Beomaster 5500, 6500 and 7000

6th January 2009

If you have owned a lot of B&O equipment then you will have probably noticed that thesse models give a slightly disappointing sound when used with passive loudspeakers when compared to some of the earlier designs. This is surprising as the power amplifier circuit is very similar to that of the Beomaster 2200, Beomaster 5000 and the Beocenter 7700 (amongst many others), all of which give very satisfactory performance.

The Beomaster 5500, 6500 and 7000 have a rather basic pre-amplifier circuit compared to that used in the original 5000 model but this is not the cause of the problems, the same arrangement is used in the Beocenter 9000 where it seems to work reasonably well. Therefore the power amplifier circuit was investigated to see what could be causing the trouble.

The only substantial difference between the Beomaster 5000 circuit and that used in the later models is the addition of four 220nF capacitors C215, C216, C415 and C416. They are not shown on all the Beomaster 5500 circuit diagrams but many sets have them. As these do not appear in any other versions of the circuit it seemed safe to remove them and see if the performance improved. With the capacitors in circuit the sound from one Beomaster 5500 was lifeless, bland and uninteresting, without them it was more similar to a 5000, engaging and lively with a full, rich tone.

It is not clear what the purpose of these capacitors is but performance definitely improves if they are disconnected. Before doing this make sure that the muting relay 3RL1 is in good condition (contact resistance less than 0.5R at rest) and that the output bias current for each channel is correctly set. [Beomaster 5500, Beomaster 6500, Beomaster 7000]