Quad 405 servicing / modding advice

[DarrenHW]

Having seen a number of members on here running Quad 405 / -2's when I spotted a 405 close to me at a fair price in it's original box I was in my car and handing over cash quicker than you could say "Have you considered the 405-2?" :doh: Everyone I've spoken to about it since says "You really should have got the 405-2" :( but I've made my bed and the 405 is staying. I'll probably try a 405-2 in the future as the 405 can always be re-homed in another location.

To add insult to injury when I got the 405 home and took the top off one of the smoothing capacitors was leaking quite badly, so I have to service it before it can make it's way into the rack.

13102

My plan now is to use one of the DADA Electronics Upgrade / Revision Kits (http://www.dadaelectronics.eu/shop/quad-upgrade-kits/quad-405-upgrade-and-revision-kit) does anyone have any experience with these kits? They seem like good value for money and come with instructions which is certainly something I'll need!

I'm also considering just using the basic DADA kit without smoothing caps and using a NET Audio Dual Mono Power Supply instead (http://www.net-audio.co.uk/405dmpsu.html), any thoughts on this?

I have also stumbled across the work documented by Keith Snook, although I'd be lying if I said I fully understand it (http://www.keith-snook.info/quad-405-mods.html) are these modifications covered by the DADA kits or is this another series of modifications and if so are they worth pursuing?

I'd be grateful for any feedback on the above, or if there are other options I've not mentioned I'd like to hear about them.

Lastly, can anyone tell me what the difference is between the 405 and 405-2, is the 405-2 really that much better and could a 405 be modified to 405-2 spec?

Once the 405 is serviced I'll use a Croft 25 (yet to be ordered) and Celestion Ditton 44's.

TIA.

[spendorman]

Assuming that the 405 is in good working order, leaking/ bad capacitors replaced, my view is that with reasonable load loudspeakers, and the 44's probably are, the biggest gain in sound quality is to replace the op amp with the one used in the 405-2. It is easy to do as the op amps are in sockets, just get them the right way round. Cost, very little.

My 405 was greatly improved by this, could not really detect any difference to a 405-2 on various of my speakers, inc. 63's 57's Tannoy DCs, BC1's etc.

[DarrenHW]

the biggest gain in sound quality is to replace the op amp with the one used in the 405-2. It is easy to do as the op amps are in sockets, just get them the right way round. Cost, very little.

My 405 was greatly improved by this, could not really detect any difference to a 405-2 on various of my speakers, inc. 63's 57's Tannoy DCs, BC1's etc.

The DADA kits come with OPA604 which seems to be a popular budget choice, would you agree with the use of these OP Amps?

[spendorman]

The IC in the 405-2 is the TL-071, when I bought them they were about 70p each, suspect that even years later they won't be much. These are what I put in the 405-1.

Later, I changed the TL-071's in the 405-2 for Bur Brown ones, forget the number, no difference noted. Not quite so easy to change in the 405-2 as the originals are soldered in. I fitted sockets for the chips, like in the -1.

I've not checked up, but will the OPA604 go in without any changes? The TL-071's will.

It's just that the original op amp in the 405 was not very good, I don't think the difference in sound quality of any of the recommended replacements will be easily detectable.

[dantheman91]

Hi Darren

Original 405's are good as i have one but has been overhauled and sounds like new there was a few problems when i first got it with caps they have been replaced and the OP amps Alex mentions have also been done 405's are near bomb proof and are a fine amplifier in working order theirs many mods to be done and many pre's match it i'm using the pride & joy 33 sounds great to me.

[DarrenHW]

The IC in the 405-2 is the TL-071, when I bought them they were about 70p each, suspect that even years later they won't be much. These are what I put in the 405-1.

Later, I changed the TL-071's in the 405-2 for Bur Brown ones, forget the number, no difference noted. Not quite so easy to change in the 405-2 as the originals are soldered in. I fitted sockets for the chips, like in the -1.

I've not checked up, but will the OPA604 go in without any changes? The TL-071's will.

It's just that the original op amp in the 405 was not very good, I don't think the difference in sound quality of any of the recommended replacements will be easily detectable.

Alex - It looks like the OP Amps in my 405 are soldered directly to the board:

13103

I'll order some sockets to reduce the risk of damaging the OP Amps when installing. The OPA604 come as part of the DADA kit which does make revisions to the boards, these may be to accomodate the OPA604's which is what makes me question whether this is along the lines of the Keith Snook mods?


