What is a single-ended amplifier?

Single-ended amplifiers, whether made with triodes (as in the single-ended triode, or SET, amplifier), pentodes, or solid state devices, entered the high-end consumer audio consciousness a couple decades ago, and they continue to have a particular pull for a certain camp of audiophiles. This may lead the rest of us to wonder whether these folks are onto something that we should pay attention to. However, there seems to be some confusion regarding what exactly single-ended amplifier are. So I thought I’d try to clear things up a little.

So, what exactly is a single-ended amplifier?

It might be easier if we first cover what isn’t a single-ended amplifier.

The vast majority of audio power amplifiers that aren’t based on switching technology are what we call “push-pull” amplifiers. In a push-pull amplifier, the power delivered to the speaker is handled by two devices (or two sets of devices). To oversimplify things somewhat, the two (sets of) devices are in parallel but effectively operate with inverted polarity — hence the “push” and “pull”. Such a design results in a vast reduction of a class of harmonic distortions, significant improvements in output stage efficiency, or both.

In the case of transformer-coupled push-pull amps, the two (sets of) devices are driven in inverse polarity and their outputs are appropriately summed via the output transformer. In transformerless solid state designs, the two (sets of) devices are complementary, meaning one is based on NPN or N-channel silicon and the other on PNP or P-channel silicon.* The devices are driven from a common source but are configured so they saturate in opposite directions. Both of these approaches yield the same linearity and efficiency benefits.

In a single ended amplifier, there is, if you will, only “push” or “pull”. Only one device (or set of devices) delivers current to the load with no fancy distortion reduction or efficiency improvements involved. Such a topology may seem appealing for its apparent purity. However, the apparent purity comes at a high cost, which we could, and eventually might, take a deeper dive into. But that won’t do much to help answer the question posed in this post.

You may have noticed that I’ve limited the above discussion to output stages. Completely independent of the technology used in the output stage, the stages preceding the output can be single-ended or not. The use of single-ended driver and voltage amplification stages is entirely unremarkable, even in modern designs. Therefore, the conventional and accepted use of the term “single-ended” in consumer electronics is to identify the technology used in the output stage.

It might also be worth noting that the “single-ended” moniker has nothing to do with power supplies. A power supply that generates positive and negative voltages is referred to as “bipolar”; one that generates only a positive voltage is properly “unipolar”. There are many single-ended amplifiers using bipolar supplies as well as push-pull amplifiers using unipolar supplies. In fact, the vast majority of push-pull tube amps and many early push-pull solid state designs use unipolar power supplies.

Finally, all single-ended amplifiers are Class A. Push-pull amplifiers are  predominantly Class AB, but they can be Class A, AB, B, and a host of others. High-end Class A push-pull amplifiers are not uncommon.

So how do I know if a design is truly single-ended?

Owing to the renown that SET amplifiers developed in the 1990s and 2000s, there are now designs available in the market that are labeled as “single-ended” but fall short of the accepted definition. Leaving aside for the moment whether a single-ended amplifier is right for you, how can you be sure that a design you’re considering actually is single-ended?  Here are a few guidelines:

  • In an all-tube amp, if there is only one output tube, it almost certainly is a single-ended design.
  • If there is more than one output tube, it may still be single-ended. Check to see whether the tubes are driven from the same source and connect to the same tap on the output transformer if you can. If they are, it’s probably single-ended.
  • In a solid state amplifier, if the output stage uses both “N” and “P” devices, it almost certainly push-pull (not single-ended).
  • Even if there are only “N” or only “P” devices in the output stage, it still might not be single-ended. It’s possible that one set of devices is configured to be “quasi-complementary.” Proceed with caution.
  • Single-ended output stages are power gluttons, and so solid-state designs, even those that generate moderate power, will require substantial heatsinking. A “single-ended amplifier” that has the same heatsinking as an equivalently rated Class AB push-pull amp almost certainly is not single-ended.
  • For hybrid designs, there are so many possible variations that it’s almost impossible to state useful guidelines.

In a future post, I may talk about whether or not single-ended amplifiers have a “sound” and whether the “sound” is desirable. For now, I hope this helps to clear up more confusion that it creates.

*A now largely obsolete variation of this is the quasi-complementary output stage, where both (sets of) devices are the same type but one is connected so as to mimic the properties of a complementary type. There are tube amplifier designs that attempt to operate the output stage in quasi-complementary mode as well, but they are quite rare.

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