tubes. Could have been a cell - this weekend I'm going to set it up the same and try keying a radio and cell by it.

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    Default Re: Weird Noise Thru Amp

    Could it be due to a voltage sag in the batteries? It sounds to me like everything was going fine for most of the service then you turned it off when it first happened and then turned it back on again after a few minutes and the hiss was gone. Then after a few more minutes, it started again, all over the course of an hour. What else changed with your system?

    When you run your test, try your radio/cell test when you first fire up the system and if nothing happens, then run the test again after an hour or so trying to recreate the same circumstances. I'm kind of curious...

    Len B.
    Clearwater, FL

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    poor excuse for anything Charlieshafer's Avatar
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    Default Re: Weird Noise Thru Amp

    Yeah, I'm wondering if Len is on to it. As the batteries wore down they lost just enough juice to affect something. A quick turn off allows the batteries to level off and "recover" (overheating of batteries?) and then a shorter interval before failure. I've never had a cell phone or radio call interfere, and our venue is right by the fire station and a cell tower.

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    Registered User Mandobart's Avatar
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    Default Re: Weird Noise Thru Amp

    The battery (deep cycle 35 H-hr AGM sealed lead-acid) is only a few months old. I've used it several times for much longer gigs. I found I could run the amp about 7 hours before I got the low voltage alarm on the inverter (~11.5 VDC). The battery voltage is still well above that (over 13 VDC with everything shut off). I fully charged it up before heading out to the woods for the wedding. The inverter is a 200 W Cotek pure sine wave.

    I haven't recharged the battery since I got back, wanting to recreate the original setup as much as possible. My multimeter shows 12.8 VDC with the inverter on and powering the amp now. No noise of any kind making cell calls right next to it. Can't find the old walkie-talkies I used to have to try if RF does anything.

    I will let you guys know if I find anything.
    Last edited by Mandobart; Jul-08-2016 at 6:35pm.

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    Default Re: Weird Noise Thru Amp

    I have a 28 a/hr battery and did 4 hrs straight with dual monitors bridged amp... I also use a solar panel to trickle power back in..

    old cel phones used to cause glitches in our rc cars with fm radios (am could be affected by lots). they would search for service (our track was way up the mountain). There was a big to do about that at one point until we went to 2.4ghz radios. maybe your phone is too new
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    Registered User Mandobart's Avatar
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    Default Re: Weird Noise Thru Amp

    One thing I forgot to mention that could matter - at the wedding gig I set up my aux speaker cab and amp at greater separation than usual, so I coupled two cords (one an actual twin conductor speaker cord and one a single conductor plus shield instrument cord) together with one of these phone jack couplers Click image for larger version. 

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    Also the pastor's mic was further from my amp than I usually have it so I coupled two XLR mic cords together with one of these XLR couplersClick image for larger version. 

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    Everything was set up on the forest floor, which was pretty dry. My inverter was and is ungrounded - it has a hot neutral and ground, with the neutral isolated from the ground which just goes to the inverter chassis, but the chassis is not connected to earth ground. I'm thinking I should bring a mini-ground rod (like a metal tent stake) lugged to a cable to actually ground everything. Even so, the ground of itself wouldn't explain why everything was fine except for the two instances, and I can't replicate the noise now.

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    Default Re: Weird Noise Thru Amp

    If you go into an electrical box the neutral and ground are connected in the same place. The ground is a safety in case someone wired something wrong or something happens there is a second ground. I don't think you need a rod into the ground.
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    Registered User Mandobart's Avatar
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    Default Re: Weird Noise Thru Amp

    Quote Originally Posted by pops1 View Post
    If you go into an electrical box the neutral and ground are connected in the same place. The ground is a safety in case someone wired something wrong or something happens there is a second ground. I don't think you need a rod into the ground.
    The whole reason for grounding is to ensure in the event of a fault, the fault current has a direct path to ground so we don't become that path to ground. The NFPA 70 (NEC) code requirement for a service entrance panel is for the neutral and ground to be bonded for residences and businesses. That panel is required to be grounded to a solid earth ground with a rod driven deep into the earth beyond the drip line of the eaves (using the cold water pipe for panel ground has not been accepted practice for many years, although hot, cold and DWV pipes are required to be grounded too). For subsequent downstream distribution panels off the same service, the ground and neutral are required to be isolated to prevent ground current loops. For the same reason you don't want a generator or inverter to have the ground and neutral bonded together - you never want current intentionally flowing to ground.

    The inverter comes with a lug to connect to your system ground - these are designed for marine or RV application in a permanent installation. If a fault occurs in the inverter or my amp when plugged in to it the chassis could become energized and without a ground connection I would become the path to ground for fault current. So a ground rod would provide some safety which I don't currently have.

    Noise issues in audio equipment are sometimes caused by improper grounding as well, though they are usually a constant hum vs a random burst.

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    poor excuse for anything Charlieshafer's Avatar
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    Default Re: Weird Noise Thru Amp

    Ok, dying battery is out. Ground is out. I'm not thinking RF frequencies unless there was some strange military installation underground there. Cable connection maybe? Maybe someone just shifted and stepped on one? If it can't be replicated, it's too strange to worry about.. We did have one gig where the guitarist kept moving around, stepping on his cable, and unplugging himself from the D.I. mid song.

