Rebuilt Quad 405 susceptible to mains noise - any suggestions?

[ImpMan71]

Hi,

Last year I bought new boards for my 405 off the online auction site and completely rebuilt them to the new Dada spec.

I also recapped the PSU and used new bridge rectifiers, changing the supply to now be wired as dual mono. I have used lots of smoothing caps across both the secondary and primary windings of the transformer and also across the main smoothing caps, but to no avail!

I'm really pleased with the sound from the new boards, however it is now plagued by pops through the speakers when any lights or anything is switched on around the house (fridge cutting in and out etc.).

It did not used to do this and my Lecson AP1 and Meridian 103D do not suffer from this.

Please can someone advise a remedy?

Many thanks,
Colin

[RothwellAudio]

What is "the new Dada spec"?
I don't like the sounds of smoothing caps across the primary and secondary windings of the mains transformer - you have to be very careful about caps across the mains.
The pops could be caused by a burst of RF being picked up by the amp or by a spike on the mains. Unfortunately tracking down the reasons behind faults like this can be very difficult. First off, could you post a circuit diagram of the changes you made?

[spendorman]

All the standard Quad amps that I have used, II's, 303, 405 and 405-2 have been pretty well immune to mains noise.

Were your boards the Chinese ones, the Dada ones, or something else?

[Light Dependant Resistor]

I would simply rewire it to return it to exactly the way Quad made it.

A 405 schematic shows the secondaries to be a parallel arrangement in each half with 4 secondary windings
about a grounding centre tap. I suspect you have ended up with unequal voltages or unequal halves
and problems arising from the centre tap being displaced slightly. Quad were concerned with how earthing
was achieved with the 405, hence R2 on the original schematic separating power supply earth from chassis earth.
Back to back diodes probably would have been a better choice.

You should have DC rails at 52 volt positive, and 52 volt negative.

As other contributors have pointed out capacitors across mains primaries also involving a transformer is not a good idea.

The biggest fault with the 405 was mediocre zener supplies to compound what was a fairly cheap op amp installed.
Those who had owned 303's up to 1976 must have wondered what Quad was up to releasing the 405, as the 303 was
dead quiet, and some would say still is, their stand out amplifier

A standard 405 is always noisy, usually on its left channel. A LM6171 op amp is a nice device to replace the 301 or 071 device.
A set of Net Audio Mk 3 boards should be on your shopping list, once the transformer is wired back to normal.

[spendorman]

"Those who had owned 303's up to 1976 must have wondered what Quad was up to releasing the 405, as the 303 was
dead quiet, and some would say still is, their stand out amplifier"

I certainly did, have had a 303 for years, then bought the 405 just after it was released, didn't find it noisy, but did not really like the sound. It sat on the shelf for years until the 405-2 came out. It used TL071's, so bought a couple for a few pence, put them in the 405, and it transformed it. Later bought the -2 boards from Quad, didn't really notice any difference.

A few years ago replaced the 071's with some Burr Brown op-amps (forget the number), and modified the zenner circuit feeding the op-amps, not sure if I can detect any further improvement.

Amp is still quiet though. I did fit a push button on-off switch on the back panel in place of one of the rubber grommets. Very useful.

[RothwellAudio]

This page has some very good information and would be a good place to start
http://www.keith-snook.info/quad-405-mods.html
His mods are quite extensive and may be more than you want to take on, but the page is certainly worth a read.
BTW, I still don't know what the Dada mods are.

[Light Dependant Resistor]

This page has some very good information and would be a good place to start
http://www.keith-snook.info/quad-405-mods.html
His mods are quite extensive and may be more than you want to take on, but the page is certainly worth a read.
BTW, I still don't know what the Dada mods are.

Yes makes an interesting read, but there only so much that can be done to original boards. Layout on boards
as we both know can be critical, and whilst Keith suggests some good ideas, it always comes back to
modifying a fairly well designed board that has worked reliably the way Quad intended.

I would instead invest in the Net Audio Mk3 boards and be happy - as you won't ever get anything better
for a 405.

[DSJR]

Net Audio basically re-design the boards I gather and I'm deeply suspicious of one or two of their re-designs... Dada keep the original, but just update caps and so on which would be pretty knackered by now anyway. Not sure the 405 op-amps need to be as way-out as the ones suggested as too wide a bandwidth can apparently cause oscillations the crude originals miss. The OPA134 seems to be a good more modern compromise and still goes out to a few MHz.

