Upgrading a Quad 405-2

Ed Howarth

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A few weeks ago I bought a Quad 405-2 off eBay. I didn't know what serial number it was but it was obviously quite late as the photos showed it had phono sockets and a IEC passthrough socket on the rear. I fancied getting the soldering iron out and doing a few of the many upgrades and mods that are documented on the internet.

I still don't know what the serial number is, because when it arrived the sticker on the bottom was torn and unreadable. But it has the latest amplifier boards in it, the 12565-7 with date code from 1985. The amp worked well when received. No undue hum (which is a common problem but easily sorted by renewing offending electrolytic capacitors).

What makes the amp particularly easy to work on is its use of standard components - they, or their equivalents, are readily available today. It needs no setting up when components are changed - there are no pots or trimmers at all. It has a simple case and chassis, and the power amps are on 2 widely spaced populated single sided boards which sit vertically. They even provide rubber blanked holes in the backplate so a screwdriver can access some difficult to reach screws. No need to use a right angled screwdriver. Most of the case is taken by a very large transformer and smoothing caps.

There are two papers itemising extensive upgrades, by Keith Snook and Bernd Ludwig, and a third by Nick de Smith which takes the best and most effective mods from the other two. It was this latter one that I followed. More modern op-amps, altered feedback circuitry, power supply improvements, new caps, new speaker posts and quite a few other tweaks to the amp circuitry. It also has a list of required components with Farnell's stock numbers which is very convenient. 

My music and electronics room doubles as a sewing room as you can see. And if you really want to know, the fat lady outside is called Gerty. 

lgWXEqq.jpg


These are the amp boards with the mods finished. All very neat on the component side, but a tad messy on the other. Caused by an extra transistor to stop switch off thump, a re-routed feedback capacitor and some decoupling caps.
krnezEu.jpg


M1wb7J4.jpg


The transformer has separate secondaries, so the opportunity is taken to completely split the PSU components that follow, so fitting 2 new bridge rectifiers, 2 sets of smoothing caps and new earthing post. Decoupling caps all over the place. It involves quite a bit of metal bashing (and judicial use of a shoe horn to get 4 caps in where there were once 2). The white stain at the far left of the front panel shown in the next photo is some missing black anodizing, looking like extreme heat damage. Its position coincides with where one of the the power amp's heatsink mates which is a bit worrying, but either the offending amp board has been replaced with a very similar one from the same era, or it has some other cause.
Lk7ukv9.jpg


And this is the amp with the boards back in, all the wiring to the power supply completed and bypass caps in place. And the lovely iconic heatsink now refitted.

lrDw1Gh.jpg


It should be going to Kegworth if there's enough space after I've squeezed a pair of Quad 2912's in the animal trailer along with all of Tony_J's stuff. Do I or do I not bring Gerty? Anyone fancy their chances?

 

bencat

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Hi Ed I should have a pair of upgraded (not by me thats for sure) Quad 405-1 units as the power amps for my Active speakers it would be great to hear how your amp sounds. Out of interest what are the smoothing caps you have used in this ?

My original Quad 405-1 which was worked on by Net Audio many years ago has a pair of Elna Cerfine Caps . The latest Quad -40 which is being worked on by Fidele Audio will have a pair of Nichicon Caps which Peter of Fidele says are very good (finding a pair of Elna Cerfine units to match was not possible.

 

Ed Howarth

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@Graham67

Thanks. Yes I am. There's a certain crisp feel to snares, cymbals and guitar strings which I haven't heard with my previous amps. I didn't really listen to this one for any length  before doing the improvements, but it's sounding very fine now. 

 
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Ed Howarth

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Hi Ed I should have a pair of upgraded (not by me thats for sure) Quad 405-1 units as the power amps for my Active speakers it would be great to hear how your amp sounds. Out of interest what are the smoothing caps you have used in this ?

My original Quad 405-1 which was worked on by Net Audio many years ago has a pair of Elna Cerfine Caps . The latest Quad -40 which is being worked on by Fidele Audio will have a pair of Nichicon Caps which Peter of Fidele says are very good (finding a pair of Elna Cerfine units to match was not possible.
Hi Andrew. I've been following your adventures with Quad!  The caps are Kemets. Like These. Recommended by the guy whose write up I was following. I would like to think they are reasonably ok at £18 each x 4. 

