QUAD 405 amplifier

The QUAD 405 is a 2x100W power amplifier. At its introduction by the Acoustic Manufacturing Co. it was a real innovation because of the Current Dumping principle to correct the distortion of a class-B amplifier. A lot has been written about this principle, I won’t repeat that here. I got this amplifier as defective but it turned out easy to repair.

The QUAD 405 is a compact and heavy amplifier with a heatsink at the front. It delivers 2x80W in 8Ω or 2x100W in 4Ω. QUAD’s minimalistic and compact design style makes this amplifier look simple and timeless. It has no knobs or other controls, the controlling must be done by the pre-amplifier. In 2018 I bought a set consisting of a QUAD 34 preamplifier and a QUAD 405 power amplifier. The person selling them said the amplifier had a strong hum and a short-circuit had occurred plugging it into a power strip. I hoped the cause of that was in the power strip. The price was right and I took the amplifiers home.

This amplifier is really heavy because of the large power transformer. When I took the chassis out of the cabinet, I noticed that the reservoir electrolytics were slightly bulging, but they had not leaked yet. On the inside of the top cover, there were burnt-in dust stripes indicating that the amplifier had been quite hot inside for extended periods. Heat is a well-known cause of problems with the QUAD-405: the small cabinet with limited ventilation leads to a high internal temperature, which tends to wear out some of the components. Even though the heat sink on the front dissipates most of the heat of the output stage, the power transformer can’t get rid of its heat easily, especially when the amplifier is standing in a closed piece of furniture. The internal hot-air circulation deposits burnt dust stripes. I replaced the electrolytics, although their capacitance and ESR measured still reasonable.

When I connected the amplifier, there was a distinctive hum in spite of the replaced buffer capacitors. I found out that in both channels a small electrolytic decoupling capacitor in the input stage was dried out. These were plastic-encapsulated electrolytics, notorious for drying out.

After replacing them, the amplifier was just working fine.

I have been using this amplifier in my living room since. On occasion, I used took it to bring loud music on a party. At these occasions, the heatsink became hot but the amplifier kept working hard for a number of hours without problems. This is a fine piece of audio gear.