Why do i need apre-amp & what does it do?

Quad 405 is a very fine power amp. But its minimum sound input option will be around 500 millivolts and it probably only has one input and no volume or tone controls.

The matching Quad 33 preamp was often paired to a 405 and contains those options -- volume and tone, filters, input switchable between phono, tuner, aux, cd and a low level out put to feed to a tape deck. Most importantly at the time when these amps were produced, the preamp contains a circuit which boosts the tiny output from a turntable's pickup cartridge to more usable levels and applies equalisation.


There are other preamps which will work with a Quad power amp, and the 33 I mention is not the best performing Quad you can match to yours -- I believe the later 44 model was better sounding.

I want to connect Quad 405 to a PC (HTPC). I want to use the volume control of the PC, so I don' t want a large pre-amp. Will a small phono pre-amp do?

If you want to connect it ONLY to your PC you can do so directly without a preamp. The pc volume control may not be convenient i.e. it gets loud to fast so you may need to adjust the levels of the sources in the PC audio control panel to compensate.
The phono preamp mentioned is for the playback of vinyl LP records with a turntable and would be completely useless to you. You could use a passive volume control instead of a preamp.
If you wanted to connect other sources such as TV you would need a preamp to switch between the them. The preamp would also provide more input to the amp that would give you more maximum volume and better dynamics.
Try direct first. Connect with the amp unplugged from the ac and make sure you have the volume all the way down before you start. Bring the volume up slowly Do not use the EQ functions except with great care as too much EQ can damage speakers.

So I don't need to worry that the signal is to low without a preamp... but I should that the signal is to high and that it may damage my quad 405 of my speakers?

In that case, would it be wise to solder a pair of resistors in the cable connecting the pc to quad 405? Or should I take any other measure to minimise the risk of not using a preamp?