Sub-woofer Crossover ?

Dear Sunil,

You are right in your understanding. But the Subs have an option of bypassing their cross-over - which is what you should use when connecting it to AVR and use its cross-over only for pre-out from stereo.

Another point (not related to this) - Subs also have the option of by passing their amps i.e., being able to be used as passive-subs or just the drivers.

Regards

Sanjay

Doc,
Do all subs have the provision of bypassing its amp ??

All the decent ones at least !

Regards

Sanjay

and i guess for doctor saab, decent ones will be something which is a luxury item for someone like me

Just kidding.

Abhi, u have experience with roarrs. do u know if they have a bypass switch in them.

No I havent checked out this feature yet. I just go and listen...

If the AVR's crossover is set at 80hz and the subs crossover is set at say 50 hz, what sounds do really comeout of the sub is it below the 80hz freq. or below the 50 hz freq. And if it is below the 50 hz freq then what happens to frequencies between 50 hz and 80 hz.

The sound that comes out of the SUB would be min(SUBxover, AVRxover). Start with 80Hz AVRxover and either direct mode or maximum (typically 200Hz) SUBxover. Typically this works well for most scenarios.

- Debu

SDhawan schrieb:

:D All the decent ones at least ! Regards Sanjay

Are you sure on this ? most subs that I have seen have a cuttof freq knob. the usual recommendation is to keep that freq above the one se in the avr to avoid loss.

ie if the AVR is crossed over at 80 hz, the subs cutoff is kept at around 100 etc...

the reason being the many subs do not have a Line level input and only have a Low leve or Pre input into the amplifier. hence even for stereo the pre amp is connected directly to it. unless there is a crossover, there is no way of filtering out the higher frequencies..

Dear Arj,

The Boston sub PSV-500 that I am using has following options:

-Input using Sub Cross-over - adjustable from 50-150 Hz (meant for input from stereo or other equipment with do not have LFE filter)
-Input that bypasses the sub. cross-over (so just the cross-over of AVR is being used)
-Rt & Lt Speaker level input (then it works like a passive sub)
-Phase switch
-Level control

And I think that if any entry level sub like Boston has these option, better Subs may have them too. But that's my assumption

Regards

Sanjay

the much more expensive MJ acoustic subs do not have that

Neither do the much higher end RELs and nor the Martin Logan depth !!
I presume it is a brand specific option..

If Sunil intends to use his AV-Receiver's built-in digital crossover, he should set his subwoofer's crossover frequency to its highest (in case the subwoofer does not have an internal crossover bypass), and vice-versa.

The subwoofer crossover would have a fixed slope, i.e., roll-off beyond the crossover frequency. For example, it can be a 12 dB/octave slope. Likewise, the AV-Receiver's built in digital crossover too would be programmed to imitate the slope of an analog crossover, say again a 12 dB/octave slope. If you use the AV-Receiver's crossover, and do not keep the subwoofer's crossover frequency sufficiently high, the subwoofer's crossover slope would interpose with the AV-Receiver's crossover slope. The result would be unintended dips and gaps in the lower frequencies, and complications in getting the integration between subwoofer and main-speakers right.

The Lithos Terra-150 subwoofer I use, sans all electronics, and instead goes for an acoustic solution for the crossover, i.e., push-pull design integrated into a band-pass enclosure. I found this solution to be distortion free, and clean-sounding. The trade-off is that you have to live with a fixed crossover, which is a little high at 150 Hz, but at the same time integrates well with most bookshelf / monitor speakers. There may be a slight frequency bump though if your main speakers go very low (whether there would be any, and if so how much, again depends upon the characteristics of your room). The only adjustments you can do to phase are 0 & 180 degrees by doing a swap between the "push-driver" & "pull-driver", i.e., the L & R channel speker wires coming out of the subwoofer amplifier (which is to be stacked separately in the equipment rack)

Sometime back, I did some experimentation with an external analog adjustable crossover (which was supposed to be phase coherent, 24 dB/octave, Linkwitz-Riley type), trying to keep the crossover between my Studio-1s and Terra-150 anywhere between 50 Hz to 80 Hz. Despite my best efforts, I did not get better integration between the sub & the main-speakers in any of the crossover frequencies while using an electronic crossover. Finally, I removed the external, adjustable, analaog crossover from my equipment-chain, and gone back to the acoustic solution offerred by the Terra-150.

