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Why Do I Keep Blowing JBL 033 Tweeters?????

WaynerN said:
Passing along some DC?
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dc offset is in the high 30s low 40s mv. I have it written down in my notes but don't feel like looking up the exact numbers. I did spend a long time selecting transistors to get it as low as I could. One channel is in high 30s, the other is in low 40s millivolts. I was told this should be ok unless someone has a reason to think otherwise.

here's a link to the resto done on the amp. It's long, so scroll to post #94 on page 5 if you want to skip all the b/s.
http://audiokarma.org/forums/index.php?threads/sae-mark-iiia-repair.673115/page-5

I will try running the amp and look for any issues @ high frequencies.

I was having issues hearing some distortion while repairing the amp and testing it. It turned out to be digital noise from CDs burned from FLAC files and not the amp. This noise is difficult to hear and was only on some of my music. Difficult to describe but I would hear it during a "spacious" part of a song off in the background. Not sure if continuing playing these CDs would cause tweeter damage.

thanks for your help everyone. hopefully I can find a problem if there is one.
 
kwape said:
as I mentioned, amp is sae mark IIIA, @ 1khz sine wave into 8 ohms puts out 118wrms. amp is rated @ 120wrms per channel. I usually set the gain pots on the amp @ 3/4s volume. Never turned my pre amp past 1/3 volume with this amp.
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FWIW, position of the volume and gain controls are only indicative of relative power. They are not representative of absolute power, meaning turned up 1/3 does not mean you are at 1/3 power. Could be more, could be into clipping, could be less. Depends on several factors.
 
whoaru99 said:
Wouldn't the tweeter's crossover cap block DC?
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Yes. If it is properly functioning. Some caps fail as opens, some fail as shorts.

If the crossover cap fails as a short, that would pass everything, including DC, but the bigger issue would be the lower (and thus more DC-like) frequencies.
 
jblmar said:
I would suspect the amp. The fact that the same amp destroyed a few 033s makes me highly suspecious.
I would get the amp checked out before continuing to use it.
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I suspect the amp too, but the cap on the tweeter could be bad. Perhaps both have a problem. Then we never did ask the OP how loud he's been playing the system either. Amp in oscillation...???
 
jblmar said:
I would suspect the amp. The fact that the same amp destroyed a few 033s makes me highly suspecious.
I would get the amp checked out before continuing to use it.
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WaynerN said:
I suspect the amp too, but the cap on the tweeter could be bad. Perhaps both have a problem. Then we never did ask the OP how loud he's been playing the system either. Amp in oscillation...???
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The fact that its only one channel tends to point to that amp / preamp channel. Is the preamp passing DC to be amplified by the amp? Is the Amp in oscillation, or shifting DC offset despite a good quiescent value? Has something in the reworked crossover gone awry? Is there a problem at the source, digital or analog, that is producing hash, distortion, UHF garbage, etc. that is getting into the system? Is there an RF leak coming in somewhere due to IC's or connection problems acting like antennas?
 
ok, when I thought it was the amp now I'm not sure. I was getting some severe distortion @20khz into 8 ohms but now the amp is warmed up a little it seems ok. Or I was just doing something wrong, I am clearly a novice here.

here's what I was looking @ the first time @ 20khz

IMG_1907.JPG



now @ 20khz

IMG_1908.JPG


well not sure how fine this is, only getting 17.6vac channel A (38.7wrms), 18.7vac Channel B (43.7wrms). At 20khz into 8 ohms (sine wave)



@ 18khz was fine till 98wrms channel a, 105.8wrms channel b.
 
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let me know what you guys think. I know the amp runs colder than I would like and the bias was an issue during the repair since it uses bias diodes that are not easily adjustable (requires a couple hours of surgery to change resistor R5). Not sure if the bias being too low would be causing my issues. May need to continue this in solid state....
 
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That is serious oscillation which is not only going to fry tweeters, it is burning output power and can overheat everything in the output stage. If your output transistors fail and dump the rails into your speakers, the voice coils will burn out. First, you did check your preamp to make sure it isn't outputting that mess, right? I suspect the amplifier output stage, but one needs to be sure. Don't let the amp sit on your workbench like that oscillating away. It can fry.

The diodes are being used for bias voltage dropping from the rails, right? Two of them? Make sure any resistors used are within spec.

Did you replace any of the output transistors? I'm wondering if you did, and the newer ones were a lot faster than the original ones, and thus have bandwidth where the originals would not oscillate.

Transistor conductance changes with heat, so it sounds like output stage issue.

