Popping circuit breaker S100

[John Vanderlee]

Allen 6000X installed earlier this year. Main and Antiphonal - six amplifiers S100.
Changed Circuit breaker, still NG. Changed Amp, still pops it. Usually does not happen until after extended playing like 90min. Checked wiring and speakers all Ok. So last time while unplugging signal live, the CB popped again. I reset it twice with time in between. The 3rd time the board under the amp went up in flames! Again replaced the amp. All OK for 2 weeks, yesterday breaker went again. Amp also uses muting input.
Help!
A local Allen tech suggested somesort of HF oscillations going on which overdrive the amp. Could that be killed with a small capacitor accross the output or input? Not big enoughto kill freq response but suppress the oscillation.
Any thoughts or suggestions?

[Admin]

A HF oscillation certainly could cause the amp to draw excessive current. The question that needs to be answered is why is it occurring. The source could be the input signal, the amp itself, or the load on the output. Since you've replaced the amp, that leaves the input and output.

The easiest thing to do would to scope the output to see if there is an oscillation in the first place. If so, I'd swap inputs with another amp and see if the problem moves as well. If not, try swapping the speaker cables on the outputs. If you don't have a scope, you'll have to just do the swaps and wait and see.

While it might be possible to suppress a potential HF oscillation by placing a suitable small capacitor across the input or output, it would be better to identify the source of the problem first.

[John Vanderlee]

I bought a service manual off EBAY. Guess what, a factory note says running output channels together in the the same conductor will cause oscillations, overheating and the breaker to open!
Aargh! We ran 4 wire conductors for each 2 channels ☹️
This means adding another 300ft of cables to replace one of the pairs!
Still wonder if a small cap would fix it.

[myorgan]

I bought a service manual off EBAY. Guess what, a factory note says running output channels together in the the same conductor will cause oscillations, overheating and the breaker to open!
Aargh! We ran 4 wire conductors for each 2 channels ☹️
This means adding another 300ft of cables to replace one of the pairs!
Still wonder if a small cap would fix it.

John,

Probably not since the oscillation is occurring on the output side between the two pairs. Even if you added a cap, the pairs running inside the same conductor will also cause distortion, as well as an overheating S-100. That happened with my MOS-2, but not to the extent yours is.

Hope that helps.

Michael

[Admin]

Still wonder if a small cap would fix it.

If that were the case, why wouldn't Allen have done it? This type parasitic oscillation is more likely with longer cable runs and is due to capacitive coupling between the pairs.

[John Vanderlee]

I was afraid of that, no other choice but to eliminate half of the pairs and run new cables. Dang!

John

[myorgan]

I was afraid of that, no other choice but to eliminate half of the pairs and run new cables. Dang!

John,

My reaction exactly when I found it out. I also suggest using 14ga. for longer runs instead of 16ga. In my case, the runs were slightly over 100ft.

Michael

[don60]

A small capacitor would probably make the problem worse since the cause of the oscillation is wiring capacitance in the first place.

Before you get into a major rewiring job, try adding some ferrite chokes in the speaker wires at the amplifier end. I was able to kill a similar oscillation that was overheating some but not all of the amplifiers driving a nine-pair cable.

I chose toroidal cores, part number 5975001801 by Fair-Rite, and passed both speaker wires through the core in the same direction for a total of two turns. Running the wires together in this manner creates a magnetic field cancellation for differential-mode signals (the desired amplifier output) while causing choking action on common-mode signals (the oscillation). You may need to revisit this configuration depending on the exact mechanism that is causing your oscillation, but a sufficient amount of inductance ought to put a stop to the problem without affecting the high frequency audio response.

A scope would really be helpful here to figure out whether oscillation is occurring and at what frequency, and to verify that the extra inductance conclusively kills the oscillation.

Allen made a blunder in the S-100 design, and as you noted this problem and its "fix" are well known among Allen people. Properly designed audio amplifiers, especially ones with completely separate power supplies, do not oscillate on long speaker runs or shared speaker cables.

[John Vanderlee]

We used 12 gauge. But the run to the antiphonals is over 100 ft. :(

- - - Updated - - -

I think it is a goof as well. Commercial sound systems run wires hundreds of feet. Actually looking at the amps there might already have been a hint of a problem, as the output already has a toroidal choke on the output.
Scoping would have to be done really fast as the amp rapidly overheats once it decides to go south. I' m currently repairing 3 blown amps. Fortunately the parts are not hard to find.

