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S.I. Audio Cult/I integrated amp reloaded

20.09.2011..
..

Designing an OTL that works, and good, it is not that simple. Things can get more difficult if you want to do it without using any feedback on the gain stage, without the output capacitor and without damaging anything when there is a short-circuit in one power tube.

What many designers as prodromal to a good sound consider, is into this amplifier.

The first choice is taking out the most critical component from the electrical and hence musical point of view: the output transformer. Here is a fine OTL.

Beloved and hated by many audio folks this strange creature - when well made - proves that, after all, the other amplifiers are lacking in something.

From a technical point of view, together with the absence of the output transformers, the SI Audio Cult/I amp here reviewed, does not have the output capacitor to protect the loudspeakers and for this reason, it turns out to be an OCL too.

This aspect is indeed very qualifying in an OTL. I will tell you why. Because the purist philosophy of such circuit, that enhances the skills of transparency - considering the absence of the output transformers, which ALWAYS determinate a sonic decay - will be partially neutralized by the output capacitor, which, although excellent, ALWAYS carries a certain “filth” in the signal. This is due to a thorough design of the circuit diagrams (otherwise everybody could do it), and in particular to the adoption of a “floating mass” connection. The presence of direct current in the output, raises the value of this connection that becomes absent in the output clamps of the amp, assuring the total safe of the user and the health of the loudspeakers coils.

Another refinement in circuitry is the matching of the power tubes with the White Cathode Follower, where White is the name of the inventor, in the ’40s.

This solution consents to lower remarkably and in an elegant way the output impedance of the gain stage, which - in an OTL - cannot get any benefit from the transformation ratio of the output transformers. This is because this circuital choice consists essentially of a Push Pull with a cathode follower pursued by a current sink.

The power supply stage is exuberant as there is a separate chassis with a 2000 VA transformer with five secondary windings, each of them dedicated to a power supply circuit specific for every circuitry stage. One of these secondary is dedicated to the preamp stage and such choice approaches its functioning to a preamp/power amp match, with the advantage of a very short and optimized connection.

Excellent are the electrolytic capacitors for the BHC filtering, which are very performing and reliable. Same thing for the other passive components contained in a valuable circuitry card that are of a quality that exceeds the selling price.

Speaking of the tubes, there is a preamp stage with the ECC83, a phase inverter with the E88CC and as stage driver the ECC82: all triodes. Everything is optimized in order to keep contained and with a correct spectrum the distortion flow. Unfortunately, this is almost impossible to realize with solid-state components.

The gain stage envisages the Famous Russian tube 6C33: obvious choice for an OTL if you consider the very low impedance and the very high capacity of current supply, which allows also the driving of any loudspeakers with important phase values and critical impedance.

 

by Paolo Di Marcoberardino
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