There may be a slight frequency bump though if your main speakers go very low (whether there would be any, and if so how much, again depends upon the characteristics of your room).

My conclusion:

If I have to choose between the distortion due to phase-shifts, summing, and other problems electronic crossovers introduce in subwoofers, and a possible slight bump in the lower frequency spectrum sans all distortion owing to an acoustic crossover, I am better off choosing the latter, whether in stereo or surround.

Comments are welcome.

By the way, it is generally held that if one has an option to use either AV-Receiver's digital crossover, or the subwoofer's analog crossover, it is better to use AV-Receiver's crossover, as a digital crossover would introduce less distortion than the cheap electronic crossovers put into active subwoofers. While a digital crossover may suffer from a poor algorithm, it is by and large distortion /noise free, and in any case, by now, these things have been perfected by most audio brands making AV-Receivers.

kspv schrieb:

There may be a slight frequency bump though if your main speakers go very low (whether there would be any, and if so how much, again depends upon the characteristics of your room). My conclusion: If I have to choose between the distortion due to phase-shifts, summing, and other problems electronic crossovers introduce in subwoofers, and a possible slight bump in the lower frequency spectrum sans all distortion owing to an acoustic crossover, I am better off choosing the latter, whether in stereo or surround. Comments are welcome. By the way, it is generally held that if one has an option to use either AV-Receiver's digital crossover, or the subwoofer's analog crossover, it is better to use AV-Receiver's crossover, as a digital crossover would introduce less distortion than the cheap electronic crossovers put into active subwoofers. While a digital crossover may suffer from a poor algorithm, it is by and large distortion /noise free, and in any case, by now, these things have been perfected by most audio brands making AV-Receivers.

Hi, nice to have you back!
Have you ever tried the digital crossover built into computer soundcards like the n-Force series? They offer to filter out bass at a variable frequency ranging from 30Hz to 500Hz. Do you understand how they work? Would they offer a decent solution to back up the bass from a bookshelf?

Unfortunately, Lithos' acoustic solution to crossover design is rare. I have not come across any other products using that design. The "cheap electronic crossover" seems to be the norm in subwoofers. Does the Terra offer true bass? Can you actually feel the different types of bass separately: guitar, double-bass, organ etc?

Will keeping the crossover setting of both the AVR & sub to the same level solve the problem ?? i.e. AVR @ 80hz and Sub also @ 80 hz instead of increasing the sub's xover to full.

SUNILYO schrieb:

Will keeping the crossover setting of both the AVR & sub to the same level solve the problem ?? i.e. AVR @ 80hz and Sub also @ 80 hz instead of increasing the sub's xover to full.

Sunilji, that might not be a good idea!
I know that it is the most intuitive thing to do
BUT it will result in the crossover slopes getting added around 80Hz and becoming too steep for proper integration. I will leave Kspv to confirm what I have said.

Cumulative effect of two low-pass filters, having the same crossover point, placed in sequence, should almost be like that of a brickwall filter. It will most probably bring in distortion issues. Also, the high-pass filter here(of the AV-Receiver), routing the >80 Hz signal to the main-speakers would be of a gentler slope (different order) than the cumulative effect of the two sequential low-pass filters. That should make a smooth integration between the main-speakers and the subwoofer difficult.

Have you ever tried the digital crossover built into computer soundcards like the n-Force series? They offer to filter out bass at a variable frequency ranging from 30Hz to 500Hz. Do you understand how they work? Would they offer a decent solution to back up the bass from a bookshelf? Unfortunately, Lithos' acoustic solution to crossover design is rare. I have not come across any other products using that design. The "cheap electronic crossover" seems to be the norm in subwoofers. Does the Terra offer true bass? Can you actually feel the different types of bass separately: guitar, double-bass, organ etc?

