Hi there,
I finally gave my big old TV away and bought a new one which is physically much smaller.
This, leads to the theme I'd like to share -- a center OB 'sub'. Combined with my original OB speakers, now in the bass, there're 3 of them -- multiple dipole subs:
The center channel 'sub' is made as V shape with 120 degree between the 2 woofers.
The center OB bass is now wired as mono, driven by a plate amp, and via a set of T-bass circuit of course. The setting of T-bass and the integrated xover of the plate amp make it work only under 40Hz.
All 3 OB's are indeed too close to the wall, but I can't do anything about it. Can't aford a bigger place.
I wired them up for only half a day, the settup is not optimum yet. Preliminary listening showed some potential but also something obviously wrong. By the RTA reading, the frequency range of the whole system extends lower and flatter, but I didn't 'feel' any improvement of the bass sound. Actually I feel it somewhat 'unfocused' on some materials.
Squatting just in fornt of the center OB, there can be a very strong sense of low bass -- feel of vibration instead of 'sound'. However I can not detect it on the listening position. I guess some cancellations happen here.
There's a knob marked as 'phase' on the plate amp with dial of 0~180 degree. I'm not quite sure its 'real' function. The difference of path lengths of the center and the main is measured as 23 cm (the center channel is closer to listener). At 40Hz, this 23 cm difference makes a 9.6 degree of phase angle shift. But where's the exactly position on the knob can give me this? It can not be examined.
By only ears and a simple RTA, I'll need more time to get it right....
I finally gave my big old TV away and bought a new one which is physically much smaller.
This, leads to the theme I'd like to share -- a center OB 'sub'. Combined with my original OB speakers, now in the bass, there're 3 of them -- multiple dipole subs:
The center channel 'sub' is made as V shape with 120 degree between the 2 woofers.
The center OB bass is now wired as mono, driven by a plate amp, and via a set of T-bass circuit of course. The setting of T-bass and the integrated xover of the plate amp make it work only under 40Hz.
All 3 OB's are indeed too close to the wall, but I can't do anything about it. Can't aford a bigger place.
I wired them up for only half a day, the settup is not optimum yet. Preliminary listening showed some potential but also something obviously wrong. By the RTA reading, the frequency range of the whole system extends lower and flatter, but I didn't 'feel' any improvement of the bass sound. Actually I feel it somewhat 'unfocused' on some materials.
Squatting just in fornt of the center OB, there can be a very strong sense of low bass -- feel of vibration instead of 'sound'. However I can not detect it on the listening position. I guess some cancellations happen here.
There's a knob marked as 'phase' on the plate amp with dial of 0~180 degree. I'm not quite sure its 'real' function. The difference of path lengths of the center and the main is measured as 23 cm (the center channel is closer to listener). At 40Hz, this 23 cm difference makes a 9.6 degree of phase angle shift. But where's the exactly position on the knob can give me this? It can not be examined.
By only ears and a simple RTA, I'll need more time to get it right....
Over 100 views and no reply ....
OK, here are some updates.
I re-adjusted the whole system -- brought up the active xover point of the woofer (on main OB), and raised the overall gain of mid-high. This gave several dB lift to all frequecies from about 50Hz and up.
So the add-in center sub would not give a 'heavy-footed' effect due to the uneven response and unsmooth transition from lower mid to upper bass to lower bass. (And now it's a mix of 3.5way and 2.1 channels.)
This made the whole system sound extended and lively. And the previous 'unfocused' phenomenon improved a lot. Now the impact and sense of vibration are stronger and more lifelike. Did I mention it sounds big? It's indeed no replacement for displacement! Big cone area works!
As to the 'phase problem', I leave that knob very close to 0 (or sometimes right at the 0). I can sense a little difference on some materials when the knob is at 0 or not. Setting it slightly off 0 is mostly a better choice with most of my collections.
I think of other problems on this. Even if the phase knob is "properly working", it's still questionable. Reflecting to the 23cm difference of path lengths, the phase angle at 40Hz and 30Hz and even 20Hz are all different! Which point should I aim for? In the end, I probably converge them in one frequency and miss all others. So, what I need should be a constant time delay instead of a phase angle. Am I right? (or wrong?)