Check earthing again - A 405 isn't designed to be earthed directly to the mains as originally conceived. The case is tied to the signal returns and the amp is usually 'earthed' to the preamp, as is the 303 I have.

Dada have a great friendly forum for just these issues. Well worth asking.

[RothwellAudio]

Check earthing again - A 405 isn't designed to be earthed directly to the mains as originally conceived. The case is tied to the signal returns and the amp is usually 'earthed' to the preamp, as is the 303 I have.
Yes, I suspect the fault has something to do with "earth" or the signal 0V. It's a much more complex subject than is often appreciated and changing any of the wiring can have unintended consequences. The amp performed without susceptibility to pops from switched lights before the mods so there should be no need to change entire boards - just get the 0V wiring right.

[ImpMan71]

Hi,

Thanks for all the replies - I believe this all comes down to earthing and I need to study it more closely. From memory I have taken the earth from the iec socket to the chasis (but I'm sure this is at it was before?) I will have a look on the Dada forum as well, thanks Dave.

I tried dropping the earth from the plug top anti surge and this resolves another earthing issue that occasionally happened, which is the left channel hisses and distorts, but goes away when the volume is turned up.

When I swapped the boards over it stays on the left, so has to either be a PSU or earth issue.

The boards are Chinese built with all new components, mainly from Farnell, or the obscure inductors from Dada.

I think it is an exact copy of the Dada V2 board (photo attached) which are considerably different from both Quad 405 MKI and MKII original boards.

I'm going to have to put the 103 back in the main system and get inside the Quad again.

I have used it with a Meridian 101 before rebuilding it and not had any issues.

It is unlikely I will be able to take a look until the weekend now. I am going to remove the caps across the transformer and see if this helps; I cannot remember now where I saw this recommendation (will have to search through my browsing history) but these have been on since I first did the rebuild, so hopefully may be the cause.........

[Arkless Electronics]

Hi,

Thanks for all the replies - I believe this all comes down to earthing and I need to study it more closely. From memory I have taken the earth from the iec socket to the chasis (but I'm sure this is at it was before?) I will have a look on the Dada forum as well, thanks Dave.

I tried dropping the earth from the plug top anti surge and this resolves another earthing issue that occasionally happened, which is the left channel hisses and distorts, but goes away when the volume is turned up.

When I swapped the boards over it stays on the left, so has to either be a PSU or earth issue.

The boards are Chinese built with all new components, mainly from Farnell, or the obscure inductors from Dada.

I think it is an exact copy of the Dada V2 board (photo attached) which are considerably different from both Quad 405 MKI and MKII original boards.

I'm going to have to put the 103 back in the main system and get inside the Quad again.

I have used it with a Meridian 101 before rebuilding it and not had any issues.

It is unlikely I will be able to take a look until the weekend now. I am going to remove the caps across the transformer and see if this helps; I cannot remember now where I saw this recommendation (will have to search through my browsing history) but these have been on since I first did the rebuild, so hopefully may be the cause.........

You may have bigger fish to fry here.... what you describe with the left channel could well be due to ultrasonic oscillation. I hesitate to recommend any cure as this is something where one really needs to be there... and with test gear etc....

[Light Dependant Resistor]

Hi,

Thanks for all the replies - I believe this all comes down to earthing and I need to study it more closely. From memory I have taken the earth from the iec socket to the chasis (but I'm sure this is at it was before?) I will have a look on the Dada forum as well, thanks Dave.

I tried dropping the earth from the plug top anti surge and this resolves another earthing issue that occasionally happened, which is the left channel hisses and distorts, but goes away when the volume is turned up.

When I swapped the boards over it stays on the left, so has to either be a PSU or earth issue.

The boards are Chinese built with all new components, mainly from Farnell, or the obscure inductors from Dada.

I think it is an exact copy of the Dada V2 board (photo attached) which are considerably different from both Quad 405 MKI and MKII original boards.

I'm going to have to put the 103 back in the main system and get inside the Quad again.

I have used it with a Meridian 101 before rebuilding it and not had any issues.

It is unlikely I will be able to take a look until the weekend now. I am going to remove the caps across the transformer and see if this helps; I cannot remember now where I saw this recommendation (will have to search through my browsing history) but these have been on since I first did the rebuild, so hopefully may be the cause.........

