SET the best?

Saki70, the speaker should be designed for tubes. See

http://www.atma-sphere.com/papers/paradigm_paper2.html

for more information.

I spent a few months tinkering with some 45-based SETs that I inherited last year. In analyzing the circuit, I realized some performance gains were possible through some simple mods and so got the amps to sound a lot better. Ultimately though I got curious what would happen if I made a P-P 45-based amp and did that as well (it makes about 4 watts and is class A). It was **instantly** better on the first attempt. IMO/IME most P-P amps are too complex and if a minimalist approach is taken SETs will not compete.

Phaelon, SETs are usually not very powerful. A big one is 15 watts, maybe 20. If you really want to hear what they do, the speaker has to be efficient enough that you never bring the amp anywhere near full power.

At full power, the THD is often about 10%! At low power levels, the distortion might be unmeasurable, and what of it there is will usually be lower orders (2nd, 3rd and 4th). At high power the high orders become involved- that is the range of the amp that tells you not to turn the volume up any higher- it *sounds* loud due to to the presence of the 5th and 7th harmonics, which tell the brain how loud the sound is.

So the efficiency of the speaker is paramount and no matter what anyone tells you, to really hear what SETs do you need high efficiency: 97-98 at a minimum if your amp makes 15-20 watts. A 45-based amp needs a speaker that is more like 107 (if you really want to hear what it does anyway). This power limitation is one reason why I don't use SETs. The bigger you make them, the less bandwidth and detail.

My speakers are 97 db and 16 ohms. I find that 30 wpc is not enough- its much easier to play the system if I have 60 watts. But the amps I use are designed to not generate any loudness cues to the human ear (unlike SETs, which rely on enhancing the 5th and 7th harmonics to get the 'dynamic' sound that SETs are known for), so I use the power without the system sounding loud. My room is 17'x 22' and is moderately lively with lots of diffusion. I do like to listen at higher levels, often reaching 100-105 db, but the system is relaxed at those levels and lacks any hint of being 'loud'.

I put 'dynamic' in quotes here and above because IME, many audiophiles when using that word are often referring to distortion of the 5th and 7th harmonics instead of actual dynamics. The way this works in an SET is the 5th and 7th harmonics don't show up until you really push the amp, which is usually on transients. So the loudness cues exist on the transients, but not in between. This is why SETs are well-known for being so 'dynamic' in a way that is out of proportion to their power. IOW its a psycho-acoustic effect.

This is why so many SET users play their systems at relatively low levels- the loudness cues created by distortion prevent them from wanting to turn it up. But if the system is designed to avoid these harmonics and has the ability to play louder, it will be very natural to do so.

Johnk, the post was worded right. The upper power limits of SETs in a way are more of a function of the output transformer that it is the tube! The bigger you make the transformer, IOW to handle more power, the less bandwidth it will have. If you try to optimize it for bass, the highs will be rolled off and vice versa.

Since many high efficiency speakers (99db+) have limited bass response, SET builders usually sacrifice the bass performance in favor of the highs, since speakers that go below 40 Hz in the high efficiency world are few and far between.

IME experience the bigger SETs also have less detail. This is based on my personal listening experiences, but you will notice that one of the revered 'king' tubes in the SET world is the type 45, which usually makes less than one watt at full power. IOW, the smaller you make them, the better they sound, which is a common theme with amps in general (although OTLs seem to be one of the few exceptions), both SET and P-P.

I've also noticed that it is exceedingly rare to compare like technologies in the SET world. People often compare say a 300b SET against a P-P EL-34 amp (with the results in favor of the SET), but how often is a 300b or other SET compared to a P-P DHT class A amp of the same power rating? IOW to be scientific, this is where one would start if looking for real answers.

The cutoff frequency of the amp does play a role. Generally speaking, you can hear artifact associated with the cutoff (-3db point) at frequencies up to about 10 times the cutoff frequency. So if the amp starts to roll off at 20Hz, you can hear artifacts up to 200Hz.

The same is true of the HF rolloff- except that now its 1/10th the cutoff frequency. So if the amp rolls off at 20KHz, you will hear artifacts down to 2000Hz.

This is why designers try so hard to get bandwidth!

IMO if there is a subsonic rolloff, you hear it as a diminution of bass impact. It can also be measured quite easily- square wave tilt is the measurement. Having played around with LF cutoff frequencies a lot (our amps are full power to either 1Hz or 2Hz), all I can say is its easy to hear once you know what you are looking for provided you have speakers with bandwidth into the low 20s.