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    Default Re: Weird Noise Thru Amp

    An inverter on a boat has no ground to earth, I put one on a tour boat a couple of years ago and just grounds to the boat, which sits in the water, that you could say goes to ground. A tube amp with only two prongs one side is chassis ground the other hot, if they get reversed you can get a big shock if you touch something plugged into another amp with the opposite ground. A grounded cord from an amp these days should not get reversed and while you may feel the need for a ground rod, I have never had a problem without, nor do I think it was causing the problem you were having.
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    Registered User Mandobart's Avatar
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    Default Re: Weird Noise Thru Amp

    Yes I agree the ground or lack thereof would not cause this intermittent noise. Boats and vehicles don't have an earth ground, but the electrical systems are grounded to the hull/chassis. A fault that puts voltage on an exposed, normally non-energized part puts you in danger - you will get shocked if you touch it and provide a current path to the hull/chassis. If there is an existing lower resistance path for fault current to the hull/chassis you are much less likely to receive a shock.

    I've found in 12 volt systems the two biggest electrical problems are grounds where there shouldn't be and a failed ground connection where there should be one.

    This is all basic stuff for a guy with significant training and experience in residential, naval and power plant distribution systems but it can be mysterious to a layperson. I still find enough improperly wired systems, cut off ground plugs, etc. to make me never take code compliance for granted when plugging in at a new venue.

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    Default Re: Weird Noise Thru Amp

    One thing I forgot to mention that could matter - at the wedding gig I set up my aux speaker cab and amp at greater separation than usual, so I coupled two cords (one an actual twin conductor speaker cord and one a single conductor plus shield instrument cord) together with one of these phone jack couplers

    You should never use a shielded instrument cord as a speaker cord. The shielding caused variable resistance which may harm your speaker. Always use a twin conductor speaker cord.

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    Registered User Mandobart's Avatar
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    Default Re: Weird Noise Thru Amp

    Not sure what would make the shield resistance vary - if so you would expect problems when using an instrument cord on an instrument as well, where the signal is in the mV range. of course the impedance will vary with frequency in every case. The main reason not to use an instrument cable for a speaker is in high audio output applications because the shield is not designed to handle higher power like an actual conductor.

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    Default Re: Weird Noise Thru Amp

    Would you get this interference from a nearby unshielded or unsuppressed electric motor, or an alternator?

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    Registered User Mandobart's Avatar
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    Default Re: Weird Noise Thru Amp

    Well thanks all for the suggestions and tips. I was unable to replicate the noise from a week ago. One of my common phrases when troubleshooting problems like these on my sub in the USN was "intermittent problems require intermittent troubleshooting..." after I'd been up 20 hours trying to fix an intermittent fault in the reactor instrumentation.

    So its back on the charger for my inverter to get ready for the farmer's market on Friday (probably won't need it - they have AC power there).

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    poor excuse for anything Charlieshafer's Avatar
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    Default Re: Weird Noise Thru Amp

    I'm glad it's not happening anymore, but I hate that not knowing thing.

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    Default Re: Weird Noise Thru Amp

    Quote Originally Posted by Mandobart View Post
    Well thanks all for the suggestions and tips. I was unable to replicate the noise from a week ago. One of my common phrases when troubleshooting problems like these on my sub in the USN was "intermittent problems require intermittent troubleshooting..." after I'd been up 20 hours trying to fix an intermittent fault in the reactor instrumentation.

    So its back on the charger for my inverter to get ready for the farmer's market on Friday (probably won't need it - they have AC power there).
    It is irritating, sometimes you just have to wait for things to fail properly, maybe not the approach you want on a nuclear sub though

    Last week I went to practice, plugged in and no noise, bought it all home plugged in and it all worked so I just though wrong plug wrong hole. Went back yesterday, plugged in and no noise, so I filched another amp and plugged into that, noise. Today I tried again at home, plugged in and no noise, getting somewhere maybe it's the red lead, after all the black was the one I used last night into the borrowed amp. Nope it's the black lead that isn't working, I'm still a bit wary though, normally you crackles with an intermittent lead like that.

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    Registered User Mandobart's Avatar
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    Default Re: Weird Noise Thru Amp

    The noise came back today - outdoor gig at a Farmer's Market. Temperature was about 93 F out, with the amp (Carvin AG300) in full sun. Plugged in to the 120 VAC receptacle there at the market (not using my inverter). Played two half hour sets, turned the amp off in between sets. Near the end of the second set the noise (harsh static) came through. I turned off the amp and unplugged my tuner pedal. Turned back on, played a few minutes and the noise returned. Turned amp off, unplugged mic and mandocello, turned the channel volume knobs all the way down. Turned back on, and the noise was there whenever I turned up the master volume off of zero.

    So I know the noise is not coming from a cable, mic or instrument, because it is there with nothing plugged into it. No noise or problem at all for the first set and most of the second, so I think it is temperature-related as the amp heats up. During normal play the channel volume knobs were both a little less than half way up. The master volume also a little less than half-way. Going to contact Carvin for help.
    Last edited by Mandobart; Aug-12-2016 at 4:01pm.

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    poor excuse for anything Charlieshafer's Avatar
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    Default Re: Weird Noise Thru Amp

    Yup, makes sense. Not sure how, but I'd say it's in the amp...

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    Default Re: Weird Noise Thru Amp

    Could be a capacitor going bad. You can tap on the components with the amp on with a WOODEN chopstick to find the offender. Be super careful, keep one hand away from the amp and be sure the chopstick is made of wood.

    Of course, contacting Carvin will be easier.
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    Default Re: Weird Noise Thru Amp

    Results of an unintentional experiment - I forgot and left my amp on overnight (indoors, in my finished basement) and all day today while I was at work. Nothing plugged in to it, all channel and master volumes turned to minimum. Plugged in my mandolin to it and turned up to normal volume. Everything sounded great with no hiss or unwanted noise. So I'm really thinking its only a problem when I play outdoors in high temperatures - which is at least half of my gigs! No word yet from Carvin, just junk email asking if I want to buy something else...

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