Article · Published 2026-10-18
Why Demograf Audio amplifiers use output transformers
Almost every tube amplifier has an output transformer between its tubes and its loudspeaker terminals. A small family of designs, called OTL, leaves it out. This guide explains what an OTL amplifier is, why tubes normally need a transformer at all, how OTL designs manage without one and what that costs, and why every tube amplifier from Demograf Audio is built with output transformers.
What is an OTL amplifier?
OTL stands for output transformerless. In an OTL tube amplifier the output tubes drive the loudspeaker or the headphones directly, through a coupling capacitor or a direct connection, with no transformer in between. The idea is not new: OTL power amplifiers for loudspeakers were developed in the 1950s, and the circuits associated with Julius Futterman are well-known early examples. Today OTL designs are found mainly in tube headphone amplifiers and in a small number of loudspeaker amplifiers and monoblocks.
Why do tube amplifiers need an output transformer?
A power tube is a high-voltage, low-current device. It works into a load of thousands of ohms, at hundreds of volts and tens to hundreds of milliamps. A loudspeaker is the opposite: a low-voltage, high-current load of 4–8 Ω nominal, sometimes less. Connect a typical output tube straight to an 8 Ω loudspeaker and almost none of its voltage swing becomes useful power.
The output transformer solves this. Its impedance ratio is the square of its turns ratio, so a transformer with 25 turns on the primary for every turn on the secondary transforms impedance by 625 to 1: an 8 Ω loudspeaker then looks like about 5,000 Ω to the tube. At the same time the transformer isolates the loudspeaker from the high DC voltage on the tube, and separate windings or taps can match 4 Ω, 8 Ω or 16 Ω loads.
The transformer has costs of its own. It limits bandwidth at both ends of the range, adds phase shift and some distortion of its own, and its core must not saturate, which in a single-ended amplifier means a gapped core and a large, heavy part. Its quality decides much of the bass, the treble extension and the character of a tube amplifier.
How does an OTL amplifier work without one?
An OTL design has to bring the output impedance of its tubes down close to that of the load. The usual methods are these:
- Low-impedance power triodes. OTL designers favour tubes made originally for regulated power supplies, such as the 6AS7G and 6080 double triodes and the 6C33C, because their internal resistance is far lower than that of typical audio output tubes.
- Many tubes in parallel. Paralleling tubes divides their output impedance and adds their current. A loudspeaker OTL amplifier commonly uses several output tubes per channel, and large OTL monoblocks use more still.
- Push-pull circuits. Most OTL power amplifiers use push-pull arrangements, such as the totem-pole or the circlotron, so that the standing currents of the tubes balance and no steady current flows through the loudspeaker.
- Negative feedback. Many OTL designs use negative feedback to lower the output impedance further.
Even so, the output impedance of an OTL loudspeaker amplifier commonly remains higher than that of a transistor amplifier, and the power it can deliver falls as the load impedance falls.
What loudspeaker impedance suits an OTL amplifier?
Higher and flatter is easier. An OTL amplifier delivers more power and keeps better control into a higher impedance, so OTL makers commonly recommend loudspeakers of 8 Ω nominal or more, and many owners prefer 16 Ω designs or loudspeakers whose impedance does not dip far below nominal. A 4 Ω loudspeaker with deep impedance dips is generally a poor match. Some owners add an impedance-matching autoformer between amplifier and loudspeaker, which brings a wound magnetic part back into the signal path.
Headphones are a different story. Many dynamic headphones have an impedance of 250–600 Ω, a load that suits a small OTL stage well, which is why so many tube headphone amplifiers are OTL. Low-impedance headphones, and many planar magnetic models, are a harder load for the same reason that low-impedance loudspeakers are. In a tube amplifier with an output transformer the headphone output is usually taken from a transformer winding, which suits low- and high-impedance headphones alike; our guide to amplifiers for headphones and loudspeakers explains more.
