Wednesday, November 16, 2005

Quad 405 Tansformer rumble ....

Some older 405's have a problem with the transformer. It produces a rumble (not in the speakers but in the transformer itself).

There is no way to fix this problem except by replacing the transformer.

Of course you can order a new transformer from Quad but this is a quite expensive solution. Therefor this alternative: one can fit a toroidal transformer (300VA 2x 35V). This also offers the advantage of better stability and somewhat less noise, limiting the cost to about 90 Euro.

The only problem: to fit it in you will need a special mounting plate. I found one on Ebay in the UK, Stuart Adamson had them made by a steel manufacturer. I also had to move the fuse because it was in the way (see the picture).

But now my 405 works fine again ;-)

PS. The transformer you need is a Toroidal 2x 35 or 2x 36 Volt, 300 VA (not less!). Make sure it will fit in.


A 300VA toroïdal transformer and the mounting-plate are now available in the Dada Electronics Webshop.


Anonymous Anonymous said...

Hello, what is the good voltage for a new toroidal transformer?


3:25 AM  
Blogger Stefaan said...

The ideal voltage of the transformer to get +/- 50 Volt, as foreseen in the original design is:

50V / squareroot(2) = 35,36 Volt.

So 2 x 36 Volt @ 4 Ampere will do perfect.


9:38 PM  
Anonymous Anonymous said...

Merci Stefaan,


2:11 AM  
Blogger KostisK said...

I have a modified 405 with net audio dual psu. My concern is whether the physical dimensions of the toroidal transformer aren't compatible ? Any opinions ?

2:41 PM  
Blogger Joost Plugge said...

Hello Kostis,

If you use a single new transformer to connect the double Psu, you will lose some advantage of the double supply. But it is possible. It will sound better than the standard completely single supply. The best option is two smaller transformers to connect the double supply. Two transformers of 2x36V 2 amps.

Regards Joost Plugge

12:44 PM  
Anonymous Anonymous said...

I wonder if you know what VA rating the original 405-2 transfomer has? (300VA?) Is the transformer in the 405-1 the same as the 405-2, BTW?

6:25 PM  
Blogger Stefaan said...


The original 405 transformer is a double 2x35V 2A transformer so this makes 280VA in total.

The 405-1 and the 405-2 have the same transformer.


6:32 PM  
Anonymous Anonymous said...


Thanks for this useful post, I have transformer noise in my 405-2 :-(

You say 2 transformers would be better for a dual supply.

I can only find:

2x35 120Va 1.71A
2x35 160Va 2.29A

Would a pair of either do the job and would it be better to have the 1.71 or 2.29 amps in the 405.



3:57 PM  
Blogger Stefaan said...

Hi, Joel,

2x 120VA is not enough, it would limit the maximum power and/or the transformer might be overloaded.
2x 160VA is a better idea if you can find them small enough to fit.


4:05 PM  
Anonymous Anonymous said...

Great Post Guys, I had the same problem and I used the transformer from Amplimo 78018 (300 VA) cost : 66 euro.

With two simple tie reps it is fixed in the chasis

6:56 PM  

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