Single-ended or push-pull: what changes in the sound and the speakers

Every tube amplifier has to settle one question at its output: does a single tube carry the whole music signal, or do two tubes share it? The answer divides tube amplifiers into single-ended and push-pull designs, and it shapes their power, their distortion, their weight and the loudspeakers they suit. This guide explains the difference in plain terms, describes how each is commonly heard, and sets out how we build our own amplifiers.

What is a single-ended amplifier?

In a single-ended output stage one tube, or several identical tubes in parallel, amplifies the entire waveform: both the positive and the negative half of every cycle. To do that without cutting off part of the signal, the tube must conduct all the time, so a single-ended stage always works in Class A. When the output tube is a triode, the design is called SET, for single-ended triode. A “Class A single-ended” amplifier is therefore not a special variety: Class A is the only way a single-ended power stage can work.

The tube’s standing current, which flows even in silence, also flows through the primary winding of the output transformer. That direct current magnetises the core, and to keep the core from saturating it is built with an air gap. The gap reduces the winding’s inductance, so to keep a full bass the transformer needs more turns and more iron. A single-ended output transformer is therefore larger and heavier than a push-pull one of the same power, and much of the cost and weight of a good single-ended amplifier sits in it.

What is a push-pull amplifier?

In a push-pull stage two tubes, or two groups of tubes, work in opposite phase. A phase splitter feeds them mirror-image signals: as one tube draws more current, the other draws less. Their outputs are combined in an output transformer whose primary winding has a centre tap, with one tube on each half. This arrangement has three consequences that matter.

• The DC cancels in the core. The standing currents of the two tubes flow in opposite directions through the two halves of the primary, so their magnetising effects largely cancel. The transformer needs little or no air gap and can be smaller for the same power.
• Even harmonics cancel. Distortion products of even order, the second harmonic above all, cancel in the transformer to the extent that the two halves are matched. Odd harmonics, mainly the third, remain. Hum from the power supply that reaches both tubes equally is reduced in the same way.
• The stage can leave Class A. Because each tube can hand over to its partner on large signals, a push-pull stage can run in Class AB, where each tube stops conducting for part of the cycle. That raises efficiency and power considerably. A push-pull amplifier can also run in pure Class A, with both tubes conducting all the time.

Class A or Class AB: what is the difference?

In Class A the output tubes conduct over the whole cycle and never switch off. Nothing hands the signal from one device to another, so there is no crossover region, but the idle current is high and most of the supply power becomes heat. The textbook ceiling for a transformer-coupled Class A stage is 50% efficiency, and a real single-ended triode stage delivers well below it.

In Class AB each tube of a push-pull pair conducts for more than half of the cycle but less than all of it. At low levels it behaves like Class A; on louder passages each tube rests for part of the cycle while the other carries the signal. Efficiency and power rise, and the tubes and power supply work less hard for a given output. The price is a transition region where one tube hands over to the other. Careful biasing and matched tubes keep it small, but it changes the character of the distortion.

Single-ended vs push-pull: side by side

How do they sound? Single-ended triode vs push-pull

Listening impressions depend on the circuit, the output transformer and the system at least as much as on the topology, so treat the following as tendencies that owners and reviewers commonly describe, not as rules.

• Single-ended triode. Commonly described as immediate and intimate, with a dense, colourful midrange, natural timbre on voices and acoustic instruments, and fine detail at low volume. Its limits are commonly described as a softer grip on the bass and audible strain when it is pushed beyond its power.
• Push-pull. Commonly described as having firmer, deeper bass, larger dynamic scale and a cleaner presentation at high levels. Some listeners find it less vivid in the midrange than a good SET; others find that a good push-pull triode amplifier combines much of both.

Part of the explanation can be measured. A single-ended stage keeps its second harmonic, which is commonly heard as fullness rather than harshness, and its distortion rises gradually rather than suddenly. A push-pull stage cancels that harmonic and has more power in reserve, so it meets loud passages with less strain. The interaction between the amplifier’s output impedance and the loudspeaker also plays a large part, as the next section explains.

