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Location: North Island New Zealand
Posts: 1,757
I'm Chris.
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
Posts: 18
Had a quick look on the Quad Spot website and found an interesting piece about floating earths;
http://quadrevisionspot.blogspot.co....-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/3216793081...%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 )
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
Location: North Island New Zealand
Posts: 1,757
I'm Chris.
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
Posts: 18
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
Posts: 18
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
Posts: 18
If you scroll down the page linked below, I believe reference is made to these boards;
https://quadrevisionspot.blogspot.co.uk/
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.
Barry
Posts: 18
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