What wattage is my amp actually putting out?

oberon567

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Jan 4, 2015
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I am based in India, and one of the most common A/V companies here is called Ahuja. They make a lot of speakers and amplifiers and so forth. However, being India, sometimes things can be confusing.

Most amps have simply an output, for high or low impedance. If an amp is rated at 100w @ 8 ohms then you can do the math to figure out the wattage at other impedance levels, generally accepting that unless specified an amp can live happily in the 4 ohm - 16 ohm range. This all makes sense and a little math later, everyone is happy. But India doesn't always allow things to make sense.

I am looking at the SSA-100M amplifier (http://www.ahujaradios.com/ProductSpecification.asp?prd_id=61&id=2&sc_id=6) . It is listed as a 100W amplifier. But if you were to look at the back of the amp, there are separate output terminals for 4ohm, 8ohm and 16ohm low-impedance speakers as well as 70v and 100v high impedance speakers.

So, the question is, what is that 100w rated at?Is it 100w @ 8 ohms? If so, then it would be ~200w @4 ohms and 50 watts @ 16 ohms.... But if that is the case, why separate terminals? And how do the high-impedance terminals come into play?

The literature is no help, and emails to support as of yet unanswered. The literature simple says not to exceed 100w on the high-impedance lines, and not to use high and low impedance terminals at the same time. The literature also suggests always under-loading the speakers, so even if maxed out the amplifier cannot damage the speakers.

So this is what I am thinking... each of those terminal strips gives 100w of power *to the impedance listed on that terminal*. That means the 4 ohm terminal gives 100w @ 4ohm, the 8 ohm terminal gives 100w @ 8 ohm, and the 16 ohm gives 100w @ 16 ohm. However, the amplifier cannot regulate the impedance, it simply sends out power. If this is true, it means that if I connect a 4 ohm load to the 16 ohm terminal I can actually get ~400w of power. On the other end of the spectrum, if I connect a 16 ohm load to the 4 ohm terminal I am only getting 25w.

Does this make sense to anyone else? What else could this configuration possibly mean?

The thing that really leads me to believe this is that in the literature they show a diagram for connecting four 20w speakers in a parallel/series array that ends up being a 80w, 16 ohm load,and they say NOT to connect to the 16 ohm terminal because the amp's power is greater than the speakers' needs, and instead to connect to the 8 ohm terminal to under-load the speakers. This would only work if my hypothesis is correct, that every terminal delivers 100w of juice to its advertised impedance, and you can do the math and determine that the 8 ohm terminal provides 50w of power to a 16 ohm load.

The biggest factor working against this hypothesis is that you would think they would then advertise this as a 400w speaker, since it can technically output 400w at 4 ohm.... So why advertise as 100w?

Thoughts?
 

makkem

Distinguished
Hi
The only time I have seen impedance selection is on valve amps .
However it is possible on solid state and implies that a transformer is being used in the output stage this means that each output will be rated at 100W as each output comes from a different tapping of the transformer.
So a 4 or 8 or 16 ohm speaker would get 100W if plugged into the correct output.
They are correct that 80W of 16 ohm speakers would be overloaded by the amps output of 100W.
And no it cannot output 400W,transformer outputs do not act like solid state ones though it will output more than 100W if a 4 ohm speaker is plugged into the 16 ohm output but not 400 w more and furthermore excessive output has a good chance of blowing the output transformer.
 

makkem

Distinguished
Hi
The only time I have seen impedance selection is on valve amps .
However it is possible on solid state and implies that a transformer is being used in the output stage this means that each output will be rated at 100W as each output comes from a different tapping of the transformer.
So a 4 or 8 or 16 ohm speaker would get 100W if plugged into the correct output.
They are correct that 80W of 16 ohm speakers would be overloaded by the amps output of 100W.
And no it cannot output 400W,transformer outputs do not act like solid state ones though it will output more than 100W if a 4 ohm speaker is plugged into the 16 ohm output but not 400 w more and furthermore excessive output has a good chance of blowing the output transformer.
 

oberon567

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Jan 4, 2015
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Thanks!

