Monoblock MA15 Mk 2 Early

The Radford MA15 series of amplifiers was a well-regarded line of vacuum tube audio amplifiers produced in the 1960s.

There were four versions:

1. MA15 Mk1: This was the original version of the amplifier.
2. MA15 Mk2 Early: An updated version of the original design.
3. MA15 Mk2 Late: A further refinement of the Mk2 design.
4. CMA15: This was a kit version of the MA15 amplifier.

The Radford MA15 Mk2 Early Monoblock was an improved version with significant enhancements:

• Output power: 15 watts (maintained)
• Circuit design: Revised for better performance
• Phase inverter: Changed to pentode-triode configuration
• Feedback: Increased to 27dB of negative feedback
• Output transformer: Improved design with first resonance at 150kHz
• Input stage: Added step network for high-frequency stability
• Feedback network: More complex R/C step network across the feedback resistor
• Performance: Significantly lower distortion, especially at high frequencies
• Power delivery: Able to deliver rated power up to 20kHz
• Phase shifts: Reduced at high frequencies
• Stability: Carefully tuned for stability with higher feedback

The Mk2 version demonstrated substantial performance improvements, particularly in distortion reduction and power delivery across a wider frequency range.

Output Transformers
The key difference between the output transformers of the Radford MA15 Mk1 and Mk2 models lies in their design and performance characteristics:

Radford MA15 Mk1 Output Transformer

• Resonance Frequency: The Mk1 output transformer had a first resonance around 60-70kHz.
• Performance: This design resulted in higher phase shifts and distortion at high frequencies, limiting its ability to deliver full power at the upper end of the audible spectrum.
• Design Complexity: The transformer was less advanced, which contributed to reduced stability and slightly inferior sound quality compared to the Mk2.

Radford MA15 Mk2 Output Transformer (T2063)

• Resonance Frequency: The Mk2 output transformer was significantly improved, pushing its first resonance to approximately 150kHz.
• Performance: This allowed for reduced phase shifts, better high-frequency stability, and lower distortion across the frequency range. It also enabled the amplifier to deliver its rated power up to 20kHz.
• Design Complexity: The improved design contributed to a more refined sound and greater technical performance, addressing the limitations of the Mk1.

These enhancements in the Mk2's output transformer were a critical factor in its superior performance compared to the Mk1.

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