OTL vs transformer-coupled: the trade-offs
| OTL | Transformer-coupled | |
|---|---|---|
| Output stage | Low-impedance tubes, usually several in parallel, in push-pull | Any power tube, single-ended or push-pull |
| Wound part in the signal path | None | Output transformer |
| Bandwidth | Not limited by a transformer | Set largely by the transformer’s quality |
| Output impedance | Relatively high; power falls into low impedances | Set by the circuit and the windings; taps for 4, 8 or 16 Ω |
| Loudspeakers | Higher, flatter impedance preferred | A wide range, matched by the winding |
| Headphones | Well suited to high-impedance models | Low and high impedance, through the winding |
| DC at the output | Must be controlled by the circuit and by protection | Blocked by the transformer |
| Heat and tube count | High; many output tubes | Depends on the design; usually fewer output tubes |
| Weight | No output transformer to carry | Heavy transformers, especially single-ended |
What owners commonly like about OTL
OTL amplifiers are commonly described as fast, open and transparent, with extended treble and a directness that their owners attribute to the absence of a transformer. Without one, the amplifier’s bandwidth is set by the circuit rather than by the windings.
What owners commonly find difficult
- Heat and power draw. Many output tubes running at high current produce a lot of heat for the power delivered.
- Tube count and matching. More tubes means more to match, monitor and replace, and more bias settings to check.
- Loudspeaker choice. The preference for higher impedance narrows the range of loudspeakers that work well.
- Bass control. With a higher output impedance, the bass on some loudspeakers is described as less firm.
Is an OTL amplifier safe for loudspeakers?
A well-designed one is, but the question is fair. In a transformer-coupled amplifier the loudspeaker is isolated from the high DC voltage of the output stage: a transformer cannot pass direct current from its primary to its secondary. In an OTL amplifier the loudspeaker is connected to the output stage itself, through a capacitor or directly. In a direct-coupled design, a failed tube or drifting bias can send direct current into the voice coil. Careful OTL designs therefore include protection, such as output fuses or circuits that detect DC and disconnect the loudspeaker through a relay, and their makers ask for periodic bias checks. The supply voltages inside are high, as in any tube amplifier, and service belongs to a qualified technician.
Why Demograf Audio amplifiers use output transformers
Every tube amplifier in our range drives its loudspeakers through output transformers. The reasons follow from the way we build them.
- A single-ended stage needs one. Our amplifiers are single-ended, in pure Class A, as standard: on the 45, 2A3, 300B, 3C24 and G-811 triodes, on the 845, 211 or 805 in Endymion, on GM-70 Triode / GK-71 Pentode transmitting tubes in Ether, and on 4E27 pentodes in Tantal Senior. These are high-voltage tubes made to work into loads of thousands of ohms, and a single-ended stage connected directly would also send its standing current through the voice coil. A push-pull build, available to order, keeps its output transformer too.
- Ordinary loudspeakers and headphones. With output transformers our amplifiers are wound for 8 Ω loudspeakers as standard, with 4 Ω or 16 Ω available to order, and every one of them is built to drive both loudspeakers and headphones.
- The transformer is part of the voicing. The transformers in our amplifiers are custom-wound; in Tantal Senior, for example, the M034 interstage and M062 output transformers set the character of the circuit.
- No DC at the loudspeaker terminals. The transformer keeps the loudspeaker isolated from the high-voltage supply of the output tubes.
An output transformer does ask something of the loudspeaker in return: a single-ended amplifier pairs well with sensitive loudspeakers with an even impedance curve. Our guide Single-ended or push-pull explains how the topology changes that, and loudspeakers for single-ended amplifiers describes what to look for.
Which Demograf Audio piece fits
- High-sensitivity loudspeakers and headphones from one chassis: Solaria (45, 2 W), Eurybia (2A3, 2 W) or Atlas (300B, 5 W).
- Electrostatic headphones as well as loudspeakers: Tantal (3C24, 3 W), with a built-in STAX-type output; see also amplifiers for electrostatic headphones.
- Larger rooms or less sensitive loudspeakers: Helios MkII (G-811, 15 W), Endymion (845, 211 or 805, 15–25 W), Ether (GM-70 Triode / GK-71 Pentode, 25 W) or Tantal Senior (4E27, 30 W).
- Low-impedance loudspeakers that a tube amplifier finds hard: our solid-state Bellerophon (Class AB) or Hades (Class D), both stable into 4–16 Ω loads.
For headphone-first listening, see our headphone amplifiers. Every amplifier is made to order by Dmitriy Gubchenko in Kazakhstan, with its own measurement report. Tell us your loudspeakers and headphones, and we will suggest the amplifier and the output winding that suit them.