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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
75 posts
 

There are some missing blanks here, but virtually no one is doing what you are proposing, and for good reason.

If you intend to get a powered subwoofer (most do), those typically have their own internal low-pass filter within the plate amp and feature various connectivity options (line and speaker level) and gain controls, so no external filtering is necessary.

If you're intending to go with a passive subwoofer, the Crown XLS series amplifiers have their own internal DSP with an adjustable 24 dB/octave low-pass filter.

sfpeloton OP
57 posts
 

The goal would be to use an active subwoofer with DSP and a low pass filter; however, I would like to keep the analog signal from my preamp going straight to the speaker amp with a passive in line hi-pass filter. 

If there are good reasons for not doing it this way, I would be interested in learning them. 

ted_b
75 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
3,949 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.

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