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While deciding on a 10" speaker for a 175w solid state (acoustic) amp, someone told me that a 50w (Em lil Buddy) speaker is plenty. He put forth this formula, based on the assumption that speakers are rated in tube watts.
30 tube w = 80 analog w = 150 digital w
Is this true? Is there such a formula? Or is it a bunch of bs?
(ps - I don't know what he meant by "analog watts" in the middle.)
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07-16-2018 12:17 PM
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Watts is watts. It's a simple calculation, W=VxA, watts equals volts times amperes. Math does not change.
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It stems from falsely equating sound loudness with power production. In addition, amp companies use non-standardized ratings for their amps ( continuous vs momentary).
From wikipedia "Audio Power" article:
Matching amplifier to loudspeaker[edit]
Charles "Chuck" McGregor, while serving as senior technologist for Eastern Acoustic Works, wrote a guideline for professional audio purchasers wishing to select properly sized amplifiers for their loudspeakers. Chuck McGregor recommended a rule of thumb in which the amplifier's maximum power output rating was twice the loudspeaker's continuous (so-called "RMS") rating, give or take 20%. In his example, a loudspeaker with a continuous power rating of 250 watts would be well-matched by an amplifier with a maximum power output within the range of 400 to 625 watts.[27]
JBL, which tests and labels their loudspeakers according to the IEC 268-5 standard (called IEC 60268-5 more recently) has a more nuanced set of recommendations, depending on the usage profile of the system, which more fundamentally involves the (worst case) crest factor of the signal used to drive the loudspeakers:[28]
- For "carefully monitored applications where peak transient capability must be maintained, a system should be powered with an amplifier capable of delivering twice its IEC rating." As an example, a studio monitor rated at 300 watts IEC, can be safely driven by a 600 watts (RMS) amplifiers, provided that "peak signals are normally of such short duration that they hardly stress the system's components".[28]
- For "routine application where high continuous, but non-distorted, output is likely to be encountered, a system should be powered with an amplifier capable of delivering the IEC rating of the system". This includes most consumer systems. "Such systems can often be inadvertently overdriven, or can go into feedback. When powered with an amplifier equal to their IEC rating, the user is guaranteed of safe operation."[28]
- "For musical instrument application, where distorted (overdriven) output may be a musical requirement, the system should be powered with an amplifier capable of delivering only one-half of the IEC rating for the system." This necessary because, for example, an amplifier normally outputting "300 watts of undistorted sinewave" can reach closer to 600 watts of power when clipping (i.e. when its output is closer to a square wave). If such a scenario is plausible, then for safe operation of the loudspeaker, the amplifier's (RMS) rating must no more than half the IEC power of the loudspeaker.[28]
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Great read.
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To expand on my first post, watts is watts, but ratings are a mess. There is no standard for rating amp output power, and that's the cause of the confusion. A manufacturer can rate its amps using maximum continuous output, maximum output before distortion starts, maximum peak power, or something in between. Solid state amps tend to be rated at maximum continuous power, and are set so that they produce clean tones up to maximum volume. They don't overdrive easily, and if they are overdriven the souind is not pleasant. Tube amps, OTOH, tend to be rated at maximum output before distortion starts, so if you are willing to accept distortion, you can drive the amp beyond its rating, fairly easily. That's why people think tube amp watts are louder than solid state watts. Watts is watts, but tube amps can often produce much higher wattage than they are rated for. The only way to tell how many watts an amp is producing is to measure it. The output doesn't necessarily comply with what the manufacturer has rated the amp to produce. Since there is no rating standard, we have to take manufacturers' ratings with a grain of salt, especially for tube amps. Solid state amps tend to be rated fairly accurately, but tube amps are all over the place, usually rated very conservatively. Thus you need to be careful when selecting speakers for tube amps. Personally, I think the speaker should be rated for ~twice the tube amp's rating. Thus, if the amp is rated at 50 watts, I'd put in a speaker that is rated for at least 100. For a solid state amp, equal ratings should be fine.
