relay causing oscillation

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Here a weird one.

Last weekend I got an amp design I have been working on for a while finished and up and running. It's nothing particularly fancy. It's an all bjt class A/B with a diff i/p attached to a current mirror, a buffered voltage amp stage and a complementary output stage using 2N3055 and MJE2955's, cos that's what I had kicking around.

I had some strange conditional stability with it at first. It behaved itself perfectly most of the time, but for medium to high output power levels, it oscillated for a tiny part of the +'ve half of the output sin wave and then behaved itself perfectly throughout the rest of the cycle. I got rid of that by modifying the feedback network slightly and adding 50 Ohm base resistors to the output driver transistors and now it works fine and sounds great.

What's puzzling me now, is that the amp works fine whatever load I connect it to, resistors (8, 4 and 2 Ohm) and a number of different loudspeakers. However, when I connect the output up to a relay I got for the protection cct it goes completely mental, oscillating wildly rail to rail, regardless of what (if anything) is connected to the output of the relay or whichever state the relay is in.

The only thing I can think of is that the relay is presenting a capacitive load. The amp has a Zobel Network on the output but I don't have an output inductor on it.

Any similar experience with relays anyone ? Any suggestions ?

Describe the protection circuit

Is it as simple as current sense via a resistor? I would like to get an idea of the type of protection circuitry you are using to drive the relay coil.

While this applies more to industrial applications -- sometimes if you are using a comparator you need to introduce a little "hysterisis" into the circuit to quiet things down.

As a temporary solution, just fuse it until you can get to the bottom of the problem. Fuses are hardly the optimal solution, but at least you'll be able to listen to your new handiwork.
I've had also problems with relays in the past (bad contact, short circuited cotacts, etc...)I just removed the circuit; I'm not pleased with relays in amps, although it's a good way to cut off the speakers from the amp.

I remember me protection circuits with triacs, I don't know if they have influence on the sound, does anyone here?

I think there has been a thread before about relays in amps.

Thanks for the comments. I did a search for relays in this forum and didn't find anything like I described above.

I'm using one of the protection IC to control the relay. It has switch on delay, power down mute, output sensing and a relay driver. Unfortunately I can't remember the IC number off the top of my head and I don't have the data sheet to hand. I heard about them from a thread on this forum but a quick search just now didn't reveal the post.

Thinking about it, I never tried to connect the output sensing cct without the relay so it could be that. The IC is connected to the o/p via an RC low pass filter.

It's probably whatever you've got wired across the output that is the problem. Check whatever you've got connected with an ohmmeter. What you've described is really whacky.
Is it possible that you have wired the relay coil (i.e. not the contacts) directly across the amplifier output?
Yep, whacky is exactly what it is. I don't get it at all.

When I first saw it happen I thought I had wired the relay incorrectly so I checked my wiring and it's fine. The relay operates fine from the control IC, switching on and off as it should. I'm no stranger to electronics but this is one of the oddest things I've ever seen and completely unexpected.

I should get to have another look at it over the weekend and perhaps I'll spot something then. I think a good look at the sensing cct for the relay controller might be in order. Thanks for the suggestion jackinnj.

I guess the short answer is to avoid dc on the speakers. I'm really using the relay to protect the loudspeaker rather than the amp. It only costs a few euro (and a bit of hassle) to replace the output devices, especially seeing as I am using nice cheap ones for this. The loudspeakers, on the other hand, are a lot more costly to fry.

My thinking in choosing to use relays was partly suspicion of my own work. If I make a mistake in a calculation and the o/p tx cook themselves, so what but I can't afford to go replacing speakers drivers very often. Ultimately I intend to get an amp design up and running to my satisfaction and then make multiple units for bi/tri-amp, so this is partly an exercise in proving the concept. Relays just seem to be the easiest way to go, especially seeing as there are driver IC's to take all the effort out of the design. Triacs, which hugobross suggested and protection ccts I am suspicious of on the grounds of affecting the sound.

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