Question Crossover filtering

Feb 27, 2019
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Are crossover filters intended to affect the signal on the whole circuit, or just part of it? I've got this low-pass filter on my subwoofer, and for the life of me I can't figure out how to wire this woofer in series without filtering the highs out of the signal.
 

Sonic Illusions

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Based on what you stated, your sub has no amp with it's own filter. Your sub will be wired in parallel with your 'satellites.' Only the low-pass inductor filter will be in series anywhere between where the + connection is from your amp to the sub. I'd place it closer to the sub. You can bi-wire the sats from the amp's outputs, with a high-pass filter connected in series to the sats. You must use the low and hi-pass filter, in order to prevent your amp from being loaded with too low an impedance. Know what your amp can handle (4 or 8 ohms) and what your sub and sats are. Choose the filter values according to the impedance. Here's a good chart. If you choose a simple 6db/octave slope (one inductor for low pass, one cap for high), then you want maybe a 500Hz or so gap between the crossover points. Otherwise, the summed output of a single crossover point will most likely have a significant peak in output at that point. Hope this helps. Questions?

https://www.parts-express.com/resources-crossover-component-selection-guide
 
Feb 27, 2019
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These are some really good responses, I always appreciate it. I believe I've had the satellites and the subs wired in parallel, with the filters wired properly in series, but the low-pass still affected the satellite. I'll rewire again according to what you're saying and see if I can get it going. The setup is to look like this, yes?: View: https://imgur.com/a/pyV2Yc9


That amp there is 8-16 Ohms w 2 channels. I'm assuming this just means one channel for left and one for right, as in the top and bottom outputs are consolidated into one channel.

Sonic Illusions, I wonder what you mean when you say to bi-wire the sats? I just looked it up, and I don't know how that could apply when my sats only have one pair of inputs. Also, could the placement of these devices be messing with the signal? I've got the low-pass filters secured to the back of the subs.
 

Sonic Illusions

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I should have said bi-wire sats and subs (sorry!), which is basically what I did with the reference floor-standing speakers I built. The sub with passive filter in the cabinet is connected directly to the amp with it's own input and the MTM with it's own passive filters in the same cabinet is also connected directly to the amp.

That drawing basically looks correct. It appears that you're using 'speakers A' and 'speakers B' which is essentially bi-wiring - running a separate low-pass and separate hi-pass from the same amp channel output. I'm a little confused by the + and - going through the filters on your diagram. If you're using a simple coil as a low-pass filter (6db/oct), the coil cannot be connected across the output (parallel). The resistance of the coil itself would present too much load on the amp, so it seems as if we're missing something here. Also, the same slope of 6db/oct using a simple capacitor as pictured is not proper; the negative side should be a straight connection for both high and low pass. Simple minimum phase-shift filters like this are typically in series on the positive side and will not affect the other filters on the same channel.

A pic of your crossover filters would help. Loading your amp with 4 ohm sats may cause it to run a little warm, but it should be okay as connected. Every octave down on the bass end of the spectrum requires about 4X of amp power. So the fact you're loading that frequency band at 8 ohms means it's easier on your amp. The only issue you may have is that since the sats are 4 ohms and drawing more current from the amp, they will play louder than the subs, depending on their sensitivities (in db). There are solutions to this, but power is wasted. For example, if your subs have a sensitivity of 91db and the sats are 88 or 89, then you may be happy with them as is. There seems (as drawn) to be no way to control the sub and sat volume separately.

Re: messing with the signal: Only the inductors' placement can affect the signal with electromagnetic induction, by inducing 'cross-talk' if they're too close to each other.
 
Feb 27, 2019
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Thanks, Sonic Illusions. I am confused by this piece:
the negative side should be a straight connection for both high and low pass
This means that no negative wire runs through the filter? I just route the positive wire through and it's good? I assumed to hook up the negative on each side because the crossover has two pairs of terminals, labeled + and -. You are also saying that I cannot wire passive filters in parallel? How would I accomplish a sub and a sat running from the same channel?

I appreciate the assistance, I really need to get this set straight so I can begin to learn more. I'm only experimenting with the Parts-Express shelf crossovers: https://www.parts-express.com/parts-express-80-hz-low-pass-8-ohm-crossover--266-442 .
 

Sonic Illusions

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Okay, now we're getting somewhere! Your low pass is a 12db/oct filter and a good quality one. Yes, you're correct... If you look at the schematic, linked on that page, you'll see that the negative runs straight through; there is nothing between the input and output on the negative side. So the negative side of the signal does not 'run through' the filter. Yes, use the negative terminals on your filters.

"How would I accomplish a sub and a sat running from the same channel?" This is an easy one: Imagine they're both in the same cabinet, like mine are. I don't have to bi-wire them, because there's a metal strap on the + and another on the - side so I could connect with one cable. But I designed the inputs to be bi-wired and I choose to do so.

"You are also saying that I cannot wire passive filters in parallel?" Previously knowing nothing about your filters, I considered the simplest type - 6db/oct, which is a single coil (LP) and single cap (HP). These components are only connected in series. With 12db/oct and higher order filters, there are components connected across the + and - in strategic locations in the 'circuit' and they should not affect other speakers connected to the amp on the same channel.

Did that help?

Now here's the curve-ball: Remember when I mentioned minimum phase shift filters? Here's an decent article I found that touches on this (part pasted below). Using higher order filters causes phase-angle changes in the connected speakers. Take two identical speakers and switch the polarity on one. Then sit in the center and crank it up. It may make you sick; it does me! They cancel each other out and there's no bass, because they're 180 degrees out of phase. With your rig and the 2nd order filters, there will be a change in phase. It may or may not be as drastic as 180, but it will be there. You can experiment with changing the polarity on your sats, or rotating your subs 90 degrees. A lot depends on room placement and room placement is one reason powered subs have phase adjustments.

