Ravensbourne Stereo

Quadrifilar wound transformers, like those used in the Rogers Ravensbrook and Ravensbourne amplifiers, present some unique challenges and considerations in amplifier design:

1. Winding complexity: Creating four precisely matched windings (quadrifilar) is more challenging than standard transformer designs. This requires careful manufacturing to ensure proper phasing and balance between windings.
2. Isolation requirements: The transformers need to provide electrical isolation between the input and output stages, allowing the output transistors to "float" relative to the power supply. This demands careful attention to insulation and winding layout.
3. Signal integrity: Introducing transformers into the signal path can potentially impact audio quality. Designers must carefully optimize the transformer characteristics to minimize distortion and maintain wide bandwidth.
4. DC offset management: Unlike direct-coupled designs, the transformer coupling makes it impossible to use servo feedback for stabilising DC offset at the output. This necessitates periodic manual adjustments to maintain optimal performance.
5. Biasing complexity: The unique topology requires careful biasing of the output stage. The Ravensbourne, for example, had four preset adjustments that needed periodic checking and adjustment.
6. Thermal considerations: The transformer design affects how heat is distributed in the amplifier, potentially impacting long-term stability and reliability.
7. Size and cost: Quadrifilar transformers are typically larger and more expensive than simpler designs, which can impact the overall amplifier design and pricing.
8. Limited component options: Specialised transformers like these are not as readily available as standard components, potentially limiting design flexibility or requiring custom manufacturing.

While the quadrifilar transformer approach offered some potential benefits, such as improved matching between positive and negative signal halves and reduced crossover distortion, these challenges likely contributed to its limited adoption in amplifier designs compared to more conventional transistor topologies,

Gary Cowans, 11.Oct.24

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Rogers took an innovative approach with their Ravensbourne and Ravensbrook amplifiers by using quadrifilar wound driver transformers instead of the more common complementary transistor arrangement.
The key points about this design choice:

Unique topology: The quadrifilar wound transformers allowed Rogers to use a split-supply output stage with two identical NPN transistors, rather than complementary NPN and PNP pairs.

Potential advantages:

• Improved matching between positive and negative halves of the signal
• Reduced crossover distortion
• Simplified biasing of the output stage
• Potentially better thermal stability

Technical implementation:

• The transformers provided electrical isolation between the input and output stages
• This allowed the output transistors to "float" relative to the power supply
• Four separate windings on each transformer enabled precise control of signal phasing

Mixed reception: While innovative, opinions on the sonic results were mixed. Some found the sound smooth and refined, while others felt it lacked transparency compared to more conventional designs36.

Historical context: This approach was relatively uncommon in solid-state amplifiers of the late 1960s/early 1970s, setting Rogers apart from competitors.

Legacy: The Ravensbourne is remembered as a well-constructed amplifier, though its sound quality was sometimes debated. The design represents an interesting transitional period as manufacturers moved from tubes to transistors.

While the quadrifilar transformer approach had some potential benefits, it also introduced complexity and potential drawbacks. As transistor technology improved, most manufacturers ultimately favoured more conventional complementary output stage designs in subsequent years.

Gary Cowans, 11.Oct.24

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