Double Bass AI interrupted output

[Scott Freeman]

At a gig Friday night, one guitarist was a no-show, and unfortunately he owns the powered speakers we ordinarily use for FOH. Instead, I used my Acme B1 (4 ohm each) cabs powered by an Acoustic Image Clarus for FOH. I was doubling, and used one channel, and the other was fed with a Mackie mixer for vocals and keys. The sound was ok, but there were repeated drop-outs in the signal, lasting from 1 to 5 seconds. The lights on the front panel did not go out, and both channels were affected. Has anyone had issues like this, or know what might cause it? The AI has been very reliable up to now, and I would hate to be forced to replace it.

 

[Michael Glynn]

Driving half a band at 2 ohms might be pushing it a bit too hard. Does your model have a limiter switch?

From the Series 4 manual:

"Under extreme playing conditions such as high volume or when
driving a low impedance speaker load, the amplifier may be pushed
beyond its limits. When that happens, you will hear high distortion and
you may even cause the overcurrent protection circuit to operate
resulting in an interruption of sound. The power amp is protected
against short circuits in its output. When the amp tries to deliver too
much output current, the overcurrent circuit will operate to prevent
damage. The result is that the power amp is turned off for a second or
two to keep it from delivering too much current. The limiter switch,
located on the rear panel of the amp head, is provided to allow the amp
to operate under these conditions with lower distortion or without
having the overcurrent circuit kick in. The limiter reduces the peak
signal that is driving the power amp, thereby reducing its peak output
with the result of lower distortion. If you find yourself having to play in
extreme conditions that cause the amp or combo to have too much
distortion or even signal interruption, engage the limiter and see if that
helps the situation. "

 

[Scott Freeman]

Thanks! That sounds exactly like what was happening. And I swear, I read the manual, but years ago. I'm not sure if the limiter is engaged, but I'll check when I'm back home.

 

[Ryan in PDX]

A quick look at the clarus spec sheet shows a 4Ω minimum load. Two 4Ω cabinets in parallel means a load of 2Ω. "Not ideal," at best.

 

[Scott Freeman]

No, I believe all of AI's amps had a 2 ohm minimum. I've used these cabs for 20+ years, 10 years with this amp, and never before had a problem. It's one of the reasons I went with this design when I wanted to get rid of the lead sled power amp. We were evidently pushing the amp to its limits, though. Probably should have lowered the gain from the mixer. And now I've been reminded of the limiter switch.

 

[Scott Freeman]

To close this loop, I checked the rear of my amp and the limiter was switched off. Lesson learned, and I love talkbass!

 

[yodedude2]

"No, I believe all of AI's amps had a 2 ohm minimum. "

Not so; I long ago owned one that was rated down to 4 ohms (a series ii model i.i.r.c.), although rick answered my question about it by saying it should be safe with three 8-ohm cabs, i.e., 2.67 ohms, but that if it were pushed beyond its limits that the protection circuitry would kick in.

the situation described in the o.p. sounds exactly like what a couple of my a.i. amps have done. both amps (different models) were rated at 2 ohms; one time was pushing three 8-ohm cabs at basically wide-open, and the other time was an outdoor gig where the amp was being overheated by direct sunlight.

 

[Ryan in PDX]

No, I believe all of AI's amps had a 2 ohm minimum. I've used these cabs for 20+ years, 10 years with this amp, and never before had a problem. It's one of the reasons I went with this design when I wanted to get rid of the lead sled power amp. We were evidently pushing the amp to its limits, though. Probably should have lowered the gain from the mixer. And now I've been reminded of the limiter switch.

View attachment 7302682

I stand corrected!

 

[Jeff Kissell]

I had a thermal overload shutdown on a series 3 Clarus from direct sunlight. I was running a single 4 ohm cab at moderate volume but the amp was way too hot to touch when it cycled off.
Lesson learned, I always have a heavy white bar towel on any outdoor gig now. Also sunscreen!

 

[james condino]

I use a 900 watt AI Focus head with a pair of 8 ohm ACME B1s, sometimes a pair of B2s, and for the big festival stages, a B4 and a B2. Regularly when something goes wrong with the other folks, on quieter gigs I use it as a small PA: 2 instruments into the main channels and then 2 more using the effects returns. The only time I have ever had an issue is as stated above, when they are in direct sunlight. A white sheet of paper (unfavored real book tunes!) / pizza box / bar towel solves the problem. If it shuts down from overheating, a 15 minute set break is enough to reset things.

Has anyone ever blown a fuse on an AI head? I've had a half dozen different models over almost 20 years and in 400+ gigs, never blown one, yet I always bring one on the job.