Usually push-pull configurations make use of two “complementary” types of transistors – “N” type and “P” type. Unfortunately, the specifications of those devices are NEVER exactly complementary! This creates some very “nasty” types of distortions in a push-pull amplifier.
As the interstage transformer inside the CanEVER LaScala PowerAmp creates two “mirror images”, it is possible to use two power transistors of exactly the same type. CanEVER Audio uses two lateral N-channel MOSFETs of the latest generation. This type of MOSFET does not need any kind of feedback for the thermal stabilization!
As all stages inside the power amp are running in pure class A mode, no distortions are created in the crossover section. Specially designed BIAS control circuits manage the symmetry of the signals of the tubes and the MOSFETs. The result is a perfect symmetry in the processing of the audio signal, even if some SPECs of the active element are different. The BIAS for all stages is fix without feedback. The individual BIAS is independent from the power generated, the impedance of the connected loudspeakers and the processed audio signal!
What makes the LaScala PowerAmp truly unique are the microprocessor driven “Bias Control Circuits”. These constantly control all stages of the amp. Without the firmware to run these microprocessors, a design like the LaScala PowerAmp would be impossible.
A basic problem in any push-pull configuration is that the two active components in these circuits (whether tubes or transistors) NEVER come with exactly the same specs, even if they were paired before and, last but not least, they always work at a different temperature because of the position at the heatsink. As a result, an amp like this creates additional distortion, which need to be avoided.
Inside the LaScala PowerAmp, the microprocessor-based BIAS CONTROL CIRCUITS manage the current and the voltage stage constantly, to make sure that all parts of the push-pull circuit always work in a perfectly balanced and symmetric way. Important: The BIAS CONTROL CIRCUIT does NOT affect the audio signal NOR create any feedback loop at the audio frequencies! It works below 0.1Hz, or, in other words, with a time constant of 10 seconds.
As a special feature, the microprocessors running the BIAS modules offer the functionality to switch the output power of the CanEVER LaScala PowerAmp between 50W and 100W per channel even while switched on. As the electric circuit is “floating”, it is possible as well to switch (bridge) the amp from 2-channel STEREO mode into one channel MONO mode. Of course, in this mode the LaScala PowerAmp delivers DOUBLE the output power compared to that in the 2-channel STEREO mode.
On the left the "tracking" BIAS Control Units inside the LaScala Power Amp
The result is a perfect symmetrical design with a minimal amount of components in the signal path, which minimizes odd order harmonics usually “hurting” our ears during listening.
The LaScala PowerAmp is made of two amplification stages only – one for voltage and one for current. Please note that there is NO CAPACITOR and NO FEEDBACK implemented in the complete signal path!
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!