Wednesday, April 26, 2006

Velleman K4700 Schematics

Several people asked me to put the Velleman K4700 protection-circuit schematics on my blog. So here it comes.

This circuit is indeed the ideal replacement for the (lousy) protection-circuit in the original Quad 405. It does not only eliminate completely all power-on or -of "Plops" it also gives the best protection for the speakers against DC-current. The 405 has the nasty habit to put -50 Volt DC on the speaker-outputs if one of the power-transistors is broken...

I build this protection-circuit in most 405's and 405-2's I upgrade.

There is no use at all trying to "reconstruct" the Velleman protection-circuit as it is sold for +/- 20 Euro by Velleman with excellent components. See



Anonymous Anonymous said...

Is there any modifications required to the Quad circuit to de-plop it with the Velleman K4700 or is it a simple(relatively) disconnect and remove old protection circuit, fit in the K4700, and attach appropriate leads?

7:43 AM  
Blogger Stefaan said...


No modifications are necessary on the 405 circuit-boards.

In the 405-1 the Velleman-kit will replace the clamp-circuit that was mounted on the speaker-outputs, in the 405-2 (where the clamp-circuit is integrated on the boards) it can stay where it is.

Kind regards, Stefaan

1:02 AM  
Anonymous Anonymous said...

Second anonymous question.

The Velleman site suggests that the mains connection should be made after a Power-on switch. Most 405's don't have a power switch and are powered up from the controller (44 or 34). Should one be installed or should the mains connection just be made as is.

Thank you for your kind blog and your generous time.

3:27 AM  
Blogger Stefaan said...

I use an external power-switch for the complete installation.

Or you can use the switched power-outputs on most Quad preamps.


2:13 PM  
Blogger Nextage said...

Could you give the distance from the edge of the lid where to fix the Velleman K4700 ?
I think you have that distance written somewhere.
Thank you


10:30 AM  

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