 

Ed Howarth

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Great caps those kemets, how did you get on with the clearance on the lid?
It was fairly close, probably 4 or 5 mm. But there is a layer of very hard foam on the underside of the lid so I knew I was ok even if the case suffered a hard impact. I used clamps to hold them in place.which means the bottoms are hard against the case bottom. 

 

mikejt

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Hi Ed,

Maybe you (or someone) could help me.

I'm in the process of searching for caps for my Quad 405-2, same rev as yours, as the original ones are cracking and leaking and I'm looking for advice.

I've looked at Nichicon Muse ES Bipolar for C17 and was looking at the Elna Silmic 11 for the other small caps but 220/100v is physically quite big as decouplers soldered to back of the pcb so looking at others,

I guess you just went for the Panasonic  ECA AM series,  as Nick De Smith, and was wondering if you're happy with those?

I suppose I could have a mixture of Panasonics for the decouplers and Silmic for signal path if I can figure that out.

I've read through all the stuff I can find on the web and asked questions of Nick De Smith as his pcb images didn't reflect the changes which he had proposed, eg Rewiring C8 or fitting transistor to power the IC, sadly no answer. He's a busy chap I suppose.

I'm two minds whether to go for the dual power supply or 2 x 15000 to make life easier.

Any ideas welcome.

Mike

IMG_20200204_175404 -resized.jpg

 

Ed Howarth

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To be honest, squeezing those 4 smoothing caps in there was really difficult. The base of the case had to be drilled to take bolts for the clamps and it was literally mm accuracy. Looks like there's plenty of space between the caps on my photos, but what you can't see is the thickness of the clamps touching each other. Be very sure that they are all going to go in before embarking on a twin power supply!! I seem to remember that getting in to solder onto the lower bridge rectifier was a bit of a pain as well.

I got most/all the components from Farnells. Some are subject to minimum order but nothing to break the bank. Was totally happy with the component selection, thank goodness. Nothing worse than that "If only I'd....." feeling.





Order Code


Qty Ordered


Mftr. Part No


Your Part No


Manufacturer / Description




1178437


2



OPA134PAG4


 


TEXAS INSTRUMENTS  OPA134PAG4  Audio Power Amplifier, 1 Channel, ± 2.5V to ± 18V, DIP, 8 Pins




1077303


2



808-AG11D-LF


 


TE CONNECTIVITY  808-AG11D-LF  IC & Component Socket, 8 Contacts, DIP Socket, 2.54 mm, 800 Series, 7.62 mm, Beryllium Copper




2675417


5



KBPC2502


 


MULTICOMP  KBPC2502  Bridge Rectifier Diode, Single Phase, 200 V, 25 A, Module, 1.1 V, 4 Pins




8766860


5



ECA1AAD330X


 


PANASONIC ELECTRONIC COMPONENTS  ECA1AAD330X  Electrolytic Capacitor, 33 µF, 10 V, AD Series, ± 20%, Radial Leaded, 5 mm




1200765


5



B32561J3154K000


 


EPCOS  B32561J3154K000  DC Film Capacitor, 0.15 µF, 250 V, PET (Polyester), ± 10%, B32561 Series




8767254


5



ECA1VAM101X


 


PANASONIC ELECTRONIC COMPONENTS  ECA1VAM101X  Electrolytic Capacitor, 100 µF, 35 V, AM Series, ± 20%, Radial Leaded, 6.3 mm




9520678


2



FSCEX 120PF 1% 630V


 


LCR COMPONENTS  FSCEX 120PF 1% 630V  DC Film Capacitor, 120 pF, 630 V, PS (Polystyrene), ± 1%, FSC/EX Series




8767521


5



ECA1JAM101X


 


PANASONIC ELECTRONIC COMPONENTS  ECA1JAM101X  Electrolytic Capacitor, 100 µF, 63 V, AM Series, ± 20%, Radial Leaded, 8 mm




1572893


4



ALS30A103DE063


 


