Quad 405-2 Stereo Power Amplifier (1979)

Name: Quad 405-2
Brand: Quad

A hundred watts a side, no controls, no meters - and a circuit that made the transistor amplifier behave itself.

Overview

Quad had an engineering idea and chose to refine it rather than replace it. The original 405 came out in 1975. The -2 revision followed in 1979 as part of the 400 series, the same family that included the 303 and the 33 preamp. The revision wasn't cosmetic. According to the available documentation, the -2 improved stability, reduced distortion and enhanced reliability over the first version. The factory literature also says it puts more VA into a wider variety of loads than the original, without giving up anything in performance or dependability.

That's the pitch, anyway. What makes the 405-2 interesting to anyone pulling one out of a cupboard today is the topology underneath: current dumping, Quad's patented approach to the transistor amplifier's oldest problems. The idea is a hybrid — a small, high-precision Class A error amplifier runs the show, calling on a high-current output stage to supply the muscle. The small amp carries the error signal, and as long as the big output transistors land within the target window, the little amp fills in the remainder accurately. Quad's own manual describes it plainly: the reproduced quality depends solely on the small amplifier, which because of its low power can be made very good indeed. Crossover distortion, quiescent current adjustment, thermal tracking, transistor matching — the manual claims all of it simply disappears.

The 405-2 is rated at 100W per channel into 8Ω, and it's happy with 4Ω loads. Period coverage in HiFi-Stereo (June 1985, p.66) confirms that figure. Frequency response is quoted as 20 Hz - 20 kHz. Input sensitivity is 0.5 V for full output, which you need to know if you're pairing it with anything other than a period Quad control unit. Distortion figures change depending on whose sheet you read. The article literature cites under 0.005% typical at 1 kHz into 8Ω, while other documentation puts THD under 0.01% up to 100 W. Either way, it's low.

The 405-2 has no controls. None. Quad's manual states it carries no controls and may be mounted out of sight in a cabinet or any other convenient location. It's a black box that makes the signal bigger and otherwise keeps quiet.

Key Features

Current Dumping with a Single Op-Amp Front End

Here's how the circuit works. A low-powered Class A stage handles fine detail while a high-powered Class B section supplies the current, and the small amplifier corrects the big one continuously. Period documentation notes the circuit uses a single op-amp for low-distortion gain and DC correction, which is a simple front end for an amplifier like this. Per Quad's manual, the payoff is that the amplifier can use extra-tough output transistors without impairing program quality, and it can run at higher-than-normal temperatures without compromising reliability. When operated at high power levels it is quite normal for the heatsinks to be hot to the touch.

Load-Sensitive, Time-Dependent Protection

The 405-2 uses a protection circuit Quad designed specifically for it - a load-sensitive, time-dependent current limiter rather than a conventional load-line arrangement. The amplifier will deliver an instantaneous maximum of 8.5 A into any load. If the load is too reactive or sits substantially below 4Ω, the limit reduces smoothly and progressively so the output devices stay inside their safe operating area. In a dead short that figure settles at 3.1 A, and it climbs back gradually once the signal drops or the short is removed. The manual is plain about the limit: shorting both outputs simultaneously for an extended period will overheat the amplifier and eventually break it.

Dual Mono Layout and Toroidal Supply

The two channels are built as separately as the chassis allows. A toroidal transformer feeds dual regulated rails, and the dual mono layout minimizes crosstalk between channels. Construction is compact and board-mounted with minimal internal wiring. That keeps the signal path short and, according to the service literature, contributes to the amplifier's well-documented reliability and ease of servicing. The modular approach is the reason these amplifiers get repaired rather than scrapped.

Single-Channel Operation

Less known: the 405-2 can be strapped into a single-channel amplifier delivering in excess of 200 watts. Wired in parallel, that's 200 W into 4Ω. Wired in series with a suitable 1:1 centre-tapped transformer, it's 200 W into 10-16Ω. In parallel operation the input level to each channel has to be trimmed with a preset potentiometer until the two outputs match exactly for accurate load sharing.

The DIN-Only Input Problem

The 405-2 left the factory with a single DIN input for both channels, presumably intended to be used with a 44 preamp. That's fine if you own the matching Quad control unit and the 4-pin lead that came with it. If you don't, it's an obstacle, though a manageable one. The chassis already has the holes for RCA sockets, and a technician can make the modification without butchering the back panel. Budget for it if you're buying a stock unit to use with modern sources.

Historical Context

The 405-2 sits at the point where Quad's amplifier design philosophy hardened into doctrine. Peter Walker, Quad's founder, held that amplifiers should remain sonically invisible, and the company's motto was the closest approach to the original sound. Current dumping was the engineering side of that: remove the variables, let the small amplifier's quality dominate, and stop the output stage from imposing its character on the result.

As part of the 400 series, the 405-2 became one of the backbones of high-end British audio systems in the late 1970s and early 1980s. It was most often found sitting under a Quad 44 preamplifier and alongside an FM3 tuner. That pairing of 44 and 405-2 is still the configuration collectors seek out, and it remains appreciated for its sound and its build reliability.

Collectibility & Value

The 405-2 carries a collectibility rating of 7/10 - sought after, not rare. Pricing is muddled. One source quotes a market range of $300-$600 USD. Recent completed eBay sales tell a slightly higher story, running $417-$625 with a median around $509 across eight sales in the past year. The October 2026 chart shows a low of $301, a high of $683, and a latest sale at $683. The market has walked past the $300-$600 range. Asking prices on active listings run far higher - $1,300 for a 405-2 bundled with a Quad 44, $1,299 for a multi-voltage example with its original box - but those are asks, not sales.

Condition and service history drive the price. Original condition, documented service, matched channel performance, and the original packaging and documentation all move the number, and professionally recapped units command a premium. The known failure points are the usual ones for gear this age: electrolytic capacitors in the power supply and feedback circuits, output transistors that can fail if those capacitors degrade or the protection is compromised, and noisy pots and connectors that develop crackling. The recommended service is straightforward - full recapping of the power supply and signal path, inspection and testing of the output devices and driver stages, cleaning of controls and connectors, and verification of bias and protection operation.

Parts are not a problem. Rebuild and recap kits are actively sold, PCB heat sinks show up for pocket money, and original Quad amplifier modules (M12565-6) circulate as spares. Aftermarket clone boards are widely available too. Those are not Quad equipment and shouldn't be confused with the real thing, whatever their sellers claim.

Market Value

┌──────────────────────────────────────────────────────────────────────────┐
│ QUAD 405-2                                                      Nov 2026 │
├──────────────────────────────────────────────────────────────────────────┤
│ High      $683                                    Latest            $683 │
│ Median    $509                                    Trend                — │
│ Low       $301                                    Volume           ~3/mo │
├──────────────────────────────────────────────────────────────────────────┤
│   $700 ┤                                                                 │
│        │                                                                .│
│        │                                                .                │
│   $529 ┤                                                                 │
│        │                    .              .              .              │
│        │             .         .                                         │
│        │                                                                 │
│   $300 ┤.                                                                │
│        ├─────────────────────────────────────────────────────────────────│
│    vol │█            █      █  █           █            █ █             █│
│        └─────────────────────────────────────────────────────────────────│
│         2026-05                      2026-06                      2026-07│
└──────────────────────────────────────────────────────────────────────────┘

eBay completed sales. Excludes accessories, parts, and non-working units. Hover points for details.