Topic: Can somebody explain wattage and speaker efficiency? (tube vs. SS content)

[lds490]

How come tube amps are all rated around 25 watts per channel?  I mean, my consumer-grade Yamaha receiver has 135 watts per channel.
When I was a kid, I kept frying my speakers because I ran out of headroom when I turned the volume up.  The solution to my problem was getting a Carver power amp with 200 watts per channel.  So, my experience is the more watts, the better.

I assume that tube amps are loud enough.  So, how do they do it with such low power?  I have a vague idea that the answer is related to speaker efficiency, which in turn is related to amps.  Can anyone explain?  So, does that mean that some speakers can't be used with tube amps, but work great with higher-wattage ss amps?

Sorry for asking dumb questions.

[Frank M]

No, those are good questions.   

A tube amp can be designed to deliver 1 Watt max or 1,000 Watts max.  The Vacuum Tube Logic Wotan, for example, delivers 1,250 Watts per channel.  The problem is that that higher-powered tube amps are disproportionately expensive.

Unlike solid-state amps, tube amps require an output transforer (OPT) to interface with loudspeakers.  The more power that transformer has to handle, the larger and more expensive it becomes.  The quality of the OPT has a lot to do with the resultant sound, so typically you want the best trannie you can get your hands on.  Some manufacturers wind their own transformers BY HAND, further increasing the cost (i.e. the Audio Note Ongaku costs about $90,000 and uses silver-wire hand-wound OPT's).

Unlike solid state amps, tube amps are high-voltage devices.  The capacitors in the power supply operate at hundreds of volts (instead of tens of volts) meaning they pack a serious whallop.  This is the main reason that tube amps sound more powerful than their rating--they have NO problem handling sharp transients.  The dynamics of a solid-state amp can seem lackluster by comparison.  A 100 Watt tube amp will sound much more powerful than a 100 Watt solid-state amp.

You're right about efficiency.  Over the past 10 years there has been an increase in the number of flea-powered tube amps on the market. 300B, 2A3, and 45 single-ended triode amps deliver approximately 8, 3, and 1 Watt respectively.   These absolutely have to be matched to a sensitive loudspeaker with a relatively high impedance (horns, for example).  From what I gather, the goosebump factor with these systems is unmatched.

Finally, the main reason people like tube amps is because of their "special" characteristics.  They sing in a way that few solid-state amps can match (harmonics, baby, harmonics!).  Because of that, we simply don't mind having less power. 


edit:
An anomaly among tube amps are the OTL's, the output-transformerless amplifiers.  The early models were unrelaible and unstable, but apparently the currect designs are fine.  Here's a link to a technical page at Atma-Sphere:

http://www.atma-sphere.com/papers/otl.html

[BobW]

Finally, the main reason people like tube amps is because of their "special" characteristics.  They sing in a way that few solid-state amps can match (harmonics, baby, harmonics!).  Because of that, we simply don't mind having less power. 

Yes, and too much capacity (watts) reduces the even-order harmonic output.
Hence, a 150 watt tube amp may actually sound much less "sweet" than a 17 wpc tube amp.

Interesting story. A government contractor developed an extremely flat audio amp for telecommunications.
Perfect reproduction fron 10 Hz to 30 kHz.  No phase distortion, THD nearly zero.
They were very pleased with their creation and anticipated a huge side benefit in commercial sales to audiophiles.

To cut to the chase, no reseller would buy it. It sounded too "flat", too "brittle", not bassy enough, lacking image, blah, blah, blah....
Coloration in sound is where it's at. 
Everyone seems to want a "musical" amplifier.

BTW, transistors, and especially op-amps are faster than tubes.
The lack of a transformer helps this along.

The inter-electode capacitances, thermionic noise, and a host of other "bad" tube characteristics seem to be what makes them so sonically appealing.

IMHO, to simulate the sound, roll-off the top end, add a very, very small amount of pink noise, and add 2nd order harmonics to taste.

Also, do NOT confuse speaker efficiency and sensitivity.
These are two distinct charateristics.

