Amp wattage question

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gmiller1122

gmiller1122

Addled but happy
I have a '71-72 Univox U65RN amp. In one old catalog, it states this amp has 15 watts; in another, it states 30 watts. On the back of the amp, it states 50 watts. Which one is right? Are there different types of wattages?

It sure sounds like a 50 watt.

I asked this question at the Univox site, but no one has answered yet!


:(
 
Manufacture's did Watts in several ways 50 watts rms per channel/ 50 watts total rms which is what alot of them do today. If you buy a 50 watt system today it is usually the combined RMS output of all the channels. I do not know when they actually started doing the combineing rms totals as far as the year some manufactures did and some didn't.
 
Wattage is also rated for the ohm load of the speakers. Are they giving different wattage based on different ohm loads?
 
You will also see, peak wattage and other wattage ratings. The only one that means diddly is RMS rating. The same amplifier output will give you different wattages depending on the load. I would suggest finding out what compromises your output stage, tubes or trannies, and determine what their output is from their specs. Does it have two or four tubes, trannies, in push pull etc.
 
Thanks for the info, folks.

What does RMS stand for? One original catalog from 1972 I've seen online states "15 Watt RMS."

Another states that the amp has 10 transistors.

Yet another states "30 Watts music power."

There is a 50 watt, 12" speaker in the amp (not original) and the back of the amp reads "117 volts, 50/60 mhz"

There's no mention of Ohms anywhere.

It's a single channel amp. Does that affect the RMS rating (whatever that is)?

Thanks again! I really like the tone of the amp and would just like to learn a little more about it.

:D
 
RMS means Raw Mean Square which if I remember correctly is .714 of your peak wattage. 30 Watts music power means nothing. Can you find out the transistor
numbers for the output stage. You could open the amp up and look. If you can get these numbers we might be able to figure out the wattage from there specs. If there are two output transistors they are probably in push pull, if there are four they are probably two in push pull. They will be the biggest transistors in the amp and most probably mounted on heat sinks. The speaker rating in ohms will affect the wattage delivered and this should be included in the info needed to determine the output. The standard rating for musical instrumen speakers is 8 ohms. The amount of channels does not effect the wattage output. The 117 volt 50/60 Mhz is what it needs to run and tells us nothing about the wattage output.:D
 
dragonworks said:
RMS means Raw Mean Square
Click to expand...

Being new to the forum, I will defer to those with greater seniority. But for what it's worth, I was always taught that RMS means "Root Mean Square." It refers to the process of taking a series of numbers, squaring them all, calcuating their average (i.e., mean), and then taking the square root of that average. Thus you have the "[square] ROOT" of the "MEAN [i.e., average]" of the "[sum of the] SQUARE." Statistically, the root mean square of a series of numbers gives you a better estimate of the center of the bell curve than a mere "average" because the process diminishes the relative weight of any measurements that deviates significantly from the norm. You could think of it as a "weighted average."

Of course, RMS may mean something completely different in the context of maximum wattage of an amplifier. I always thought it referred to the maximum weighted avage (using the root mean square formula) wattage the amplifer could or should produce. But admittedly that's just wild-ass speculation on my part.
:D
Kelby.
 
Kalby, you are correct, I dont know why I typed Raw, must have been a Freudian slip, maybe I was eating sushi or something.
 
Im afraid that the 50 watt rating near where the AC goes in is just how much consumtion of power that amp uses up. root means square is 71% of half that so 15 watts is the magic no.


Peak to peak 50watts

Peak 25 watts

Rms 15 watts.
 
I dont believe you can determine the wattage from the power consumption?
 
15 watts is the magic no.
Peak to peak 50watts
Peak 25 watts
Rms 15 watts.
Click to expand...


Darrin -

Can you explain that? I get the RMS 15 Watts now, but how do you figure the 25 watt peak? And what's peak to peak?

And why is it that it seems so easy to list the wattage of a new amp in a catalog, but it is so difficult to figure the wattage of this old thing? :D
 
Well knowing the wattage is interesting enough, but knowing the SPL available for this amp when driving a specific cabinet or cabinets over its range of volume settings is more useful.
 
