Usable Power, SS Watts vs Tube Watts, and the First Watt??

Lancer X

Lancer X

Member
Hi folks. I'm new here, so please bear with me. I am struggling to understand what I am looking for in a stereo power amp.

At the moment, I am listening through a pair of recently-acquired ADS L880 (89 dB efficiency). According to the feedback on these forums, these speakers should not leave me wanting, especially considering my moderate listening volume goals.

We're currently playing vinyl through a little Chinese tube phono preamp, into a little Chinese Class D power amp. This Fosi BT30D amp claims 50W per channel max output. I don't know that that would translate to in true RMS rating??

Through these speakers, it seems like even 20W should be plenty loud, but it isn't. I can dime the amp and it isn't overly unpleasant. (Our living room area is open to the dining room and kitchen, so this is a decent sized space.) Almost leaves me wanting more, if I were home alone and in the mood to rock out.

Now, I come newly to your strange hifi world from the guitar planet. In recent years, I've discovered tube (valve)-driven guitar amps, and I will never, ever go back to SS. (In fact, I became interested enough to build a 5E3 Fender Tweed kit, and am now working on a custom 1-2W practice amp mod of that same circuit.)

My little 5E3 Tweed makes about 14W, and they are mighty watts. It would blow the doors off most 60W SS guitar amps and sound better doing it. So, I've understood for some time that not all amp "watt ratings" are created equal.

Steering this convo back to the stereo hifi world, I eventually want to build my own Class A tube power amp. But what should my goal specs be?? It seems that most audiophile stereo tube amps are claiming about 8W per channel. But even 50W per channel in my current amp doesn't seem sufficient (although I do get that my speakers aren't all that efficient).

Some Googling has landed me on pages such as this one, where the author discusses "phony watt" claims, "usable power" and the elusive "first watt". (Of course he is also in the business of selling 8W tube amps, so YMMV.) Still, my experience with my trusty 14W Fender Tweed has me believing that he's onto something here...

Please do chime in. How many watts per channel am I looking for in a Class A single-ended tube amp to drive these L880s?? And how should those watts be measured/calculated? Can't wait to learn from you all. I think there's enough meat here to get you enthusiasts waxing poetic. :D

PS - I am also sending that little Fosi amp back. It's actually a 2.1 channel amp (50Wx2 + 100W passive SW). Since those 100 SW watts are wasted on my 2 channel setup, I've instead ordered this Fosi model BT 20A, which is a 2.0 design claiming 100W max per channel. It should suffice until I figure out my dream amp.)
 
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With guitar tube amps the speakers are probably more efficient. My son uses a 10 watt single ended head amp I built for him using a 5881 output tube. He drives two Jensen 12" speakers with alinco magnets that gets really loud.
Without a RMS rating for that Chinese class D amp, who knows its real output.
I dought your getting 50wpc with a 24v wall wart power supply. A watt is a watt no mater if it's SS or tube. I have a 17wpc tube amp that gets pretty loud with 93db rated speakers. The lower the output, the more efficient the speakers need to be. An 8wpc tube amp isn't going to get you the volume your looking with those speakers.
 
bigaltx24 said:
How big is the open area and is there a lot of furniture, drapes, etc?
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It's a pretty big space. Vertically, it's a chalet-style house, so the ceiling is peaked. Probably 16 ft high at the peak, and maybe 10 ft at two of the walls? Horizontally, the footprint of the whole space is probably 18 ft wide X 36 long?

The two speakers are located in opposite corners of the short wall, in the living room area. They have pretty direct line of sight (line of hearing?) clear into the dining room and kitchen. The LOS passes two upholstered armchairs, a couch, and area rug. All other surfaces are hard, with a few pieces of wooden furniture along the walls. The long outside wall is almost entirely glass.
 
Lancer X said:
. Since those 100 SW watts are wasted on my 2 channel setup, I've instead ordered this Fosi model BT 20A, which is a 2.0 design claiming 100W max per channel. It should suffice until I figure out my dream amp.)
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I recently purchased a Fosi BT20A-S (just a silver faced version) for a buddy to use on his screened in porch. Yeah, it says 100W/ch. But I imagine that's 100W peak for a microseond or something. It does fine for what he's doing, running a pair of full range speakers at patio volume levels and he loves the little thing (BT was one of the primary reasons I chose that amp). Before I took those speakers to him, I had them hooked up to a Crown D75A that's rated at 40 wpc @ 8 ohms.

There's no comparison between the 40 wpc Crown and the "100 wpc" Fosi, but he wanted BT and the tone controls so I didn't just take the Crown with me. Don't let the advertised power rating be your only guide. I'm not an expert on guitar amps, but I suspect the distortion figures of that mighty tube 5E3 might render it as a bad choice for a hifi amp. And tube amps typically don't like hard to drive speakers, especially single ended designs that are already not going to be powerhouses with the amps most people use like 2A3 or 300B.

