RCF ART312A Protect mode or not?

[radioman1033]

A little background on my situation. I used a pair of ART312A's at a Civil War Reenactment this last weekend. The speakers were used in a symposium tent to amplify a single mic for guest speakers. Saturday's event went without incident. That evening I covered the speakers with plastic trash bags to keep the mildew off even though sound system was in very large tent.

The next morning I powered up the system. One speaker came up. The other never came out of protect mode (red led). Recycling power did no good. I noticed a slight odor coming from the rear of the enclosure so I pulled it from service.

I brought it home and decided to dig into it. This is the first time inside. I did study an older thread on this forum http://music-electronics-forum.com/t28333/ so I had a little head start as to what I should look for. I removed the amp module and disconnected the speaker wires. I started sniffing around the module and looking for something burned to no avail. I stuck my nose into the enclosure and the odor was strong. I suspected the voice coil was fried and I was right. Open coil.

I saw one fuse blown on the main board. I replaced the 4 amp fuse and powered the module up. Protect LED went out normally. I hooked up another woofer and powered up. Sounded perfectly normal. However there was a distinctive faint popping sound out of the driver. First it sounded like a regular pattern of a ticking sound. Then it started sounding intermittent. Popping and frying but very low level. I haven't put a scope on the output yet to see if it was symmetrical or asymmetrical. This could tell me if the problem was coming from a previous stage (AC) or if it was coming from one of the driver transistors before the main output transistors. Note: tweeter circuit is quiet and works fine.

The popping could be coming from a leaking capacitor, bad solid state device or even some contamination of flux under a surface mount device. Something that wasn't properly cleaned in the board level assembly. I have seen this before in troubleshooting noise like this.

I downloaded and studied the Bass-Amp schematic and noticed something interesting in the protect circuit. All the protect circuit does, which is U12A and B, is upon powerup, mute the audio until the speaker output line stabilizes and once at zero, unmutes. This is not a protect circuit. It just mutes audio until output is zero or if at any time the output swerves from zero DC. A failure of the complimentary output stage will not protect the driver upon powerup or anytime during use by the protect circuit.

At least this is my understanding. Nothing will protect the driver should there be a failure of the output circuit except the fuses. In this case something failed and the output circuit went full negative, blew the voice coil and then blew the fuse.

All of my professional power amps have relays in the speaker line to disconnect the speaker from the output stage should a DC offset appear on the output.

Now for my questions: What protects the driver from a failure of the output stage? Why is it important to mute the audio only when powering up besides a little distortion? Anyone else have a similar experience?

I'll be digging into the circuit with a scope this week to see if I can find the culprit.

Rick

[The Dude]

Without a schematic, it's hard to discuss the protection circuit or anything else. Do you have one you can post?

There are a couple of contradictory statements in the post. First you say, "This is not a protect circuit. It just mutes audio until output is zero or if at any time the output swerves from zero DC".
Then you say, "A failure of the complimentary output stage will not protect the driver upon powerup or anytime during use by the protect circuit".
Either I don't understand your statements or,......

If the protection circuit is able to sense DC on the output, it will go into protect when it senses DC. So, if there is a failure in the output section of the amp, it will most likely put out DC and the amp will go into protect.

If the speaker's voice coil is fried, it's quite possible that it was getting DC, or at least pulses of DC. If that's the case, either the protection circuit isn't fast enough or it's not working (maybe a stuck relay?). I wouldn't leave the speaker hooked up until you verify with a meter that there isn't DC reaching the speaker. That popping sound you are hearing could be DC and/or could be the amp going in and out of protect. Monitor the output sans speaker with a meter or scope and see what's going on.

[radioman1033]

Sorry for the confusion. A link to the schematics is on the link referred from a previous post in my original post. It is the second set of links to the schematics on that previous post. First set of links is broken. Here it is again if you can't find the post.http://music-electronics-forum.com/t28333/

Like I stated, it is not a protect circuit at all. From what I see in the schematics it only mutes the audio until the output stabilizes. At any time, even before the protect LED extinguishes, full voltage is applied to the output stage (via a slow fet turn on) and the speaker. Should there be a failure of the complimentary output stage the speaker will blow and/or the fuse will blow. If the protect LED goes out, and a DC level appears on the speaker, the protect circuit ONLY mutes the audio. Big deal. The speaker will blow anyway. If a fuse blows, let's say the positive voltage, the full negative voltage will appear on the speaker thus frying it.

There *is no* protect mode that I can see. There *is no* relay to interrupt the output stage to the driver should DC appear. It just mutes the audio.

I have taken precautions with the test driver using a lower amperage (2A) fuse in series with the driver.

Strange design.

Rick

[The Dude]

http://music-electronics-forum.com/a...a-bass-amp.pdf

http://music-electronics-forum.com/a...ilter-card.pdf

Reposting direct links for convenience

[radioman1033]

http://music-electronics-forum.com/a...a-bass-amp.pdf

http://music-electronics-forum.com/a...ilter-card.pdf

Reposting direct links for convenience

Thanks Dude.

[The Dude]

Ok, haven't had time to look at the schematics yet, but if it mutes the audio instead of disconnecting the output, your right- gotcha. It isn't really a protection circuit, but a mute circuit.

