QUAD 405 updates and clones

Hi, I've been into Quad 405's over the last 30 years or so.
I'm not saying they're the worlds best amp - but the originals are/were exceptional value, and the Chinese clones are too, with a bit of work.

The traditional criticisms of these amps are very easily overcome to make a great classic amp.
The current limiting was the major one, and lots of work rounds have been proposed. The simplest and best of which, in my opinion is to just throw the components in the bin.

I use them because I think they sound good, and have a 3 way active system with dual bass drivers. So need 4x stereo amps for my main speakers.

I've got 2 original ones, 1 original refitted with chinese boards, and 1 which I made as a student in the 1980's from boards I etched myself.

All of these, I've modified in what I consider a 'sensible' manner.
1) using recognized improvements made on later amps
2) deleting the current limiting
3) not redesigning the amp into something different
4) easily done on the existing boards, Quad or clone; don't need new boards.

The diagram is my summary of it.
Please bear in mind I'm not an Elec Eng, just a hobbyist. But I have done these exact mods to about 10 boards, and had them running well for years.
 

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Quad 405 rebuild

Morning John Hi,

Just last weekend my brother gave me his 405 that he built for himself in the the mid-80s. He reckons there is a fault on the boards (he thinks it is the input) possibly caps and IC, but I noticed that R30 & R31 were decidedly worse for wear. Very burnt and not looking very healthy including the through hole area of the PCB. I suspect there is way too much current being pulled through them. I'm not terribly clued up on electronics, so would benefit hugely from someone who knows them well.

Do you have a full BoM of the components that would be recommended to bring the boards up to modern spec. I will make new PCBs for the rebuild, and make new heatsink mounts etc. The toroidals are still perfect for the job. I also want to install the speaker protection as this amp has none. I would love to try this amp out driving my pair of stacked Quad ESLs. But only once the amp is tried and tested, with the speaker protection added.

I have added a picture of the one PCB as currently installed. Both have the resistors in the same state.

Regards, Kevin
 

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Hi OldDIY,

When I do rebuild I will definitely lift the resistors off higher. I will also make sure the PCBs are 2 oz. copper, and if possible slightly larger to permit better positioning. I will be replacing all the caps in the PSU as well and will use better cabling techniques. I'm not young (59) but I am more than prepared to learn from you informed gents. Thanks once again for your input.


Kevin
 
Hi Ferret,
It looks like one of R30/31 has been changed already because they don't match. The burning could be from before.
These run hot anyway - and dissipate over 1W each as normal.
Changing them for 2.5 or 4W - the biggest which will fit well would be good.

I don't have a BOM for this job, but I've got some handwritten notes I can post a pic of later.

Your boards look ok - if the tracks are in reasonable shape, I'd reuse them.
You need to change all electrolytics and anything damaged. Renew the heatsink compound etc.
Building new boards up will be maybe 3x the work of fixing the old, and not guaranteed to work first time every time if you've not done it much.
 
Added below is a visual guide to making these upgrades to the Quad 405.
This is for the early 405 mk 1's but also the Chinese clone boards on ebay etc.

Components in the Greyed boxes can simply be cut off the boards and thrown away. No desoldering needed.
 

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Hi,
There are many ways of upgrading this amp, which make it excellent for the price and effort put in.
I followed the work of Bernd Ludwig, Keith Snook, and De Smith.
If you google these guys names + quad 405; there's a gold mine of information.

Where I differed is mainly in deleting the current limiting circuits completely.
These were the source of most of the criticisms of the original, and most amps don't have them. So rather than going for a re design, or work round, I just scrapped them.

Apart from this you need to replace all Electrolytics; and bypass them for more supply rail decoupling.

Change the op-amp for a more modern one, and update with mods made to later versions of the amp. These are - add D13, move C8 leg to Tr2 side of C7/R13, change C11 to 1000pf. Some or all of these updates are already done on later boards - it depends which version you have.

The other significant thing is reducing the gain to modern Digital Audio levels. This is done by altering R4/C4/C2/R6, according to the diagram or instruction.

Those are the main ones, then there are a few minor ones, which are shown on the diagam and mentioned elsewhere.

Again, the intent of this is to make an excellent amp with reasonable time, effort and expense. The Quad 405 was one of the biggest selling 'high end' amps, and very popular with professional Sound Engineers. It still does a great job.
 
I see what you mean.
In the original, R6 is 330k, and I reduced it to 100k, it's possible to go further but you also need to modify R4/C2 to keep time constants similar.
As I've left it, the gain is ok, but if you drop R6 much more you have to make other alterations to keep the gain of the amp high enough.
So for me, I'm happy with where it is. Others including Des Smith have measured THD&N with a similar layout, and found it to be very good already.
 
Feedback bridge

You already reduced the original feedback resistor, so the improvement was already made. Tweaking the feedback bridge might be the only option left for this great but already ancient amplifier. Low idling current is also one the great specs, Peter Walker incoporated.
 
Yes, I was a bit tempted to do that.
But I run 4 stereo amps, so that means 8 boards to do and they're very good already.
So there comes a time when you say 'enough', lets just enjoy the music.
I'm happy I reached that stage now.
I ended up putting opa627's in them, which I thought made an improvement, but it's hard to be sure with subjective things like that.
They've been running very well without fault for about 2 years now, so I'm not inclined to mess with them any more, except if I blow a board up, or when the Electrolytics start to age...
 
Thank you for putting this one up, a good read. I have toroidal chokes in stock, 6.8uH 6A rating, I cannot find the current rating of the original Anco chokes used OEM, are these going to be beefy enough? I see the Anco part was +-20% tolerance, is there any advantage in closer tolerance? Last question, I see 6.8uH used in many projects on the net but in the OEM parts list and drawing they are 6.9mH, what gives with that?
 
Hi Ian,
The 6.8 value is obviously on the standard E12 series.
In practice, Quad used various values over different generations of these amps.
Between 6.9 and 22uH are listed for L4 (and L3 where it is used)
These values are not very critical at all - just being for HF stability and not really doing anything at audio frequencies. In fact on my boards, I've shorted them out.
L2 is the important one - being part of the error correction circuit.