Amp volume drops significantly while playing

[Msegbass]

A few weeks ago I was jamming with a friend of mine and my amp (ampeg svt 450 which I’ve had for 12 years) started to lose power in the middle of a song. It would drop to around 10% volume, then after I stopped playing for a minute it would go back to normal. This lasted all night. When I tried to turn on a few days later the amp would not power on at all. No lights or sound would turn on.

I ordered a new amp (darkglass alpha omega 500) and played for a few weeks at low volume at home without any problems. I brought it to the same friends house last night and after a few hours of playing, the amp volume dropped to around 10% of what it was in the middle of a song.

Does anyone have any ideas what the problem could be? My theory is that it has to do with the power in my friends basement. What do you think?

 

[willsellout]

Could be. How are you EQ’ing your amps?

 

[Msegbass]

Could be. How are you EQ’ing your amps?

Eq pretty flat, bass and lower mid up a bit, treble cut a bit

 

[Old Garage-Bander]

Hi. Welcome to TB. For this tread it might be helpful to know where you are located.

When you go to your friend's house do you just take your head and maybe use a different cab that he has there?
Or is there anything else different with the equipment you use or how it's set up there?

Power is very unlikely, though it was my first thought as well... then I thought more about it.
Your location (country) might be helpful.

For the power to bad enough to cause what you have experienced, I'd think there would be a lot more things at your friend's place that wouldn't be working right. Lights dimming, large appliances not working right...
Modern amps are pretty rugged when it comes to withstanding power problems.

If you were playing into the wrong speaker load, or a defective cabinet with an intermittent problem, I can can see these things possibly happening to your amps. Even a bad speaker cable can cause problems. You'll need to do a lot of detective work to discover what is really going on here.

Any time you experience problems with equipment, it's always best to shut down and get the problem resolved rather than continuing to play. Your SVT, being older, may have been more susceptible to whatever is causing this. But until you find out what's really going on, I wouldn't be subjecting your Darkglass to any of it.

 

[Msegbass]

Hi. Welcome to TB. For this tread it might be helpful to know where you are located.

When you go to your friend's house do you just take your head and maybe use a different cab that he has there?
Or is there anything else different with the equipment you use or how it's set up there?

Power is very unlikely, though it was my first thought as well... then I thought more about it.
Your location (country) might be helpful.

For the power to bad enough to cause what you have experienced, I'd think there would be a lot more things at your friend's place that wouldn't be working right. Lights dimming, large appliances not working right...
Modern amps are pretty rugged when it comes to withstanding power problems.

If you were playing into the wrong speaker load, or a defective cabinet with an intermittent problem, I can can see these things possibly happening to your amps. Even a bad speaker cable can cause problems. You'll need to do a lot of detective work to discover what is really going on here.

Any time you experience problems with equipment, it's always best to shut down and get the problem resolved rather than continuing to play. Your SVT, being older, may have been more susceptible to whatever is causing this. But until you find out what's really going on, I wouldn't be subjecting your Darkglass to any of it.

The cab is the same in both places. Could the cab be causing the amp problems? It is a fender rumble cab rated at 4 ohms but it is older and has seen a fair amount of gigs and transportation. We are in Canada. I will change all cables and try it but don’t want to try too much until I know what’s going on

 

[Old Garage-Bander]

The cab is the same in both places. Could the cab be causing the amp problems? It is a fender rumble cab rated at 4 ohms but it is older and has seen a fair amount of gigs and transportation. We are in Canada. I will change all cables and try it but don’t want to try too much until I know what’s going on

OK. Thanks for the extra info. I wouldn't expect power in Canada to be bad enough to cause any of this. If you were in East Buhgrania (made up place but with a dodgy power grid ) then maybe.

Absolutely, a problem with the cab can cause this. Being on the road as it's been makes it even more likely.
Yes, rule out the easy stuff like cables and whatever, but definitely get that cab checked out. For starters I'd be looking for an intermittent wiring problem inside the cab. Many amps will go into some kind of self protection/shutdown mode when they see an improper load.

As far as it seeming to only happen at your friend's (though I think you said your SVT died at home?) this could be coincidence. If it happened a lot, and only when you played at your friend's place, I'd suspect something going on there. But only a couple or three times, could just be the luck of the draw.

 

[seamonkey]

Also, if you have an active bass, check the battery in it.

