AMP popping!

[bishopsnet]

I have a Fender Chorus Combo solid state 2x12 amp. Ever since I have owned it, it randomley makes a loud popping noise when I turn it off. I have tried unplugging the guitar and cable, I have also tried turning all the amp controls to 0. It still does it about 70% of the time. How can I fix this?

Pete

[vonFrenchie]

All of my amps have done that. I have never found anything wrong with them. I just tell myself that its the initial power-up of the speaker. Its like the starter motor in your car. Its loud then your car settles down after that. I'm most likely wrong but I'm not worrying about it.

[gfxbss]

im w/ von on this one. every amp ive ever owned did that. i acctually have a fender 2x12 also, and it pops every time....

[bam]

I think every solid state amp in this planet does. Hmm.. Even the amp in my car.

[sumgai]

All,

Solid state amps "pop" at turn-on or turn-off due to some circuit design constraints. I'll try to keep this non-technical, or at least, if you understand tone circuitry, this shouldn't be a problem for you.

In these amps, there is a dual voltage power supply, positive and negative of the same voltage (usually between 30 and 70 VDC, but it can go higher). These two voltages feed through the power amplifier transistors, and meet up at the common point that goes out to the speaker. In theory, positive and negative should cancel each other out, and there should be no DC voltage at the speaker, only the amplified signal. However, no pair of transistors is ever completely balanced, hence the exact DC voltage at the common meeting point won't be zero. We call this remainder an "offset voltage".

Now, we don't want DC to appear on our speakers, right? (Why not, oh Great and Powerful Oz?) Because an AC signal moves the cone forward and backwards, that's easy to understand. So if a DC voltage appears, what happens to the cone? It moves in the direction of that voltage's polarity and stays there. The cone can now only move a small amount further in that direction, and the net effect is, your speaker now sounds like the Human Fly: "Help me, help me." It ain't pretty.

You can demonstrate this for yourself. If you've never done so err now, you should take the time to hook up a low voltage battery (cell) directly to a speaker. You'll hear a thump each time the cell is connected and then disconnected. (Perhaps a single cell will be weak, but don't use a 9 volt battery, that's a pretty heavy thump. Two cells should be more than loud enough.) If you're using a 10 or 12 inch speaker, you should also see the cone move, and plant itself near one end of travel. Reversing the polarity will move the cone to the other end of its travel.

This action is the same as what happens if even a few tenths of a volt DC offset get into a speaker. Even a small amount of displacement from center will cause a cone to sound like it's being hit with the Ugly Stick.

Now, I'm sure that you all recall that a capacitor passes AC signals, and blocks DC voltage. In the case at hand, we usually insert a large capacitor (to pass low frequencies) between the common meeting point and the speaker lead. This allows the signal to pass through, but blocks the DC from getting out to the speaker. Unfortunately, it takes time for a large capacitor to charge up to full value (and later, to discharge to fully empty (or nearly fully empty)). During that time of transistion from one state to the other, some DC is passing through. (OK, for you purists, it's not, but the difference in AC potential between the two sides is significant - for all practical purposes, the differential is effectively DC.) And therein lays the rub. The capacitor is intended to prevent tone-robbing DC from getting to the speaker, but it takes time to get up to speed, in order to do it's job. During that 1/2 second (or more), we hear a thump for the above noted reasons.

If the thump is obnoxiously loud, your amp may be in trouble. If every amp of that model sounds the same (too loud), then I'd suspect the design itself. Perhaps the engineer, in an attempt to cut costs, used poor quality output transistors, and just as likely, built a poor power supply with little or no significant regulation. At that point, no matter how big the capacitor is, the thump will always come through. In fact, the size of the capacitor has no effect on the amount of the thump itself, after a certain value. After all, if the cap is too small, then the tone goes down the toilet. And at that point, thump or no, who's gonna buy the amp in the first place?

There you have it, straight from one horse's mouth. ;D


sumgai

[vonFrenchie]

That explains why my amp would push the speaker coils out and then they would vibrate a tad. It make this awful buzz and the cone would be pushed almost to its max. I guess it was just getting a DC load. I didnt know what to do so I took the amp into my local music shop.

Solid State... it pops... I dont really care. It lets me know that it should be working fine.