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DIY subwoofer high pass filter after preamp

Hoping to get some sage advice from some of the brilliant members of this audio forum. I am interested in trying a subwoofer in my system. I am not really interested in ultra powerful bass, but I am quite curious to see if taking some of the low frequency work away from my monitor speakers can further enhance clarity. I am also interested in improving the soundstage and filling out the music more.

Details of my system are in my profile but the speakers are Dynaudio Heritage Special monitors. I do not like the idea of using the subwoofer DSP for high pass filtering and would like to implement the simplest, highest quality solution without breaking the bank. After doing a little research it seems one simple passive approach would be to run the sub in parallel via one set of preamp line out connectors and then use a simple high pass capacitor introduced between the second set of preamp line outs and my amplifier.

I was thinking I could have a go at doing this myself. The plan would be to buy a high quality capacitor and solder one end of it directly to a male RCA plug I would then solder the other end of the capacitor to a female RCA plug and shrink wrap and jacket the whole thing together. I would then just have a filter/adaptor at one end of my interconnect that would connect directly to either the amp or preamp.

A few questions:

1) Is this feasible or even a good idea? 

2) What value capacitor would I need for roughly 80 Hz filtering with a 37 kOhm, 220 pF input impedance on my Audionet Amp1 V2? My speakers are 4 ohm, but not across all frequencies. 

3) Would I need matched capacitors for each channel and to within what % of each other?

4) Can I solder the capacitor leads directly to the connectors in this way? Most capacitors look to have enough wire for this. 

5) Anything else I should consider with this plan.

Thanks in advance for input.

 

sfpeloton
ted_b
74 posts
 

The frequency divider typically used in powered subwoofer plate amps is analog and not a DSP. Whereas a simple series capacitor gives a lazy 6 dB/octave slope, the filter in subwoofer plate amps is higher order - usually 18-24 dB/octave.

For example, with a REL T/7x series subwoofer, you will see both low and high level inputs. The low level takes an RCA cable from one channel of your preamplifier as the input. When using the high level (speaker level) inputs, the subwoofer input takes a lead from the (+) terminal of each channel of your speaker amplifier output, combining these internally and using this as the input to its internal amplifier. In either case, the analog fullrange signal from your preamp still goes straight to your speaker amp as it does presently.

sfpeloton OP
57 posts
 

Thanks for taking the time to provide that information. My take on what you wrote (this is not my area of expertise) is that if I were to use a capacitor the way I proposed, that would result in a shallow attenuation slope that would possibly be hard to match with the slope of the low pass subwoofer filter? That does sound like a possible downside.

On the second point, I understand the full range signal still goes to my speakers in your example, but I am wondering if it isn't beneficial for the speakers to only see a high pass filtered signal. If they see full range, doesn't that mean you give up some of the benefit in mid/high-end clarity from shifting the low end exclusively to the sub?

I am not tied to this idea. I saw some information on the GR Research website and elsewhere describing this approach, so I thought I would inquire more before I jumped in.

 

pindac's avatar
pindac
3,946 posts
 

Possibly timely info: on HiFi Shark, I have seen Velodyne Plate Amps, of which one is their Digital Design. How difficult to implement is not known.

gdaddy1
519 posts
 

As Teb-b points out, the REL sub has availability to connect to the 'high level' input. The big advantage of using this connection is that if you have a good dynamic sounding amp you'll get the same dynamic to the sub. Going out through the pre-amp you won't get any of the flavor of the amp.  It's a perfect match to what's being fed to the speakers.

Talk to REL. The end reult will be a fuller, more dynamic sound running full signal to the main speakers and 'high pass' to the sub.

I would NOT run the signal through any other electronics for any reason. Let the speakers do their mid-bass perfectly fine and set the sub crossover low, around 40hz. should put you near the sweet spot. Simple.

gdaddy1
519 posts
 

If they see full range, doesn't that mean you give up some of the benefit in mid/high-end clarity from shifting the low end exclusively to the sub?

Your speakers start to cut off @42hz. There's a good quality mid-bass they produce. It's part of the design. There would be little, if any, increase in clarity by high passing to a higher level and cutting out perfectly good mid-bass that those speakers do so well! Let them breath.

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