SET v. Pushpull

I’ve owned many amps...from an Electron Kinetics Eagle to an Acurus A250...Forte Model 55, etc...all great sounding. Just before the Had amp I was using a factory modded tube Jolida 502p which also sounded great. Just all fun all the time...I became curious about Dennis Had’s retirement projects and one popped up on Ebay and I grabbed it...it was built only a few months before I bought it so I figured it might work...it does, it’s amazing sounding for reasons hard to describe other than it sounds like music to my ears, and initially made me sit up and go "WOW." I used it with supposed 91db Silverline Prelude speakers initially but switched to a pair of 99db Klipsch Heresy IIIs and a tube preamp and that combo is brilliant (with 2 SS A/B REL subs...so there's that). It’s said no SS amp is free of odd harmonics (maybe a new Nelson Pass "SIT" is, but I haven’t heard one of those), and single ended class A has no "switching" distortion, but meh...so what...I did enjoy many SS amps anyway, still use ’em in live sound stuff and keep one (a clean and punchy Adcom "mosfet") in the rack for outdoor deck speakers. I use push-pull tube guitar amps as well as a class A single ended Burriss Royal Bluesman...all work swell, the Burriss is simply amazing sounding though. There ya go!

@atmasphere  +1 If people want to make 85 dB @ 4 Ohm or lower speakers they need to cough up and make active versions or just make them more efficient. People listen to speakers at show rooms with amps that are not practical at home. They get the speaker and then the sticker shock of the amp required hits them. 

In the lower to mid end of consumer/audiophile (a term!) gear speakers should be 92 dB @ 8 Ohm stable or higher. Else they need to come with a warning to help ordinary folks like me. New people like me are totally stunned at shows / show rooms when we see amps that cost twice the amount of speakers or more sometimes. The speakers sound great at the dealer but once they are home it is a different matter.

I understand that we need things at every price point however.

Low powered amps need efficient speakers, but beyond that I found that my little 12 watt per side SE tube amp (Dennis Had Firebottle HO) simply sounds astonishingly better than any other amp I've heard, tube or SS. But then I don't get out much...

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.

 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. 

@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

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!

Many excellent replies! 
You really want to do some listening to a variety of tube amps. Many tubes sound quite different and of course designs of a given tube do as well. There are a number of push pull amps that allow you to roll EL34s, 6550s and a few other variants.

SETs are a broad group and many are very low power(e.g. <10w). You really need a super friendly speaker(e.g. horn) to get the most of those. It all comes down to matching speaker+amp. 

A couple of personal experiences might be of help. I loved the sound of a number of SETs I heard, but I didn't like many of the horns and other speakers that were the best fit. That led me to 845 SET monoblocks from Dehavilland which run at 35w and allow you to open up your speaker options to many conventional speakers in the 93db neighborhood. Silverline La Folias were my best match with them. Merlin VSM Series also a good match, though lighter in the bottom end. 

When I had the opportunity I ran a pair of Atma-Sphere MA1 OTLs, which are another entirely unique class of tube amps that do many things better than most others again IF AND ONLY IF paired w/the right speakers (i.e. moderately efficient but more importantly with a friendly impedance curve. Some folks say they conquer that issue using items like Speltz Autoformers, to open up the speaker options to theoretically almost any impedance. Personally, the autoformers were not my cup of tea. OTLs had speed and clarity superior to most of the other tube amps I've heard of all other types. 

For me you've got to find the speaker you want first and then ask what amps will be a good fit with it, and consider a wide variety of topologies based on listening. If you can't audition, buy used so you can change gears without losing your shirt. Cheers,
Spencer

A number of good points have been made above. An important one that Geek101 mentioned that warrants additional emphasis, IMO, concerns impedance swings.

An impedance curve for the LS50 can be seen in Figure 1 of Stereophile’s measurements. It can be seen that the impedance varies from a low of about 4 ohms at 200 Hz to about 13 ohms at about 1.5 kHz, before descending to around 6 ohms in much of the treble region. That impedance characteristic coupled with the small size of the speaker and its woofer (see the next paragraph for the relevance of that), as well as its low sensitivity (84.5 db at 1 meter for an input of 2.83 volts), makes it clear that the speaker was designed with the expectation that it would usually be used with solid state amplification.

With a few exceptions, nearly all solid state amplifiers have an output impedance of a tiny fraction of an ohm. The interaction of that very low output impedance with the LS50’s impedance variations over the frequency range will work in the direction of augmenting the mid-bass response, relative to the response at higher frequencies.