 

[Msegbass]

OK. Thanks for the extra info. I wouldn't expect power in Canada to be bad enough to cause any of this. If you were in East Buhgrania (made up place but with a dodgy power grid ) then maybe.

Absolutely, a problem with the cab can cause this. Being on the road as it's been makes it even more likely.
Yes, rule out the easy stuff like cables and whatever, but definitely get that cab checked out. For starters I'd be looking for an intermittent wiring problem inside the cab. Many amps will go into some kind of self protection/shutdown mode when they see an improper load.

As far as it seeming to only happen at your friend's (though I think you said your SVT died at home?) this could be coincidence. If it happened a lot, and only when you played at your friend's place, I'd suspect something going on there. But only a couple or three times, could just be the luck of the draw.

Ok thank you! They both happened at the same place, but that’s also the only place I use that equipment at high volumes for an extended period of time.

 

[Msegbass]

Also, if you have an active bass, check the battery in it.

I was using a passive bass.

 

[Slevage]

You should have the impedance of the speaker cabinet verified, especially after playing it for a while.

Note that I said impedance, not resistance. (Multi meters can not measure impedance).

 

[Old Garage-Bander]

Ok thank you! They both happened at the same place, but that’s also the only place I use that equipment at high volumes for an extended period of time.

Ah ha! Another piece of the puzzle is revealed!

Yes, operating at higher levels means the amp is working harder and producing more heat. Many amps use temperature to trigger protection circuits.

This certainly can be due to a cab problem, and now that we know what we know, it may not be an intermittent problem.

Something to try, if you are willing...
Do you know how to test speakers using a battery?
If so, remove the grill cloth and do the battery test on the full cabinet, at the input jack. See if all four speakers move. And if possible see if they move by the same amount compared to one another and in the same direction.

Alternately, you could try watching the cone movement while playing, but this might make it a little harder to observe what's happening.

If you have a bad speaker (voice coil open, for instance) this will change the impedance of the whole cabinet.

Otherwise, get your cab checked over by an amp tech.

 

[Old Garage-Bander]

You should have the impedance of the speaker cabinet verified, especially after playing it for a while.

Note that I said impedance, not resistance. (Multi meters can not measure impedance).

Good point. Heat related issues might be at hand, especially if one speaker is failing. Under load testing is always preferable, as well as doing it over a period of time.

And also correct that that Multimeters do not measure impedance. However someone with experience can interpret impedance of a speaker or cab that result from using a Multimeter.

I doubt that most techs will have an audio impedance meter. There is little call for knowing exact cab/speaker impedance in the repair side of the business. The stated impedance of a speaker or cab is not necessarily, or for that matter, consistently exact. It is nominal only, just ballpark so to say. But any good tech will know other means to verify the proper operation of a speaker or cab.

 

[beans-on-toast]

I suspect that the problem is related to a bad connection in your speaker cable or cabinet wiring. For starters, change the speaker cable and see.

If the wall voltage is sagging, you can measure it with one of these inexpensive monitors. Go to amazon and search for DM-55-1.

 

[CTW68]

I suspect that the problem is related to a bad connection in your speaker cable or cabinet wiring. For starters, change the speaker cable and see.

This happened to me. A solder joint in the speaker cable went sort of half-bad, and my volume would drop but not cut out entirely. Worth a look.

 

[Slevage]

Good point. Heat related issues might be at hand, especially if one speaker is failing. Under load testing is always preferable, as well as doing it over a period of time.

And also correct that that Multimeters do not measure impedance. However someone with experience can interpret impedance of a speaker or cab that result from using a Multimeter.

I doubt that most techs will have an audio impedance meter. There is little call for knowing exact cab/speaker impedance in the repair side of the business. The stated impedance of a speaker or cab is not necessarily, or for that matter, consistently exact. It is nominal only, just ballpark so to say. But any good tech will know other means to verify the proper operation of a speaker or cab.

I agree that the exact impedance isn’t the issue. A significant change after playing it would be relevant to the OP’s issue. Normally, z increases with heat. A faulty coil could present a lower z when heated if the coating of the mag wire is compromised.
I’m unfamiliar with an audio impedance meter, but a basic LCR meter would work for this evaluation. The DCR function is much more accurate than the typical multi meter set to measure resistance.l at the low end of its range.
I would measure inductance with an LCR meter to evaluate the speaker.

