I recently purchased a classic Quad 405 power amplifier to use as my main stereo amp. It sounded fantastic even before my overhaul, read on for the gory details and, for some extra reading, check out my Quad 405-2 review.
The Quad 405 is revered by many, even 40 years after its release. It’s not hard to see why, the 405 does an amazing job of just getting out of the way and playing music. This is a truly special amplifier, don’t let the typically understated English looks fool you.
Mine is actually a 405-2, but the 405 and 405-2 are quite similar. They are masterpieces of clever, understated design and showcase the less-is-more approach of Quad’s legendary engineer, Peter Walker.
I can’t think of a mainstream commercial amplifier with fewer parts than the Quad 405. Sure, they used an IC in the front-end to keep the parts count down, but the driver stage is just two TO-220 devices operating in class-A. The output stage is another two N-channel TO-3 devices per channel. No PNP-NPN device mismatching here!
The driver and output transistors are thermally coupled to one block of aluminium. They are also board-mounted, eliminating unnecessary wiring. Add in a sprinkling of passive components of very high quality and you have the elegant simplicity that is the Quad 405 amplifier board.
Much has been written about the remarkable Quad 405. There is a great review and some background info, here. One of my favourite resources is Keith Snook’s fantastic Quad repository. Keith goes into great detail about his own journey with the 405 and breaks things down into two large and very detailed 405 pages. The first covers the original 405 and its various iterations, the second covers the 405-2, and its various iterations.
Keith’s site is a wealth of information and I encourage you to visit and spend some time reading and studying the schematics. Keith has painstakingly redrawn them into one document showing the changes at each version of the board. You’ll also find the service manual along with all revisions and parts lists, very handy indeed.
Overhaul and Improvement
OK, so what did I do to mine? I tend to utilise a similar, less-is-more approach to my work, not senselessly changing parts that are perfectly good. To that end, transistors, resistors and most other parts remain as factory. They are of excellent quality and perfect, so why change them?
My 405 needed of some TLC though. The seller told me it was ‘restored’ so I was keen to see what that actually meant. Restored means something very different to me obviously, I guess this is why I have so many customers, but in this context, its annoying. The amp was also dropped during shipping, causing panel damage. I claimed for this through eBay and was refunded full purchase price, plus shipping.
Apart from this, mine has two different amp modules, one version apart. Quad made something like 10 revisions of the 405/405-2, improving them as they went. This is of no real consequence in a dual mono amp like this, but one must have blown up at some point. Quad had a very simple repair policy – order a new module and screw it in place, job done.
My amp contained a few original and dead caps, some old chewed-out screws and other bits and pieces that I decided to sort out. Let’s have a look inside…
Further Quad 405 Mods…
The 405 sounded much better after replacing the old board-mounted capacitors. I decided against changing the TL-071 op amps for a number of reasons. Firstly, I’m against the forum-driven, non-scientific craze of just swapping op-amps because someone said they should sound better. What’s better, and smarter, is to try to understand the reasons behind the choices originally made. Then one can see if there is a better, modern replacement.
For noise performance, low DC offsets and low distortion, the FET input TL-071 is an excellent choice, and still a great op-amp, despite what you might read. It might be bettered by the TL-2071, so I bought a couple of those and will try them at some point.
The remaining important mod was to replace the original filter capacitors with modern, higher-spec parts. STC branded 10,000uF @ 63V parts came factory-fitted. I replaced these with premium Kemet 15,000uF @ 63V capacitors. The Kemet parts are better, will sound better and are rated a longer service life. Before doing so, I examined the increased inrush current to ensure this was within limitations of the diode bridge.
The original STC filter caps that came out measured quite badly. Capacitance was OK, but ESR was quite high for large caps. We should see an ESR of very close to 0 Ohms for large caps, measured using a proper ESR meter. The STC caps were around 0.5 Ohms, way too high. The new Kemet parts were as close to zero as you can get, at around 0.01 Ohms, plus they have 50% greater capacitance, so this is a really worthwhile improvement.
There are a couple of improvements I’ve yet to make. The first is to replace a couple of ceramic caps in the critical bridge circuit with lab-grade styroseal capacitors. Second is to possibly replace the TL-071 input op-amps with the modern TL-2071 equivalent.
Finally, the Chassis…
Because of the damage incurred during shipping and also because the original screws just looked old, I decided to replace them with new zinc-plated parts. New fasteners were easy to find at my local fasteners supplier, yours too I’m sure. They made a big difference to the appearance of my Quad 405-2.
The last thing is a new side panel to replace the dented one, ordered from Quad UK and on its way. That isn’t here yet and so does not feature in these images – yet!