For power amplifiers equipped with tubes a normal speaker can not usually be directly connected to the output tubes, normal output tubes are high impedance devices that need to be connected to a load impedance in the kohm range to give high power.
Normally an output transformer is used to transform the low impedance of the speaker to a more suitable load-impedance for the output tubes. As an example a suitable load impedance for the popular 300B tube can be ~2.5 kohm so the output transformer should in that case transform the 8 ohms of the load speaker into 2.5 kohm.

To make a good output transformer is not easy and the output transformer is always the limiting factor for the quality of sound in a transformer coupled tube amplifier. The transformer limits the frequency response and also introduces distorsion. The additional phase shift in the transformer make it very difficult to use feedback and at the same time make the amplifier stable, (this is one main reason why feedback has got bad reputation for negatively affecting the sound)

OTL (Output TransformerLess) amplifier

There are other ways of designing an output stage so the output transformer can be excluded, amplifiers designed this way is usually called OTL's (Output TransformerLess). This technique is quite old and there was OTL's designed already in the twenties, these amplifiers was intended to be connected to speakers with higher impedance in the range of 500 - 1000 ohm's. Although today almost all speakers have impedance of 4 or 8 ohm or sometimes 16 ohms, earlier it was not so standardised and speakers wher made with impedance's of 4, 5, 8, 10, 16, 32, 40, 200, 500 and even 1000 ohms. It is more difficult to make speakers with very high impedance's as they have to use very fine wire in many turns so soon the lower impedance's dominated and now 4, 8 and 16 ohm is the de-facto standardised speaker impedance values.

Sound quality of OTL amplifiers

Some people have the misconception that sound from different amplifiers have minimal differences, some even say that "all amplifiers sound the same". Nothing can be further from the truth! We recommend everyone to do a serious listening test comparing different kind of amplifiers, for instance bipolar transistor, MOSFET transistor, Single ended triode, (SET), Penthode push-pull and OTL type. I is quite easy to hear the difference and decide what kind of amplifier you prefer.

If we shall describe the sound you will hear from our OTL amplifiers we want to describe it as clear, wide and open, both bass and treble frequencies gain clarity compared to traditional tube amplifiers. Compared to push-pull tube amplifiers our OTL amplifiers give a clearer sound with better transient response. Compared to SET amplifiers our OTL amplifiers give a clearer sound without beeing tireing as the sound from transistor amplifiers. If you like the sound from SET amplifiers we urge you to listen to one of our OTL amplifiers, you will find that the sound gain one more dimension without anything else being lost.

OTL circuits
Today mainly two different circuits are used in OTL amplifiers, Futtermans single ended circuit and the Circlotron. After thorough research during more than 20 years and testing of many different circuits for OTL amplifiers we have choosen to use a variation of the Futterman circuit.

This circuit has following advantages over the Circlotron:

Lower output impedance, useable also with 4 ohm speakers
Simpler power supply
Output grounded
Useable power achievable already with a few output tubes


The circuit we use also have a major advantage over the standard Futterman circuit in that the output impedance is ~4 times lower which means that fewer tubes need to be used.

Many commercial OTL amplifiers use many parallell connected output tubes in order to achieve high output power and low output impedance. These designs usually exhibit problems with thermal runaway and also other problems due to the fact that even matched tubes will usually have differences in parameters after some time due to un-even aging.

In accordance with our "minimalistic" design philosophy we use as few tubes as possible in our OTL amplifiers, this is possible because our circuit choice and selection of output tubes. In our OTL-32 100W monoblock where we will use parallell connected output tubes we also use special circuitry that assures that there is no risk for thermal runaway even when the output tubes are aging.

OTL amplifier safety

We fully realise that customers would not appreciate either that their expensive speakers burns out or that they get an electric shock if they are accidently touching the speaker tersminals. Even so there are some OTL amplifiers on the market where these risks are obvious.

Our OTL amplifiers have a built in circuit that assures that there is no risk of burning your expensive speakers or any risk for electric shock. Our speaker protection consist of ultrafast fuses with additional circuits to eliminate the known demetrial effect of ordinary fuses. By applying a modified Futterman circuit the risk of electric shock is completely eliminated, our amplifiers are safe to use also with headphones.