Preamplifiers

Mystere ca21 Stereo Tube Preamplifier

ARTICLE INDEX

The Design

The ca21 uses dual triodes (6SN7) in its single gain stage design, and is, therefore, signal inverting (you will need to reverse the speaker connections if your power amplifier does not invert the signal). It also uses a tube (5AR4) as the full-wave rectifier in its power supply. So, this is an all-tube product, like its brother, the pa21.

The circuit of the ca21 uses a variation of a cathode follower, called the White cathode follower.

Here is a schematic of a generic cathode follower, capacitor-coupled (in other variations, there is a resistor at the input rather than a capacitor). Its name is derived from the fact that the cathode is connected to the output instead of the anode (plate), so the cathode follows the input. The red arrows outline the signal path. The main advantage of this type of circuit is that it produces a low output impedance.

cathode-follower-schematic-indirectly-heated-cathode-with-arrows

In the White cathode follower, there is a second tube (it could be the second triode in a dual triode, which is the case for the 6SN7). While a cathode follower does not have any gain, the White cathode follower does provide additional idle voltage so the stage can be biased more into Class A.

white-cathode-follower-indirectly-heated-cathodes-schematic

Here is a pin-out schematic for the 6SN7 (first diagram) and the 5AR4 (second diagram). The 5AR4 is a twin-diode.

6SN7-schematic-pin-out

5AR4-pin-out-schematic

Shown below is the basic schematic for the Mystère preamplifiers. Only one channel is illustrated here. It is a variation of the White cathode follower, and is called a Series Regulated Push-Pull configuration, or SRPP.

The bottom line is that the ca21 only has a single gain stage, which is beneficial, because it means less things in the signal path. In fact, there is only one capacitor in the path, at the output (this prevents DC from getting into the signal that is sent to the power amplifiers). The preamp circuit does not use any negative feedback.

mystere-ca-21-preamp-generic-schematic-600-pixels

From the factory, I received the following description of the circuit:

"The input circuit is a non-decoupled SRPP, direct coupled to the output cathode follower with a constant current source in the cathode, which is a simplified variation of the White cathode follower, providing an output impedance of just below 600 Ohms but at the same time without the White's relatively poor input overload voltage rating. Our specific circuit can cope with much higher signal voltages, making it more linear, hence less distortion, and considerably more resolution and transparency, because no feedback is employed. The only "feedback" is local cathode degeneration, not decoupled, to maintain high linearity. Decoupling is only done when lots of gain is required, and involves a bypass capacitor on the cathode resistor."

The ca21 is very heavy, at 45 pounds, due to the power supply transformer, but also because of two massive power supply inductors that are encased on the top of the chassis next to the main transformer.

With power supplies, they can simply have a capacitor filter at the output (usually, there are several), which optimizes the power supply for high voltage deivery, or it can also have chokes (inductors), which provide steadier current under varying load conditions.

First, here is a schematic of a full-wave rectifier with capacitor filter.

full-wave-rectifier-dual-triode-single-capacitor-filter-schematic

Below is the same power supply, but with a choke added to the circuit.

full-wave-rectifier-dual-triode-single-choke-filter-schematic

Here is the power supply with two chokes, as is the case for the ca21.

full-wave-rectifier-dual-triode-dual-choke-filter-schematic

The front panel has the input selector, which lets you choose from among four input pairs of RCA jacks on the rear panel. On the right is a stepped (24 steps, via precision resistors) volume control, shown below. Outputs consist of a single pair of RCA jacks. All the jacks are gold-plated, and good build quality.

mystere-ca-21-preamp-volume-control-potentiometer