In addition, I'm not sure about what exactly the built-in 2nd order xover of the plate amp and the T-bass circuit do to the final acoustic phase. So I have no idea if the 3.5way structure itself is coherent or not. Let alone this 'tiny' path length difference. I'm basically shooting in the dark here. And, afterall, related to the wavelengths of 40Hz and down, 23cm is just so small. Maybe I should just ignore it?
Any comments?
OK, here are some updates.
I re-adjusted the whole system -- brought up the active xover point of the woofer (on main OB), and raised the overall gain of mid-high. This gave several dB lift to all frequecies from about 50Hz and up.
So the add-in center sub would not give a 'heavy-footed' effect due to the uneven response and unsmooth transition from lower mid to upper bass to lower bass. (And now it's a mix of 3.5way and 2.1 channels.)
This made the whole system sound extended and lively. And the previous 'unfocused' phenomenon improved a lot. Now the impact and sense of vibration are stronger and more lifelike. Did I mention it sounds big? It's indeed no replacement for displacement! Big cone area works!
As to the 'phase problem', I leave that knob very close to 0 (or sometimes right at the 0). I can sense a little difference on some materials when the knob is at 0 or not. Setting it slightly off 0 is mostly a better choice with most of my collections.
I think of other problems on this. Even if the phase knob is "properly working", it's still questionable. Reflecting to the 23cm difference of path lengths, the phase angle at 40Hz and 30Hz and even 20Hz are all different! Which point should I aim for? In the end, I probably converge them in one frequency and miss all others. So, what I need should be a constant time delay instead of a phase angle. Am I right? (or wrong?)
In addition, I'm not sure about what exactly the built-in 2nd order xover of the plate amp and the T-bass circuit do to the final acoustic phase. So I have no idea if the 3.5way structure itself is coherent or not. Let alone this 'tiny' path length difference. I'm basically shooting in the dark here. And, afterall, related to the wavelengths of 40Hz and down, 23cm is just so small. Maybe I should just ignore it?
Any comments?
Hi CLS, I like the look of what you have done. I take it that the subs are solid enough not to cause any vibration problems to the TV. What speakers are yhou using in the mains and the subs now?
I have four Beta 15As but am concerned they run out of xmax so quickly. Maybe I should forget sims and just do it. Right now I am listening to two open baffles with four 6" speakers in each, not ideally located on the baffle. They are on an old JVC amp which has loudness comp. switch which is on. they are rated at 1.5 W each speaker and the voloume is on 2.5 on a dial which goes up to 10 or 12. I am about 36 feet away in another room and can hear them quite loud. Stan Getz is sounding sooooooo good. So maybe I can disreguard xmax issues on the Betas.
jamikl
I have four Beta 15As but am concerned they run out of xmax so quickly. Maybe I should forget sims and just do it. Right now I am listening to two open baffles with four 6" speakers in each, not ideally located on the baffle. They are on an old JVC amp which has loudness comp. switch which is on. they are rated at 1.5 W each speaker and the voloume is on 2.5 on a dial which goes up to 10 or 12. I am about 36 feet away in another room and can hear them quite loud. Stan Getz is sounding sooooooo good. So maybe I can disreguard xmax issues on the Betas.
jamikl
Thanks for the compliment I'm now using Eminence Sigma Pro18 on center sub and also on main baffle. Actually the center sub still vibrates. (Its overall depth is very shallow) After all, the cones and the air they pump is massive. Luckily there're foam pads under the seats of TV for some damping, so it's not seriouly shaken. I've once built a W baffle, with face to face arrangement, it cancels the vibration much more effectively. However this V baffle is better looking
I'd say don't worry about the Xmax. My woofers barely move most of the time. The motion can only be seen within very short distance. When the motion is visible, I'd begin to worry if I'm bothering the neighbors. It's no way to see them move at the listening position. They're just so loud with so less moves. Even with your slightly smaller 15"ers, I think it's no problem at all. Just go for it.
