2016 December 22      Audio,   Electronics      Reviews, Teardowns, & Guides

Introduction


A reader asked me how to troubleshoot an amplifier that makes a loud popping noise every time it's powered on.

This is actually a common problem with guitar amps, stereo amps, and other devices that send an amplified signal to a speaker.  Let's see if we can figure this out.

Just a word of caution here.  Amplifiers have deadly voltages.  Tube amps are especially dangerous (500+ volts), but a transistor amp can kill you just the same.  Amplifier components can still carry lethal voltages when they are unplugged!!! 

Remember, you don't have to touch a capacitor directly to get a lethal shock;  the caps are connected to other stuff in the amplifier.  Treat everything as if it's lethal, because it might be.  Don't work on electronics if you don't know what you're doing.  Disclaimer.

Now let's see what we've got here.



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In This Article


The Problem

Capacitors?

The Power Switch

Checking Connections

Semiconductors

Power Delay Circuits

Conclusion




The Problem


You turn on the amplifier and it emits a loud pop.  It sounds like it's amplified to full volume.  Awful!

Many home stereo amps actually have delay circuits to minimize this problem, but not all of them do.  It seems to be a lot more common with guitar amplifiers, especially low-cost or vintage ones.  Also you will encounter it a lot on those big power amplifiers and pre-amps made for cars.

Most amps do emit a mild "thump" sound when turned on.  When there's something wrong with the amp, the sound can be much more harsh.  Let's try to understand why.


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Capacitors?


In the Vintage Electronics Guide, Part 2, we looked at re-capping and why it's important.

Will re-capping fix the loud popping sound at power-on?

There's a chance it might help.  It probably won't solve it completely.  The root cause is likely to be elsewhere.  That said, capacitors can develop punctures through the dielectric material.  Once a pinpoint arc occurs, it creates a track where there can be more arcing later.  The capacitor might actually function normally "most of the time", but it could arc over under certain conditions. 



On this vintage guitar amp, I desoldered the electrolytics and they tested good.  (I really want one of these.)  The thing is, any of the caps could still have internal pinpoint arcs.  ESR is great for knowing which caps to replace, but in a situation like this where there's arcing, you might just want to do a full re-cap anyway.

Arcing can be caused by inrush current.  I believe this can put quite a bit of stress on the capacitor dielectrics.  What happens is that inrush current causes something called "ringing", which is basically unwanted resonance between the caps and the inductive components of the circuit. 

Ringing generates high voltages, much higher than the applied voltage.  Therefore, ringing isn't good for the capacitors or much of anything else.  So, I think it's possible that old caps could worsen the popping sound.

Something that's a little tougher to fix is unwanted signal coupling.  A high voltage pulse due to ringing in the transformer stage can couple capacitively to the signal-amplifying portions of the circuit.  This type of thing often happens when they try to jam stuff together too close, so they can fit all the components into whatever item they're trying to make.  In other words, some of these mysterious noises could be happening because of the circuit layout.  That might not be happening, but it's worth considering.


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The Power Switch


Many guitar amps have rocker switches for the On/Off.

The internal switch contacts may be arcing.  Amplifiers have inductors (transformer, etc.).  A collapsing magnetic field on an inductor will generate high voltages.  When you open the switch but the contacts are not entirely separated, the voltage can arc. 

Arcing across switch contacts has been well-known since the days of the telegraph.  You can get some serious arcing when you suddenly collapse a magnetic field.

Now, what about when you turn on the amplifier?  Closing the switch doesn't collapse a magnetic field, because there is none yet.  Or is there? 

Notice that when you turn the switch on rapidly, the pop sound is worse.  Activate the switch very slowly, and it might not be as bad.  Why is that??  Probably it's because of inrush current to the capacitors. 



The rapid inrush can cause resonance with various inductances in the circuit, including the transformer.  (Electronics engineers correct me if I'm wrong here, but I'm fairly sure that a full-wave bridge rectifier can still have ringing.)

Now, maybe the switch was already damaged by arcing.  That can worsen the effects of ringing.  Did you know that a spark gap is a type of high-voltage regulator?  The gap distance controls the breakdown voltage.  That means it can't arc over until the voltage gets high enough. 

As contacts start to erode, it could make the spark gap a bit wider each time.  We're talking microns here, but it has an effect.

That means the voltage will have to build up higher before it arcs over.  Then the arcing sound can get sharper and louder over time. 

So basically, when you have a circuit where there's ringing, and you've got spark gaps created by various things, the problem can get worse as the spark gaps widen.  And I can just about guarantee you that cheap amps are going to "ring" more than expensive ones, as a general rule.

Here's an early step in troubleshooting an amp like this.  Try bypassing the power switch with a new one.  The old switch may not be the primary cause, but a bad one can make the problem worse. 


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Checking Connections


First check anything that's connected by prongs, plugs, jumpers, etc.  If it can be unplugged, look for tarnish or crud on the electrical contacts.  Clean them.

When you have a faulty connection, that's another possible spark gap.  Arcing = loud pop through speaker.

Make sure to check the speaker connections, too. 