In contrast, tube amp output impedances differ widely among different amps. Some have low output impedances, in the area of 1 ohm or even less, while some have output impedances of several ohms or even more. And if the LS50 is driven from a high output impedance the interaction of that output impedance with the speaker’s impedance swings will result in a tonal balance that is not what the designer intended, and in particular is likely to be weak in the mid-bass.

You indicated that Reichert obtained good results pairing the LS50 with an LM518ia. Undoubtedly a major reason for that was that the LM518ia has a considerably lower output impedance than most SET amps, especially from its 4 ohm tap. Per Stereophile’s measurements of that amp:

The output impedance varied slightly with frequency and load impedance, but was approximately one-quarter the nominal value of each output-transformer tap: 3.8 ohms (16 ohm tap), 2 ohms (8 ohm tap), and 1.1 ohms (4 ohm tap).

These impedances are relatively low for a single-ended-triode amplifier.

So I would definitely not extrapolate Reichert’s finding to other SET amps, or for that matter to many push-pull tube amps.

Also, regarding power capability, keep in mind that what will **dramatically** affect how much power is required is the dynamic range of the music that is listened to (i.e., the **difference** in volume between the loudest notes and the softest notes). Many and perhaps most pop and rock recordings are dynamically compressed to a dynamic range of less than 10 db, meaning that less than 10 times as much power is required to reproduce the loudest notes as to reproduce the softest notes. While classical symphonic recordings may have dynamic ranges of 30, 40, or (in a few cases I have measured) as much as about 55 db. Those correspond, respectively, to 1000 times, 10000 times, and about 300000 times as much power being required to reproduce the loudest notes as to reproduce the softest notes (and those are not typos). So in those cases if the volume control is turned up high enough to hear the soft notes at reasonable levels vastly more power will be required than would be necessary for recordings having minimal dynamic range, such as most pop and rock recordings.

Finally, regarding the Joseph Audio Pulsar, Stereophile’s measurements show it to have impedance characteristics that are more tube-friendly than the LS50’s, but its sensitivity is even lower than the LS50’s, making power capability an even more important consideration.

Good luck. Regards,
-- Al

https://parttimeaudiophile.com/2018/03/30/review-mcintosh-ma252-tube-solid-state-integrated-amplifie... 

Did he state how far is he sitting from them?. His room also plays a role. 22 watts for 83 dB is low. You want your amp to be operating at 50% or lower to achieve peaks of 97 to 100 dB swings given the impedance swings of your speakers. 

May be hybrid integrates?. 

Guys, thanks for these responses - to add:  My budget is going to be $5K or less, and because of real estate issues I don't have room for tube separates, or monoblocks, - need to go with an integrated.

Herb Reichert at Stereophile has paired a SET design, the Line Magnetic 518ia with the KEFs, with good results.  22 Watts, supposedly. 

No budget was mentioned. As one data point, I have heard the LS50 paired with Prima Luna Premium HP amplifier (the 70wpc model) extensively and I thought the sound was excellent albeit a bit bass shy.

A nicely restored pair of Dynaco Mk III's will work well with your LS50's. PP 6550's and excellent transformers - 60 + watts from a proven classic design! Competitive in SQ with today's much more expensive tube amps!

@wolf_garcia guilty!.

To claim all types of "good" amps adjusted for level sound the same is ridiculous. A 5 point penalty is charged.

At 85 dB sensitivity I wouldn't even go with any tube amp, at-least in my case. 

Not sure when people say SET watts are more than SS watts, but watts are watts. It is just that when driving to spec SET distortion is more pleasant then SS. But why listen to distortion (I prefer not to)?.

Pulsars are 83 dB efficient?. Not sure why specs on website does not talk about Pulsars sensitivity?. At least it says impedance does not get lower than 6 ohm that is good.

50 Watts will get you to 95 dB at 8 feet for 83 dB speakers.

If you get a good amp that puts out the watts your speaker needs for your listening needs without breaking a sweat you should not hear a difference. Basically non-existent distortion for all the watts your music needs. This is difficult at loud levels for fair moderately efficient speakers. What I have learned is to not to drive an amp to its spec.

If you want to go in the SET direction then may be pick a speaker that is close to 98 dB or even 100 dB sensitivity?. Then 10 to 20 watts SET will get you there given impedance does not swing. It is a bad idea to pick speaker after choosing an amp, in my limited opinion.

I do own Line Magnetic 508ia and it puts out 48 Watts for a price that is hard to imagine!. It sounds great at moderate levels on 92 dB paradigm studio 40v3 (when compared to Marantz SR7005). It seems it may not be good enough for new set of speakers I got myself, so I am on the fence with this class A tube thing.

If you can do true A/B test of AMPs, after level matching the SPL level,  you should not hear any difference at normal listening levels for good amps SET/Push-pull/Class A/ClassAB or Class D etc.