[BCostigan]

Frank M..........great info.  +T

[Evil Taper]

Nothing can compare with aunthentic and pure tube sound though.  Don't be fooled by wattage on stereo gear because the circuitry on alot of higher watt gear is highly ineficient.  If you go to a car stereo store you can but a 2500 watt subwoofer amp, but it's only around 70% efficient so there is massive loss of wattage before it hits the speakers.  Companies moved away from tube amps because it's cheaper to build a tubeless amp and there are no tubes to break so there should be less maintenance.  A tube amp rated at significantly less wattage than an alternative unit may be just as loud and will definately sound better.  I'm highly biased on this topic though because I'm in love with pure tube sound in all applications.  IMHO you just can't beat those 70s albums recorded with entirely analog studio setups.  Even listening to some older recordings, Grateful Dead from '77 comes to mind, you'll hear a warmth and richness to the sound that just isn't there anymore.  The highly talented modern electronic music producers like Richard D. James use mostly analog instruments and initial recording gear to achieve that unbeatable sound.  Go to your local Guitar Center and plug the same guitar into a tube amp and then a standard and see for yourself how different it sounds.

[ducati]

How come tube amps are all rated around 25 watts per channel?  I mean, my consumer-grade Yamaha receiver has 135 watts per channel.
When I was a kid, I kept frying my speakers because I ran out of headroom when I turned the volume up.  The solution to my problem was getting a Carver power amp with 200 watts per channel.  So, my experience is the more watts, the better.

Actually, the reason you blew your speakers is you turned your initial amp up so loud that it started clipping.  To make it very simple, clipping is when the tops and bottoms get chopped off smooth analog waveforms.  What you are left with looks like a smooth waveform with a square top.  This is because your amplifier just cannot produce the amplitude you are asking of it...  And it sounds bad, real bad.  Not to mention it fries speakers faster than you can believe.  I would guess 99% of all fried speaker drivers are due to amp clipping.

I assume that tube amps are loud enough.  So, how do they do it with such low power?  I have a vague idea that the answer is related to speaker efficiency, which in turn is related to amps.  Can anyone explain?  So, does that mean that some speakers can't be used with tube amps, but work great with higher-wattage ss amps?

More to the point of your original question, the interplay between watts and "efficiency" is but one part of the game.  Efficiency is usually discussed in terms of how many decibels of sound are produced from a speaker at 1 meter with 1 watt of power.  This gives us a reference point.  So when people say "my speakers are 88db efficient" this means that they produce 88db 1W/m.  If the next guy has "90db" speakers, his will produce more volume with a given watt.  This is a rough guide, and all things being equal high efficiency is better than low as you spend less $$ in amplification to achieve the desired result.

As I said, this is but one part of the game, so on to the next part of your question (tube vs. ss and speakers).  When thinking about a tube amp, you really need to think about impedence.  Impedence is the resistance the speaker presents to the amplifier at a certain frequency (it changes radically when you sweep from low to high freqs).  Low impedence, less resistance, will demand more current from an amplifier.  High impedence demands less current, and all things being equal presents a more benign load on the amplifier.  This is particularly problematic for tube amplifiers for a few reasons, but the big one is alluded to above--the transformer coupled output that a tube amp has is both a strong and a weak point of a tube design.  These transformers are optimized for a certain impedence, and usually have multiple taps.  Say 8 ohms and 4 ohms.  If your speaker presented an ideally flat 8-ohm impedence (none do) the tube amp would sing at it's happiest.  In reality, as I said, the impedence varies greatly with frequency.  When matching speakers to a tubed amp, all things being equal, you really want the most benign impedence curve possible.  One with huge dips and valleys will cause wildly differing frequency response when fed by a tubed amp.  In contrast, most SS amps are NOT transformer-coupled, and can deliver whatever current the speaker is asking (to the limits of the design, power supply, etc).  This is why speakers that present "difficult loads" (either very low impedence or wild swings) are often powered by solid state amplifiers.