Peak to peak means measuring the output of the amp at the highest levels it is capable of putting out for a brief moment. The peak refers to the tops of a sine wave you might have seen pictures of from an ocilliscope. But amps can't produce this constantly so a more useful rating is RMS which is calculated using an averaging technique mentioned above by dragonworks. As for music power, that was just a meaningless term used to try to make the amp seem more powerful back in the days before these things were regulated by law. I once saw a cartoon where an amp was advertised as having 5000 wattsMWR and in fine print it defined MWR as Mike's wattage rating. The reason some amps sound louder than others of the same rating is because of power supply issues. Some power supplies can push a little extra when called upon to do so but some are maxed out because they were designed to do just enough but no more. It's usually a matter of expense because a power supply is one of the more expensive sections of an amp.
 
If thats a tube amp, post the output tube types and how many and Ill tell you roughly how many watts it is. Probably solid state, eh?
 
Hate to bring out the math, but here is the basic formula:

Power = RMS Current * RMS Voltage

Then if we substitute Ohm's Law

Current = Voltage / Load Resistance

and assume the amp can always supply enough current we get

Power = RMS Voltage * RMS Voltage / Load Resistance

The RMS Voltage in this case is the maximum time averaged voltage the amp can supply. It depends on two things - the maximum voltage the amp's output can swing and the shape of the waveform.

For a sine wave RMS Voltage = 0.707 * Max Voltage Swing

For a square wave RMS Voltage = Max Voltage Swing

So let's say we have a power amp who's output stage can swing plus or minus 40 Volts and we're driving an 8 Ohm speaker load. Sine waves are usually the standard waveform because they better approximate music content, so we'll use that formula. The power output of the amp is then:

Power = (0.707 * 40V)* (0.707 * 40V) / 8Ohms = 100 Watts

Now if we switch to a 4 Ohm speaker and assume the amp can still supply enough current we get:

Power = (0.707 * 40V)* (0.707 * 40V) / 4Ohms = 200 Watts

So the power output depends on the fixed maximum voltage swing the amp is capable of and the speaker load attached to it. BUT, not completely. In some case you may see specs like this:

Power Output:
100 Watts into 8 Ohms
150 Watts into 4 Ohms

You'd expect to see 200 watts for 4 Ohms, right? Well this spec illustrates an amp with limited current capacity. It can't reach it's maximum voltage swing with a 4 ohm load because it can't supply enough current to do it. So, why do some of those relatively low power amps in the hi-fi stores cost and weigh nearly as much as battle tanks? Because they have massive power supplies and arrays of output transistors providing huge current capacity such that their specs look very much like an ideal amplifier:

Power Output:
100 Watts into 8 Ohms
200 Watts into 4 Ohms
400 Watts into 2 Ohms
800 Watts into 1 Ohm

Oh, and that "peak to peak power" is kind of a bullshit rating calculated from a square waveform. In that case we could say: "Peak to Peak Power" = 40V *40V / 8Ohms = 200 Watts. But almost nobody plays music which looks anything like a pure square wave.

Hope this helps. :)

barefoot
 
Just thought of this.

The "peak to peak" power rating does illustrate a good point. If you crank things up too much you clip the waveform and it starts too look more and more like a square wave. So, you're effectively doubling the power going into your speakers. But a square wave is essentially just a sine wave with a bunch of high frequency harmonics which sharpen out the corners. So all that extra power is going into the higher frequencies - i.e. it's going into your tweeters. This is why tweeters are usually the first drivers to blow when you're maxing out your system.

barefoot
 
Thanks, guys -

Hey, Barefoot -- did you take a lot of Math classes or what? ...Jost joking :D

Savage -- yes, it's solid state. I haven't had a chance to look at the transistors yet. Hopefully this weekend.

Thanks again...
 
gmiller1122 said:
Hey, Barefoot -- did you take a lot of Math classes or what? ...Jost joking :D
Click to expand...
Well, I'm a physicist by education, so I have taken a few math classes here and there.;)

barefoot
 
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