And, please also try T-bass circuit on your OB bass:
http://www.diyaudio.com/forums/showthread.php?s=&threadid=130679
I'm now using Eminence Sigma Pro18 on center sub and also on main baffle. Actually the center sub still vibrates. (Its overall depth is very shallow) After all, the cones and the air they pump is massive. Luckily there're foam pads under the seats of TV for some damping, so it's not seriouly shaken. I've once built a W baffle, with face to face arrangement, it cancels the vibration much more effectively. However this V baffle is better looking I'd say don't worry about the Xmax. My woofers barely move most of the time. The motion can only be seen within very short distance. When the motion is visible, I'd begin to worry if I'm bothering the neighbors. It's no way to see them move at the listening position. They're just so loud with so less moves. Even with your slightly smaller 15"ers, I think it's no problem at all. Just go for it.
And, please also try T-bass circuit on your OB bass:
http://www.diyaudio.com/forums/showthread.php?s=&threadid=130679
Yes, LT is 12db/oct driver equalization which moves the poles. For example Eminence Alpha has Fs=41Hz and Qt=1.2, with LT we can tune it to Fs=20Hz and Qt=0.7. Perfect!
http://linkwitzlab.com/filters.htm#9
So in an OB we use dipole equalization (6db/oct) plus LT to obtain flat response to very low frequency.
I have heard of T-Bass, but it seems to complex where the active xo solution is more precise and simple.
http://linkwitzlab.com/filters.htm#9
So in an OB we use dipole equalization (6db/oct) plus LT to obtain flat response to very low frequency.
I have heard of T-Bass, but it seems to complex where the active xo solution is more precise and simple.
I've not yet tried a LT, can't comment.
In my active xover, I once used very low xover point (40Hz, or even lower) with higher gain (+12dB) to offset the baffle loss, and fine tuned the whole thing with a digital EQ. This brought me a frequency response quite flat and very extended, but I was not so happy with the real world sound quality. Until I tried the T-bass.
It's actually not that complex and really worth a try.
In my active xover, I once used very low xover point (40Hz, or even lower) with higher gain (+12dB) to offset the baffle loss, and fine tuned the whole thing with a digital EQ. This brought me a frequency response quite flat and very extended, but I was not so happy with the real world sound quality. Until I tried the T-bass.
It's actually not that complex and really worth a try.
Not at all actually. Driving OB woofers takes very little power. My 50watt gainclone copes really well driving Eminence Alpha. Note that ALpha has high Q=1.21 at Fs=41Hz, which peak is flatten so in fact it takes more SPL with LT.
Of course I agree with you that more is better and 18" is better than 15". Everyone knows that except the ministry of war and finance.
Of course I agree with you that more is better and 18" is better than 15". Everyone knows that except the ministry of war and finance.
In my application, these OB woofers eat not much.
Now with T-bass circuit, the impedance is approaching 1 Ohm in particular range, so I use SS amp. Before I tried T-bass circuit, I used tube amps for the OB bass, even with SE 300B for quite some time
At that time, the SE 300B could also made some room-filling and window-rattling bass. (Not so tight and controlled as present SS amp with T-bass, though) I've never clipped any of the amps, even on that 6W SE tube power which was dealing with several 18"ers.
OK, maybe my apartment is small and the window frames are in bad shape... :lol:
Well, fine with me anyway.
When I hooked up the 1 Ohm load (thanks to T-bass) to my old SS Hafler, I worried a little, as the user manual says "do not connect to loads much lower than 8 Ohm". Hmm... I thought to myself, what the xxxx, just give it a try.
Then it turned out very well. The cheap old little SS amp is doing just fine with that 1 Ohm load. No problem at all. Playing pretty loud for several hours, it's still only warm on the heatsinks. (It's only rated as 60W x 2 output @ 8 Ohm load, no data for other load impedances. Its power transformer is maybe 300VA or so, and with tiny filter caps. A totally no-looker for low impedance loads. No internal photo should be published in reviews for such amp, if any.)
