© 2017 by Q-tron Audio. All rights reserved.

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About Q-tron Audio

Q-tron was founded already 1976 by my brother Ulf and myself Hans Beijner to develop and produce advanced electronics. Throughout the years we have developed and produced special designs for the Swedish Military and Industry. Both of us have a background at Ericsson where my brother worked with sales and support and myself with electronic circuit design and product management.

My interest for vacuum tubes started early and I built my first tube amplifier at the age of 15 and soon I also tried several OTL circuits. When I discovered the 6C33C tube I started to seriously design a better OTL amplifier which was finished in its first form 2000, this has since been refined and improved by e.g. adding distortion cancellation and active noise reduction. 


Consultancy services, design of audio vacuum tube amplifiers. 

 

We can provide tube amplifier design for pre-and power amplifiers with transformers or OTL type.

Consultant services range from schematic design with detailed component choice to delivery of complete and tested prototypes.

Contact us now for a detailed discussion of your requirements.

 

About OTL amplifiers

Why has most tube amplifiers an output transformer?

For power amplifiers equipped with tubes a normal speaker can not usually be directly connected to the output tubes, most output tubes are high impedance devices that need to be connected to a load impedance in the kohm range to give high power. 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  difficult to use feedback and at the same time make the amplifier stable, (this is probably  one reason why feedback has got a bad reputation for negatively affecting the sound) 

 

OTL (Output Transformer Less) amplifier 

There are other ways of designing an output stage so the output transformer can be excluded, amplifiers designed this way are usually called OTL's (Output Transformer Less). This technique is quite old and there was OTL's designed already in the 1920's, 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 where 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 with 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), Pentode 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 being tiring 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 chosen to use a variation of the Futterman circuit. This circuit has following advantages over the Circlotron: 

 

  • Lower output impedance, usable also with 4 ohm speakers

  • Excellent balance giving lower distortion, with feedback disconnected our PA12 amplifier give 2nd order distortion 55dB below main signal, this is better than any other OTL design.

  • Simpler power supply

  • Output grounded

  • Usable 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 parallel 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 uneven ageing. In accordance with our "minimalistic" design philosophy we use as few tubes as possible in our OTL amplifiers, this is possible because our choice of circuit and the selection of output tubes. 

Unique benefits of Q-tron Audio OTL amplifiers

We believe that our OTL amplifiers offer substantial benefits compared to OTL amplifiers from other manufacturers:

  • Lower distortion, due to our unique driver stage we have been able to reduce distortion by more than 10 times achieving <0.01% at 1W and <0.1% at 10W in our 25W design, (PA12), note! we reach these distortion figures without the use of excessive feedback.

 

  • Lower noise, By using active noise cancellation we reach a noise level better than 100dBA below max power.

If you compare you will find that these figures for distortion and noise levels are better than in any other OTL amplifier currently or earlier offered in the market. Even though we are using 25dB lower feedback than original Futterman amplifier we achieve much better performance

  • Higher reliability, our OTL design allows for low idle power and high output power using only few tubes thereby improving reliability. We have not had one tube failure in any customer amplifier in more than 7 years of production, our prototype has been in use since 2004 without power tube replacement.

  • Our pre-amplifiers are designed to have high performance including very low noise and distortion to match the performance of our power amplifiers. The pre-amplifiers are equipped with a high performance volume control with discrete resistors and 64 positions that are switched using relays with gold plated contacts. 

 

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 accidentally touching the speaker terminals. 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  a circuit that sense any level of harmful DC voltage on the output and in that case switch of the amplifier, as additional protection ultrafast fuses are also used. By applying a modified Futterman circuit the risk of electric shock is completely eliminated, our amplifiers are safe to use also with headphones.