Connecting multiple speakers to one Amplifier output

My understanding is that this is just a nominal value. AIUI, the impedance of a loudspeaker varies a lot over the frequency range anyway. Likewise that of an amp. Bu I imagine that those who've actually built one could give a more extensive answer (and indicate what happens with a mismatch).

It would be better not to go lower, though higher isn't a problem. If the speaker impedance is too low, the power supply could run a bit short of current and clipping could occur below the full rated output.

'Matching' was important with valve amplifiers, too high an impedance would cause an increased voltage swing at the anode(s), potentially causing damage, particularly if completely open circuit at the transformer secondary.

I don't think 6 Ohms has ever been a standard speaker impedance, typically 4, 8 or 15 were normal, or 100 Volt line. But a nominal 8 Ohm speaker might dip to 6 Ohms at some part of the audio spectrum.

Interesting change with transistors, then: the Quad 303 claimed to be "unconditionally stable" with short-circuit or open-circuit at the speaker connection. How did they manage that?

Indeed. A friend of mine who was a mobile DJ would happily disconnect and pack up all the cables and components of his rig, and the Quad happily continued to play on however many, or few, speakers were still connected as he did this. It never complained.

Adding more components.

It was normal to ensure that one's audio amps would take fully shorted outputs without blowing up. As well as all shades in between including capacitors across the output, and diodes to the supply rails to clamp inductive transients.

Mots amps have a small choke in series with the loudspeaker and an RC pair, in series, across it to ensure all this.

Plus temperature sensing circuitry and VA sensing circuitry built in to protect the output transistors from thermal runaway and secondary breakdown.

Getting all that lot stable and working without compromising the ability to deliver low distortion power across the whole spectrum into a rated load was about 40% of the design and development.

But it was important to ensure that badly wired up speakers simply stopped the kit from working temporarily, not permanently...

Because there was no transformer. With no load on the secondary, a valve output transformer behaved as a large inductor, and varying the current through it generated voltages that could be much higher than the anode could tolerate. Even today, if you drive a relay with a transistor you need to include a normally-reversed diode across the relay coil, to absorb transients which might be high enough to damage the transistor when the current turns off.

The trouble is that a shunt diode slows down the opening of the relay which may or may not matter. Avalanche rated transistors are a good solution that does not need any extra parts. Ideally the back emf should be allowed to get as high as reasonably possible for fast opening and an avalanche rated transistor or a suitable parallel resistor will allow that. Once the back emf gets to about five times the operating voltage there is no further benefit in opening times for the relays that I have tested.

John

I assume you tried swapping the speakers...

Maybe you mean there is no output from one of the channels.

I would chuck it if that won't offend the donor.

Most of the modern amps have an easily replaceable IC as output device - they are eminently repairable. Ebay it for 'spares or repair' for around £50

It is not recommended to put speakers in series as each one then does not see a low driving impedance, and so might show undamped resonances.

Why not use just the one speaker, as another one won't make the sound any better, and reduce total power to half if it's in series.

You've got to know what's wrong and how to fix it. and £50 is still quite a lot of money.

But it's also a sound system. There's an unnecessary premium charged for those, to dissuade you from buying new :-) It all balances out.

This one is easy. The first thing you check, is the two fuses :-)

Paul

Thanks, I have persuaded the Amp to work in stereo (bad speaker connection on the amp).

I would still be interested in other opinions.

PURE Scirocco 550.

The only spec, is the one printed on the back of the unit.

If you look at some of the Texas Instruments integrated Class D chips, a sample item produces 40 watts into one channel (so you'd need two chips to build a stereo), and the speaker rating on that chip is 4 ohms and 8 ohms (as "design examples"). That gives you some idea how they ended up with 6 ohms nominal.

Putting two 6 ohm speakers in parallel would be 3 ohms.

But if you can figure out exactly what chip is used for each channel, you might be able to narrow it down further. I just wanted to see, ballpark, what a 40W integrated Class D could do. And this gives you some idea.

TPA3106D1 ---------

Paul