SET v. Pushpull

Tom,

You have been given very good input here. Almarg's points about impedance was definitely spot on (as usual in his posts): both as a key speaker attribute when considering to match with SET, and in showing your current speakers aren't the best fit with SET. The main points were around impedance variability over the frequency range. I also want to point that impedance magnitude is very important for SET matching as SET amps are constant current sources.

Most other amps are constant voltage sources, so as you halve the speaker impedance they deliver twice as much power, or half as much power as you double impedance. You've probably amps specs saying 200W into 8ohm and 400W into 4ohm. Here's the simplified math behind that:
Power (P) equals voltage (V) times current (I). P=V*I.
Also V=Z*I, where Z is impedance.
You can replace in the first one: P=V^2/Z. If an amp is constant voltage, power is inversely proportional to speaker impedance. Double Z, halved P.

On the other and SET are constant current, so in P=V*I you can replace into P=Z*I^2. As you double speaker impedance the amp delivers twice the power.

Look into speaker designs by SET amp manufacturers. For example Coincident Technology. Their speakers are 95dB-ish sensitive and 10 ohm, low variation impedance. Just looking at the 95dB sensitivity might lead you to believe they aren't good match for a 300B 8W, but the designer had that amp in mind when designing. BTW, note these aren't horns. While horns are indeed a good match for SETs, not only horns are suitable for SETs.

Also, another consideration often brought up by Ralph from Atmasphere: SETs produce extremely low distortion provided you run them at say 25% of their rated power, and low distortion up to 50% of their rated power. Distortion grows fast as you approach rated power. So while it's true SETs produce more distortion, that is true at their rated power, and a well designed system that incorporates SET should take into consideration running them as easily as possible. Hence it's also true SETs produce very low distortion, with the added benefit of producing more pleasing harmonics when they do.

Bottom line: it's all about execution. No blanket statement can be made. SETs could be better or worse. Your existing speakers aren't a good match for SET. But you also mentioned they are too small for your room. Maybe you want to go SET knowing you will replace your speakers with SET friendly later. Or replace your speakers first and then find an amp to match.

Sorry for the length!

@Lewinskih01, thank you kindly for the nice words. And for the generally excellent and informative post. But a little of it isn’t quite correct.

SETs are not constant current sources, which of course would mean that they provide an amount of current that remains essentially constant as load impedance varies, as long as they are operated within the limits of their maximum voltage, current, power, and thermal capabilities. They, and also push-pull tube amps, are not constant power sources, either. What is fair to say is that to a very loose approximation most of them behave as constant power sources, and how loose that approximation is depends on their output impedance. While a few others that have unusually high output impedances loosely approximate behaving as constant current sources, to a degree that depends on their output impedance.

As we have said, nearly all solid state amps maintain voltage that remains essentially constant into varying load impedances (for a given input voltage to the amp), as long as they are operated within the limits of their maximum voltage, current, power, and thermal capabilities. (Certain First Watt amps that have been intentionally designed to have high output impedances are notable exceptions). While SET and push-pull tube amps, when operated within their capabilities, will deliver more voltage and less current as load impedance increases, while delivering less voltage and more current as load impedance decreases. But the product of voltage and current in those cases will not remain constant for most such increases or decreases.

To see that, consider a tube amp to be an ideal voltage source (having an output impedance of zero, and outputting a voltage V) in series with a resistor, denoted as R1, that is equal to the amp’s output impedance. And consider the speaker impedance to be R2.

Per the voltage divider effect the voltage appearing across the speaker will be (V x R2)/(R1 + R2). The current provided to the speaker will be V/(R1 + R2). Plugging various values for R1 and R2 into those formulas, and calculating the product of the voltage and current provided to the speaker, will illustrate my point.

For example, consider an amp having an output impedance of 2 ohms (which is actually a good deal higher than the output impedance of the 4 ohm tap of the Line Magnetic SET amp that was referred to earlier), and speaker impedance that increases from 4 ohms at one frequency to 8 ohms at another frequency. You will find that both the current and the power that are delivered into 8 ohms are significantly less than what is delivered into 4 ohms, assuming the amp is operated within the limits of its maximum capabilities into both impedances.

Best regards,
-- Al

 One thing I would point out is that some of the distortion from push pull comes from the crossover region which can be more objectionable to your ear. SET distortion is mostly even ordered and usually is perceived as pleasant. But you do want to run SET very lightly in which case it’s one of the lowest distorting topologies around.  In my experience, running very efficient easy load speakers, I preferred SET over push poull and 0TL over SET by no small margin. 
 And in my experience SET running a proper load is not warm and fuzzy. It’s quick light and dynamic. I just found O TL to offer an even better picture into the musical event. Hard to describe. 

So much depends on topology!
*Some* pushpull amps have crossover distortion problems but most do not. What push pull amps often do have trouble with is the so-called 'first watt'. Many do have higher distortions in that area, but its usually not crossover distortion, although that is often what gets trotted out as the explanation.
SETs have a distortion character where as the power level is decreased, the distortion linearly decreased to unmeasurable. Some push-pull amps are like that too- ours are, and in general I would suspect any fully differential push-pull amplifier to have a similar quality if designed correctly.
SETs often do not employ loop negative feedback! This accounts for both their high distortion at full power and also their higher output impedance. The tradeoff is that first watt- which tends to have lower distortion on this account. That can be done with push pull amps too, but it is trickier.
Generally speaking, a sensitivity or efficiency of a loudspeaker that is less than 85 db is bordering on criminal- it gets nearly impossible to find an amplifier that simultaneously makes enough power and actually sounds like music in most average rooms. Exceptions might be if the speaker is only heard nearfield (within 3-5 feet) or the room is quite small. You just wind up having to push the amplifier too hard, and in the case of an SET this really won't bring out the strengths of the amplifier!
Someone already mentioned my comments about SET amplifier power above so I won't repeat them. But here's a thing in a nutshell:Tube amplifier power whether SET or not is expensive so its to your amplifier investment dollar's advantage to use a speaker that is more efficient and easy to drive. In particular its to your advantage to avoid four ohm loads as all amplifiers (tube, solid state or class D) make more audible (and unpleasant) distortion into lower impedances! If high quality reproduction is your goal, again your amplifier investment dollar is best served by a higher impedance (8 ohms or more) loudspeaker. If sound pressure is your goal you have a 3db argument for going with 4 ohms, only you have solid state.