SET v. Pushpull

@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

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