Hi Darren

Original 405's are good as i have one but has been overhauled and sounds like new there was a few problems when i first got it with caps they have been replaced and the OP amps Alex mentions have also been done 405's are near bomb proof and are a fine amplifier in working order theirs many mods to be done and many pre's match it i'm using the pride & joy 33 sounds great to me.

Can you tell me what else should be replaced with the OP AMP (and can anyone recommend a UK source for the TL-071's, I can only find these from Mouser), the DADA kits include new capacitors, resistors and diodes are these just to allow a change of OP AMP or components that should be replaced (i.e. electrolytic caps) within the context of servicing?

[dantheman91]

Can you tell me what else should be replaced with the OP AMP (and can anyone recommend a UK source for the TL-071's, I can only find these from Mouser), the DADA kits include new capacitors, resistors and diodes are these just to allow a change of OP AMP or components that should be replaced (i.e. electrolytic caps) within the context of servicing?[/QUOTE]

Hi Darren,

You could try asking Alan (Firebottle) as he done the work on mine and done a professional job the guy knows the in & outs of the 405 boards. :)

[spendorman]

Alex - It looks like the OP Amps in my 405 are soldered directly to the board:

13103

I'll order some sockets to reduce the risk of damaging the OP Amps when installing. The OPA604 come as part of the DADA kit which does make revisions to the boards, these may be to accomodate the OPA604's which is what makes me question whether this is along the lines of the Keith Snook mods?



Can you tell me what else should be replaced with the OP AMP (and can anyone recommend a UK source for the TL-071's, I can only find these from Mouser), the DADA kits include new capacitors, resistors and diodes are these just to allow a change of OP AMP or components that should be replaced (i.e. electrolytic caps) within the context of servicing?

I had heard that some 405 did not have sockets for the IC's, you must be unlucky, all the ones that I've had here had sockets. I did the 071 replacement for several friends.

Cricklewood Electronics are reliable.

[spendorman]

They're cheaper than years ago, I want my money back!

http://www.cricklewoodelectronics.com/Cricklewood/search.php?mode=search&page=1

[DarrenHW]

I had heard that some 405 did not have sockets for the IC's, you must be unlucky, all the ones that I've had here had sockets. I did the 071 replacement for several friends.

Cricklewood Electronics are reliable.

Would these be the ones (http://www.cricklewoodelectronics.com/Cricklewood/product.php?productid=14463&cat=237&page=2)? When you replaced these did you change any other components or could I just replace the OP Amps and smoothing caps? What about the wiring, I've read quite a few posts that suggest the internal wiring should be replaced?

Edit. I see you've beaten me to it, thanks for the link.

[spendorman]

Would these be the ones (http://www.cricklewoodelectronics.com/Cricklewood/product.php?productid=14463&cat=237&page=2)? When you replaced these did you change any other components or could I just replace the OP Amps and smoothing caps? What about the wiring, I've read quite a few posts that suggest the internal wiring should be replaced?

Edit. I see you've beaten me to it, thanks for the link.

I am fairly sure that those are the very ones I used, no other components need to be changed. I believe that you can just replace the smoothing cap and IC's. But have you got a working amp to start with?

You mentioned caps leaking. I would replace those first, see if amp is OK, then replace IC's

If wiring is OK, leave it.

[spendorman]

Would these be the ones (http://www.cricklewoodelectronics.com/Cricklewood/product.php?productid=14463&cat=237&page=2)? When you replaced these did you change any other components or could I just replace the OP Amps and smoothing caps? What about the wiring, I've read quite a few posts that suggest the internal wiring should be replaced?

Edit. I see you've beaten me to it, thanks for the link.


I only changed the op amps in the 405-1, no other components or wiring. Just pulled old ones out, pushed new ones in. Sorry, must answer questions rather that writing what I think.

[spendorman]

One thing I just remembered, check the fuses and fuse holders on the boards. They can corrode/weaken/break. replace if necessary.

[DarrenHW]

I am fairly sure that those are the very ones I used, no other components need to be changed. I believe that you can just replace the smoothing cap and IC's. But have you got a working amp to start with?

Yes the amp is working.


You mentioned caps leaking. I would replace those first, see if amp is OK, then replace IC's

If wiring is OK, leave it.