[don60]

JV, I just looked at the underside of an S-100 that I have sitting around. I see one pitiful-looking ferrite bead on the "high" speaker lead. A bead that size is not going to give the inductance that you need to kill this oscillation. Before you run hundreds of feet of cable, I really suggest you try the part number that I mentioned above. It is not a bead but a toroid, and it will allow you to adjust the inductance by simply winding more turns around the core.

I recommend running both speaker wires around the core in the same direction as a first attempt; if that approach does not work, you can reverse one of the wires; if that does not work, you can put a separate toroid on each wire. I suspect that the first approach will do the job since the mechanism causing the oscillation is probably a common-mode signal coupling from one amp to another.

[John Vanderlee]

Thanks Don60. Certainly worth the try. I' ll let you know what happens, Would certainly beat the alternative!

[John Vanderlee]

Just ordered a bunch of these from Mouser.
Don, just so I am sure, do you run both the hot lead AND the common through the bead?

Thanks!

[don60]

Yes, I did. But my customer's amplifiers are being used in bridge mode and have both terminals floating with respect to system ground. I see that Allen ties the common speaker terminal directly to chassis ground, so you might need a slightly different configuration on the chokes to suppress the oscillation. I would say to be prepared to experiment a little. Don't be afraid to take two or three turns through the core--it is large enough to accept a fairly large cross section of wire.

I was lucky on my first try and cured a problem that had plagued the organ for 12 years since it was installed. At least in my case nothing ever blew up--the affected channels ran very hot and created a hissing in the speakers.

[Innocentti]

The S-100 have a feedback circuit that senses the 'back EMF' from the speakers - this circuit will induce erroneous output from any spurious signals on the speaker cables, but obviously works correctly when the signal is a result of the original output of the amplifier.

As you have replaced the amplifier and the replacement amplifier is also tripping, my conclusion would be that the trip is being caused by the other amplifier on the shared cable (especially if it occurs when no output is audible).

As Don has now correctly stated the S-100 has a 'ground' output and a 'hot' output. In a four core cable the conductors have a 'square' relationship to each other. I would speculate that in the cable connected to this amplifier you have both amplifier 'hot' conductors next to each other (on the same edge of the square), which will make the capacitative/inductive linkage greater than if they were diagonally opposite to each other.

Try swapping the conductors at both the amplifier and speaker ends of the cable (only on one amplifier!).

Otherwise try swapping the other amplifier.

[don60]

The The S-100 amplifier uses a conventional complementary-symmetry output stage that presents a very low source impedance to the speaker output over the usable frequency range of the amplifier. I also see both a dc and an ac feedback loop that further lower the output impedance, linearize the amplitude response, and stabilize the bias point of the output stage. The combination of these mechanisms makes the speaker output look like a nearly ideal ac voltage source that is highly resistant to changes in load conditions. Rather than causing some sort of problem with externally-induced signals on the speaker output, feedback makes the amplifier more robust against changes in load.

The fun begins at higher frequencies, of course. Then all sorts of additional parasitics appear, from unwanted phase shifts to little feedback loops within the amplifier (especially involving the output transistors, which have large device capacitances).

I have honestly never heard anyone finger the lay of the conductors in a two-pair cable as a factor in causing or not causing feedback beyond the usual advice that twisted pairs are more resistant to crosstalk. If using diagonally opposite conductors kills feedback while using adjacent one causes it, the amplifier design is fatally flawed and needs attention. No one in the field is going to fuss with choosing diagonally-opposite conductors for the hot wires in a cable--a system that fussy is a bad system!

Another thought is that a big feedback loop involving two amps sharing a speaker cable requires a loop closure somewhere at the amplifier end as well as the cable end. How is signal getting from one amplifier input to the other? In the case that I investigated, the three overheating amplifiers shared a nine-pair speaker cable, but they also shared a three-channel shielded cable running from a mixer board to the amplifier inputs (which were physically close). I would bet money that the loop closure involved capacitive coupling in that input cable, especially since no other amplifiers ever showed the problem.

JV, could you have a similar situation on the inputs of your amplifiers? Are two or more sharing a cable? Sharing a (poor) chassis ground connection?