The Acoustic Guitar Forum

The Acoustic Guitar Forum (
-   Acoustic Amplification (
-   -   Weird Noise thru my Amp (

Mandobart 07-05-2016 06:20 PM

Weird Noise thru my 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 (with the tuner pedal out of the chain this time to eliminate it and the second cord as possible noise sources) 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!

varmonter 07-07-2016 06:29 AM

A hiss or a shhhhh sound is usually a signal chain related.
A buzz or static is ground loop.a CB radio transmission
Would sound like "breaker one nine" in that it's
Frequency related and either you would hear it or not.
I would check signal chain to make sure you are
Not adding too much in one path.check to make
Sure eq highs did not get accidently bumped.

Mandobart 08-12-2016 04:07 PM

It's Back....
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.

Any other Carvin users experience this problem?

BTF 08-12-2016 09:37 PM

You may have a faulty capacitor, especially a chip's monolithic bypass capacitor on one of the preamp digital chips. Capacitors are sensitive to temperature, and when a bypass chip goes faulty or dodgy, the result is often a very loud rushing noise.

Another possibility might be a faulty solder connection. As the temperature inside the amp chassis increases, a broken connection will separate. Then, left to cool, the crack will contract and the circuit will function near normal levels. Our shop used to see loads of faulty solder connections, especially when wave soldering became the norm.

Just offering. Good luck!

varmonter 08-14-2016 03:54 PM

Now that i have reread this post the capacitor issue
Seems more likely.dont capacitors charge up and
Discharge.this would explain the delay in time
It takes to hear your noise.the cap charging.

Mandobart 08-15-2016 07:18 AM


Originally Posted by varmonter (Post 5033695)
Now that i have reread this post the capacitor issue
Seems more likely.dont capacitors charge up and
Discharge.this would explain the delay in time
It takes to hear your noise.the cap charging.

Thanks for the reply - but just how long do you think it takes a cap to charge??? I currently work as a technical trainer at a power plant and I can't let a gross conceptual error slide without attempting to correct it. I was also an electronics tech in the USN for over 8 years. Capacitors are basically used in RC (resistor-capacitor) or RL (resistor-inductor) circuits. The charge time is determined by the time constant, which depends on the value of the resistance and capacitance in the circuit. It takes five time constants for a cap to charge or discharge. In most RC circuits the time constant is in the micro to millisecond range. Holy crap for a cap to take an hour to charge up it would litterally have to be bigger than a house!!!! They just don't use caps like that in any consumer electronics.

jseth 08-15-2016 12:36 PM

First thing I would do is to try the amp using normal AC household current... I'm guessing that the issue is the inverter/amp/temperature chain...

Mandobart 08-15-2016 03:29 PM


Originally Posted by jseth (Post 5034415)
First thing I would do is to try the amp using normal AC household current... I'm guessing that the issue is the inverter/amp/temperature chain...

That's a good suggestion, and the first thing I did after getting home from the first time this happened back in July. As you already read in my earlier post, I could not reproduce the problem at home.

As I'm sure you read in my follow up post from Friday, the problem returned when plugged in to the 120 VAC at the venue (farmers market). No inverter was used when the problem returned. Again, when I got back home the problem was gone.

I'm really hoping to hear back from Carvin but until I do I'd sure like to know if any of the dozens of other Carvin AG 200 or AG300 users on this forum have experienced anything similar.

Carruth 08-15-2016 03:59 PM

Never had a problem with my AG300.

Mandobart 08-15-2016 10:35 PM

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...

jseth 08-16-2016 01:16 PM

Yes, if you are playing in direct sunlight and it's warm>hot, THAT will definitely affect the amp's output... I don't know the technical jargon for it, but I have experienced issues when my amp/PA head has gotten really warm...

Some sort of "sun-shield" and a whisper fan would probably keep it cool enough...

Mandobart 08-19-2016 04:17 PM

Here's another update: I played another Farmer's Market Gig today with my Carvin AG300. Plugged in to the AC receptacle there - again not using my inverter. Outdoors, Carvin up on a speaker stand, did not use an extension cabinet this time. Temperature was ~90 F and we started out in the full sun at first. I brought along a small fan which I placed on top of my amp, blowing straight down onto the amp. Played for about 2 hours, 15 minutes without ever turning off. Plugged my mandocello into channel one and a SM58 into channel 2. All volumes set at about half way. No problems at all, no unusual noise, everything sounded fine. It is either a single one or combination of the following that may explain why I had no problems today:

1. Temperature was a little cooler (90 F vs 93 F last week)
2. Cooling fan on top of the amp
3. Did not use the extension cabinet which draws a little more power from the amp
4. Moved an umbrella to shade the amp after about an hour
5. I brought along a friend's acoustasonic as a spare if my amp flaked out

I'll keep posting on this, more for my personal log than anything else but hopefully this info is useful to someone else at some point as well. Never did hear back from Carvin on this subject - they just keep sending me ads.

