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Quad 405 Amplifier Service & Refresh

Readers will know how much I like the Quad 405 amplifier. In this article, I look at the Quad 405 amplifier service process and what’s involved when a 405 visits Liquid Audio.

Talk about great-sounding amps, this particular Quad 405 is completely original, with its original capacitors and connectors. The new owner wanted me to freshen her up and fit some RCA connectors, so that’s what I did. Let’s look briefly at what’s involved in a Quad 405 amplifier service like this.

By the way, if you want to read or find out more about the legendary Quad 405, check out my Quad 405 restoration article and my comprehensive review.

Refreshing a Quad 405

Keep in mind this is a Quad 405 service, not a full restoration. Service focuses on reliability improvement and bang-per-buck sonic gains. I start by carefully inspecting the 405 and looking for any signs of parts failure or earlier poorly executed work.

Quad 405 Amplifier Service
Lovely, simple, elegant, retro – what’s not to love about a 100 watt per channel amp this small and cool-looking?!
Quad 405 Amplifier Service
That’s all there is inside. Less really is more with the 405 and these amps prove it. Centrally, there’s a large, high-quality potted transformer, to the left of which lie two decent-sized filter capacitors. These capacitors will stay in place for this service.
Quad 405 Amplifier Service
So English, this beautifully executed wiring loom really is a blast from the past. If only equipment were still made like this.
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Note the crappy looking ERO caps, one covered in I don’t know what. These always fail, in this 405, two out of four measured leaky and had ridiculously high ESR.

Thermal Interfaces

The next step involves removing both amplifier modules and cleaning all the old thermal compound from modules and heatsink.

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Four screws hold each module in place, remove them and the connectors, and the module just pops out. All the old thermal paste must be removed.
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This is how both surfaces should look before adding fresh silicone thermal paste to the module and reinstalling it. I always check the tightness of the heatsink fixing screws, before reinstalling the modules.
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In a Liquid Audio Quad 405 amplifier service, the modules come out for a little freshening up. Depending on what my customer asks for, I’ll do more or less at this stage of the overhaul. In this case, the original ERO capacitors were absolutely dead.

I pay careful attention to fasteners. In this amp, some of the transistor mounting fasteners were not tight, and the module fasteners were barely finger tight. I remake the solder joints under each transistor after doing this, to remove stresses. Lastly, I clean each board, removing flux where necessary.

If the customer wants something more like a restoration, I’ll remove the output devices, possibly replace them, and remake the thermal interface with silicon thermal pads.

Quad 405 Amplifier Service
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Close-up of a module. Note the Toshiba 2SD424 transistors, superb devices. The ERO caps are the cylindrical maroon coloured things. Also, note the original LM301 op-amps. I replaced these with TL071 which Quad used in all later iterations of the 405. The TL2071 is an ever better replacement and I’ve gone further with my own 405-2 and others where full restoration is requested.
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Finished module with new, high-spec caps in place. I use super-low ESR, low impedance, high-temperature replacements in all my work.
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The other module before …
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… And now recapped
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A better view of the new TL071 op-amp I’ve used as a replacement for the LM301. It’s important to use FET-input op-amps like the TL071, or TL2071 which is better again, in the 405, to avoid problems.

RCA Connectors

Finally, as part of this Quad 405 amplifier service, I installed some RCA connectors on the back panel. Whilst not strictly necessary, the owner requested it here.

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Finally, you can see the modules reinstalled in this shot, and the new panel-mount RCA sockets, waiting to be soldered to.
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Here they are, connections soldered in place.
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I now always connect the common ground connection to chassis ground, to avoid any possible DC offset at the outputs. This is not a big deal but helps the amp run cooler and sound better.
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As you can see, with the RCA sockets installed like this, they basically look as though the amp came this way. It does take up the service holes used to remove screws holding the module in place, but with the correct screwdriver, access is still no problem.

5 thoughts on “Quad 405 Amplifier Service & Refresh”

  1. From where do you take the signal to the RCA connectors?
    From the DIN connector, or the module connector?

    1. Hi Fredrik, as mentioned in there somewhere, I remove the DIN from the circuit. In most cases it won’t be used again and the extra connections and wire are counterproductive.

  2. Hi the laced wiring loom in 303 and 405 series amplifiers are atypical to UK Government contract equipment of the 60s and 70s. I suspect Quad or whatever their name was back at their begining made stuff for the Military under tight Government specifications

    1. Hi Bryan, yes and typical of all really good (ie non-consumer grade) electronics from that era in fact. It has a real aviation vibe to it and as a test and measurement equipment collector, I can tell you that this was the norm across a range of really good electronics from back in the day. Marconi, Hewlett Packard, Tektronix etc all used this type of looming, making equipment like this a real pleasure to work on.

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