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Amp popping
Old 1st March 2014
  #1
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3 Reviews written
🎧 5 years
Amp popping

Hey slutz
Been a while... Hope no ones been trollin each other..

I have a bass amp (swr-350, ax7 pre and SS power) hooked up to many 15s. It makes a little pop when it's turned on (trollololol) but has started making a big pop when it's turn off.

Any ideas on how to rectify this? I've considered rewiring the fan defeat switch as a speaker mute, but would rather treat the problem and not the symptom.

I don't have a schematic handy
Old 1st March 2014
  #2
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You might have some DC offset going directly into the speaker when the amp is turned on. That's easy enough to check with a volt meter.
Old 2nd March 2014
  #3
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So measure the speaker cable for a dc voltage spike when turning the amp on? Would it be the same when turning the amp off (this is where it's really noticeable, when turning the amp off, also problematic becauae it wasnt doing this a few weeks ago.

What would dc offset on the speaker out be a symptom of?
Old 2nd March 2014 | Show parent
  #4
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Richard Crowley's Avatar
 
🎧 10 years
Quote:
Originally Posted by enginefire ➑️
What would dc offset on the speaker out be a symptom of?
It is a symptom that something is drifting in the amplifier circuit and is in serious need of diagnosis and repair. You are lucky that the symptoms are gradual. It is much more common for a power amplifier to fail in a catastrophic manner, rather than giving early-warning symptoms that it is sick. And catastrophic failures typically take out many expensive components in modern circuit designs. I would certainly stop using the amp to protect it (and the speakers) from further destruction.
Old 2nd March 2014
  #5
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Thanks Richard that's what my mind was telling, my wallet didn't want to listen.
I'll measure tonight for the offset and then swap out the amp for a few days if that's the case.
Old 2nd March 2014 | Show parent
  #6
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Quote:
Originally Posted by enginefire ➑️
What would dc offset on the speaker out be a symptom of?
Bad design? Individual parts drifting out of spec?

If the amplifier is direct coupled all the way through, then any DC offset at the input gets amplified all the way thru to the output, meaning the problem may actually exist previous to the output stage. They are called DC amplifiers for a reason. The first thing to check is whether the amp, or any portion of it, is direct coupled.

A simple solution is to add coupling caps between the output section and speakers. Maybe they are already there and have become leaky. Of course add caps is heresy to some purists, and will slightly change the sound. Personally, I'd first find where the problem is. An electrolytic coupling cap somewhere in the circuit that has become leaky is one common cause of rising DC offset. I'd recap the electrolytics if they are getting old just to remove the possibility of they have become leaky.
Old 2nd March 2014
  #7
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🎧 10 years
But if it was designed for direct-coupling and it only started popping recently, then putting in a capacitor is only masking the symptom. Like putting a BandAid over melanoma.
Old 2nd March 2014 | Show parent
  #8
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Quote:
Originally Posted by Richard Crowley ➑️
But if it was designed for direct-coupling and it only started popping recently, then putting in a capacitor is only masking the symptom. Like putting a BandAid over melanoma.
Yes, I covered that. I suggested that purists usually find this solution disgusting. Read my post better next time.
Old 2nd March 2014
  #9
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Richard Crowley's Avatar
 
🎧 10 years
It is not a question of "purist". It is a question of glossing over what could be a catastrophic problem.
I respectfully disagree with the suggestion of side-stepping the problem until it blows up.
Old 2nd March 2014 | Show parent
  #10
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Quote:
Originally Posted by Richard Crowley ➑️
I respectfully disagree with the suggestion of side-stepping the problem until it blows up.
Where did I specifically side step? I just laid out the available alternatives. Some better than others.

I specifically said... " Personally, I'd first find where the problem is." That is not a statement that side steps the issue.
Old 3rd March 2014
  #11
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3 Reviews written
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ok I took the DC offset measurements:
when the amp is powered on it is a steady -0.1 volts
after I turn the power off, it swings to a high of +1.6 volts and then steadily drops.

If the volume is at zero when the amp is turned off, there is no popping.

Does this shed any light on the possible problem?
Old 3rd March 2014
  #12
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🎧 15 years
Hi
Yes, a stage BEFORE the volume pot has an 'issue'. This at least gets into the realm of a 'cheaper' repair.
Decent amplifiers have relays that disconnect the speaker until a while after the supplies have stabilised and before they collapse significantly at switch off.
Having 0.1 Volts DC on a speaker is not brilliant but 'OK' but as the 1.6 Volts is 'transient' (it goes away after a short time) it is simply being 'impolite' but not damaging.
Correctly specified capacitors in the right places are your friends!
Matt S
Old 3rd March 2014
  #13
Gear Addict
 
Minion's Avatar
 
🎧 15 years
Im no expert but maybe your amp has a soft start or speaker relay that is failing .... When you turn your amp off the PSU capacitors start to discharge and if Positive or negative caps discharge at a different rate it can cause a temporary dc offset at the output which could cause a popping sound ... the speaker relay connects and disconnects the speaker at turn on/off so the DC offset doesn't reach the speaker .....
Old 3rd March 2014
  #14
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🎧 15 years
The original post is a trifle cryptic in mentioning AX7. If it is indeed a 12AX7 valve preamp stage feeding a Solid State power amp then you need to be fixing it 'pronto' as a complete failure of the cap coupling the valve to the volume pot then power amp WILL toast something.
Maybe there is a 'pop eliminator' circuit (soft mute)? that has failed or on the way out which is not suppressing the preamp output before it hits the volume control?
A circuit would help loads!
Matt S
Old 3rd March 2014
  #15
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I will dig up the schematic, i had found it at one point.
yes, ax7=12ax7, no intention of being cryptic!
Old 4th March 2014
  #16
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3 Reviews written
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the 1st schematic in the pdf is the right version.
i dont think it has any relays or soft start functions.
Attached Files
File Type: pdf Bass 350 Schematics.pdf (600.4 KB, 256 views)
Old 4th March 2014
  #17
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3 Reviews written
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On closer inspection, the second schematic looks good too, in fact better, the redraw is much clearer.
I am suspecting the power supply caps 6800/80v as they look a little physically swollen (pic tomorrow with luck)
Old 6th March 2014
  #18
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3 Reviews written
🎧 5 years
So I tested the voltages for the ICs, they look good at plus minus 15.
The problem is definitely in the preamp section.

Any suggestions on how to go about finding the bad part?
Old 8th March 2014
  #19
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3 Reviews written
🎧 5 years
So I brought the amp to my tech today, he put a 0.01 cap across the power switch and voila no more pop. He explained that the pop was from extra power in the power transformer from inductance. For what it's worth.
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