I've used the word tend multiple times, because there are no hard and fast rules, IME. I'm sure anyone can find exceptions to these tendencies. Guitar amplifier power ratings are, to put it mildly, a total mess. Caveat emptor.
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But there are also standard ways to measure amp power, and when these are used, there is still a "tube watts vs solid state watts" phenomenon (at least according to my understanding). This stems from the different distortion behavior of (some) circuits. With lots of exceptions and caveats, tube amps tend to compress and somewhat preserve the waveform rather than hard clip it relatively far into the range in which are distorting. OTOH, solid state amps tend to "hard" clip the waveform almost immediately at the onset of distortion. The effect of this is that tube amps tend to have more usable clean headroom than solid state amps. So to get a (at least pscyho-acoustically) clean signal, you need less power in a tube amp than a SS amp.
Originally Posted by sgosnell
John
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I don't think it's actually less power, but less rated power. Headroom and power are not the same. Tube amps do handle being overdriven more gracefully than solid state amps, although I don't like the overdriven tube sound as much as some. But there isn't the abrupt unpleasant distortion that solid state amps can present. IME, most of the solid state amps I've heard don't distort much, if at all, even at full volume. This is easy enough to control through component design. Tube amps tend to go into at least some distortion well before the volume is full up. This is a design decision, and tradition is very involved. But the bottom line for me is that with the same cabinet and speaker, the same output from any type amp will give the same volume, because watts is watts. The sound will be different for each amp, of course, whether pure or distorted, but the volume, with the same measured power being delivered to the speaker(s), should be the same. But in the real world, the exact power output will be almost impossible to reproduce, and we really have no idea what the power output really is. It will seldom, if ever, be the full rated power of the amp. As guitarists, what we want is a pleasing sound, with adequate volume. But those are seldom, if ever, the same to different guitarists. Thus the plethora of amplifier choices available, and the opinions about them.
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And besides the watt rating, there is also the ohm (resistance) rating to consider. You never want to operate an amp without a speaker, and ideally you want a perfect match of the speaker and the amp resistance values. If that is not possible, tube amps shouldn't operate with a speaker that has a higher resistance than it should (say a 16ohm speaker instead of an 8ohm one), cause you can damage the output transformer and tubes. Solid state amps on the other hand shouldn't work with a speaker of a lesser impedance than optimal (say 4ohm instead of 8ohm) cause they will overheat and might take damage, mostly the power transformer.
Manufacturers tend to overbuild for that, specially the old fender amps, but still it is safer in the long run to match things.Last edited by Alter; 02-21-2020 at 07:09 AM.
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This thread is great reading. I can't contribute to the substance except by pointing out that speaker sensitivity is an often-overlooked factor which influences the loudness more than double-digit differences in the rated amp output. But there's also a persistent bias (no pun intended) in favor of tube amps. A highly respected guitarist known for his quest for the perfect tone, and for his stable of boutique amps from Dumble to Carr, speaks of Tube Amp Talebans and now tours with a pair of Quilter 101Rs. Modesty prevents me from saying which speaker cabs he hooks them to.
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I agree with an earlier comment that tube amps can put out much more than their rated power if driven into distortion. But I’m pretty sure you won’t find thst formula in any electrical engineering text. It makes no sense. Originally Posted by Woody Sound
Years ago I bought a 100W Fender Twin Reverb which I didn’t realize was loaded with two 15W speakers. One speaker developed coil rub and had to be reconed. But the other speaker lasted many loud gigs without problems. It’s risky, but possible.
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With the caveat that I've watched a tech do it, but not done it myself, yes, measuring power is an art, not a science. Originally Posted by sgosnell
That said, take a "40 watt" Fender tube amp and a "40 watt" Roland Cube, as measured by the same tech with the same test signal, scope, etc. Now plug both into the same speaker (assuming they put out the same power into the same impedence without impedance matching problems...) . Set the tone controls flat, the volume at 1 (assume a linear pot ...), and play a maj7 chord. Repeat, turning the volume up a notch at a time, until the chord becomes a little distorted (but still somewhat usable) That will happen lower on the dial with the Roland. Keep turning the dial until the chord becomes unusable. That will happen later on the Fender. Perform the same experiment with a "100 watt" Roland Cube and the same Fender. Those thresholds will happen at about the same points on the dial on both amps.