Output impedance and loudspeaker matching

An amplifier with a high output impedance does not hold the voltage at the loudspeaker terminals constant. Its frequency response follows the loudspeaker’s impedance curve: where the impedance rises, at a bass resonance or around a crossover, the level rises a little; where it dips, the level falls. The amplifier also has less control over the cone’s own movement, which is heard as looser bass.

A triode has a relatively low internal resistance for a tube, but a single-ended triode amplifier usually runs with little or no negative feedback, so its output impedance stays relatively high. Its damping factor, the ratio of loudspeaker impedance to amplifier output impedance, is commonly in single figures. Push-pull amplifiers, especially those with pentodes or beam tetrodes, usually use negative feedback and reach a lower output impedance. That is why choosing a topology is really choosing a loudspeaker.

• For a single-ended amplifier, look for high sensitivity, roughly 93 dB and above for a 300B and higher still for the 2 W triodes, and a fairly even impedance curve at 8 Ω or more. Single-driver and horn-loaded loudspeakers are classic partners. Our guide to loudspeakers for single-ended amplifiers goes into detail, and Power triodes for single-ended amplifiers shows how sensitivity relates to power.
• For a push-pull amplifier, the range widens: less sensitive loudspeakers, larger rooms, multi-way designs with complex crossovers, and loads that dip to low impedance.

The output transformer winding matters too. Where an amplifier offers 4 Ω, 8 Ω and 16 Ω windings, the one closest to the loudspeaker’s real impedance is usually the right choice for both power and control.

Bias and tube matching

A related question is how the output tubes are biased. With cathode bias, often called automatic bias or self-bias, a resistor in the cathode circuit sets the operating point and the tube largely looks after itself; it is common in single-ended amplifiers and needs no adjustment. With fixed bias a separate negative supply sets the grid voltage; it can draw more power from the same tube, but the bias needs checking as the tubes age. Tantal Senior, for example, uses fixed bias. A push-pull stage also needs its two halves in balance, so it is built with matched pairs, and in fixed-bias designs each tube’s current is set and checked. A single-ended amplifier has no pair to balance, which is one reason owners find tube changes simple.

Which one is right for you?

• Choose single-ended if your loudspeakers are sensitive, your room is small to medium, and your music centres on voices, acoustic instruments and small ensembles, where midrange colour and low-level detail matter most.
• Choose push-pull if your loudspeakers are less sensitive or have a demanding impedance, your room is large, or you play large-scale orchestral music, rock or electronic music at higher levels.
• Consider the loudspeakers first if you want the single-ended sound but your present loudspeakers are not sensitive enough: a more sensitive pair is often the more effective change.

How Demograf Audio builds single-ended and push-pull amplifiers

Every tube amplifier from Demograf Audio is built single-ended, in pure Class A, as standard. That is how Dmitriy Gubchenko voices them, from Solaria on the 45 and Eurybia on the 2A3 to Endymion on the 845, 211 or 805 and Ether on GM-70 Triode / GK-71 Pentode transmitting tubes. A push-pull build is available to order, for less sensitive loudspeakers, larger rooms or more difficult loads. The topology is agreed with you before the build starts, together with the output transformer winding: 8 Ω as standard, with 4 Ω or 16 Ω available.

Each amplifier is made to order and comes with its own measurement report. It is made in Kazakhstan by Dmitriy Gubchenko and measured before it ships. The steps are described on how to order.

Which Demograf Audio piece fits

• Loudspeakers of about 98 dB and above, such as our three-way Orpheus at 103 dB: Solaria (45, 2 W) or Eurybia (2A3, 2 W), single-ended.
• Loudspeakers of about 93–97 dB, such as Cassandra MkII at 95 dB: Atlas (300B, 5 W), or Helios MkII (G-811, 15 W) for more headroom.
• Less sensitive loudspeakers or larger rooms: Endymion (845, 211 or 805, 15–25 W), Ether (GM-70 Triode / GK-71 Pentode, 25 W) or Tantal Senior (4E27, 30 W), all single-ended.
• Loudspeakers that need more than single-ended power, or a favourite tube such as the 300B or the 845 with more headroom: a push-pull build to order.

Browse our amplifiers for loudspeakers, or read why Demograf Audio amplifiers use output transformers. Tell us your loudspeakers, your room and the music you play, and we will suggest single-ended or push-pull, and the tube to build it around.