 

[Old Garage-Bander]

I agree that the exact impedance isn’t the issue. A significant change after playing it would be relevant to the OP’s issue. Normally, z increases with heat. A faulty coil could present a lower z when heated if the coating of the mag wire is compromised.
I’m unfamiliar with an audio impedance meter, but a basic LCR meter would work for this evaluation. The DCR function is much more accurate than the typical multi meter set to measure resistance.l at the low end of its range.
I would measure inductance with an LCR meter to evaluate the speaker.

Yes, any change that could damage an amp would be significant and relevant. (Removing my Captain Obvious hat now. LOL) And I agree that the faulty voice coil due to heating the insulation of the wire to the melting point would present a change of impedance. But maybe not so much of a change due to adjacent coils vs the whole thing being shorted. But if the insulation melts at one place it can reduce the coils resistance (increasing the current) and that might cascade to multiple failure points throughout the coil.

For an open, you only need one fault anywhere in the the electrical path to the coil. Not as likely that the coil itself would open, but anywhere from the tinsel to where that speaker's wiring is common with other speakers can also throw everything off impedance-wise. With four speakers in a cab the possibilities grow in numbers.

I believe both amps are solid state. Based on that, and that most solid state amps tolerate no load (open) vs the load being too low, (shorted, or nearly so) I have to concede that a shorted voice coil, or even multiple speakers with shorted, or partially shorted coils is the more likely scenario. But I would need to know more about how each amp behaves under both circumstances. "Suffice it to say, the cab is the likely culprit." quote - Capt. Obvious.

Impedance meters are not in as widespread use as are LCR meters. An audio impedance meter could incorporate LCR measurements, and also account for frequency, and then display the result directly as impedance. If you use an LCR meter to measure the speaker's inductance you can do the math to determine impedance at any given frequency. An audio impedance meter would be something I'd expect to see more on the test bench for someone designing speakers rather than in an amp repair shop.

 

[Slevage]

This happened to me. A solder joint in the speaker cable went sort of half-bad, and my volume would drop but not cut out entirely. Worth a look.

A bad solder joint = R in series with the load, limiting current to the speaker.

 

[Slevage]

Yes, any change that could damage an amp would be significant and relevant. (Removing my Captain Obvious hat now. LOL) And I agree that the faulty voice coil due to heating the insulation of the wire to the melting point would present a change of impedance. But maybe not so much of a change due to adjacent coils vs the whole thing being shorted. But if the insulation melts at one place it can reduce the coils resistance (increasing the current) and that might cascade to multiple failure points throughout the coil.

For an open, you only need one fault anywhere in the the electrical path to the coil. Not as likely that the coil itself would open, but anywhere from the tinsel to where that speaker's wiring is common with other speakers can also throw everything off impedance-wise. With four speakers in a cab the possibilities grow in numbers.

I believe both amps are solid state. Based on that, and that most solid state amps tolerate no load (open) vs the load being too low, (shorted, or nearly so) I have to concede that a shorted voice coil, or even multiple speakers with shorted, or partially shorted coils is the more likely scenario. But I would need to know more about how each amp behaves under both circumstances. "Suffice it to say, the cab is the likely culprit." quote - Capt. Obvious.

Impedance meters are not in as widespread use as are LCR meters. An audio impedance meter could incorporate LCR measurements, and also account for frequency, and then display the result directly as impedance. If you use an LCR meter to measure the speaker's inductance you can do the math to determine impedance at any given frequency. An audio impedance meter would be something I'd expect to see more on the test bench for someone designing speakers rather than in an amp repair shop.

An inexpensive ($400) LCR meter is a 4 lead device that can be set to 50,100,1k,10k,100k Hz. Very handy in a coil winding shop. A voice coil breakdown in the OP’s case could have originated in the early manufacturing of the coil or the mag wire itself. This is just one possibility of the volume drop. Relative inductance from cold to hot should show no change.

 

[mike buzzetti]

Resistance is typically about 75% of the rated 8 OHM inductance IME , not exact but close enough to test for a bad cab.

 

[Thumpin6string]

If it has an effects send and return, put a patch cable between them and see if that fixes it. If it does, it's dirty jacks. A little Deoxit on a plug pulled in and out a few times should fix it. Also, a dirty headphone jack can cause this.