Preamp hum.

[mschwilson]

Hi.
I use a Perreaux PMF2150B power amp with matching SM2 pre.
I have two slight problems.
The first is that when the pre is on top of the amp,I get loads of hum.
I got round this by moving the pre away,but I would like to know if there is a way to get rid of it rather that just make do.
The transformer is a huge block type,and the top of the amp is made of what appears to be some kind of cardboard with cork on the inside(search me why).
Anyway,the second problem is that when switching the amp on,it "sucks" the bass cones of my speakers in(quite normal)but a grating noise,rather like the noise you get from a noisy volume pot comes from the cones.
I recently re-coned the drivers and know they are pretty much perfect,and have also tried other speakers with the same result.
This is only brief and after the initial surge,cannot be heard again.
The amp works perfectly and I hammer it regularly,but is this a sign of something about to give in.
Please help.
Thank's,
Simon.

[AndrewT]

Hi,
the amp is not working perfectly but it may be working normally.

It has a large DC output offset at start up, which is pretty common.
Most manufacturers will either eliminate the temporary offset or fit an output relay to hide the symptom.

You will find it easier to hide it.

Your reconed speakers are not pretty much perfect.
From your description it sounds like the voice coil is scraping against the pole piece or top plate. This is probably due to an offset in the voice coil from true geometric centre.

Recheck your cone setup. If glued at both surround and spider then you have to fathom out a fix. If it is bolted at one or both you should/may be able to recentre the voice coil more accurately.