If you happened to be one of those few misguided folks who followed this blog, you may have noticed that….there hasn’t been any for quite a while. My excuse is that I have to get enough steam up to vent my spite on unsuspecting pieces of electronic equipment, and, more often than not, their manufacturers.
The Ampeg BA300, whether in the 1x 15″ or the 2x 10″ versions, seems, from the various forums I’ve read, to be leaving a trail of very disgruntled musicians in its wake. Many of them having had these amps switch themselves off, mid-phrase. The amp then switches itself back on again after a leisurely 10 seconds or thereabouts, and then, if the volume settings remain the same, does it all again. Although the amp itself seems not to suffer any embarrassment, the same cannot be said of the bass player.
I’d like to be able to say I’d successfully repaired one of these, but the honest statement is that I’ve done no such thing. But I have spent many hours inside one, and can let you in on a few conclusions.
The first mystery is why any builder of amps can think that it’s a great idea for the amp to switch itself off if too much power is demanded of it. The traditional answer to that problem is that the amp goes into clipping distortion, the player hears that, (not pleasant) and turns it down. If the music demands a heavy distortion, the amp just doesn’t go any louder if turned beyond it’s rated power output.
This amp is a ‘clever’ amp. This means that it’s complicated, not necessarily for any good reason except for, maybe, profit boosting purposes. It is a ‘class D’ amp. The ‘D’ stands for ‘dismal’ in my book. But it actually means that it uses a modulated carrier signal (of about 450 kHz if memory serves). There are different versions of the modulated carrier technique; this one uses ‘pulse width modulation’.
They also use a switchmode power supply. It’s a lighter alternative to a traditional isolating transformer, but it has its own problems.
The power amp section is unusual, and not a bad idea. It uses a bridgemode technique, which means that the speaker is driven from both sides, instead of the usual hot drive to positive and ground to negative. The power rail voltages can be much lower than in the traditional configuration, and this results in better efficiency in the power amp section.
There were two faults showed up on this amp. The first was easy and fairly obvious. It didn’t work. With certain faults in mosfet devices, (these are !RF640 used in a push-pull configuration) an output signal can be produced which disappears when there is no load (speaker) connected. This amp has four !RF640 mosfets. Each pair produces a push-pull output, and the two pairs are out of phase; and applied to the speaker at either side. This was the bridgemode arrangement described earlier.
These devices were replaced and…It worked!
Un…fortunately I found out that a transient (a ‘pop’ or a slap on bass) switched it off. It returned after a delay and would do the same thing again. Etc…etc…
The ‘protection’ (if that’s what it is; one could argue that it’s musical assassination) switches off the carrier signal mentioned earlier.
Clever? Yes, but not very sensible.
Time for tea.