Dummy loads on amps

[smoguzbenjamin]

I am officially confused. If I need a 65W speaker for my prospective 50W amp, and I wanted to change the apparent impedance so the amp doesn't get killed, my brain goes 'add a resistor in series or parallel. Is there sucha thing as a 75W resistor? :lol: Or is there a different approach?

[Mike Burgundy]

wattage and impedance are two different things.

Impedance is the AC variant of resistance: how much the signal is, well, impeded. If the amp wants 8Ohms, you hook up an 8 Ohm speaker, two 4's in series or two 16's in parallel.
Impedance has no bearing on how much power a speaker can handle.

Power ratings (wattage) on speakers designate how much power it *can* handle for sustained times. You don't have to feed it it's maximum all the time obviously.
On amps, it's maxiumum possible output power - same story.
So: on a 50W amp, you can easily hook up a 65W speaker without modifications, as long as you match impedances - but you don't have to match power. You could also hook up a 100W speaker (no way you'll blow that up) or even a 10W speaker, just keep the amp turned down low.

So: what's the speaker's impedance? What's the amp's output impedance?
hih

[Peter Snowberg]

I think it's important to note that an 8 ohm speaker is only 8 ohms at some frequencies. At other frequencies it may have an impedance of 400 ohms or more.

Just my 1 cent.

Take care,
-Peter

[Mike Burgundy]

...and if you do want to make a dummy-load, yes, there are really heavy-duty resistors. Using arrays of 10W resistors on a heatsink is common as well.
Conrad has a 100W 8.2Ohm resistor for €12,50 or something.

[smoguzbenjamin]

Ah... complicated.

If I used the LM3886 (which I think I will) for a power out stage,  the datasheet says 8ohms load. So here's my dilemma. I plan on making that amp stereo, but if I for some reason want to use mono, I need to take two 8ohm speakers and make them seem like 8ohms total, while they are both on. I could wire them in series with a total impedance of 16ohms, or in parallel with a total of 4 ohms.... I was wondering if I could just get a 16ohm resistor in parallel with the two speakers and make it look like 8 ohms... but yeah I forgot about that its resistance varies with frequency... hence the name "impedance".... dang :roll:

I heard a guy say today that he had some 300W speakers with an 8ohm independace :mrgreen: That was funny

[smoguzbenjamin]

Oh and mike, I think I mis-formulated the question, I know that a speaker has a Wattage rating for its maximum, otherwise how would you go about turning the volume down It's more the impedance I'm trying to figure out.

[The Tone God]

If you look at the spec sheets alittle closer the LM3886 can run from 4-16 ohms impendence without adjustments although you will not get the same output power.

You could make the amps switchable meaning you can run them bridged for mono operation or you could just switch off one amp and wire the speakers together.

Just some thoughts.

Andrew

[Nasse]

Was just testing my LM3886 amp few days ago and connected it to 4 ohm speaker for a while. Played for some time and pushed up the volume few times, sounded o.k. Later I left the amp on for 15-20 minutes with no signal, just the almost unnoticeable background hiss from my new cheapo Behringer mixer.  I got back to the room and noticed some funny smell (the heat sink was dusty :oops: ) and the heatsink was very warm. Indeed it was much warmer than before, I have used the amp occasionally for my 15" selfmade subwoofer with cheap chinese 8 ohm speaker. I powered it immediately down, thought I have something wrong with the amp.

I remebered having read something about the chip is optimised for 8 ohms, and some internal protection goes on at supply voltage near max and with four ohm speaker load. I connected it for my 8 ohm speaker (it was under some junk upstairs, carried the 15" ´s heavy box down the stairs :x ), and the excess heat generation was gone, amp worked just like dream no problemo. Dunno if the speakers caused the high temperature. Maybe there is some info on datasheet, maybe it´s time to read it again... But that what just happened to me, it might be just something wrong with my circuits :wink:

PS just etched successfully two circuit boards for two another amps, I have the chips and suitable transformer ready for it. I printed the pattern with laser printer on inkjet photo paper, and the heating process was successful at first try, nothing what a sharpie could not fix. I used smaller Elektor layout. Read somewhere that some layouts have hum problems (some ground loops?)

[smoguzbenjamin]

'My' design is at +/-35v and the datasheet doesn't mention the power handling at 4ohms. So I guessed that it wasn't up to that.... I'll see, bedtime now. G'night everyone