Output Transformer/Capacitor
& OTL/OCL Circuits

Transformer and capacitor output coupling:

There are two common coupling methods in tube amp circuitry; one is capacitor coupling and the other is transformer coupling. Both methods are used to separate the AC music signal from a circuit’s DC operating potential. Capacitor coupling is the traditional, low-cost method. It is somewhat stable but transfers only voltage, not wattage (i.e., not real energy). With transformer coupling, about 90% of real wattage is transferred (there is still a transformer loss of about 10% of wattage, while voltage is not affected). Several models of Allnic preamp use transformer coupling.

While transformer coupling is superior to capacitor coupling, all the advantages of transformer coupling depend on the quality of the transformer and on the choice of tubes. Allnic Audio manufactures its own transformers and uses what it views to be the best core material, permalloy. Allnic Audio’s unprecedented, wide (16 Hz ~ 50 kHz, -3db), low distortion, and ultra-flexible (up to 20 kHz square wave response) output transformer helps Allnic Audio realize the ideal transformer coupled pre-amplifier. Gain tubes are also carefully selected for three critical factors for function with the permalloy output transformers: high gm, low internal resistance, and high mu. This is one reason why Allnic preamps often use tubes that are not commonly found in line-stages produced by other manufacturers.

In addition, Allnic’s power supply circuit has “under 1% voltage regulation”. The voltage regulation of the transformer is the percentage change in the output voltage from no-load to full load. To get the best performance out of your transformer, you need the lowest possible voltage regulation. The voltage regulation of Allnic’s power supply circuit is under 1%. It yields an improved power transfer and lower regulation factor, resulting in increased current speed and better dynamics.

Finally, one of the other benefits of transformer coupling is that it can facilitate low output impedance. Low output impedance is critical to the design of a good pre-amplifier, and Allnic’s circuitry achieves constant low output impedance across all frequencies.

Output transformer-less and output capacitor-less (OTL/OCL) coupling:

As noted above, output capacitors and output transformers are used to separate the AC music signal from the DC operating potential. If this is not done, the latter will destroy the power amplifier, the loudspeakers, or both. However, these two coupling devices also introduce their own influences on the transmission of the musical signal, adding their own character (colouration), increasing distortion, and consuming small signals, all as predicted by physical efficiency theory. In addition, they also limit signal dynamics. With superior transformer coupling combined with other carefully designed circuitry, these issues can be mitigated but not entirely eliminated yet, nevertheless, still yield absolutely outstanding sound quality.

For these reasons, the elimination of output transformers and capacitors has been considered the best solution for the design of the most accurate amplifiers. Removal of these two coupling devices from the signal path theoretically achieves the best results possible:

⦁    No colouration of the musical signal
⦁    The lowest possible distortion
⦁    Extremely detailed expression, natural harmonics and subtle musical decay
⦁    The widest musical dynamics
 

Of course, the success of the OTL/OCL circuit will also depend on the quality of the design of other circuit elements, not to mention the tubes employed in the different gain stages. For example, some circuit experts might be concerned about DC potential in the music signal in an OTL/OCL design. However, the chance of this problem occurring can be 100% reliably eliminated by doing away with the potential for DC in the signal path. To do this, Allnic uses a “floating power supply circuit”, a design which deals with the theoretical issue by simply not creating any significant DC potential in the first place.

Other elements of the power supply, its means of rectification and voltage regulation and the method and materials used for output attenuation, for example, are critical to maximizing the potential of the OTL/OCL output circuit. The Allnic OTL/OCL circuit is basically SEPP, “Single-Ended, Push-Pull”, so a “cancellation circuit” is required to adjust the balance of signal between the upper and lower power output tubes’ input grids. Allnic uses an extremely sophisticated “Active Balanced Positive and Negative Feedback Circuit”, in which the 6AN8 pentode, for example, perfectly controls the circuit’s operation with extremely low distortion and incredible speed.

In Allnic’s OTL/OCL preamp output circuits there is no coupling device between the preamplifier and the power amplifier; that is, the final gain/output tubes are directly connected to the power amplifier. Combined with optimization of other elements of the devices’ complete circuit design, the benefits of the OTL/OCL circuit listed above become a reality with the Allnic OTL/OCL circuits.

Two examples of Allnic transformer coupled and OTL/OCL line-stage preamps

 

Let’s focus on two preamps using the two design approaches and relatively close in price, the transformer coupled L-7000 and the OTL/OCL L-8500, priced at $16,500.00 and $13,500.00 respectively.