Because of this "voltage source" characteristic, the 300- series amplifiers double their power demands every time a loudspeaker impedance is cut by half. For example, the no. 336’s continuous rated output is 350 watts per channel at 8ohms; 700 watts at 4ohm; 1400 watts per channel at 2ohms. Again, many manufacturers understate in their literature the 8ohm wattage so the 4ohm and 2ohm wattage looks to be doubling. It’s only on independent tests this shows up the 8ohm to be higher and the 4ohm or 2ohm not doubling. Levinsons specs: for 334. 300 series 8ohm=125w 4ohm=250w 2ohm=600w
Stereophiles tested specs 8ohm=139.9w 4ohm= 242.4w 2ohm= 430w As you can see they don’t double, when tested.
Cheers George
|
nutty The amp doesn’t double, as tested by Stereophile. That you believed it did, because of the ML literature. https://forum.audiogon.com/posts/1492322 Saying this the ML's are a fine amp and have enough current to drive probably everything, just that they don't double their wattage for each halving of impedance, they come as close as I would expect, no amp can double all the way down to 2ohms. Cheers George |
Pass Labs is another that truly doubles down
Many manufacturers understate in their literature the 8ohm wattage so the 4ohm and 2ohm wattage looks to be doubling. It's only on independent tests this shows up the 8ohm to be higher and the 4ohm or 2ohm not doubling. Here's what I mean, both taken from Stereophile on one of the latest 60 .8 series. Manufacture: Specified as putting out 60W into 8 ohms and 120W into 4 ohms (both 17.8dBW), Stereophile measured: The XA60.8 considerably exceeded that power, delivering, at 1% total harmonic distortion (THD), 150W into 8 ohms (21.8dBW, fig.4), 240W into 4 ohms So you see 150w into 8ohms but only 240w (not 300w) into 4ohms, where's the other 60w gone???? Cheers George |
PS AUDIO 200C 200 to 8 ohms, 400 to 4 ohms, 800 to 2 ohms, I call BS on this, just advertising, there are always losses, independent non affiliated bench test would impress me if it did. As that’s like "perpetual motion" Show me one (any) Stereophile tested amps that can do this as they nearly always give those 8-4-2ohm wattage figures when they test amps. Except strangely not in the PS Audio 200C review, no tested wattage figures, why?? You figure it out. Or they an advertiser? https://www.stereophile.com/content/coda-technologies-s5-power-amplifier-measurements Cheers George |
Most tube amps are the equivalent of a graphic equalizer
Couldn't have said that better +1, there are the odd one that's almost flat with speaker loading, but they are a rare breed. Cheers George |
prevailing wisdom about Forests needing a lot of horsepower to sound their best. With this type of impedance and -phase angle, to they are EASY to drive, https://www.stereophile.com/images/archivesart/TotFofig1.jpg I would even say a 20w SE tube would do it. And they can’t drive ****! And here’s what Stereophile measured test concluded: "(fig.1) revealed it to be a very easy load for the partnering amplifier to drive. —will hardly stress even the most current-challenged tube amplifier." Cheers George |
absolutely astonishing when the F7 was used for a short time with 87 db sens. Totem Forests. It won’t peel paint but the sound produced is "to die for".
Here is a classic example of a good amp that doesn't need too much current to drive this easy Totem Forest load, but the same amp on a hard load that dips down much further in impedance and negative phase angle would be very handicapped. https://www.stereophile.com/images/archivesart/TotFofig1.jpg Cheers George |
More than a few people over the years on these pages have said only those SS amps which double down in output power as impedance drops are truly special or worthy amps. Eg., 200 @ 8ohms; 400 @ 4 ohms; 800 @ 2 ohms; etc.
To put it more simply: No amp will exactly double all the way, anyone who advertises this is stretching the truth. Ones that can come close to doubling (usually good bi-polar output amps) all the way to 2ohms, can push good current into hard to drive speaker EG: most Wilsons especially Alexia ( .9ohm epdr in the bass) and many other brands. This is however not an indication of good sound quality, but your half the way there with a "very" important part of the equation to get it, but your half the way there with a very important part of the equation to get it. It's up to you then to sort these out with best one sound wise, as you taken care of the drive (current) factor. Cheers George
|
If so, is there anything that can/should be done from a practical perspective to prolong the longevity of a class A amp?