Thank you for the above paragraph, it inspired me to look further into the DADA kit. The kit seemed like a cheap and easy way to get the 405 ready for service but further investigation shows it's not that great value at all. The price for the caps from DADA is €45.00, I'm pretty sure these are the same (http://uk.farnell.com/kemet/alt22a103cd063/cap-alu-elec-10000uf-63v-solder/dp/9348255) £19.94 from Farnell! As you suggest, I'll probably start by replacing these first then the OP Amps you recommend. I say "probably" as I also found these (http://uk.farnell.com/vishay-bc-components/mal205658103e3/cap-alu-elec-10000uf-63v-snap-in/dp/1165503) which I'm also pretty sure are the caps used in the NET Audio Dual Mono PS, I'm now trying to find a DIY article for this.

Cheers,

[spendorman]

Yes the amp is working.



Thank you for the above paragraph, it inspired me to look further into the DADA kit. The kit seemed like a cheap and easy way to get the 405 ready for service but further investigation shows it's not that great value at all. The price for the caps from DADA is €45.00, I'm pretty sure these are the same (http://uk.farnell.com/kemet/alt22a103cd063/cap-alu-elec-10000uf-63v-solder/dp/9348255) £19.94 from Farnell! As you suggest, I'll probably start by replacing these first then the OP Amps you recommend. I say "probably" as I also found these (http://uk.farnell.com/vishay-bc-components/mal205658103e3/cap-alu-elec-10000uf-63v-snap-in/dp/1165503) which I'm also pretty sure are the caps used in the NET Audio Dual Mono PS, I'm now trying to find a DIY article for this.

Cheers,

Perhaps have a look at Cricklewood, see if they have some suitable capacitors. My 405-2 needs some smoothing caps.

[DarrenHW]

Perhaps have a look at Cricklewood, see if they have some suitable capacitors. My 405-2 needs some smoothing caps.

These were all I could find on Cricklewood (http://www.cricklewoodelectronics.com/Cricklewood/product.php?productid=19153&cat=0&page=1) but it's not the easiest site to use so I may have missed some? I'm not sure whether it makes a difference but these have a 9.4A ripple current whereas the BHC's used by DADA are 4.97A, I don't know what this even means let alone whether or not it matters :doh:.

[spendorman]

These were all I could find on Cricklewood (http://www.cricklewoodelectronics.com/Cricklewood/product.php?productid=19153&cat=0&page=1) but it's not the easiest site to use so I may have missed some? I'm not sure whether it makes a difference but these have a 9.4A ripple current whereas the BHC's used by DADA are 4.97A, I don't know what this even means let alone whether or not it matters :doh:.

I don't think the small difference in ripple current would matter, but will these caps fit in?

[DarrenHW]

I don't think the small difference in ripple current would matter, but will these caps fit in?

I don't know, not a huge amount of info on Cricklewood, but I think they say the height is 80mm?

[spendorman]

I don't know, not a huge amount of info on Cricklewood, but I think they say the height is 80mm?

Well, get your measuring stick out!

It's mainly the height you have to worry about, but dia would be useful, perhaps give them a phone call.

[DarrenHW]

Well, get your measuring stick out!

It's mainly the height you have to worry about, but dia would be useful, perhaps give them a phone call.

Measuring stick out!

Stock Caps - H ~ 83mm (to top of posts) D ~ 50mm.
Samwha Caps - H 80mm, D 51mm.

They would seem to be a drop in replacement, I agree a phone call would be a good idea. Maybe I'll order some and forget about the Dual Mono for now, I'd really like to go Dual Mono but even though the circuit seems simple I don't have the tech knowledge to tackle it myself.

[spendorman]

Measuring stick out!

Stock Caps - H ~ 83mm (to top of posts) D ~ 50mm.
Samwha Caps - H 80mm, D 51mm.

They would seem to be a drop in replacement, I agree a phone call would be a good idea. Maybe I'll order some and forget about the Dual Mono for now, I'd really like to go Dual Mono but even though the circuit seems simple I don't have the tech knowledge to tackle it myself.

Still be careful of the connections on top of the caps shorting on the top cover. Perhaps order the caps, IC's, IC holders, fuse holders etc. all together from Cricklewood, only one postal charge I believe, but it's a while since I ordered from them.

[DarrenHW]

Hi Alex, do you know if this would be a suitable replacement for TL071CN, http://uk.farnell.com/texas-instruments/tl071cp/ic-op-amp-jfet-low-noise-dip8/dp/1103001 it's TL701CP. I'm going to go with the BHC caps from Farnell as they have solder terminals and I'm concerned about damaging the original very hard wiring, I guess it's tinned inside the outer when it was originally soldered which will make using the screw terminals awkward.