Also look for broken or cracked solder joints on the board.  These can act as miniature spark gaps.  Carefully re-solder any that are obviously bad.  If you see any frosted or grainy-looking ones, re-solder those.  One thing I learned from building Tesla coils:  it's the spark gap that helps the voltage climb way up higher than it would have.  So, loose connections could be creating just the right situation to generate high voltage in your guitar amp.  Inductance + capacitance + spark gap = miniature lightning bolts.  (Sometimes.)

Always check the amplifier's ground connections.  Faulty ground can cause voltages to be where they shouldn't.  And again, spark gaps.  I've seen bad ground connections cause a lot of weird stuff, sometimes quite severe.

Sometimes there will be a connection to ground where it doesn't belong.  This could even cause arcing somewhere else in the circuit.  On this guitar amp, there's a rectangle of insulating material underneath the circuit board.  But guess what: they made it too narrow!  So there's this whole section of circuit board where something unwanted could short it to the chassis.  Mind you, some of the solder connections underneath are supposed to connect to chassis ground.  But some of them aren't!



Believe it or not, I mostly fixed the "loud popping" problem by insulating that section with electrical tape. 

The amp now makes a mild "thump" sound when it's powered on, where before it was a loud "CRACK!!"


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Semiconductors


Finding bad semiconductors on a circuit board can be tedious.  A better-quality amp can have multiple transistors, diodes, JFETs, etc.  One of them could be going bad.  Voltage spikes (a.k.a. "spike transients") can damage semiconductor junctions, making them more prone to arcing next time.  You would not know when this happened.  If the arcing happens after the gain or volume control portion of the circuitry, it's going to be at full volume. 

This could be happening intermittently when you turn on the amp or turn it off. 

A transistor is used as a type of electronic switch.  If you have a magnetic field on an inductor, and you keep switching that on and off, there's going to be kickback voltage. 

Now, if you have a poorly designed circuit, you can have high voltage going over and through your power transistor.  You might wonder how I can be sure of that.  Here's how.  One time I was testing out a high voltage circuit.  There was, uh, a mishap.  Who would ever have known that a 2N3055 could have twenty or thirty thousand volts on the metal case and still function?  I sure didn't know that, up until that point.  But the transistor is actually what kicks the voltage up into the kV range, because it keeps switching on and off the current supplied to an induction coil.  So the magnetic field collapses repeatedly, and you get high voltage arcing.

Well in that example with 30kV on the transistor casing, I had to make a capacitor out of aluminum foil and acetate transparency sheets, tape it together, and connect it to earth ground.  Problem solved, but without that shunt capacitor, the 2N3055 might not have kept working.  I think I might even still have that transistor, but I wouldn't use it for good electronics.

Anyway, yes, transistors can have unseen damage.  They're not really "digital" devices.  They can keep working, but at some reduced capability. 

You can get a transistor tester for cheap;  definitely have one of these in your electronics toolkit.  You'll have to desolder the transistors to check them.


These are essential if you work on radios, amps, etc.  They also test ohms and capacitance.  Just get one..


My cheap guitar amp is a very simple example.  "Q1" looks like a common small-signal transistor.  "Q2" looks like a MOSFET, but it's actually a TDA 2030A integrated circuit.  It's basically a high-wattage op-amp.  Something like this can definitely go bad, just like any other semiconductor device. 




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Power Delay Circuit


Many amps will make a mild "thump" noise no matter what else you do.  They're just designed that way.  These are candidates for a power delay circuit. 

The basic delay circuit keeps the output circuitry, or the speaker, disconnected until the power is fully on.  That means the speaker will be unable to play a "thump" or "pop" sound, because that sound can't get to it!  After that unwanted signal has had enough time to dissipate, the speaker gets connected by the relay circuit.

You can buy kits or ready-assembled boards to do this.  They're not expensive;  try this link.  I don't know that they all use the same method to solve the problem, but the end result is that they're supposed to get rid of speaker pop.  If you are good with electronics, you should be able to figure out how to wire these into your existing amp.  (I haven't tried one yet, so I can't comment on specific wiring.  Hopefully they have instructions.)

Even if you're going to use a power delay circuit, you should try to correct any loud popping noises first.  (Or find someone who can safely do the repair for you.)  Get the noise down to a tolerable level, then install the delay.  Otherwise a really loud "pop" could mean there's something wrong that would eventually damage the amplifier.  Remember, ringing generates high voltages, and high voltages can kill semiconductors, capacitors, and other stuff.  So when someone tells you "the popping sound can't hurt the amplifier", send 'em here to read my article!

To be fair, though, that resonance can take a while to destroy stuff.  Remember that 2N3055 transistor I told you about...


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Conclusion


This was a look at troubleshooting that loud "pop" you might hear when powering up your amplifier. 

Sometimes, the methods I described can reduce the popping sound but not eliminate it altogether.  Many amps make a moderate sound when powered on or off;  if that's still annoying to you, it's always possible to connect a power delay circuit.  Then you can eliminate the unwanted sound altogether.

In this article I've tried to provide some knowledge that you might not find anywhere else.  If you found this article helpful or even just entertaining, please shop for any of your gear through the links on this page.  It's the only way I can keep this site on-line and bringing you helpful articles like this one.



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