I tried the following today: I removed R10 (2k) thereby reducing the gain of the input stage by 6dB. The instability occurred at approximately the same input level (around -9dB). Can this tell us anything?

Sympatico is actually a rather old and proven design.

What you are describing does not actually fit the description of motor-boating that I am familiar with. Its actually a form of clipping.

Classical motor-boating is a self sustaining low frequency harmonic that grows on its own. That is not what I see you describing.

If actually looks to me like it could be that one half of the LM4780 is dead. That could be from operating it as you did.

I will look a bit closer at your measurements when I have time.

How is the other amplifier behaving?

Please measure the DC voltage across C5 and C6.

Yes shorting the back of the package (with a screw or otherwise) to GND can kill/damage the LM4780 because it is not insulated.

Overloading it is less likely to kill it.

It could also be something else, it difficult to say.

Ok, thats good.

One guess is you are just asking for more current than the AMP is designed to deliver. Sympatico is not designed as a subwoofer amp for certain. It is more for normal full range use. :)

Make sure your rails are not sagging with the signal under test. If they are you will need more capacitance.

Also make sure your not asking the LM4780 to deliver more current then it was designed for. Refer to the datasheet.

You may also want to decrease the gain. Up to you.

Cheers!
Russ

Edited by user Friday, June 01, 2012 3:43:31 AM(UTC)  | Reason: Not specified

Are you measuring the differential voltage? Or the voltage to GND at one end of the output?

Remember the outputs are not ground referenced. They are referenced to each other. This means you need a scope that can do truly differential measurements to get an accurate voltage

I keep staring at my amps, but no bright thoughts of mine have yet emerged. I have sent PM and email to Brian. Awaiting response.

To add to my previous post, I have now unscrewed the heat sinks from both amps and performed a visual inspection: the LM4780 of the right channel amp (on which I reported flying sparks in post 21) has pin 11 melted (V-, shared with several other pins). This can put into question test results reported afterwards in post 23 which were based on this amp although everything seems to be same as before.

Thankfully, my left channel amp appears unscathed from any improper handling on my part; results reported in post 33 are based on this amp. As I indicated early on, both amps seem to go unstable when the sine output reaches around 20V pk-to-pk. As a further answer to Russ' post 30, I did check the LM4780 datasheet and I'm positive that I'm well under the maximum current capability; a quick ohm's law calculation with 10V voltage and 6 ohm impedance confirms it. I'm also positive that heat sink was tightly screwed on.

I could send any time you like my left channel amp for a quick test if you would be willing to check it. A side by side comparison test with your own amp might quickly show any problem I might have created with mine and would confirm whether I installed something incorrectly (doubtful at this point) or damaged one of the ICs through improper handling. In the mean time, I will order a new LM4780 part to repair my right channel amp and for the time being I'm running my subs with my 20-year old Nakamichi 50W TA-2A Receiver and preamp bypassed at comfortably high levels.

Finally, I would like to apologize in advance for any measurements mistakes and damage I have caused. Even though I'm far from an experienced DIYer, this is not my first project. I built one year ago the Linkwitz Pluto project using bare PCBs for the active crossover and featuring bridged LM3886s for the woofer amp. Generally speaking, my soldering skills and electrical measurements skills are not totally lacking. I'm also an electrical engineer by education, but I have only designed circuits very early on in my career and only for a short period of time. :)

Best Regards,
Martin

Edited by user Friday, June 08, 2012 11:52:20 PM(UTC)  | Reason: Not specified

Perhaps it was all a power calculation mistake on my part then. Beginners mistake :(

My application does not require to push the amp very hard in normal circumstances. The instructions I followed to build the sub and its associated active filter that is inserted at my preamp output recommend an amp 125W per channel to have some capability in reserve. Most of the time, I really don't listen very loud. The "motorboating" sound (I'm using quotes because I don't want to insinuate it is really that, as was established in previous posts) first occured when I was watching a movie. The active filter behind the amp boosts the signal quite a bit to extend the bass down to 20Hz; I've also measured a DC offset at the output of the filter which I really need to eliminate. Perhaps these factors combined to push the amp beyond its normal limit and hence the loud pops.

After this first occurence and the initial measurements I made early on, I switched to testing with pure tones directly from a volume-controlled source. If I was driving the amp too hard during those tests, it could have been because I was not relating correctly the calculated output power and current to the measured output voltage. I also now think that the gain of the amp is set too high and I should be removing R10.

I will revise my power calculation (feel free to share with me your closed form equation).

The Sympatico is not at all designed to be pushed to clipping. It is also not really designed as a subwoofer amp, you would be better off with an amp designed for that purpose. It has no protection circuitry to deal with the nasty things that can happen when one side of the bridge tied amp looses steam (thus depriving one half of the fully symmetrical feedback). That is why you saw the nasty effects that you did. If the current protection circuitry decides to trip because of high junction temp or excessive current you will get some nasty behavior as you did.

The good news is the very likely on the damaged amp all you need do is replace the LM4780, at least it's worth a shot.

I am always glad to help.

I appreciate the tone with which you have presented you questions and findings.

People like you will always get further with me than the people who lose their temper. :)

Edited by user Saturday, June 09, 2012 8:21:49 PM(UTC)  | Reason: Not specified