[Oldpinkman]

Apologies if this repeats stuff - I have skimmed through.

Op amps are an obvious easy upgrade - generally the upgraders rank soldered as a "super" upgrade compared to socketed. Not sure I am persuaded on either front, but most choose to solder

I have soldered the spades, and whilst its hard to A:B back and forth on this, we all did it at PT 20 years ago, and so had the chance to compare then. I suspect the benefit is "new and clean" so just unplugging and plugging back may have the same effect.

If its old, it probably wants a complete recap. In particular C5 on the board that gets hot has a life of less than 10 years, and you will get a lot of hum if it is below optimal

Lowering the input gain significantly improves the SNR and is fairly straightforward

Deal with the wiring loom, which scrunches signal cables up with mains. Run new well-shielded signal cables direct from the board to phono sockets on the chassis, in place of the DIN arrangement, and route them away from the mains cables.

Beyond that modifications focus on the accuracy of the bridge, and the electrolytic in the signal path - but I don't know the detail - I have a "man what does" for me.

It's a great amp when well sorted

[DarrenHW]

Apologies if this repeats stuff - I have skimmed through.

Hi Richard, thanks for posting :thumbsup:




Op amps are an obvious easy upgrade - generally the upgraders rank soldered as a "super" upgrade compared to socketed. Not sure I am persuaded on either front, but most choose to solder

I have soldered the spades, and whilst its hard to A:B back and forth on this, we all did it at PT 20 years ago, and so had the chance to compare then. I suspect the benefit is "new and clean" so just unplugging and plugging back may have the same effect.

I intended using sockets to prevent any potential damage to the OP Amp/s or board due to repeat soldering and to facilitate relatively easy A/B testing. It had crossed my mind that adding a socket in the signal path would be less than ideal and once the final OP Amp is decided upon I'll remove the socket and solder directly.


If its old, it probably wants a complete recap. In particular C5 on the board that gets hot has a life of less than 10 years, and you will get a lot of hum if it is below optimal

Lowering the input gain significantly improves the SNR and is fairly straightforward

Deal with the wiring loom, which scrunches signal cables up with mains. Run new well-shielded signal cables direct from the board to phono sockets on the chassis, in place of the DIN arrangement, and route them away from the mains cables.

Beyond that modifications focus on the accuracy of the bridge, and the electrolytic in the signal path - but I don't know the detail - I have a "man what does" for me.

I do also intend on doing this most likely with one of the DADA kits once the 405 is fully operational again.


It's a great amp when well sorted

So it would seem, there's certainly a lot of love for it on here. Where did you go with your mods / do you have an opinion on the Dual Mono arrangement?

[Light Dependant Resistor]

To my ears the best sounding op amp for the 405 is the Analog Devices AD847, give it a try.

[DarrenHW]

To my ears the best sounding op amp for the 405 is the Analog Devices AD847, give it a try.

Thanks for the suggestion, would this one be appropriate (http://uk.farnell.com/analog-devices/ad847arz/ic-op-amp-50mhz-300v-us-8soic/dp/2305605)? Is it a drop in replacement or does it require any modification to the circuit?

[The Barbarian]

As this

http://www.alibaba.com/product-detail/Dual-to-Mono-Op-amp-module_658220252.html

[DarrenHW]

As this

http://www.alibaba.com/product-detail/Dual-to-Mono-Op-amp-module_658220252.html

Thanks for the link Andr'e, forgive my ignorance but is this required to fit AD847 and if so can you tell me why? Is it because it's SOIC not DIP, apologies if this is a stupid question.

Edit: I've googled this now, I didn't realise OP Amps came in different shapes and sizes :doh:

[spendorman]

These look interesting, anyone any experience of them?

Item No. 321493959780

http://www.ebay.co.uk/itm/QUAD405-CLONE-MJ15024-stereo-100W-100W-boards-kit-heatsink-2pcs-/321493959780?pt=Vintage_Electronics_R2&hash=item4ada888464

[The Barbarian]

Daz:
You can get it in 'S08' & 'PD8' packages

[Light Dependant Resistor]

Hi
Yes you will need the DIP8 version of the AD847
http://uk.farnell.com/analog-devices/ad847jnz/ic-op-amp-high-speed-dip8-847/dp/9604707

as this will then fit in turned pin sockets replacing the TL071, just observe correct orientation that Pin 1 of the op amp usually with a small round indent on the top case, is Pin 1 on the socket.

