SET v. Pushpull
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: 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 |
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 |
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