Hi
As far as I knew the Chinese 405 clones are always copies of series 1 405's , but if they are now copying Dada boards lets see exactly what you have there.
Hence referencing the exact schematic and the actual physical board and what has been modified, will help us help you. If you can make a drawing and
provide images of what changes you made, we will all be a bit clearer.

What I referred to earlier, due to changes you made with transformer, imbalance now in the transformer earthing is quite likely as well as signal
earthing which others have alluded to.

Cheers / Chris

[ImpMan71]

Had a quick look on the Quad Spot website and found an interesting piece about floating earths;

http://quadrevisionspot.blogspot.co.uk/2007/01/installation-instructions-405-dual-psu.html

I know my earths are bolted uninsulated to the heat sink as well as the case, so will address this at the weekend, when I remove the caps from the transformer.

I'm afraid I do not have a schematic. My electronics knowledge is limited and the thing I like about Quad 405's is it is a bit like painting by numbers as the boards are so clearly marked. I was supplied with a parts list which had the component locations on which I followed to the letter.

The boards are still for sale on Ebay;

http://www.ebay.co.uk/itm/321679308112?_trksid=p2060353.m1438.l2649&ssPageName=STRK%3AMEBIDX%3AIT

I am convinced this issue is not with the boards, as stated earlier it stays on the left even when the boards are swapped over (and this issue goes away when the earth is disconnected, just the the pops remain which are driving me nuts :doh:)

It has been working fine (apart from the popping) since last March, I'd just really like to iron this out!

I did make several changes to the original MKI boards as per Keith Snook's website (op amp, zenner voltage increased, new caps, input gain reduced) and a few other bits) successfully, but then got a bit too ambitious and got something very wrong - vapourising some tracks on the board and wiping out the output transistors. I managed to undo this mistake and take it back to where it previously was as I was unable to see what I had done wrong.

It worked fine till about 2 years later when the same channel was cooking hot! This is why I went ahead with brand new boards and components.

Cheers,
Colin

[Light Dependant Resistor]

Had a quick look on the Quad Spot website and found an interesting piece about floating earths;

http://quadrevisionspot.blogspot.co.uk/2007/01/installation-instructions-405-dual-psu.html

I know my earths are bolted uninsulated to the heat sink as well as the case, so will address this at the weekend, when I remove the caps from the transformer.

I'm afraid I do not have a schematic. My electronics knowledge is limited and the thing I like about Quad 405's is it is a bit like painting by numbers as the boards are so clearly marked. I was supplied with a parts list which had the component locations on which I followed to the letter.

The boards are still for sale on Ebay;

http://www.ebay.co.uk/itm/321679308112?_trksid=p2060353.m1438.l2649&ssPageName=STRK%3AMEBIDX%3AIT

I am convinced this issue is not with the boards, as stated earlier it stays on the left even when the boards are swapped over (and this issue goes away when the earth is disconnected, just the the pops remain which are driving me nuts :doh:)

It has been working fine (apart from the popping) since last March, I'd just really like to iron this out!

I did make several changes to the original MKI boards as per Keith Snook's website (op amp, zenner voltage increased, new caps, input gain reduced) and a few other bits) successfully, but then got a bit too ambitious and got something very wrong - vapourising some tracks on the board and wiping out the output transistors. I managed to undo this mistake and take it back to where it previously was as I was unable to see what I had done wrong.

It worked fine till about 2 years later when the same channel was cooking hot! This is why I went ahead with brand new boards and components.

Cheers,
Colin

Hi Colin

What a mess, Look at the schematic provided at the eBay site it is not representative of the boards also shown at the ebay site

Suggesting boards different to the schematic- a great place to start ! But lets take a closer look.

where for instance are Network 1 and Network 2 on the schematic Were you supplied with N1 and N2 ?
Per the schematic they have borrowed the bootstrap capacitor C10 from earlier schematics and
assigned the wrong value of capacitance just for good luck ! The resistors R20 and R21 which are part of the current dumping bridge,
per the schematic at the eBay site they are incorrectly connecting.

Now the boards you have ( if they are the same as the eBay site ? ) Following the traces for C12 which in proper boards is the zobel capacitor should
connect to R39 - but no it looks like on your boards it ends up at L2 bypassing R39 altogether joining in with a ground connection
L2 is part of the bridge and on your boards should not be grounding, Given there is no label for ground who knows ?
but and worse C8 part of the bridge connects straight to C12 then where does R38 also part of the bridge connect ? absolutely nowhere
as each side of the double sided board where it should connect to L2 is open circuit !!