Ahhh... I have no idea about how an output transformer might work.

Since you say they do not act like solid state transformers (which I don't really understand, either), and since you say it cannot output 400w to a 4 ohm load on the 16 ohm channel, then I am to assume the formula I have found elsewhere:

Amplifier Output = Amplifier Watts * (Amp Ohm rating / Actual load ohms)
(ie, Output = 100w * originally rated at 16 ohm/ 4 ohm current load, 100 * (16/4) = 400w @ 4 ohm)

does not apply in this case.

In that case, do you have a formula or way to know what the wattage output for each of the various terminals is, assuming your load impedance is not the specified one of that terminal? (So a formula, or at least guide/suggestion, to know know how many watts a 8 ohm load can pull from the 16 ohm terminal, OR, how may watts will the 8 ohm terminal be able to provide to the 16 ohm load, so I can know just how much they are suggesting I under-load the speakers? If the above formula was correct, the 8 ohm terminal would provide 50w to a 16 ohm load, but it seems that formula may not apply so...)

Also, how can excessive output blow the output transformer? The output from the amp's side stays the same, right? Or is this basically the same way that a normal amp, rated for 4 ohm - 16 ohm loads, shouldn't run a 2 ohm load, not because the math says there isn't enough juice but rather because the amount of grinding away the amp will have to do to meet the needs of the load can burn up the amp? This would basically mean that the allowable impedance variance is smaller for each of these terminals.... where a regular amp allows 4 ohm - 16 ohm the 16 ohm terminal on this amp might allow 10 ohm - 16 ohm, or something like this? So running a 4 ohm load from the 16 ohm terminal might damage the 16 ohm terminal, but it might be okay to run the 8 ohm load from the 16 ohm terminal? Again, do you have math to help point toward limits?

Since someone knowledgeable is here, I am also interested to know how the speakers can be overloaded... I mean, I see the potential for it, 100w is more than 80w, I get that... But the amp does not always have to be putting out maximum wattage. Normally just because an amp is rated at say 100w, if you have the master volume on the amp below 0 it is not giving out full power, and you can use that to make sure you don't hit any limits. Right? It's the same here, yeah? Basically by suggesting under-loading they are trying to dummy-proof someone from having everything turned up all the way all the time and hence overloading the speakers. But, at the same time, you can also damage speakers by continually under-loading them, or at least various forums on the internet would suggest. Most sources seem to suggest, for live sound, that it is better to have an amp that is anywhere from 1.6x - 2.5x more powerful than your speaker array, to account for clipping (not for continual use). Any reason that doesn't apply here?

Thanks for your help!


 

makkem

Distinguished
A lot of questions.
The manufacturer is avoiding liability by stating that the 100W amplifier should not be used with 80W of speakers.
If the amp is used as a PA amp as intended then it would be fine to use with 80W speakers as the volume levels would never be continuously high enough to cause damage.

Forums advocating using a higher amp power than the speakers are generally talking about domestic audio equipment ,though this would also be fine when considering PA equipment.
However live amplifiers are sometimes used as guitar amps.

I used to work as a sound engineer for live groups in the 70's and Marshall used to sell a 100W amp with a 4 x 12 cabinet which contained 4 30 Watt Celestion speakers for use with guitars.
With heavy usage it was quite common for the voice coils of the speakers to fail,so a 100 Watt amp was blowing 120 Watt of speakers.
Why ?
On doing tests I found that because guitarists tended to overdrive the input to achieve that lovely Marshall sound and have the volume up full and use a lot of sustain on the guitar that the amp was actually delivering 375 Watts to the speakers.
The solution was to use two 4 x 12 cabinets with a power handling of 240W which was enough to prevent damage.
So you needed 240 W of speakers for a 100 W amp.

I am not aware of a formula for mismatching speakers to outputs on an output transformer suffice to say it is not something that should be done if at all possible.
I do know that it is not a linear formula like the one for output transistors that halving the impedance doubles the power.
 

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