Thanks. Keep them biased right, Pass recommends palm hand test on the hottest part of the heatsinks being able to keep them pressed for at least 6 seconds (no brickies hands) any shorter and it’s running too much Class-A, and also keep them in a well ventilated spot. Cheers George |
I’m also not convinced that the drawbacks of feedback are so bad it should be completely avoided. A little story about feedback. An output transformer "GOD" wound up some very nice massive cores for a friend of mine who was his buddy cost no object, interleaved to the hilt. I ended up with these cores when he passed away. The transformer "GOD" I got in touch with and he got me to get some special C-Cores from Egypt where apparently is the best silicon. These transformers could be changed via the separate interleaved winding tabs for 16,8,4,2,and even 1ohm. They sounded magnificent on a big pair of 150w tube monos I had, and I decided that maybe because the transformer were that good that there was maybe no need for feed back to help them have a lower output impedance. So I put on a variable feedback pot that could be changed from 30db down to zero. Adjusting the volume for the change in gain, it was clear as a bell that zero feed back was by far the best and because the transformers were that good the bass remained tight and dry. So that proved to me, feedback is good in tubes to get better bass with lower output impedance, but it does have a negative effect on the rest of the music. Cheers George |
The good watts are pretty much always the clean ones
Correct, you need good watts, but also good current to back them up, otherwise you end up with a "tone control" instead a of an amp that trys to remain dead flat into varying impedance's. Cheers George |
The more I’ve dug into amp designs and uses the less important watts seem to be. Correct, you just have to listen to a pair old well serviced huge Class-A Mark Levinson ML-2 monoblocks, these were only 25w!!!! into 8ohms, yet could almost double that wattage all the way down to 1ohm, which meant bags of current. These are one of the most magnificent amps you will ever hear. Nothing could touch them on a pair of ESL57’s, and they could drive anything, even the .9ohm Wilson Alexia’s maginficently, just not party levels, remember they are still only 25w. Cheers George |
good engineering demands speakers that are easy to drive This would be great, maybe in the future with different construction techniques, motor design, materials ect, but for now easy to drive speakers usually have other permanent compromises, that can’t be fixed driven by the right amp. At least the expensive hard to drive speakers are just that because they present no compromises except one, being asked to be driven with amps that can drive them with the current that's needed. Cheers George |
Do you know if the Sabrinas and later model Sash’s have similar IMP traces as the Wilson’s you just mentioned here? Both are hard to drive, not quite as bad as the Alexia, but both to sound at their very best in the bass would need an amp with good amount of current ability. https://www.stereophile.com/images/416WiSabfig1.jpg https://www.stereophile.com/images/archivesart/710Wilfig1.jpg
I’m hearing from this person and that, they have listend to the Sabrina and Sasha with very modest power amps, even < 100 wpc., tube amps and really enjoyed what they heard.
Yes it would sound good, I'm not saying it wouldn't. But he's not hearing them at their best with an amp with a bit of current. for the load they represent in the bass, that would make most tube behave like an equaliser (tone control). Cheers George |
It's useless to talk about 4ohm speaker and 8ohm speaker, as none are flat at that impedance across the audio band, they are very different. And the best way to treat it is to look at the lowest impedance and - phase angle and base an amp around that.
1: There can be dips in the impedance curve at certain frequencies that can be very low.
2:There are also negative phase angles at certain frequencies that can be very high.
Combine the one and two at certain frequencies and you have almost a short as seen by the amplifier "usually" in the bass.
EG:
Wilson Alexia is spec'ed by Wilson at 90db and 4ohm
Independent test measured:
I in the power region of the bass between 60hz and 110hz approx it has a nominal impedance of just 1.9ohms!, combine this with the negative phase angle of around -45 degrees and you have an EPDR (equivalent peak dissipation resistance) that the amp sees as just .9ohm!!!!!
So much for being designated as a "4ohm speaker"!!!
And the Sofia 3 is even worse than this.
Cheers George
|
Combine hard to drive speakers with most tube amps and you have a response that is all over the place.
Yeah EQ'd to the sh**-***se, some call this tube sound or euphonic's. I call it an equaliser. Cheers George |
I've NEVER seen a speaker company provide charts of impedance magnitude or phase angle.
You couldn't trust them anyway, only independent testers like Stereophile and the Miller Audio Research Company (you have to join), give you "unbiased test reports" For advertising propaganda some amp manufacturers deliberate understate the 8ohm wattage so the 4ohm looks to be doubling, you can catch them out with Stereophiles real tests, which shows far higher 8ohm and then the 4ohm guess what? it doesn't double. Cheers George |
When it comes to the perfect amp, Ralphs OTL Atamsphere’s are it.
But, and it’s a big BUT, you need the perfect speaker. One that is efficient, has a nice even resistive high impedance load across the frequency range, with virtually zero phase shift also across the frequency range. And Autoformers such as the Zero are a bandaid fix to get it, as they create other evils instead, similar to the Mac autoformers.
Full range speakers and horn speakers ("maybe Magie’s"??) are possible to have these 4 attributes, but are severely compromised in too many other areas, eg: colouration’s and distortions. Good luck finding any without these compromises.
Cheers George |
The Mac autoformer takes care that the amp drives the rated output across all loads 2,4,8, without raising distortion! (which these other brands dont do)
This reeks of MacItosh proper-gander "Good designed" solid state amps are designed so the output transistors are used in their most linear part of their curve. Cheers George |
This gives the Mac a very non fatiguing sound especially when listening loudly for long periods.
That's because they sound like a big fat sponge, no life, no dynamic contrast, that's why they're not much of a hit amongst audiophiles, but the "glitz queens" love them. Cheers George |
It makes sense to understate the ratings and show excellent measurements. Yeah as Al and I both stated, it makes it look like the amps doubling, when it can’t. Good marketing, and false advertising, take your pick. Cheers George |
However **some** designers in that situation may instead choose to publish ratings of 75 watts into 8 ohms and 150 watts into 4 ohms, which as George has pointed out may help sell amplifiers because it can lead potential customers to consider the amp as being able to "double down." Al, the big boys nearly all do it, just look at their own spec sheet proper-gander, and then compare it to actual test performed by independent testers. I’ve PM’ed you a really good audio test sight, far more in-depth testing than Stereophile’s testing. You'll be able to view 2003/4 testing but have to join to view later ones Cheers George |
So the upshot is, if you have speakers that are a kind load to the amp, then the amp is not called for to deliver current, and will stay reasonably flat in frequency response. But if the speakers are a load that asks the amp to deliver current at certain frequencies, and the amp can't, then what happens at those frequencies is the amp "sags" in power at those frequencies and therefore cannot stay flat in frequency response, and becomes a tone control instead. EG: Like the blackish grey trace shows in this graph of a tube amp that can't stay flat into a easy simulated speaker load. Which is plus and minus 6db!!! very far from sounding flat, and very mush a tone control. https://www.stereophile.com/images/1116PLPPfig01.jpg Cheers George |