Plus there were never any double sided circuit boards Quad made for the 405

No wonder you are having difficulty, these boards are a joke.

Make contact today to Quad, and order some 405.2 boards - as you are totally wasting your time on these boards.
http://www.quad-hifi.co.uk/contact.php

[ImpMan71]

Hi LDR,

Someone on another forum pointed me in the direction of these boards, as they look identical to the Dada v2 boards. The Ebay listing is worded wrongly as it is not a copy of a 405 MKII board which as you point out is single sided, but of the Dada V2 board (not sure if this is because of copyright).
Dada have recently launched a V3 board, so I'm struggling to now find anything on the V2. The pic I posted the other day, I saved off Dada's website last year. Below is a photo of my board.
I am going to look at the earthing over the weekend and let you know how I get on.

Cheers,
Colin

[ImpMan71]

Hi LDR,

Someone on another forum pointed me in the direction of these boards, as they look identical to the Dada v2 boards. The Ebay listing is worded wrongly as it is not a copy of a 405 MKII board which as you point out is single sided, but of the Dada V2 board (not sure if this is because of copyright).
Dada have recently launched a V3 board, so I'm struggling to now find anything on the V2. The pic I posted the other day, I saved off Dada's website last year. Below is a photo of my board.
I am going to look at the earthing over the weekend and let you know how I get on.

Cheers,
Colin

[ImpMan71]

If you scroll down the page linked below, I believe reference is made to these boards;

https://quadrevisionspot.blogspot.co.uk/

[Barry]

As Chris has observed, there seem to be several anomalies with the circuit diagram shown in the eBay listing. It looks as though it is based upon Quad's own drawing: QUAD 405-2 CIRCUIT DIAGRAM M12333 ISS 10, yet the thick-film networks N1 and N2 are absent.

Now it maybe that Dada have redisigned the circuit to avoid the need for this extra circuitry, and if you look on the Dada website and at photograps of the populated boards, I cant see their presence either. The Dada mods are an amalgamation of modifications proposed by Bernd Ludwig (1999) with those of Keith Snook.

Another anomaly is the suggested values of R4 and C2, for a given input sensitivity. The sensitivity of a stock 405 is 0.5v for full output, for which R4 has a value of 22k. The value on the circuit diagram shown in the eBay listing has it as 1.56k. Where does this and the other values for reduced sensitivity come from? Have Dada recalculated the values? I doubt it.

Finally there is some confusion regarding earthing, and the relation between signal ground and the chassis. Originally the 405 was intended to be used with Quad's own preamp, with chassis earthing provided by the screen signal earth back to the preamp. This arrangement was used in the Quad 22/II system. This was pretty standard at the time, but nowadays is not regarded as acceptable. However the idea was carried on with the Quad 33/303 system with the addition of direct chassis earthing made at the earth pin of the mains connector, as well as through the screen of the pre-amp connecting lead, with the two 'earths' being connected together at pin 2 of the 4-pin DIN connector. A potential earth loop can thus exist, but as others have commented no hum problems occur in practice. I have at some time used all the Quad amplifier systems up to Series 5, and have never experienced any difficulty.

With the later version of the Quad 405-2, which had RCA connectors fitted in place of the 4-pin DIN socket, the signal earth is separate from the chassis earth. The only connections made to the chassis are, according to Quad's circuit diagram M12333 ISS 10, the mains earth at the IEC connector, the centre tap of the transformer secondary windings, one end of C12 (the capacitor of the Zobel network), the 'black' speaker socket, and C15 and C16 (along with the R40, the current limiting resistor for the LED indicator). As such there ought to be no problems with earth loops.

I have read Dada's discussion of the problems when the two secondary windings are 'de-paralleled' so individual power supplies are used for the two channels, and I don't understand the problem. Surely the centre tap of the each of the two secondary windings will be earthed to the chassis as shown on Quad's schematic?

So all in all a bit of a mess. I greatly suspect the Chinese PCBs are to blame and would recommend you replace them with Dada's own and start again.

[ImpMan71]

Apologies for the delay in following this up!

I have rerouted the earths as per the recommendations on the dada website, which did not cure the issue.

Today I have introduced an IEC in line filter and also a small transformer after this - finally this issue appears to be resolved:)

Cheers,
Colin