Short version: Don't judge all solid state (or tube, for that matter) amps based on the comparison of the Fosi vs the Fender. It's apples to avocados.
 
From what I found the one you sent back has the same chip (3116) as the one you are getting, so @30 watts into 8 ohms.
 
Jeffery said:
I suspect the distortion figures of that mighty tube 5E3 might render it as a bad choice for a hifi amp.
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Hah, no - it's a notoriously unclean amp. The 5E3 is Neal Young's growl generator of choice, for example.

Jeffery said:
Short version: Don't judge all solid state (or tube, for that matter) amps based on the comparison of the Fosi vs the Fender. It's apples to avocados.
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Thanks much. I'll consider SS amp plans too then.

What about the phono preamp? Should I chase a tube-driven design, or is that so much smoke and mirrors too?
 
whoaru99 said:
Powered at 24V, presuming the supply has sufficient current capability, the 3116 chip the Fosi is based on is ~40wpc @ 8 @ 10% thd and a bit over 30wpc @ 8 @ 1% thd. And, that's datasheet optimum conditions.
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Interesting. So since the amp I sent back was spec'ed at 50W/ch and 100W for sub "max power", does that mean it would have been reserving half of its power for the sub channel? In other words, maybe I was getting ~15W/ch @ 1% THD? (And, that hopefully the new 2 channel one will be ~30W/ch?)
 
With preamps it's way more up to listener preference. Some guys swear by tube pre and phono pre, my pre and phono pres are all solid state and I like them fine.
 
Your acoustic suspension speakers really need 100+W for dynamic music especially in a large space. Getting that with tubes running class A is expensive and not necessary IMHO.

I think you'd be happy with a good quality switcher like an Ncore or Purifi based model.
 
Lancer X said:
Interesting. So since the amp I sent back was spec'ed at 50W/ch and 100W for sub "max power", does that mean it would have been reserving half of its power for the sub channel? In other words, maybe I was getting ~15W/ch @ 1% THD? (And, that hopefully the new 2 channel one will be ~30W/ch?)
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All but certainly two separate chips; one used in stereo mode for left and right, the other configured with paralleled outputs (called PBTL mode) for the sub.
 
Acoustic suspension speakers like your L880's are power hungry; that said, AS speakers are known to have better bass control and depth as well as a more lucid mid-range & treble presentation then their bass reflex/vented counterparts; specifically when overall cabinet volume is taken into account. In other words, they sound damn good.

AS speakers like lots of clean wattage to sound their best. A quality 100 wpc+ SS amp/receiver is in order here.
 
whoaru99 said:
All but certainly two separate chips; one used in stereo mode for left and right, the other configured with paralleled outputs (called PBTL mode) for the sub.
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Hmm, the manuf web page descriptions for both the 2.1 and 2.0 channel models both sport the following image. Maybe I actually got this right and will end up with 30W/ch instead of 15...

4_cf075e2d-7718-416f-b36f-5955322b630e_600x.jpg
 
If they actually used two chips in the stereo model, which I suppose is possible, but I doubt, and have them set up as PBTL/parallel mode, it does little to nothing for power at higher impedance 4 or 8 ohms. The purpose of PBTL is for more power at low impedance like 2 or 3 ohms.
 
The specs/datasheets for 3116 chip is freely available. There is no need to guess at the chip capability. The only guessing is how the often relatively poorly documented amps actually incorporate the chip(s).

And, not a ton of guesswork there. It's either stereo BTL or parallel mono (PBTL), AFAIK, and both those sets of data are in the datasheets.
 
Another point to consider is how much current an amp is capable of delivering. I had a Kenwood KM-206 that was rated at 150wpc, and it was soundly thrashed by a 45wpc Onkyo A5 that was capable of delivering far more current. I'm no expert by any means so take my experience for what it's worth.. My Citation 16 is rated at 150wpc but with the 2 massive transformers and the 4 22,000uf capacitors (10,000uf were original) it probably delivers enough current to arc weld and it has never seemed to be lacking for the oomph to drive a y of my acoustic suspension speakers.
I guess the point I'm trying to make is that a 100 plus wpc amp with a weak power supply will not satisfy your needs either.
 
opnly bafld said:
I hope the two that have recommended 100w+ for AS speakers never tell my speakers...
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I promise.

Henry would likely turn over in his grave, but I get why a mentor of mine chose to review double Advents via a Phase Linear 700 back in '72. I use a 300 watt/channel Ncore amplifier in the garage driving New Advents. It is wide dynamic range content that calls for it...ever so occasionally at that with authority. With average levels in the single digit watt range. Say this track from the soundtrack to Rogue One called "He's Here For Us" that opens the movie as we navigate through the rings of a Saturn-like planet for landing.

hereforus.jpg
 
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