[The Barbarian]

One of the reasons the old Barbarian sticks to the '303' is it has no 'IC's

:carrot:

[spendorman]

One of the reasons the old Barbarian sticks to the '303' is it has no 'IC's

:carrot:

lol! I was going to mention the 303, but didn't. Can anyone tell me why I have four of them!

[Oldpinkman]

Hi Richard, thanks for posting :thumbsup:




So it would seem, there's certainly a lot of love for it on here. Where did you go with your mods / do you have an opinion on the Dual Mono arrangement?

Mine is recapped, Burr browns, input gain lowered and bridge modified - I Know not the details, except that OJ commented most of the mods that think they take the electrolytic out of the signal path don't.

My opinion on the dual mono would be valueless. OJ commented that if it made a difference (he thinks not) it needs doing properly with separate cores, not a shared core with separate windings. His view is that all of the current supplied to the amps is from the reservoir capacitors - the transformer is only topping those up, and so there is no issue. He's a bright lad. I rely on him. ;)

[DarrenHW]

Hi
Yes you will need the DIP8 version of the AD847
http://uk.farnell.com/analog-devices/ad847jnz/ic-op-amp-high-speed-dip8-847/dp/9604707

as this will then fit in turned pin sockets replacing the TL071, just observe correct orientation that Pin 1 of the op amp usually with a small round indent on the top case, is Pin 1 on the socket.

Thanks for the link and the clarification, I think I'll give them a go :thumbsup:


Mine is recapped, Burr browns, input gain lowered and bridge modified - I Know not the details, except that OJ commented most of the mods that think they take the electrolytic out of the signal path don't.

Do you know if your OP Amps are OPA604?



My opinion on the dual mono would be valueless. OJ commented that if it made a difference (he thinks not) it needs doing properly with separate cores, not a shared core with separate windings. His view is that all of the current supplied to the amps is from the reservoir capacitors - the transformer is only topping those up, and so there is no issue. He's a bright lad. I rely on him. ;)

Who's OJ?

[spendorman]

If you are going to do some, or all of these mods, I would still suggest making sure that you have a full working amp to start with.

I think that I have read that with the 405 and 405-2, it is easy to end up with a badly oscillating amplifier. This will not do your speakers any good!

Still, in 1976 I bought a 405 brand new, not long been out. It was the first powerful (then) amp that I had owned. It was fed into speakers that I built myself to a Hi Fi News design by Martin Colloms, Mini Monitors. I followed the design, KEF B200, Celestion HF1300 and KEF T27, apart for the supertweeter, I used Coles 4001G in place of the T27, as I had some already.

The Colloms Mini Monitor is inefficient and I had been driving them with Quad II's, nice, but not quite enough power. So it was great to have the 405 with 100 W RMS instead of the 15 W from the II's.

Well, one of the B200's rapidly burnt out!

[DarrenHW]

If you are going to some some, or all of these mods, I would still suggest making sure that you have a full working amp to start with.

Agreed, first step will be new smoothing caps and I'll go from there.


I think that I have read that with the 405 and 405-2, it is easy to end up with a badly oscillating amplifier. This will not do your speakers any good!

Is there anyway to test this without specific equipment?

[Marco]

Hi Darren,

Give me a shout, mate, when you have something sorted to listen to. I’m intrigued by the Quads :)

Marco.

[spendorman]

Well, It is possible that a digital multimeter connected to the output will show a largish AC voltage if the amp is oscillating.

[spendorman]

Has your 405 got the small Crowbar PCB mounted on the back of the speaker sockets? Most did, but I think a few very early amps may not have them.

[walpurgis]

Good amps the 405/405-2. Underrated by many. I've heard them sound remarkably good in the right system. Obviously they sound just fine driving Quad 'Statics and they seem capable of driving just about anything. They are far more open, punchy and lucid than the dear old 303.

Often considered keeping one as a 'back-up' amp to drive tricky loads.

[tubehunter]

Hi all

I use a pair of these most excellent amps mono blocked (just one board in each case).
This gives the best set-up imo, a double c transformer with F&T capacitors ( http://www.rapidonline.com/electronic-components/22000-f-10-30-63v-screw-terminal-mounting-bolt-aluminium-electrolytic-capacito-50-4012 ) for the power supply to each board.
If you change the op-amp to a modern one, make sure you change the zenner diodes to 16v ones and bypass the supply with some 0.1uf caps, close to the pins.
I have bypassed the zenners with a 100uf cap and a 1uf film cap seems to work well.
You will need to replace the existing electrolytic capacitors on each board.