That's the "tube watts" vs "ss watts" hypothesis with respect to guitar amps. I agree, a watt is a watt, and it's incorrect to express the difference in amplifier characteristics in terms of power. But it's sort of a useful shorthand. It's probably more accurate to say something like "a 2x6L6 SF Fender is functionally as clean and loud as a typical 100 watt SS amp". 40 tube watts = 100 ss watts recapitulates that.
John
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OP here. Notice I am referring to a SS acoustic amp, designed *not* to go into any distortion. Originally Posted by Woody Sound
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Yes, but all amps distort. It's a matter of how much, and what the distortion sounds like, which is part of the question of how many watts an amp puts out. How much power the speaker is rated to be able to handle without getting damaged is a whole other question, but asking it sort of leads to the rabbit hole of how to measure power, and what are the implications of this for speaker selection. Bottom line, you're usually safest with a speaker rated for more power than the amp's nominal RMS wattage, but you can get away with a lower rated speaker if you don't play too loud and/or with too much bass. You can also often rely on a speaker's power handling rating being conservative and an amp's rated power output being exaggerated, which adds to the safety margin in using "under-rated" speakers. People routinely run amps with higher nominal output into speakers that are rated for less power than this without problems with guitars, especially if the amp is not constantly played at high volume. But plug a bass into it and crank it to 10 and you might blow the speaker, even if the speaker is rated to handle the amp's nominal power output (voice of experience on that). Originally Posted by Woody Sound
John
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Originally Posted by Woody Sound
And would that be why the high-end stereo amplifier builders never had this rating puzzle ? They used to simply rate their stuff, I believe, at RMS per channel @ ' x ' ohms.
Not sure they ever specified tube vs SS affecting ratings - - the message was : ' here's the output, full boat, before distortion. '
Maybe that was why I was drawn to Ampeg amps - - 'on' was 'on', and power was continuous throughout, from low to high frequencies, and bass, mid and high settings were separate from one another.
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Measuring power output from an amplifier, on a test bench, is absolutely science. You apply a known input and measure the output. It's perfectly repeatable, and that's how an amp should be rated. It's easy to monitor the output and find when the output signal starts distorting, and that output level is generally the rating assigned to a tube amp. It is very possible for the maximum output to be much higher, and it almost always is higher. However, when an amplifier is used by a guitarist, measuring output is a guess at best, because the input is unknown, and not constant. No one can pluck a string exactly the same every time, or even twice in a row. Not to mention that many people love the overdrive distortion, and always try to get it. Thus the amp often delivers more than the rated power to the speaker. And as noted, loudness and power are not linear. Speakers are different, and the efficiency of the speaker affects loudness very much. I've been ignoring that in my posts, by assuming the same speaker and cabinet for different amps. That's the only way to compare them, IMO, because otherwise it's apples to tangerines to bananas. It's easy to affect loudness by changing speakers.
All amps can distort, if allowed to, but it's a simple to design one that won't distort with the volume at maximum. The speaker may well distort, and may even blow at maximum volume, but that should also be prevented by using a speaker rated at more than the output of the amp at maximum volume. Whether to let the amp go into distortion is a design and marketing decision, as is the power rating. Some manufacturers may decide to rate their amps at less than the power it can actually provide. On some tube amps, the rated power may be produced at half volume, or maybe a little more. Some may rate them at the actual maximum output, or perhaps even a bit more. It's marketing. Do they want to be known as the most powerful amp, or the best-sounding amp?
All I'm saying is that watts is watts, and is measured the same for any amp. But the advertised rating doesn't necessarily have much to do with what the amp can actually produce. My rule of thumb is to install a speaker with a rating equal to the amp rating if it's solid state, and somewhere near twice the amp rating if it's a tube. That's because of the rating anomalies I've seen. You tend to see a lot more blown speakers in tube amps than in solid state, especially when the ratings are equal in tubes. It's not a law, just my rule of thumb, which could be wrong.
