Michael Yee Audio
Manual for the PA-3
The PA-3 is a Dual Mono 25W Class A power amplifier. The PA-3 started as an experimental design created from some mathematical experiments. The questions asked in this mathematical experiment were very fundamental, What proportions of global feedback are appropriate with regard to other feedback techniques? Should a speaker be driven from current or voltage drives?"
The sound of the PA-3 is not at all a common sounding amplifier. The best way to describe its character is "organic." One of the most unique and unconventional aspects of the PA-3 design is its differential current source output stage.
However, the PA-3 is not for every system. There are many aspects of this design that use unconventional design topologies which give it sonic benefits unmatched by conventional designs. On the other hand, there are some detrimental aspects of the design.
On one hand, the PA-3 has an organic and lifelike sound; on the other hand, it has a limited current output. Unlike most Class A outputs that switch to Class B as soon as the bias current is exceeded, the PA-3 simply runs out of current after the Class A current is used. The PA-3 is rated at 25W for an 8 ohm load. The output power is only 12.5W into a 4 ohm load. Speaker load and efficiency are important parameters in determining what speakers will work well with the PA-3.
The PA-3 uses many of these techniques in order to solve some special problems. The result is a very unique sound. There is however a significant downside to this particular implementation: The output power halves when the impedance goes from 8 ohms to 4 ohms.
The PA-3 has a current output rather than a voltage output, which causes the amplifier to interact a bit with the load, giving a bit of a supple sound. The output stage uses MOSFETs in a current output configuration rather than the typical voltage follower configuration (source follower). MOSFETs have often been associated with a soft or hazy sound. The supple sound I refer to is not like this at all, it is a much more organic sound.
Other than this, the sound has an extremely integrated harmonic structure, especially with phase-aligned speakers. This is my favorite amplifier design
The PA-3 is a typical Mike Yee Design and has:
In short, the PA-3 is a great amplifier for a person who likes a tube sound but with definition.
The PA-3 is a very unique high fidelity audio power amplifier, designed and constructed to very high standards. The PA-3 will process an audio signal from a line amplifier and drive standard 6 or 8 ohm loudspeakers.
The PA-3 is a dual mono, solid-state, 25 watts/channel amplifier designed for 4 to 8 ohm loads. It is designed from an understanding of the interaction of all design parameters. A "holistic" approach is used that defines construction for maximum dependability and consistency. Each signal path is a fully discrete amplifier circuit, carefully designed using Michael Yee Audio proprietary technology.
The PA-3 has optimal power supply "segmentation". The high current output stages are not sensitive to power supply fluctuations and are therefore un-regulated. (Regulating the output stages would constrict the current flow, instead the output stages are supplied directly from large power supply capacitors.) Whereas the feedback and control portions of the PA-3 are designed to have low current consumption, and a tightly regulated power supply is easy to design. The dual mono design includes separate power transformers for each of the channels high current stages which eliminates channel to channel interactions.
There is plenty of space between the large magnetic parts and the printed circuit card in the PA-3. This helps to eliminate un-desired interactions.
Amplifier 18" W X 16.5 D" X 5.5 H"
Power Req.: (50-60 Hz) 120 Volts 4 Amps RMS
230 Volts 2 Amps RMS
Input Sensitivity: 1.5 Volts
Frequency Response: 2Hz-20kHz ±0.02dB
Input Impedance: 50k Ohms
Output Impedance: 0.5 Ohms
Shipping Weight: 33 lbs