Looking at your pictures you seem to have a very early model possibly with no loudspeaker protection.:eek:
With my own 405s I've fitted Velleman K4700 speaker protection boards, they just fit in front of the transformer and back panel.
They give you a 6 second delay on start up and shut off immediately on power down so no nasty pops through the speakers which can appear after changing the op-amp as they'll draw more current.

Duncan

[spendorman]

Hi all

I use a pair of these most excellent amps mono blocked (just one board in each case).
This gives the best set-up imo, a double c transformer with F&T capacitors ( http://www.rapidonline.com/electronic-components/22000-f-10-30-63v-screw-terminal-mounting-bolt-aluminium-electrolytic-capacito-50-4012 ) for the power supply to each board.
If you change the op-amp to a modern one, make sure you change the zenner diodes to 16v ones and bypass the supply with some 0.1uf caps, close to the pins.
I have bypassed the zenners with a 100uf cap and a 1uf film cap seems to work well.
You will need to replace the existing electrolytic capacitors on each board.

Looking at your pictures you seem to have a very early model possibly with no loudspeaker protection.:eek:
With my own 405s I've fitted Velleman K4700 speaker protection boards, they just fit in front of the transformer and back panel.
They give you a 6 second delay on start up and shut off immediately on power down so no nasty pops through the speakers which can appear after changing the op-amp as they'll draw more current.

Duncan

Well spotted about the lack of speaker protection, I asked if this unit had it, but I failed to look at the photo.

[DarrenHW]

Hi Darren,

Give me a shout, mate, when you have something sorted to listen to. I’m intrigued by the Quads :)

Marco.

Hi Marco,

Will do and once the 405's up and running it'll be time for some shopping :eyebrows:


Has your 405 got the small Crowbar PCB mounted on the back of the speaker sockets? Most did, but I think a few very early amps may not have them.

It doesn't look like it.

13124

[DarrenHW]

Hi all

I use a pair of these most excellent amps mono blocked (just one board in each case).

This is also the direction I'd like to go.



This gives the best set-up imo, a double c transformer with F&T capacitors ( http://www.rapidonline.com/electronic-components/22000-f-10-30-63v-screw-terminal-mounting-bolt-aluminium-electrolytic-capacito-50-4012 ) for the power supply to each board.

Thanks for the link, they're beasts! I had seen 22,000 used on the Keith Snook site but not really seen them used anywhere else, definitely food for thought (just as I thought I'd finalised my shopping list :doh:) did you notice a difference from using 10,000's?


If you change the op-amp to a modern one, make sure you change the zenner diodes to 16v ones and bypass the supply with some 0.1uf caps, close to the pins.
I have bypassed the zenners with a 100uf cap and a 1uf film cap seems to work well.
You will need to replace the existing electrolytic capacitors on each board.

I think this is the modification covered by the DADA kit?


Looking at your pictures you seem to have a very early model possibly with no loudspeaker protection.:eek:
With my own 405s I've fitted Velleman K4700 speaker protection boards, they just fit in front of the transformer and back panel.
They give you a 6 second delay on start up and shut off immediately on power down so no nasty pops through the speakers which can appear after changing the op-amp as they'll draw more current.

Duncan

Yes it is early dated 1976, serial number 1170. I've just checked out the Velleman K4700, they seem put a lot in the signal path did you notice any degradation in SQ compared to the crowbar clamp?

[spendorman]

A few years ago I needed a Quad 405 crowbar board, I think it was £25 from Quad, not absolutely sure though.

[DarrenHW]

Looking at your pictures you seem to have a very early model possibly with no loudspeaker protection.:eek:
With my own 405s I've fitted Velleman K4700 speaker protection boards, they just fit in front of the transformer and back panel.
They give you a 6 second delay on start up and shut off immediately on power down so no nasty pops through the speakers which can appear after changing the op-amp as they'll draw more current.
Duncan

Thanks for highlighting this Duncan, after a bit of googling it would seem no speaker protection would be a very bad idea and the Velleman is a popular mod for the 405's :youtheman:.


A few years ago I needed a Quad 405 crowbar board, I think it was £25 from Quad, not absolutely sure though.

