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accugroove tri-112L: anyone blown speakers?


octa

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it has happened twice to me. my setup is the qsc plx-1804 into 2 Tri-112L's with Nord electro 3 & motif xs rack both running stereo into Ashly LX-308 mixer.

 

the 1st time i blew the tweeter on one Tri-112 at a smaller gig where i used just one speaker.. the smaller gig turned out to be louder than anticipated but i didn't feel it should've blown. after it was blown somehow only the 6" speaker worked until i had the tweeter repaired.

 

a few weeks ago i blew the 12" woofer on a cabinet and i'm not sure when it happened. for a bass guitar gig, i used both cabinets together with an acoustic 470 bass head & i suspect this is when it happened.. the cabinets are 8 ohm and the bass head is 4 ohm so i figured this would be a perfect combo but it did sound more distorted than i hoped. the acoustic 470 with one of the AG cabinets provides a great low volume tone for recording..

 

i'm hoping it's just bad luck but wonder if anyone else has experienced the same problem.. a couple of the groups i play with are very loud and my setup seems plenty loud but perhaps i need an even louder rig to avoid speaker repair costs/hassle..

 

 

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The Acoustic Model 470 amplifier was made in the early/mid '70s. It has two channels, a graphic EQ, reverb and tremolo. It's rated output is 170 watts rms (at 10% THD) into 2 ohms.

 

For comparison, I think this works out to be a little over 40 Watts @ 8 ohms. That's not much when you're playing an Electro with a loud band. And if you were playing organ parts, you were probably clipping 90% of the time.

 

Transistorized amps built in the early to mid seventies, in my opinion, didn't sound very good. They were low-powered, noisy (hissssss), and when they distorted, they sounded horrible.

 

Junk.

 

Tri 112L Specifications

 

I think you're beating the crap out of the cabinet by overdriving it. And with the Acoustic 470 you're feeding it distorted power from the start.

 

Use the QSC - not the Acoustic 470 and back off the volume.

 

Tom

 

"Music expresses that which cannot be put into words and that which cannot remain silent." - Victor Hugo
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I lost a crossover when the cabs were fairly new. The tweeter quit working, but it was actually the crossover that gave up. They sent me a prompt replacement and have had no problems since.

 

Comments

 

The Acoustic Model 470 amplifier was made in the early/mid '70s. It has two channels, a graphic EQ, reverb and tremolo. It's rated output is 170 watts rms (at 10% THD) into 2 ohms.

 

For comparison, I think this works out to be a little over 40 Watts @ 8 ohms. That's not much when you're playing an Electro with a loud band. And if you were playing organ parts, you were probably clipping 90% of the time.

 

Transistorized amps built in the early to mid seventies, in my opinion, didn't sound very good. They were low-powered, noisy (hissssss), and when they distorted, they sounded horrible.

 

Junk.

 

Tri 112L Specifications

 

I think you're beating the crap out of the cabinet by overdriving it. And with the Acoustic 470 you're feeding it distorted power from the start.

 

Use the QSC - not the Acoustic 470 and back off the volume.

 

Tom

 

I think Tom has analyzed your problem pretty well. Power curves vary for amplifier models (eg., for 8 ohm, 4 ohm, 2 ohm). It is possible that the Acoustic amp could produce maybe 60 watts at 8 ohms, but Tom's estimate of 40 watts is more likely. 10% THD is a lot of distortion. (Bogen PA amps of that generation were typically 5%, which was quite audible.)

 

I am not a schooled amplifier technician, but encountered problems similar to yours, and this is how I remember it was explained to me (I hope I get this right--those of you who know this stuff, please correct me if I have this wrong, so that we can all learn.)

 

When an amplifier is driven into distortion, several things happen. If you "look" at the amplified signal on an oscilloscope, with a sine-wave (which looks like an "S" on its side) it sounds smooth and pure. When you turn the amplifier up to full output, and then increase the gain beyond the point of full output, the sideways "S" on the oscilloscope gets larger, but parts of the sideways "S" get cut off as they have reached the limits of the amplifiers ability. When the sine wave hits the upper and lower "rails" (eg., the floor and ceiling) of the oscilloscope, the sine-wave gets clipped--clipping occurs. The more you increase the demand for more volume, the more the sine wave becomes a square wave and sounds like a Jimmy Hendrix fuzz tone. Thus, a square wave has 1.414 times as much energy as a sine wave--almost half again as much.

 

When an amplifier gets stressed like this for a period of time, some strange things can happen. If the power supply isn't "beefy" so that it stays stable and well regulated, it can become unstable and cause voltage spikes of up to 8 times the normal voltage, which can raise utter havoc throughout the amplifier, passive crossover, and speakers.

