The Tetra's FR-4 PCB is extra thick, 0.094 inches (inserting and pulling tubes from their sockets won’t bend or break this board), double-sided, with plated-through 2oz copper traces, and the boards are made in the USA. The Tetra PCB holds two phono-stage preamplifiers, with each phono preamp holding two CCDA gain stages with a passive RIAA equalization circuit in between. Thus, one board is all that is needed for stereo use. The boards are 6.0 inches by 8.0 inches, with seven mounting holes that prevent excessive PCB bending.
The Full kit includes two JJ E88CC and two JJ ECC81 tubes along with 24 capacitors and 80 resistors. The RIAA eq capacitors are hand-matched and tight tolerance. The coupling capacitors are CDE 1kV polypropylene 942 series. Four ceramic tube sockets and seven sets of hex standoffs with screws and rubber O-rings and user guide included. The assembled kit requires a high-voltage power supply between 240V to 280V and a 6.3V or 12V heater power supply. The PS-1 is an excellent choice. The assembled kit put out +40dB of gain.
The Tetra Sans PS uses two tube-based gain stages with a passive equalization network in between. The gain stage's topology is the Constant-Current-Draw Amplifier [CCDA], whose low distortion, fairly high gain, and low output impedance are desirable attributes in a phono preamp. The CCDA is a compound circuit that holds a grounded-cathode amplifier directly cascaded into a cathode follower. So what; what's so special about this obvious pairing? Its special status lies in the details.
Each triode sees the same cathode to plate voltage and the same load resistance and same idle current draw. Both the grounded-cathode amplifier and the cathode follower are in voltage phase, but in anti-current phase. For example, as the grounded-cathode amplifier sees a positive going input signal, its plate current increases, which increases the voltage developed across the plate resistor, which in turn swings the triode’s plate voltage down. This downward voltage swing is then cascaded into the grid of the cathode follower, whose cathode follows its grid’s downward swing, which decreases the current through the cathode follower to the same degree that the previous stage's current increased. This results in the constant current draw feature of this topology (a highly desirable feature, as the signal amplification will not alter the amount of current being sourced from the power supply and consequently not perturb the power supply, thus greatly simplifying the design consideration of the power supply).
No On-Board Power Supply
The new Tetra Sans PS is essentially the standard Tetra phono preamp, but without the power supplies portion. Why? Many already have a power supply, for example all those who own All-in-One boards, which hold high-voltage B+ and low-voltage heater power supplies. These boards hold solder pads that allow us to tap into the B+ and heater power supplies. Some own old tube equipment and can jettison the old guts. Others own power supplies, such as the Janus and PS-1 regulators. And others still own the All-in-One PCBs that hold both a high voltage and low-voltage power supply, with extra solder pads for just such an external-load use. Only four wires are needed to energize the Tetra PCB from an All-in-One PCB.
By using different tubes, different bias points, different B+ voltages, a nearly infinite number of different CCDA amplifiers can be built. But with a phono preamp, our choices are limited, as high-gain, and low-noise are our key requirements, not high-current or low output impedance. Thus, we can ignore the 6H30 and concentrate on the 6AQ8, 6DJ8, 6N1P, 12AT7, 12AV7 12AX7, 12BZ7, 5751, 6072….
For example, a 12AX7 input tube will yield a gain close to 50 (+34dB) per CCDA stage (with un-bypassed cathode resistor—more with a bypassed cathode resistor), which perfect for a phono preamp; the 6DJ8/6922, 16 (mu of 33), which is a little weak, but if a step-up transformer is used, the 6DJ8’s lower noise contribution would certainly override the concern about its low gain (and using a bypass capacitor at the 6DJ8's cathode resistor would increase the gain to over 20). The only hard stipulations are that the two triodes within the envelope be similar and that the tube conforms to the 9A or 9AJ base pin-out.
Typical tube configurations are all 12AT7 tubes, which yields a final gain of 46dB; all 12AX7 tubes, gain of 48dB; all 6DJ8/69222/7308/E88CC tubes, gain of 36dB. Of course, different tubes can be used in the same preamp, such as the 6DJ8 as the input tube and the 12AT7 as the 2nd stage tube; this is an excellent pairing, as final gain works out to 40db and the two types seem to synergistically complement each other.
The Tetra Sans PS phono stage PCB requires an external power supply for its heater and B+ voltages. The heater power supply should deliver 12.6Vdc (or 12Vdc), as the 6V and 12V tubes can be used with this voltage; 6V tubes, such as the 6DJ8 and 6N1P, can have their heaters placed in series. In contrast, 12V tubes, such as the 12AX7, cannot be used with a 6.3Vdc power supply.
The high voltage power supply voltage depends o the tubes used. In general, 200Vdc to 340Vdc is needed.
Regardless of the underling technology used, tube or solid-state, a phono preamp must undo the RIAA (Recording Institute Association of America) equalization curve or the CCIR (Comité Consultatif International des Radiocommunications) or IEC (International Electrotechnical Commission) the European versions of the RIAA) curve used in making the record. The inverse of the RIAA curve returns the signal to flat by cutting the highs and boosting the bass. The Tetra phono preamp uses passive equalization, rather than active, feedback-based equalization. The passive equalization network sits in between two CCDA gain stages.
The Tetra Sans PS phono stage’s passive equalization network holds an optional high frequency correction. Correction? The passive network assumes that the LP’s high frequencies continue to climb at 6dB per octave from 2,122Hz to infinity. But do they? Not likely, as the cutting heads and cutting power amplifiers used to make record master have high frequency roll offs that fall closer to 30kHz to 60kHz than to infinity. In other words, the passive equalization will over attenuate the high frequencies beyond the record’s intrinsic high frequency corner frequency. Adding the optional resistor, R9 on the PCB, will place a countervailing stop to this excessive attenuation.
PCB includes 16-Page User Guide booklet