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Playmaster subwoofer amp - LFE connection?

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I have a Playmaster "300W" sub amp, which I have been happily using both from amp 'pre outs', and 'lfe' outputs from various multichannel players.


I realize that if using it from the 'pre outs', it gives me good low end, but doubles up a bit on mid-bass, but I use the lowest (70Hz) crossover setting, and like the results.


But I've just become concerned about using the LFE signal, where there isn't a dedicated input for it. The amp was designed before LFE signals were invented, so the sub amp's crossover is filtering the signal again, and not giving me the unblemished LFE from, eg, the blu-ray player.


It takes L and R signals from the player, and has a 2nd-order LR filter on each channel. The lows are retained and amplified as a mono signal, the highs sent to the main amp and speakers. What I'd like to do is bypass these filters and feed the LFE signal directly to the sub amp proper.


The question is: will I need a resistor, or other bits, if doing this? I have the circuit diagram, but don't know enough to meddle safely.


I've spent a while searching, and can't find anyone else with the same question. It may be covered in Errata of a later EA article, but wouldn't know how to search for this.


Any help would be appreciated.

Edited by BioBrian
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OK, I'll be a bit more helpful, and put in the circuit diagram. This is for the filter section only; the power amp section is almost identical to the stereo version some of you have built, but has more output transistors to boost power.




To bypass the filter section, and feed the LFE signal from my Blu-Ray or DVD player directly to the amp, but still retain volume control, my guess is to switch out the other inputs, and switch in the LFE, between R55 and RV2. What do you think about that?


3 questions arise from this:


- Will the signal need further attenuation to not damage my hearing or the amp?


- Will the impedance match?


- Is there something I would need to do to avoid switching spikes if switching inputs with the amp on?


If the previous post was not intelligible, my wish is to avoid filtering the LFE signal again with this Sub Amp. As can be seen from the circuit, it is an active filter, and the highs are meant to be fed back to the Normal amp. But for quality stereo listening I prefer to keep this signal path clean: player - amp - speakers. For (eg) organ music, I like to use the Sub amp to augment the bottom, going from the amp's pre-outs to the Sub amp. With the LFE signal from movies, I'd like it to be amplified just as the engineers made it, so anticipate fitting a single RCA input socket for the LFE, and a switch, so I don't have to keep unplugging cables.


Please let me know if I'm being overly naive here.



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