Fidele Audio
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Fidele Audio
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Quad 405-2 SE Upgrade - Full Details The operational amplifiers are replaced with Burr Brown OPA227s, these op amps will produce a much more neutral sound and have very low distortion and noise. The Op Amp has RF de-coupling added to increase stability. Also the Op Amp power supply zenor diodes have added RF de-coupling. The amplifier circuit electrolytic capacitors are replaced with non-polarised and standard electrolytic (6 of per channel), and 2 electrolytic capacitors are added to the circuit boards to improve de-coupling. This removes a major source of distortion within the 405-2 and allows the amplifier to produce a much finer and clearer sound. Key resistors are replaced with low noise types in the input circuit, CC circuit and feedback circuit, this further improves the signal to noise ratio. The output bridge components are replaced with hand picked new components, including a new high current inductor. Each component value is measured and the final balance of the bridge is set by trimming a resistor value. The main power supply capacitors are replaced with standard electrolytic capacitors of increased capacitance (22,000uF), to improve bass handling, and have additional RF de-coupling capacitors added. Further RF de-coupling is added to the primary and secondary of the mains transformer. The output transistors are replaced with MJ15003 transistors and the drivers are replaced with MJE15031 transistors. The current limiting is reduced to allow up to 14 amps of transient current. The internal DC wiring is replaced with a heavier gauge of wire to better handle the current. This upgrade provides an all round improvement in mid to high end frequencies while providing a firmer bass with more presence. The input gain can also be modified by reducing the gain of the pre-amp stage. This has the effect of reducing the noise floor to over -100dB. The input for full output is changed from 0.5V to 1.5V. A complementary modification is available for the Quad 34 to increase the output to match. The Quad 44 already has this level of output. Other modern pre-amps and CD players also have a suitable level of output. |
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