HTH

[pfife]

Take the following with a grain of salt, cause I am speaking with experience from guitar tube amps, which many of the same principles have applied-  maybe someone can confirm this-

but it gets even more difficult than comparing watts for tube vs. ss - as far as guitar amps go, a 30W tube amp from one company has the potential to be louder, and way more powerful than a 100W tube amp from another manufacturer.  I have a Red Bear 120W tube amp for my guitar, and a 30W Orange amp will kick the crap out of it volume wise...

This has always confused me too...

[Tim]

great info in here....

[pfife]

great info in here....

There are some ridiculously knowledgable people in this forum, and all over this site.  I love it.

[Frank M]

Take the following with a grain of salt, cause I am speaking with experience from guitar tube amps, which many of the same principles have applied-  maybe someone can confirm this-

but it gets even more difficult than comparing watts for tube vs. ss - as far as guitar amps go, a 30W tube amp from one company has the potential to be louder, and way more powerful than a 100W tube amp from another manufacturer.  I have a Red Bear 120W tube amp for my guitar, and a 30W Orange amp will kick the crap out of it volume wise...

This has always confused me too...

Power ratings are always for a given distortion level.  It's a swag, but the 30W rating may be for a low distortion level while the 100W figure may be for a high level.  In a guitar amp, the distortion is such an important part of the sound that there's no accepted distortion level at which to rate it.  With hi-fi amps, the power rating is for 1% THD or less.

(S.W.A.G - Scientific Wild-Ass Guess)

[ducati]

Also, and I forgot to mention this...  Rated power is often not with all channels driven, at all frequencies.  Those "125W" home theatre amps can't push anywhere NEAR 125W in the real world because many are measured with only one channel driven, at one freq, and sometimes with only a pulse signal, not a constant!  Most good hifi amps are rated much more conservatively.

And think about the "clipping" I mentioned.  SS circuits clip in that very nasty, squared-off way; tubes clip by rounding the top of the waveform.  Essentially, it just stops getting larger.  Tube clipping is much softer and less bothersome to the ears.  Thus, you might be running a 30W tube amp right up at 30W with a few clips here and there, but the 125W amp that can only push 50 "real" SS watts can't be run anwhere near 50W output as your ears will keep you away from your clipping.  So maybe that 30W tube amp will get louder than the 125W SS amp after all.

FWIW, I have only 50wpc with my tubed setup, and it's way more powerful than I even need.  Room is 12x37 (big long living room/kitchen combo). 

Also, remember that it takes a doubling of watts to increase the sound by 3db.  So if I wanted 6 more DB I'd need 200 tubed watts.

One thing that should be said--big tube power comes at a price.  >100 watt tubed designs (often monoblocks) carry a great heat and weight penalty.  I entertained the idea of moving to a pair of 200W tubed monos (the larger version of my amp) just because.  My dealer talked me out of it as he said the monos would make my room uncomfortably warm--as in sweating like a mofo!!

[BCostigan]

great info in here....

There are some ridiculously knowledgable people in this forum, and all over this site.  I love it.

QFT

[macdaddy]

so with speakers that clearly say 8 ohms on the back of them (from the factory), i should be plugged in to the amp at 8 ohms, right..? i have people telling me all the time to move them to the 4 ohm taps, bt i havent done it - what would be the advantage, when the speakers are recommended to be used at 8 ohms..?


i have said it before, but im saying it again now - the playback forum kicks a$$!!!!

great info in here....

There are some ridiculously knowledgable people in this forum, and all over this site.  I love it.

QFT

qft - again.

[Lil Kim Jong-Il]

My old CJ tube amp had multiple taps.  They recommended starting with the tap that most closely matched the speaker.  Then try the others to see which sounded best. 

So I don't think it's a fixed thing.

[ducati]

Definitely not a fixed thing--try them all.  Sometimes manufacturers rate their speakers as "8 ohms" and they really end up being anything but.  And since impedence varies wildly with frequency, you may find you like the 4ohm tap better.

For my ProAcs, on my ARC, I use the 8 ohm tap.  I tried the 4 ohm and it didn't sound as good (it was really a marked difference).

But each situation is different.  It can't hurt to try.