Hello,

I want to add a subwoofer to my Beosystem 8000. I will be building the sub myself, using a plate amplifier I've had on hand for awhile. My slight predicament is that this amp has only line-level inputs, and the line-level outputs from the BM8000 are fixed level, of course.

I suppose I could use a line-level converter from the speaker level outputs of the BM, but I had a different idea that I thought might be a technically better solution.

I was thinking about breaking into the preamp outputs right before they go into the power amp boards, splitting off, and wiring those leads into the tape 2 output located at the flip-up connection terminals under the aluminum lid.

My question is; do you think the Beomaster's preamp will be able to drive this load? The input impedance of the subwoofer plate amp is 33k ohms.

Any help is appreciated. Thanks!

Jon

So you are planning to use the signal right before the poweramps, i.e the outputs of 2IC5? It is a LF353 / TL072 / µAF772 which have no problem driving such a load, and its only purpose seems to be buffering anyway.

The input impedance of the BM8000 poweramp appears to be considerably less in any case ( < 10 kohm) so you would not be altering the impedance seen by the buffer amps much. As you probably won't need the full signal swing (you'll need to adjust the sub volume anyway with some other means, I guess) I would add something like 10k-22k series resistors for these new outputs to further isolate them from the internals.

tournedos:

So you are planning to use the signal right before the poweramps, i.e the outputs of 2IC5? It is a LF353 / TL072 / µAF772 which have no problem driving such a load, and its only purpose seems to be buffering anyway.

I don't have a schematic in front of me, but I'll take your word on the IC number. Yes, that's correct - the signal right before going into the power amp modules.

tournedos:

The input impedance of the BM8000 poweramp appears to be considerably less in any case ( < 10 kohm) so you would not be altering the impedance seen by the buffer amps much. As you probably won't need the full signal swing (you'll need to adjust the sub volume anyway with some other means, I guess) I would add something like 10k-22k series resistors for these new outputs to further isolate them from the internals.

Cool, thanks! This is what I was concerned with - the preamp being able to drive both the plate amp and the BM's power amp loads paralleled together. Yes, the plate amp I have is specifically meant for subwoofer duty, and has an internal low-pass crossover, gain adjustment, and EQ and phase settings to fine tune with.

I appreciate your help. Thanks!

Jon