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There is a LOT that goes into amp/speaker design. In the 1930s-1950s, only tube circuits existed for commercial/consumer use. Moreover, power outputs for amplifier circuits were restricted to the 1-50 watt range until the late-50s. This caused designers to favor the use of speakers that featured very easily excited voice coils and speaker cones. Voice coils were wound on paper and speaker cones were 3ksp, thin paper pulp material. It took very few watts of power to excite these speakers to full excursion.
An example is the vaunted Klipschorn speaker, or the Altec 604 duplex speaker. These speakers remained very popular with enthusiasts for decades. However, they had quite low power ratings due to the fact that they were designed at a time that "hi-fi" amplifiers boasted 5-25 watts of power availability. These speakers delivered quite loud SPL volumes into the room with low wattage amps. It is quite easy to chew up either speaker with modern, high-power amplifiers. In the 70s and 80s, I witnessed high-end audio sales staff blow the high-frequency drivers on both Klipsch and Altec speakers connected to 100-200 watt amplifiers.
Modern amps, with 200-600 watts of power on tap, are typically envisioned to be employed with speakers that employ plastic voice coil forms and feature speaker cones made of tougher stuff than 3ksp paper, e.g., Kevlar (in some instances).
Well...I've blathered on enough for one message.
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So a 15 water Class A tube amp with a very efficient (102.8 db sensibility) speaker would be the equal to what in a SS amp?
I played a 15-watt Traynor tube amp tonight at a rehearsal. It was almost loud enough. Almost. No worse than a JC40 I was using for a while.Last edited by Bach5G; 07-18-2018 at 09:39 AM.
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Tube amp player here -- RMS is essentially an average output, and the peak wattage from a tube amp can be up to (so I've heard, pardon the pun) three times the RMS rating of the amp. This is with everything dimed.
As a result, I never pair a tube amp with a speaker lacking at least twice the rating of the amp (because only an idiot dimes everything).
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This might be a silly question, but is speaker wattage additive across speakers? I've thought about putting a pair of 50w cannabis rex's in my 60w Deville. Originally Posted by Thumpalumpacus
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I keep wondering if Roland is rating the new Jazz Chorus amps the same way they did with the original ones. I could not help but notice the new JC22 amp although smaller is very close in weight as the old JC55.
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Or worse, ELEVENs everything. Originally Posted by Thumpalumpacus
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Or, if it's a Fender, twelves everything...
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The following is written by a BSEE. I’m an MSEE. I mostly worked at RF frequencies but I have some audio experience. I think he’s nailed it.
Guitar Speaker Power Handling | Amplified Parts
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Kirk,
The author indeed does a clear job of explaining amp output power, distortion, speaker loads, and speaker operations.
Nice article.
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The 60w from your Dv will be split between the two speakers, and that's safe money; you've got 100w of speaker dealing with 60w of output, should be good for most everything but the large end of the gain. Where you want to make sure you're right is resistance. Matching the impedance of the speakers is more important than assuring the speakers power rating, because if you pump 100w into a 50w speaker, you're replacing a speaker, but if your speaker impedance is lower than your amp's output impedance, you could be replacing an output transformer, depending on the amp's design and quality of the OT. Originally Posted by Jehu
The less resistance the speaker gives to the amp, the more electricity flows back into the amp, and that will go back to the transformer, heating it up. Too much, and yeah, ugly smell.
If I remember correctly, two speakers wired in parallel (assuming same impedance) will halve the listed impedance of each speaker. A Blues DeVille 2x12 outputs at 4 ohms, so ideally you should have two 8-ohm speakers wired in parallel.
You can get away with impedance mismatches for a while, but I've always made sure I match them as closely as possible. I'm no expert, and if what I'm passing on has inaccuracies in it, I hope builders here will correct me.
If you'd like a further and better explanation on the topic, PM me, and I'll get you in touch with pro builders who understand this much better than I do.
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