Neutral,

I have not tried a digital crossover yet. But I observed on www.diyaudio.com, that digital crossovers, even from a brand like Behringer (which is typically a PA equipment manufacturer, and has a reputation for violating the design copy-rights of other audio companies by way of making cheap imitations), have won compliments from very knowledgeble audio-enthusiasts, and that surprises me!

As I understand, a digital crossover such as Behringer DCX 2496 would just be containing few op-amps & algorithms stored on a chip to imitate the crossover characteristics of different types of analog electronic crossovers. Since the processing power of a PC is very high, if the algorithm stored on the chip is correct, then the digital crossover of the PC soundcard should do the job fine. As you might already be knowing, digital crossovers do not add noise to the signal.

Here is a review (= take it with a pinch of salt) that sings paeons of the Behringer DCX 2496 and its superiority over analog crossovers.

http://www.newformresearch.com/digital-systems-overview.htm

The question is, if a $300 digital crossover could have so many virtues, why are our high-end audio manufacturers still preferring to go with analog crossovers? As on date, I do not know the answer for this.

However, an adjustable digital crossover like DCX 2496 would be of good use to DIY enthusiasts who are building their own boxes, to try and compare between different crossover frequencies just at the touch of a button. For example, the Behringer has mono and stereo output operating modes, all with individual crossover filter types (Butterworth, Bessel and Linkwitz-Riley) with selectable roll-off characteristics from 6 to 48 dB/octave. The delays for all inputs and outputs too, are adjustable.

The arguments against an acoustic crossover solution in a subwoofer I have read on some forums are as follows.

1.)It is inherently wasteful of the amplifier power. The amplifier (or the power-module as Lithos calls it) here, sends full-frequency sound to the subwoofer. The drivers take all of it, but give out only the low frequency sound in the end, dissipating the rest of the energy.

2.) Very difficult to get the design right. Wrong drivers & incorrect design would end up in blowing one of the drivers in the "push-pull" configuration, though the "push-pull" design is known to offer very accurate, distortion free bass. Wrong drivers & incorrect design in pure band-pass subwoofers result in "one-note bass," though the bandpass designs offer good SPLs.

3.) Fixed crossovers need to be a bit high to make most bookshelf speakers integrate with them smoothly. This may result in a slight bass bump depending upon your room characteristics. Also, for somebody who likes lots of frills on his subwoofer, a subwoofer designed with an acoustic crossover may seem too unsophisticated and "bland".

While phase coherence between high-frequency driver & mid-frequency driver is important, it has been a widely debated issue as to whether phase in a subwoofer matters? Experiments indicate that it does not matter. In any case, you generally enjoy the subwoofer frequencies in two ways:

A.) The pressure mode, or "tactile stimulation" through the movement of huge mass of air in the room, vibrations of the floor, furniture etc., which have nothing to do with your hearing, or phase.

B.) The sound mode, where the audible subwoofers waves (owing to their wave-length) will invariably be out of phase with the main-speakers, no matter what "phase-adjustment" you do on your subwoofer dials.

So perhaps phase in a subwoofer does not matter. Just see that your subwoofer is distortion-free, and the SPL level calibration between the subwoofer & the main-speakers is right. As for placement, while there are some arguments for placing the subwoofer in the front-centre (Doug Blackburn), the most satisfactory trade-off invariably is to place it in the corner (within 53 c.m.s of the corner in most cases ; don't ask me about this 53 c.m.s calculation now! )

Lithos seems to have successfully overcome many trade-offs in its ingenius (yes, you don't get to see it elsewhere except some M&K subwoofers, and this particular design of Lithos Terra-150, as I understand, is patented) acoustic crossover solution, where a musically accurate "push-pull" design is integrated into a high SPL bandpass enclosure. The Terra-150 offers very accurate and musical bass at good SPL levels. Infact the emphasis here seems to be more on accuracy and elimination of both 2nd harmonic & 3rd harmonic distortions than sheer SPL or how low the subwoofer can go.

I am told that world's biggest organ can go down to 16 Hz.

Well, Lithos Terra-150, with its -3 dB point at 30 Hz, can not go that low (assuming that low a bass would still be audible to human ear, and does not sound "slow" and sluggish in music). But what the Terra can offer you is very clean bass with an SPL sufficient for your room.