Hi! I discovered the previous homeowner(s) at some point connected their whole home audio system directly into a Boston Acoustics VRiSUB82 in the wall without any kind of crossover network. EEK!
I discovered this because it sounded like crap hooked up to the AmpliPi! That’s when I discovered I had a pair of sub speakers in a wall!
I am thinking I need a low pass filter, and I don’t need crazy wattage out because this is for a central space. I just want things to sound nice and a background level - this isn’t a theatre - just an open central location of home.
Q: If I get a low pass filter, can I combine the left and right channel out from the AmpliPI to feed it? Or will that damage the amp?
Q: Is there a way to make the output “mono” for L+R ? … or Just R?
This sub originally came with an amp (2002), but it’s not present in home… and I hesitate to spend $250 on a 20 year old amp that’s probably always on… so I’d really like to use channels from the AmpliPi if possible.
It sounds like the easiest solution here would be to add a crossover to the zone output you want to use since the AmpliPi hardware does not have any high/low/mid filtering capabilities. @jason can probably give you more specifics.
agreed, that’s what the low pass filter is.
My question is more along the lines of best way to attach it to AmpliPi – can I combine output of left and right channels? Is there the capability to bridge stereo output into mono? etc.
I got a really great answer from Jason, posting it here for posterity… to help others. My solution? Well I’m luck in that my woofer has two drivers… so I’m going to drive them separately each with their own low pass filter. I hooked it up to the “right” channel today, where both were being driven off just one channel, and it was “good enough”. But might as well do it right, and have L and R channels each woofing for those situations where the waveforms are not in sync.
Jason wrote:
So in old amps you could do this - it was called bridging. Essentially you would combine the right and left channel in series and use that to power a single speaker (IE hook up R+ to L- and then connect the speaker to L+ and R-). Unfortunately you can’t do that with class D amps [Amplipi]. They are already bridged internally, basically both the + side and - side are actively driving the speaker (as opposed to the + side driving and the - side being grounded).
The amps we use do support something called parallel bridging. Essentially you can connect the two channels together in parallel (R+ to L+ and R- to L-) and then drive a single speaker from that. It does not necessarily give you more power but does allow you to drive speakers with lower impedance since you can push more current with the two channels in parallel. We have yet to try this mode on the amplifiers. NOTE: you can ONLY do this if the amp has been configured for parallel bridging. We would need to modify the circuitry on your board to configure an amp to run in parallel bridging or PBTL mode.
If we did configure an amp in your box to support PBTL mode, we would still need to combine the left and right inputs together before the amp chip to create a single mono signal that the amp could amplify. This would require either an active summing circuit with an OP-amp or a passive summing circuit with a few resistors. We could do this on a small circuit board that would go between the amp board and preamp board.
I think there is a potential software solution we could do here too: We could make one source play the audio out as stereo, with different tracks for R and L. We could then create a mono version of that source in the ALSA config that would play out of one of the other sources (IE both R+L tracks would go to R and L). It would be a bit clunky though because you would have to select a different source for your sub and main speakers.
Another option would be you could use something like this to combine the stereo output from the line level outputs on the back of AmpliPi:
However, this solution would require an external amplifier.