This is one of the few beocentres you can do this.
The Beocentre 3500 was a Beomaster 3000 with a turntable cobbled on, quite effectively as it turns out.
This shows up because the Beocentre still has the external turntable connections that belonged to the original Bemaster 3000.
On the back of the Beocentre 3500 you will see two connections for turntables, Q1 and Q2, for differant signal levels, you will see that one is occupied by a DIN plug, (I think its Q1) and this is a jump lead to make the bridge between the original Beomaster 3000 design and the 3500.
The front panel has switches for Phono 1 and Phono 2, where Phono 1 goes to teh Q1 connector and Phono 2 goes to the Q2 connector.
All you do, is to plug the Q1 jump lead into your extra amplifier, easy as that, and of course you would need to set your Beocentre to run Phono 1 to get the turntable to power up, it wold probably be a good idea to ensure both sets of speakers are off