KEMET  ALS30A103DE063  Electrolytic Capacitor, Long Life, 10000 µF, 63 V, ALS30 Series, 11000 hours @ 85°C, ± 20%, Screw




3885318


4



EP0882-PNF


 


LCR COMPONENTS  EP0882-PNF  Mounting Clip, Nylon, No Flange, 35mm, LCR EP Series




8767670


5



ECA2AAM221X


 


PANASONIC ELECTRONIC COMPONENTS  ECA2AAM221X  Electrolytic Capacitor, 220 µF, 100 V, AM Series, ± 20%, Radial Leaded, 12.5 mm




1236669


5



MCNP35V106M5X11


 


MULTICOMP  MCNP35V106M5X11  Electrolytic Capacitor, 10 µF, 35 V, MCNP Series, ± 20%, Radial Leaded, 5 mm




8767513


5



ECA1JAM470X


 


PANASONIC ELECTRONIC COMPONENTS  ECA1JAM470X  Electrolytic Capacitor, 47 µF, 63 V, AM Series, ± 20%, Radial Leaded, 6.3 mm




1215515


15



BFC237021104


 


VISHAY  BFC237021104  DC Film Capacitor, 0.1 µF, 100 V, PET (Polyester), ± 10%, MKT370 Series




8767262


2



ECA1VAM221X


 


PANASONIC ELECTRONIC COMPONENTS  ECA1VAM221X  Electrolytic Capacitor, 220 µF, 35 V, AM Series, ± 20%, Radial Leaded, 8 mm




1215491


4



BFC236845474


 


VISHAY  BFC236845474  DC Film Capacitor, 0.47 µF, 250 V, PET (Polyester), ± 10%, MKT368 Series




1215481


4



BFC236845104


 


VISHAY  BFC236845104  DC Film Capacitor, 0.1 µF, 250 V, PET (Polyester), ± 10%, MKT368 Series




1738644


4



ROX2SJ2K7


 


NEOHM - TE CONNECTIVITY  ROX2SJ2K7  Through Hole Resistor, 2.7 kohm, ROX Series, 2 W, ± 5%, Axial Leaded, 350 V




1574390


5



MPSA42


 


MULTICOMP  MPSA42  Bipolar (BJT) Single Transistor, NPN, 300 V, 625 mW, 500 mA, 40 hFE




2114877


5



1N4744A


 


NEXPERIA  1N4744A  Zener Single Diode, 15 V, 1 W, DO-41 (DO-204AL), 5 %, 2 Pins, 200 °C
Cut Tape




 
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mikejt

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Feb 2, 2021
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Thanks for that Ed.

It's reassuring to know I don't need the audio caps Silmic 11 or Nichicon KZ to achieve a good result.

The KEMET 15,000/63V will probably fit the same clamps as existing 10,000/63 STC caps so I think I'll go that way. Although I like the dual PSU idea it's a lot more work and can't convince myself to do it.

I hope the OPA134PAG4 is an audible improvement on the TL071 and the 220/35v cap across D1 apparently stops the switchoff plop so that's good.

I'm sticking with the existing earthing arrangement with the input signal 0V screens connected to chassis at the DIN I/P skt and then continuing on to the clean earth at the PCB I/P tag. (Apparently if I change to Phonos the 0V is insulated from chassis instead) I'll stick with the Keith Snook recommendation. Chassis is connected to mains earth at the power socket.

I used to use the QUAD 33 preamp which distributed power and earth via signal lead to the 405 but that's been in the attic for many years as I built the Hart-John linsley Hood Modular preamp in the mid 90's . This has served well and is very flexible if you want to record to tape, as I did then, or listen to class A headphone sound.

I could go on but I'd better get ordering

Regards

Mike

 

rabski

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For the opamps, if there's room you could consider putting in DIP sockets, so it makes future opamp changes a lot more straightforward. I try to use sockets for any opamp or IC, as much as anything else as it avoids the need to apply heat to the actual device.

 

mikejt

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Feb 2, 2021
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For the opamps, if there's room you could consider putting in DIP sockets, so it makes future opamp changes a lot more straightforward. I try to use sockets for any opamp or IC, as much as anything else as it avoids the need to apply heat to the actual device.
Thanks for the suggestion.

