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La Scala Power Amplifier

The Output Transformers

While standard in most tube amps (beside OTL designs), there are only a few companies worldwide using output transformers, in power amps based on transistor circuits.

Most experienced engineers know the benefits of using these transformers - once skillfully designed and wounded – very well. However, to find a manufacturer skilled enough to produce them, with the same level of quality - especially with the high bandwidth needed - is not an easy task. Finally, the cost, weight and size prevent most companies from using such transformers.

The main purpose of the output transformers is their capability to “transform” the output impedance of the power amp to match with the impedance of the connected speakers. In tube amps, the output impedance of the circuit can be several hundred Ohms compared to the 4/8 Ohms of the speakers. In MOSFET based amps, there is still an output impedance of about 30/40 Ohms, which needs to be matched with that of the connected speakers.

Using an output transformer, the power transistors are loaded with a higher impedance than that of the loudspeakers. So, the power transistors work at a HIGHER VOLTAGE, but at a LOWER CURRENT with benefits in terms of distortion. Working at a lower current means less stress for the power supply and, as result of that, the rectifiers generate lower noise.

The output transformer substitutes the capacitor at the output, which is needed in almost all power amplifiers to isolate the power transistors from the loudspeakers.

The benefits for the sound are evident, because the sound energy transferred by a capacitor is less transparent than transferred by a transformer. The phase shift is also an issue to be taken into account and the matter was solved simply by designing the transformers with a bandwidth of a 75KHz ±0.5dB minimum bandwidth.

Other well-known implementations based on transformers are the step-up transformers for MC phono cartridges or all-inductors, no-capacitors RIAA preamplifiers like the famous Vendetta Research phono stage, designed by John Curl over two decades ago, or tube amplifiers where the output transformer is the key building block.

As result of the impedance matching, based on output transformers, the MOSFETs can work at a higher voltage and a lower current! The less current the circuit draws from the power supply, the less distortions are created by the switching diodes of the bridge rectifier.

Furthermore, the output transformers, in a push-pull design, as that of the LaScala PowerAmp, effectively cancel any distortion created in the power supply by the design. This effect reduces the values needed for the filter capacitors in the power supply, which in turn, decreases the current needed for charging those caps, and leads to further reduction of noise created by this process.

Since the primary and the secondary windings of a transformer are not physically connected, no DC voltage can reach the output connectors of the amp. Therefore, a coupling capacitor, usually implemented at this point of the circuit to block DC from the outputs (even though having negative effects on the sound) is NOT necessary inside the LaScala PowerAmp!

How to avoid overheating?

How to avoid overheating? - CanEVER AUDIO®

Besides the many benefits regarding sound, an amplifier running in pure Class A mode gets very hot!
To ensure a long lifespan for the electronics and a stable running amp, is a true challenge for the designer. That is why most Class A amplifiers come with huge heat sinks, which in many cases still get very hot and are unappealing to look at. For the LaScala PowerAmp the design goal was to use smarter solutions to reduce the heat from the amp.

The pictures above show a row of four heat pipes, including temperature controlled fans mounted from the underside, and four massive heat sinks mounted on top of the  MOSFETs. Looking at the LaScala PowerAmp from the outside, only the heat sinks on top are visible. The elegant style of the cabinet gives no sign that a massive Class A amp is working “under the hood”!

Following below are two pictures showing one of four heat pipes, which cool each of power MOSFETs.

The first picture shows the top side of the heat pipe showing the area where the MOSFET is glued to.

The second picture shows the bottom side of the heat pipe with its many narrow fins for effective heat dissipation. 

The third shows the low Speed Fans for the Air Circulation. The amplifier can work also with the fans powered off by setting the related parameter at the menu. The fans let only the air moves around the heatsinks. It increases the chimney effect.

With fans excluded the LaScala PowerAmp works at 15°C higher temperature with a reservoir left of 35°C. 

Conclusion

With the LaScala PowerAmp, CanEVER Audio has once again proven its skills in offering extremely innovative products to the world of high-end audio. Thanks to the ability to combine well-known and proven engineering concepts with innovative ideas, CanEVER Audio has been able to create a product with amazing sound quality, which is able to compete with the best amps on the market today.

For further information send us an e-mail, addressed to:  sales@canever.eu

 

 

Conclusion - CanEVER AUDIO®

Bibliography

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Investigation of Current Driven Loudspeakers, Dissertation presented at the 138th Convention of the Audio Engineering Society, Warsaw (2015), Schneider H., Agerkvist F., Knott A., Andersen M.A.E.

Distortion Improvement in the Current Coil of Loudspeakers, Dissertation presented at the 134th Convention of the Audio Engineering Society, Rome (2013), Pillonnet G., Sturtzer E., Rossignol T., Tournier P., Lemarquand G.

Effects of Acoustic Damping on Current-Driven Loudspeakers, Dissertation presented at the 122nd Convention of the Audio Engineering Society, Wien (2007), Bortoni R., Filho S.N., Silva H.S.

Current-Driving of Loudspeakers by Esa Meriläinen - Eliminating Major Distortion and Interference Effects by the Physically Correct Operation Method - Ed. Createspace, ISBN 1450544002, 2010.

Linear Audio by Jan Didden - Volume 12, pag. 183 - Book Review - Current-Driving of Loudspeakers by Hans Polak.

Current driving - The transconductance amplifier - Measurements.

Distortion reduction in moving-coil loudspeaker systems using current-drive technology, P. G. Mills, M. J. Hawksford, Journal of Audio Engineering Society 37 (3) (1989) pag. 129–148.