Here's an explanation of some of the issues with oscillation:
sound.westhost.com/amp_design.htm
(Rod Elliott's writeups are nicely done, but Google will turn up more on the subject than you'll likely care to read.)

Things to think about:

One: small cap between the transistor base and ground to form an RC network with the base stopper resistor to remove any RF/AM components.

Two: Output Zobel is missing or incorrect.

Three: adding snubbers to the power supply diodes and power transformer can remove noise being broadcast inside the amp and picked up in your output stage.

Could be all sorts of things, but that oscillation is bad.
 
Retrovert said:
Did you replace any of the output transistors? I'm wondering if you did, and the newer ones were a lot faster than the original ones, and thus have bandwidth where the originals would not oscillate.

Transistor conductance changes with heat, so it sounds like output stage issue.
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I did replace all outputs, using MJ15024 & MJ15025, It's a quasi-complimentary design. In fact I replaced every component in the amp except for 2 transistors and the transformers. Everything else is new, all resistors are beefed up RN or Vishay dale of higher wattage. I suspect the outputs being quicker having an issue here. I might need to send this to someone who knows what they're doing.

why was I getting only 38 & 43wrms @ 20khz? amp is rated @ 120wrms 20hz-20khz.

and I have no idea if the first pic is even accurate or I was just doing something wrong because I haven't seen that since. I will run 20khz again from cold and see if it does that again.


in regards to the pre-amp. It's new so I didn't suspect it but I will check it now to be sure.
 
Cap in crossover would block any DC from getting to tweeter.

Amp is suspect though for oscillation and crossover suspect for allowing output below normal crossover point. A mic and rta would allow OP to diagnose for the latter.
 
please ignore that first pic showing distortion on post 28. I just checked the amp from cold @ 20khz sine wave into 8 ohms and was getting 12.8vac before distortion. still that is complete ****. It's a 120wpc amp 20hz - 20khz. So I need to find out why it's doing this.

I will continue this in solid state in my original thread for this amp. I posted the schematic on that thread in the first page if anyone is interested.

http://audiokarma.org/forums/index.php?threads/sae-mark-iiia-repair.673115/

scroll down to the bottom of post #2 for schematic.


(not using a pre-amp here, just injecting a sine wave off my computer using audacity directly into the amp)

no dc (0mv) @ preamp output (model: emotiva usp-1) if there was I would be so mad because that is new and should be fine, thank goodness.
 
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kwape said:
I did replace all outputs, using MJ15024 & MJ15025, It's a quasi-complimentary design. In fact I replaced every component in the amp except for 2 transistors and the transformers. Everything else is new, all resistors are beefed up RN or Vishay dale of higher wattage. I suspect the outputs being quicker having an issue here. I might need to send this to someone who knows what they're doing.
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Your new output transistors have much greater bandwidth than the original and will be more prone to oscillation.

You might want to verify the amp has the appropriate RC circuit at the output base to filter out RF, and that the other small filters are present to remove oscillation. Also that the final zobel is present. Look at some transistor amp designs, including the link I supplied.

Also look at rectifier and power transformer noise, easily cured with snubbers.

I would suggest you not use this amp until you've resolve the oscillation issue. It will only destroy more speakers and eventually itself.

kwape said:
and I have no idea if the first pic is even accurate or I was just doing something wrong because I haven't seen that since. I will run 20khz again from cold and see if it does that again.
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Amps prone to oscillation tend to be subject to hard-to-isolate issues such as cables picking up RF or other noise, cable capacitance and speaker impedance, phase of the moon, etc.

kwape said:
in regards to the pre-amp. It's new so I didn't suspect it but I will check it now to be sure.
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You said you were injecting a sine wave so it's not the preamp. That's amplifier oscillation, which is what I always suspected.
 
thank you retrovert. I'll try to find a solution, but might need to send this one out.
 
kwape said:
thank you retrovert. I'll try to find a solution, but might need to send this one out.
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The small caps, resistors, and inductors for the RC and RLC networks to stop oscillation are inexpensive and reasonably well understood.

So why not do some experiments? You can always send it out. And you might learn more about oscillation than this magical tech, who probably isn't going to do much beyond what you can do.

You'll need to do the transistor's base and output, and then add some snubbers (RC circuits) to the diodes and transformers. All of it is pretty simple.

What one person can do another can do.
 
kwape said:
thank you retrovert. I'll try to find a solution, but might need to send this one out.
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I'd like to see how you fix this oscillations, since I never faced this problem in person... I'd follow Retrovert's advice and do it, step by step, one thing at a time, and try.
 
As far as the low output at higher frequency, have you confirmed the signal source is maintaining necessary output voltage at that frequency to fully drive the amp?
 
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