Mandobart 09-20-2016 11:17 PM

Latest, hopefully last update. I called Carvin about a month ago and reached a real person in their tech department. Since I was still in the 1 year warranty they told me to remove the power amp from the cabinet and send it to them and they would fix it. So I did that. When I took the amp out I was surprised to see great gobs of silicone RTV deposited like some kind of random incontinent rubber bug got loose in there. You can see pics here, post #31

I packaged up my amp along with a long letter explaining the symptoms I outlined above. I also stated that since the problem is temperature-sensitive, and RTV is a good thermal as well as electrical insulator, I was concerned about the non-workmanlike manner in which it was so liberally applied. I contacted the guy I spoke with and asked him directly what he thought of it.

Never did get an answer, but last week (about 2-1/2 weeks after I sent it to them) I received an email saying they fixed my amp and were sending it back. I replied with a thanks and also asked if they could tell me what the cause of the problem was and how I can prevent it from recurring. They answered "gee we don't know, I don't have the tech's notes, but we fixed it and you're all set." The amp arrived today, looking like they gobbed a bunch more RTV on it. More pics at same cafe post.

So I re-installed the amp and tested it out. Works fine, sounds just as good as ever, but I won't know until next summer if it really is fixed as our 90 F days are over here. So I give Carvin a medium-high (7 out of 10) rating for customer service - they were responsive over the phone, they kept me informed on the repair status, shipped back to me no charge. But they weren't able to tell me what the problem was and as a former nuclear submarine electronics tech I hate to see such sloppy work, and I wonder about the silent treatment when I called them on it.

RCGuitar 09-21-2016 11:43 AM

The RTV silicone (while not pretty) is used to hold the caps and power resistors in place and to keep them from moving during numerous hot and cold power ups. You don't want them popping a solder joint as they heat up and expand.

martingitdave 09-21-2016 11:59 AM


Originally Posted by RCGuitar (Post 5073227)
The RTV silicone (while not pretty) is used to hold the caps and power resistors in place and to keep them from moving during numerous hot and cold power ups. You don't want them popping a solder joint as they heat up and expand.

I agree with this. It might have been a (counterintuitive) case of too little RTV, not too much.

Sent from my SM-G930V using Tapatalk

jonfields45 09-21-2016 02:18 PM

My bet is they were not able to reproduce the problem and loaded it up with hot glue hoping that helps. Intermittent problems are a pain!

My friend's active bass had an intermittent condition where the output was distorted even with a fresh battery. I looked inside, did some of the usually checks and found nothing wrong (since it sounded perfect regardless of what I did). There were three tin plated connectors and I soaked them with Deoxit and gave myself credit for a "repair".

Well now a couple years later it did it again and again it worked perfectly on my bench. This time I decided to take the connectors apart to really thoroughly clean them. Low and behold that was all that was holding the circuit board in place and as soon as it came out I saw a broken solder joint with one of the many wires soldered to the back of one of the pots just laying there. I cleaned the connectors and fixed the solder joint. And again I've given myself credit for a repair...

I think the most important concession from Carvin would be an extension of the warranty for one year after their repair.

I've had my share of intermittent problems and it has motivated me to use the least expensive, high quality, least integrated solution for amplification so that I can relatively inexpensively get rid of the offending item rather than chase ghosts. Today that is a single Yamaha DBR10, a Yamaha MG06x mixer for acoustic gigs, and a Boss FDR-1 (Fender Deluxe Reverb modeling pedal) for electric gigs.

jonfields45 09-22-2016 07:57 AM

A few other random thoughts...

If you send something back, put a small mark somewhere with a Sharpie; that way you can tell if you got a new or refurb instead of a repair.

To differentiate new from refurb, or repaired from no problem found, look the PCB(s) over carefully for a little shiny residue (rosin). In production that is cleaned off but in rework it is rarely removed.

For this particular problem which seems to be temperature and humidity related, and infant mortality, I would guess you have an active component (all of which will be encapsulated in non hermetic epoxy cut with silica with no RTV prior to encapsulation) with a bad wire bond. You'd be shocked how cheaply non milspec electronics are made, and still work pretty well. Why they eliminated the other PCBs from being returned must reflect some experience they have with the problem, or cost containment.

I got a replacement PCB for my T5z from Taylor earlier this year and spotted the rework rosin on all three pots. Since it was free and worked, I did not complain :~).

LowCBassStudio 12-07-2017 09:03 PM

Hey if anyone is still here ... I recommended the AG300 to a student Xmas 2016, and here it is Xmas 2017 and Carvin Audio is busted outa business.

This AG300 amp has a little crackle/fizz/AM-radio whine coming out of the speaker even with input volumes and master all the way down...but NOT that loud hiss that fatocaster caught on video.

However, NO noise in the headphone output nor the stereo music input jack with my iPod sound fine. So that means the preamp section is OK. So what's the problem? And has anybody found the schematic?

All times are GMT -6. The time now is 10:45 AM.

Powered by vBulletin® Version 3.8.7
Copyright ©2000 - 2018, vBulletin Solutions, Inc.
Copyright ©2000 - 2018, The Acoustic Guitar Forum

vB Ad Management by =RedTyger=