Thanks for the info Alex, it would seem the Velleman offers better protection and it's pretty cheap too :).

[DarrenHW]

It took a few weeks to get the parts ordered and delivered but the 405 is now serviced and lightly modded.

13225

I ordered the F&T capacitors and Velleman kit recommended by Duncan (tubehunter). The capacitors are a perfect fit once the threaded bar is cut off and at 22000uF they're a major upgrade to the 10000uF stock value. The Velleman could have been made to measure for the 405 it slips in nicely behind the transformer, it took me about 3 hours to assemble the kit and it's only about £15 more to buy it pre-assembled so I'll probably order the next one assembled.

I ordered the Dada Upgrade / Revision kit with OPA604, it, in conjunction with their instructions is pretty good but I did still have to refer to the service manual where the instructions were unclear. I didn't use the supplied binding posts as these seem to be designed for use with the crow bar and didn't include any tags to solder onto. This wasn't a major problem as I intend to replace the signal cable (most likely with Van Damme Blue 2.5 as this is what I'll be using for speaker cable) in the future so will install new binding posts at that time. I also ditched the included shielded audio cable for Klotz MC5000 as this is what I'm using for inter connects, it was a bugger to use in the confines of the amp housing, I'll have to revisit this and will probably solder direct to the board once all work is completed.

I've removed the voltage selector and hard wired the windings to 240v. The original wire was replaced with 1.5mm and I'd like to increase this further but will have to replace the IEC socket and fuse holder to allow for a larger wire, I will be installing a 4mm ring main and using 4mm flex so would also like to increase the internal wiring to 4mm but I'm not sure if I'll be able to get it in there?

The 405's been up and running for 2 days now and I'm very happy with it, I built new MC5000 / MS Audio interconnects at the same time so everything's still burning in and offers me the perfect excuse to spend the weekend on my backside getting on the outside of Germany's finest beer :cool:.

Now the 405's a proven working amp my next step will be to further rework the boards (there were a few crappy looking resistors and one board's had a previous repair with some replacement diodes) inline with the Keith Snook mods, look into replacing the bridge, transistors and possibly a bit of OP AMP rolling, then source another 405 to convert both into mono blocks.

Thanks for everyone's help and advice :thumbsup:

[DarrenHW]

It's taken some time but I've eventually made some progress. I've sourced a second 405 and have fitted a board in each:

http://i1324.photobucket.com/albums/u606/drtwas/IMG_5182_zpsizsxkw41.jpg (http://s1324.photobucket.com/user/drtwas/media/IMG_5182_zpsizsxkw41.jpg.html)

This has been a major improvement over the stock 405, stereo separation has improved significantly which along with the reduction in back ground noise has resulted in a far larger and immersive sound stage. There's a lot more detail coming through and the improvement in bass weight and control is like listening to a different amp.

I still have some work to do, I've not reworked the mains wiring as I'm still looking for a solution to incorporate an IEC socket and switch without cutting into the case, one this is done hopefully I can find a good place to secure the Velleman. I now have a spare brace of boards so will now focus on modifying these using Keith Snook's schematics.

The question is, are these considered mono blocks? I know the standard Quad 405 mono block bridges the two channels which I clearly haven't done, but surely these still constitute mono block, if not what are they?

[Firebottle]

As it's only one channel (mono) with it's own power supply it's got to be a monoblock.

Sounds like a very good upgrade with the improved separation and detail. Keep us posted on further listening :thumbsup:

:cool: Alan

[DarrenHW]

Thanks Alan, that was my thought too.

[tubehunter]

Looking good, but you have the boards in the wrong side.
How are you going to fit the other two capacitors next to the existing ones? :) and if you do you'll need new 35amp rectifiers.
When you fit the MJ15003 NPN 140V 20A 250W output transistors you can remove the limiters which brings worth while gains with driveablity into low impedance loudspeakers and sound quality at all levels.

[DarrenHW]

I didn't know which side to fit the boards, I thought the other side might be a better (quieter) location so thought it might be a better place for the Velleman (I'm still not sure if that makes sense, I thought it might be more sensitive to interference than the Quad board?) and keeping the boards there keeps wire length to a minimum? I had hoped to fit the Velleman where one of the boards was but can't find a way without manufacturing a bracket.

Are you serious about board location and do you really have 4 x 22000 uF caps fitted :eek:?

I'm looking forward to hearing what the KS mods bring.