I would still scope or meter the output to actually "see" what's going on there and what the pulsing sound is. Knowing if it's DC or something else would be helpful. Also, enough DC can still work it's way through the fuse to do damage. If it was me, I would just unhook it until you verify what's happening.

[radioman1033]

Ok, haven't had time to look at the schematics yet, but if it mutes the audio instead of disconnecting the output, your right- gotcha. It isn't really a protection circuit, but a mute circuit.

I would still scope or meter the output to actually "see" what's going on there and what the pulsing sound is. Knowing if it's DC or something else would be helpful. Also, enough DC can still work it's way through the fuse to do damage. If it was me, I would just unhook it until you verify what's happening.

That's what I plan on doing. As stated on my OP if the pulses are asymmetrical, going primarily one direction, it is probably in the output stage. If the pulses or noise is symmetrical, it is most likely coming from the EQ board or first opamp in the power amp board.
Will be curious to see.
Rick

[radioman1033]

Well I had time to hook up a storage scope to the output. Sure enough a negative spike was there. Almost pure DC which I suspect it's from the output transistor(s) in the negative circuit since the input opamp U15B was clean and quiet. As you can see there is a noise level just before the pop and then it's quiet for a few moments. I can't seem to post image. It keeps bombing out so here's a link to my website where the image is.
Rick
http://www.neatcircuits.com/art312a1.jpg

[The Dude]

Oh, wow, I just noticed the SAP16's in there. If that's your problem, good luck finding any that aren't counterfeit!

Edit: Here's a thread that contains some info about using an alternate part with an external emitter resistor. It might help if you need those outputs.

http://music-electronics-forum.com/t36343/

[radioman1033]

Update!!

Hooked the amplifier module up to a load and a music source. Still had pops and crackling noise every so often. Output swung hard negative every once in awhile. And what looked like smoke coming from somewhere. Quickly shut down to see. Nothing burned so far. One thing I noticed was the surface of the SMT side of the board seemed to have a film of slime over it. Not just rosin or flux. Rubbing it with my finger it felt like a sticky surface. Almost like a coffee spill over the entire board. It was bought new from factory. It just didn't seem right. I happen to have a quart of methelyene chloride in my shop that I use to develop and clean new PC boards. I shot a stream of solvent over the board. It seemed to take most of the slime off. However the board just didn't seem like it was clean. I took a bunch of Q-tips and went over the entire board again. I kept on bringing up a brown "junk" off the board. After a good cleaning around all SMT components, I dried the board thoroughly with a heat gun.

I hooked up everything and powered up the module. I can't believe it. Completely clean audio. No noise, no pops, no hum. I cranked it up to just about clipping and ran it for a hour with ear plugs in. Not a glitch. Just pure clean audio. I plan on pushing it for the rest of the evening till I get tired.

To sum it up, never discount a contaminated circuit board. Especially one with SMT components. Some amplifier manufacturers take shortcuts or just don't know how to properly clean boards. This was a new amplified speaker that came from the factory so no one got into it before me. If troubleshooting similar problems don't start shotgunning components hoping to hit the right one. Sometimes you do more damage than good. Especially with glued down SMT parts. Make sure you have a clean board.

I have been in the electronics industry since 1972. I have troubleshot the strangest things. In this scenario, noise pulses and output saturation could be a contaminated board or even an improperly soldered component. Today's semiconductors are pretty robust. They either work or they don't at all. Noisy semiconductors are not as likely anymore. The strange popping and clicking could be a sign that contaminated aqueous flux was leaking across key bias components.

As I think back to the story in the OP, I stated that the enclosures were covered to keep humidity off the outside. Since it was a high humidity level inside the tent all night and the speakers were turned off and covered, they may have cooled down enough to draw moisture into the enclosure and aggravated the contamination on the board which led to the original problem.

As a design engineer, I have studied the protect circuit more carefully and realized that the protect circuit is NOT a protect circuit. It only mutes the audio until the plus and minus power supplies stabilize and that is all. Absolutely no protection for the speaker driver whatsoever. As soon as you power up, full voltage could be present on the speaker voice coil if the circuit isn't biased to zero.

This brings me to an idea. I'm designing a circuit that will sample the input audio and output audio. It will look for a zero cross on the input and a coincident zero cross on the output. When the zero cross occurs on the input and a very close zero cross occurs on the output the relay will close engaging the speaker. Anytime a DC voltage of either polarity is seen on the output stage when the input stage is at zero, the relay will open thus protecting the speaker.

We've already run simulations in software simulator and it seems to work. I would like to make this available to those who have amplifiers that do not have relay protection.

Will keep everyone posted on a new thread when it's ready for production. A driver that's is blown because of an error voltage is something I can't tolerate.

Rick

[J M Fahey]

Cool to know and glad you found it.

I will post here an independent protection circuit, let me find it first, which is a small half business card size PCB, with a Triac and a couple small transistors which shorts across the speaker terminals if you have more than 12V DC for more than 2 or 3 seconds.
Itīs so independent that you can even mount it in a speaker enclosure.

A fuse alone is not enough because it canīt detect DC from AC, so an efficient one will nuisance blow all the time and one which does not, wonīt protect you from DC, but placing this crowbr circuit at the speaker and adding a fuse in line before it, is a good combination, the fuse will certainly blow after the crowbar shorts speaker the line.