I need to do that as existing IC is soldered directly into PCB

One concern with IC is to avoid static electricity destroying it.

 
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Ed Howarth

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Apr 17, 2012
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128
Isle of Mull
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  1. No
I always use sockets. Not worth not doing.

Never knowingly zapped anything with static. Before I do a joint, I touch a cold bit of the soldering iron, or just touch it with the solder, held in my other hand. 

 

bigrod

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A few weeks ago I bought a Quad 405-2 off eBay. I didn't know what serial number it was but it was obviously quite late as the photos showed it had phono sockets and a IEC passthrough socket on the rear. I fancied getting the soldering iron out and doing a few of the many upgrades and mods that are documented on the internet.

I still don't know what the serial number is, because when it arrived the sticker on the bottom was torn and unreadable. But it has the latest amplifier boards in it, the 12565-7 with date code from 1985. The amp worked well when received. No undue hum (which is a common problem but easily sorted by renewing offending electrolytic capacitors).

What makes the amp particularly easy to work on is its use of standard components - they, or their equivalents, are readily available today. It needs no setting up when components are changed - there are no pots or trimmers at all. It has a simple case and chassis, and the power amps are on 2 widely spaced populated single sided boards which sit vertically. They even provide rubber blanked holes in the backplate so a screwdriver can access some difficult to reach screws. No need to use a right angled screwdriver. Most of the case is taken by a very large transformer and smoothing caps.

There are two papers itemising extensive upgrades, by Keith Snook and Bernd Ludwig, and a third by Nick de Smith which takes the best and most effective mods from the other two. It was this latter one that I followed. More modern op-amps, altered feedback circuitry, power supply improvements, new caps, new speaker posts and quite a few other tweaks to the amp circuitry. It also has a list of required components with Farnell's stock numbers which is very convenient. 

My music and electronics room doubles as a sewing room as you can see. And if you really want to know, the fat lady outside is called Gerty. 



These are the amp boards with the mods finished. All very neat on the component side, but a tad messy on the other. Caused by an extra transistor to stop switch off thump, a re-routed feedback capacitor and some decoupling caps.




The transformer has separate secondaries, so the opportunity is taken to completely split the PSU components that follow, so fitting 2 new bridge rectifiers, 2 sets of smoothing caps and new earthing post. Decoupling caps all over the place. It involves quite a bit of metal bashing (and judicial use of a shoe horn to get 4 caps in where there were once 2). The white stain at the far left of the front panel shown in the next photo is some missing black anodizing, looking like extreme heat damage. Its position coincides with where one of the the power amp's heatsink mates which is a bit worrying, but either the offending amp board has been replaced with a very similar one from the same era, or it has some other cause.


And this is the amp with the boards back in, all the wiring to the power supply completed and bypass caps in place. And the lovely iconic heatsink now refitted.



It should be going to Kegworth if there's enough space after I've squeezed a pair of Quad 2912's in the animal trailer along with all of Tony_J's stuff. Do I or do I not bring Gerty? Anyone fancy their chances?
The 405 was a brilliant concept with the current dumping and at the time was unbeatable at 100W per channel in such a small enclosure .. built by some of the best British engineers of the day .. It’s design set a standard and it’s a testament to its ability that it is still widely used today in a vast array of systems and speakers.. Think I had seven at last count ..including a pair of monos ..Widely used by the BBC as the AM16/8 monos to drive the Rogers LS5/8 monitor..Among other incarnations..My uncle often said if the BBC use anything it was probably among the best there is .. It wasn’t their money they were spending..

 

mikejt

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Feb 2, 2021
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While I was browsing the web for Quad 405 refurbs etc. I looked at Keith Snook's and Nick de Smith's web sites and interestingly the images of the 405 boards were identical apart from the date code on the output transistors. Its like spot the difference. The top one is from Keith Snook http://www.keith-snook.info/QUAD-405-2-Modification.html and the lower one from Nick de Smith https://www.desmith.com/NMdS/Electronics/QUAD_upgrades.html I found this a real puzzle. I wonder what the explanation is.

  QUAD-402-2-discrete-N1-N2.png P1040534a.jpg

 

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