From: https://www.audiofrog.com/community/tech-tips/time-alignment-part-three-delays-and-crossovers-for-tweeters-and-mids/
About half way down. Same rules apply to lower frequencies.
'If we choose higher order crossovers, the phase relationship changes. In a second order crossover, the low pass lags by 90 and the high pass leads by 90. The difference is 180 degrees. That means that the speakers are out of phase. At and around the crossover, that matters because both speakers play the same thing. If they are out of phase, that creates a dip in the frequency response and it also means that at those frequencies, we hear the locations of the two speakers rather than the combination as one speaker, just like when our right and left speakers are out of phase and we hear no center image.

Next, is the response of the sum of the two filters. Because these are second order (12dB/oct) filters, the speakers are 180 degrees out of phase. At the crossover, that's a problem and it creates a big dip in the response.'
 
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Feb 27, 2019
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Thank you, the phase was definitely an issue. I've now set up the system as shown in the picture, but with the polarity reversed 180 for the satellites. How would I incorporate a way of turning down the subs? I want a pair of lone volume knobs. I think the subs are naturally way louder than the sats because it sounds alright but I can't reasonably turn it up loud enough to get the brightness out of the sats. Also, I think these crossovers just might be garbage because plenty of sound far, far past 80 Hz is coming clearly out this sub. Maybe I should build myself a 24 dB/oct low-pass?
 
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Sonic Illusions

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Filter not working - that's odd. Since you have 2 and the odds of both being defective are slim to none, try the other if you haven't already or call Parts Express. Connect it to your sats to see if any treble is filtered. Regarding leveling, check this page: https://www.parts-express.com/cat/speaker-l-pads/306 The L-pads are limited to 100W.

So the subs louder = higher sensitivity.

Have you considered using a plate amp for your subs? If you're handy, you can fab or modify an enclosure housing to put near the subs. You don't want to modify your sub's cabinet to accept the amp, because the amp will affect the internal cabinet volume and negatively affect the sub performance. This way, your crossover issue is solved, your gain and phase control issues are solved. I do have a sub with a PE plate amp and adjust the sub roll-off to blend with the sats. I didn't filter the sats. If you consider this, you will free up more amp power to your sats.

Here's a decent amp. It has fixed, not variable phase control. For $200 you can have 200 watts of sub power. More power, more $$.
https://www.parts-express.com/dayton-audio-sa100-100w-subwoofer-plate-amplifier--300-802
 
Feb 27, 2019
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Thank you, Sonic Illusions. I have considered buying another amp for a pair of speakers, and I most likely will in the future. This is some good information.

I'd like to know if you have any pages you could link to explain the purpose of L-pad attenuators to a layman? I've been looking for some info and I have a loose understanding of their function. I would use a pair of these to reduce the amplitude of the signal sent to my subs? As it is, my issue is that the subs seem to take up so much power and it starves the sats, affecting the very upper high range. Do you think an L-pad will help to alleviate this happening?
 
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Sonic Illusions

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You're welcome.

Without knowing what amp you have, or it's capabilities, other than impedance load, it seems to me as if you need more power. Your amp isn't intended to be loaded at 4 ohms (sats), so it may be straining to deliver. When an amp strains, it clips. The clipping is most noticeable in the upper end and even a weak amp can can fry tweeters during clipping. You mentioned your subs taking up more power. If the sub is 8 ohms and the sats are 4, then 4 ohms has less resistance to electrical current and will draw more power from your amp, compared with 8. Your amp is being 'overworked' with a load beyond it's spec's in the sat range above 80Hz. If your sub really is 8 ohms and it's playing too loud with 4 ohm sats, then it's because of sensitivity (@1W/ 1.83V@1M) and not because it's hogging power. With that 4 ohm sat load, you're also asking the amp to deliver power in the lowest frequencies that take the most power to produce sound. That being said, based on what you mentioned, your amp is under-powered and the speaker/ sub output levels are unbalanced. An L-pad is an inefficient band-aid, but will work if you don't exceed it's power rating. An L-pad is basically 2 circular wire-wound variable resistors (rheostats) that maintain impedance to the amp, while attenuating the signal. They're easy to connect; you can install in the sub. Remember that bass demands the most power, so choose an L-pad that will handle it. Read the reviews AND Q&A: https://www.parts-express.com/parts-express-speaker-l-pad-attenuator-100w-mono-3-8-shaft-8-ohm--260-262#lblReviews


I recommend either powered sat speakers or a different amp and plate amps for the subs, which have gain control, crossover filters and phase adjustments. You'll be happiest with something like that.
 
Feb 27, 2019
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I have one more question. Why is it that the amp did not seem underpowered when the pairs of speakers were plugged in without the filters? The sats I have are now high-passed at 300 Hz, and the amp didn't seem to stress to play them alone, no crossover, no subs. The volume knob rested at 1/10 for a comfortable volume. Even with the subs, the sats were playing well, but it is with the filters that my amp seems stressed. What are they doing to affect the circuit?
 

Sonic Illusions

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"High-passed at 300hz"? That's a filter. By stressed, what do you mean? Typically, a "stressed" amp gets hot. Is your amp hot? Why don't you simply cross the woofer at 300hz if the sats are already filtered at 300? What is the upper end of your sub's response? For 300hz @ 8 ohms @ 6db= 4.25mH inductor. For 250= 5.09mH. Keep it uncomplicated and it'll work.
 
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