300b lovers

@atmasphere 

Yes we agree.  If you cannot drive a loudspeaker in a normal room to adequate levels with 60 watts/ch, then you really need to reconsider your speaker choice!

The least efficient speaker I ever owned was a pair of Joseph Audio RM25si Mk2, which were 89 dB and a true 8 ohm easy load.   With the 60 watt amp I could play them to FAR higher levels than I would ever listen to in a very big living room.  Now my speakers are generally in the 95-97 dB range so I could do the same with 10 watts.   The 27 watt 300b monos hardly know the speakers are connected.....

There are so many just brilliant speaker designs out there that are 89-90 dB+ and easy loads.   Why on Earth people get huge 84 dB poorly behaved speakers that require 200 watts/ch or more is beyond me.....

@donsachs Yes- those Joseph speakers were nice.

I can see 88-89 for smaller speakers. I have a little 5 Watt tube amp  I designed for desktop or a bedroom system and I use a pair of Fritz Carbon 6s with it, which are 88dB and I never run the amp out of gas (but I never play it that loud either).

But if they are going to be large there's no reason they should be hard to drive. I keep telling people that if you want to get the most out of your amplifier dollar investment, its best served by a speaker that is higher impedance and easier to drive, on account of the simple fact that the harder the speaker is to drive and the lower the impedance it is, the more distortion the amp is going to make. IOW a simple way to make any amp sound smoother and more detailed is to have it drive a higher load impedance (if all other things were somehow equal, which they never are...).

Most audiophiles do not realize how stupendously inefficient speakers are. By way of reference, 92 dB/watt/meter is about 1% efficient, or put another way, 100 watts of electricity is converted to one acoustic watt (which is plenty loud).

So where does the other 99% of these pricey watts go? Voice coil heating, which isn't great considering how tiny voice coils are, and how poor thermal coupling to the outside world is. First the voice coil has radiate its heat to the magnet, which is the closest thermal sink, then the warmed magnet has to transfer its heat to the inside of the enclosure.

Since the goal is to create X amount of acoustic watts, not a clumsy form of room heating, even small gains in efficiency are worthwhile, since less voice coil heating is occurring for given acoustic output.

Aside from outright failure, another problem with VC heating is copper's change in resistance with temperature. The resistance goes up with temperature, which might be acceptable, excerpt the time constant is fairly slow, on the order of several seconds, This creates a dynamic slurring which is pretty audible.

I should add I am completing a large-format 2-way speaker this summer, a collaboration with Thom Mackris of Galibier Designs, and an entirely separate project from Don Sachs and the Spatial Audio team. It’s a culmination of the extremely long "Beyond the Ariel" thread over on DIYaudio.com, and the first version was built by Gary Dahl, of Silverdale, Washington.

The woofer is an Alnico-magnet 416 (15" midbass) from Great Plains Audio, the successor to Altec Lansing, using Altec staff and tooling. It’s in a low-diffraction (4" radius curved edge) 4.2 cubic foot closed box. My version will have Bubinga (African rosewood) veneer on all sides.

The high frequencies are from an Athos Audio Yuichi A290 wood horn, with a to-be-determined 1.4" exit monitor-class compression driver. Crossover will be around 700 Hz, most likely Altec-style 2nd-order. The RCF 850 and 18Sound drivers are candidates. I also have a pair of Altec/GPA 288’s in house as fallbacks.

Efficiency will be a true T/S value of 97 dB/meter/watt. With a 27 watt/channel amplifier, headroom should be, in the timeless words of Rolls-Royce, "adequate". Alternatively, sufficient for a studio monitor application.

A 20-watt amplifier and 97 dB/meter loudspeaker was pretty typical for a serious high-end system in the mid-Fifties, so it’s not as weird as it sounds. It’s only weird in the modern context of 200 to 500-watt Class D amplifiers and 85 to 87 dB/meter audiophile speakers.

@lynn_olson - First, let me state that I've really enjoyed your, Don's and Ralph's discussion of amplifier design and tradeoffs.

I've been intrigued by Alan Wright's designs (I'm currently building a line stage preamp inspired by the RTP3D). You've done a good job explaining the downsides of a differential output stage. Alan was a big proponent of this approach so he obviously felt it had advantages. Since he is no longer with us to defend his design, what do you think are the positive attributes of a differential output stage in a tube power amp?