Tuesday, 2 August 2011

An active crossover for my 2.1 HiFi

Filed under: Music,Uncategorized — Michael @ 7:55

It’s been a long time since my last post. I have been doing stuff, though, and I thought I would post about a modification I did to add a subwoofer crossover to my hi-fi system. It came out pretty well, I think.
What I have is an NAD C315BEE integrated amp. This is their entry model from a couple of years ago and it has no separate pre-amp out or subwoofer out. Speakers are B&W 685s with an ASW610 subwoofer. (A ‘2.1’ system.) I was using the subwoofer’s speaker-level inputs and internal low-pass filter and the system sounded OK. But I got to thinking that the low frequencies were also going to the main speakers and even though they can’t reproduce, say, 30Hz they still get the signal. So I thought about “bi-amping” with the active subwoofer amp and the NAD and with an active crossover. I got a hold of the service manual for the NAD and found that there are actually jumpers on the circuit board between the pre-amp and power sections, all I needed to do was build in some connectors for a pre-amp out/in.
First, I went ahead and built the crossover. It is a standard Linkwitz-Riley 24dB/octave circuit set for crossover at 59Hz. (I just added 10Hz to the -3dB frequency for the 685 speakers.) Graphing the response showed all was in order, no phase problems and crossover frequency just where I’d calculated. Cool!
Now it was time to open up the NAD and see about removing the jumpers and adding the connectors. Turned out that it was really easy to work on. They even have a panel on the bottom that, when removed, allows access to the underside of the circuit board. Anyway, once I got the covers off it looked like there would be room to actually mount the crossover inside. I knew from the schematic that there was +/-17V in there which could power the crossover (it only draws like 30mA so I wasn’t worried about that).
So, this last weekend I wired it all up and was truly surprised at how much clearer the lower notes sound. Whether rock, jazz, or concert orchestra, the low-end is just more transparent and real sounding.
I attached a few pictures. The first shows the added subwoofer-out jacks (lower-left) with the location of the two removed jumpers (upper-middle, just to the right of a tall heatsink). The two jumpers just above the upper removed jumper are +/-17V. I removed them and replaced them with new jumpers with a little loop to solder to. I also drilled 5 small holes in the audio ground plane for the ground wires. The second picture shows the wiring in place. The third shows the crossover in place. The last two show the amp with the crossover and the new backside.
Thanks for looking!

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