 

That extra energy can cause heat which can change the values of the resistors and capacitors within the passive crossover, which then changes the crossover frequencies allowed to pass to the speakers and tweaters, which can cause their subsequent failure (blows them out). It can also cause on again/off again intermittent problems.

 

Of course, with an amplifier with reserve power, you don't experience any of the above problems. Instead, you just destroy the speaker by "launching" the "former" of the speaker out of the magnet gap, or melt the coil windings.

 

Like Tom said, use your newer system, which is cleaner, more stable, and more powerful. And check to see that the passive crossovers are "beefy"--under-rated crossovers can cause big problems downstream.

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i wouldn't and didn't play an electro through the acoustic 470 but the 70's P-bass i was playing (when the woofer blew) sounds GREAT through the acoustic 470. i'm just saying it definitely ain't "junk". it does seem like an organ going through this rig would sound extra junky tho i agree. .
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octa - don't take my comment personally.

 

However, you are comparing a 30 year old amplifier design with a current, state-of-the-art QSC. It's just not a fair comparison.

 

In its day, the Acoustic was one of few choices. I played a Rhodes through one for two seasons at a theme park in Charlotte. It made me hate coming to work every day. Absolutely awful!

 

I am trying hard to think of something nice to say about Acoustic amplifiers. But I can't.

 

It's Landfill fodder. An interesting doorstop. A hissing old dinosaur. A fancy boat anchor... Junk.

 

That's the best I can do. :)

"Music expresses that which cannot be put into words and that which cannot remain silent." - Victor Hugo
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heh. once again, playing any type of keyboard thru the 470 (at Carowinds?) probably sounds bad. but electric bass sounds a lot more intersting through an acoustic 470 than it would through a QSC. it just has a nice warm growly tone that to me is only second to the top end Ampeg SVT stuff, which costs too much.
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heh. once again, playing any type of keyboard thru the 470 (at Carowinds?) probably sounds bad. but electric bass sounds a lot more intersting through an acoustic 470 than it would through a QSC. it just has a nice warm growly tone that to me is only second to the top end Ampeg SVT stuff, which costs too much.

 

Understood.

 

:) Yes, Carowinds. Two seasons starting around Spring '75. I had a blast! No kidding.

 

The Rhodes I was using was beat to hell. That Acoustic amp was really brittle. The term "smooth response" was just a figment of imagination when discussing PA and backline equipment. From then until now, over 30 years, I have never heard a transistorized amp - made in that era - that sounded good.

 

My choice was a tube amp - Fender. In fact, I had a new Fender Rhodes and was playing it through a Fender Bandmaster head sitting on top of a cab with two 15" JBLs and a couple of tweeters. Allen Organ Co. designed the cabinet and used this combination. It sucked for anything resembling accuracy. And with the transistorized RMI amp that used to sit on top of it - ...well, it would be a tough choice between that and an ice-pick in my good ear.\

 

So I bought the head from a friend of mine - music major who turned from jazz to embrace classical guitar. Forget what I paid for it, but he was cryin'. It was totally tube-driven and when I connected it to those two 15" JBLs and played that Fender Rhodes through it, it was clear and (DH will love this... :cool:) oh so sweet-sounding.

 

[oldfart]Rock on. Remember to protect your ears. And try not to blow any more speakers.[/oldfart]

 

Tom

 

"Music expresses that which cannot be put into words and that which cannot remain silent." - Victor Hugo
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Tom,

 

Thank you for your comments on the old Acoustic amps--best belly-laughs of the week. I, too, had one--an Acoustic 150 Lead (110 watts, who knows what rating method). I used it to drive a Leslie 122 with a Hammond M-3 in the early 70's (with an overdrive circuit ahead of it.) It sounded pretty good for distorted (Steppenwolf) organ, or even Jimmy Smith organ.

 

I still have the amp, and had the old amp gone through by a qualified technician. For some reason, bass players now love this amp, and choose it over their own at band practices. (Go figure!) It does sound good and warm. Why, I can't explain. But it does. Better than most, by far. (But the Ampeg SVT is still king to my ears, too, Octa.)

 

Octa, you might try having the Acoustic amp modified with an "interupt" 1/4" jack so that by plugging in a 1/4" phone plug to the jack you can separate the pre-amp/pre-driver sections from the power amp. (I had a pre-amp output jack on my Acoustic amp, and had a second "power-amp in" jack installed so that I could separate the two parts of the amp.)

 

Thus, you could run the Acoustic pre-amp into your QSC amp and see how it sounds (without having a load on the Acoustic power amp.) Perhaps you'll get the best of both worlds.

 

Good luck.

 

 

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I used to have an Acoustic 150 top in the 70's, used it with 2 Dual Showman bottoms (in other words, 4 JBL D130 15 inch speakers). Sounded OK, not great, hmm, that was about 50 watts per cabinet? Perhaps underpowered by today's standards...
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