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


Monday, April 10, 2006

Upgrading the 33 power-supply voltage

Why is it useful to increase the power-supply voltage and how to do it?

With thanks to Ruud Janssen - see the link on the left in the link-table.

Originally the power-supply voltage of the 33 was 11 Volt. This choice was made at the time to allow for smaller capacitors, bigger ones wouldn't have fit in. As modern capacitors today are a lot smaller there is no problem upgrading the power-supply to 16 Volt and at the same time increase the stability by using larger caps.

This will decrease distortion, specially if the output-signal is 0,5 Volt or more and also increases the maximum voltage on the inputs with 3,2 dB to 2,9 Volt with a Total Harmonic Distortion of less than 0,1%, making the preamp better suited for a CD-player.

You will have to remove the power-supply from the set (some soldering and 2 screws).

Replace R500 and R501 for 28 Ohm resistors, replace the zener-diodes with 16 Volt 1/4 Watt. The capacitors C500 and C501 become 4700 MF 20 Volt, C502 becomes 1000 MF 25 Volt (axial) and the diodes MR501 and MR502 are replaced with 1N4002.

Don't forget to replace R300 on the Phono-preamp with 1500 Ohm when you upgrade the power-supply tension. This is necessary to keep the supply-rail in the Phono-preamp at 10,5 Volt, which gives the best performance and the lowest noise.

And you're done ;-)