I think, the cones must pump out specific amount of air to produce particular SPL. And the speaker drivers need that particular 'power' to do it. When in normal condition, high impedance peaks commonly seen in the bass range need very high voltage to get enough power.
With helps of T-bass, the impedance peak drops dramatically so the woofer can draw a lot more current (power) in the working range. Compared with the ordinary combinations of [high gain (EQ) + high driving voltage + high load impedance ], I prefer the performance of lower impedance + high current of T-bass.
As to the inevitable OB loss, I guess it's somewhat averaged out by the 'unloaded' cones which are much easier to move. Overall, I never feel OB speakers hunger for power. They are easily driven and pretty loud in my usage.
Now with T-bass circuit, the impedance is approaching 1 Ohm in particular range, so I use SS amp. Before I tried T-bass circuit, I used tube amps for the OB bass, even with SE 300B for quite some time
At that time, the SE 300B could also made some room-filling and window-rattling bass. (Not so tight and controlled as present SS amp with T-bass, though) I've never clipped any of the amps, even on that 6W SE tube power which was dealing with several 18"ers.
OK, maybe my apartment is small and the window frames are in bad shape... :lol:
Well, fine with me anyway.
When I hooked up the 1 Ohm load (thanks to T-bass) to my old SS Hafler, I worried a little, as the user manual says "do not connect to loads much lower than 8 Ohm". Hmm... I thought to myself, what the xxxx, just give it a try.
Then it turned out very well. The cheap old little SS amp is doing just fine with that 1 Ohm load. No problem at all. Playing pretty loud for several hours, it's still only warm on the heatsinks. (It's only rated as 60W x 2 output @ 8 Ohm load, no data for other load impedances. Its power transformer is maybe 300VA or so, and with tiny filter caps. A totally no-looker for low impedance loads. No internal photo should be published in reviews for such amp, if any.)
I think, the cones must pump out specific amount of air to produce particular SPL. And the speaker drivers need that particular 'power' to do it. When in normal condition, high impedance peaks commonly seen in the bass range need very high voltage to get enough power.
With helps of T-bass, the impedance peak drops dramatically so the woofer can draw a lot more current (power) in the working range. Compared with the ordinary combinations of [high gain (EQ) + high driving voltage + high load impedance ], I prefer the performance of lower impedance + high current of T-bass.
As to the inevitable OB loss, I guess it's somewhat averaged out by the 'unloaded' cones which are much easier to move. Overall, I never feel OB speakers hunger for power. They are easily driven and pretty loud in my usage.
CLS,
Have you exchanged your right and left speakers so the tweeter and midrange are on the inside of the sound stage? This arrangement often sounds better to my ears. I design 3-way speakers and put the woofers in a separate bottom structure to avoid transfering vibration to the upper mid-tweet structure, and to allow me to angle the mid-tweet inward toward the listener and angle the woofers outward toward the walls to adjust room effects.
Have you exchanged your right and left speakers so the tweeter and midrange are on the inside of the sound stage? This arrangement often sounds better to my ears. I design 3-way speakers and put the woofers in a separate bottom structure to avoid transfering vibration to the upper mid-tweet structure, and to allow me to angle the mid-tweet inward toward the listener and angle the woofers outward toward the walls to adjust room effects.
Never tried that actually. (did that before with other boxed speakers, though)
I thought, with such large toe-in (and the needs of near-wall positioning for practical reason), the inside arrangement would make the back of midrange even closer to the wall and the sides of TV (the previous TV was much larger and deeper). By the 3' width of the baffle, the difference is quite large. By this reason I keep them on the outside.
The separate mid-high and bass parts is a good idea. Will do in the near future.
I thought, with such large toe-in (and the needs of near-wall positioning for practical reason), the inside arrangement would make the back of midrange even closer to the wall and the sides of TV (the previous TV was much larger and deeper). By the 3' width of the baffle, the difference is quite large. By this reason I keep them on the outside.
The separate mid-high and bass parts is a good idea. Will do in the near future.
CLS said:
You are right. Phase and delay are two different things. You could try something, when your wife is not at home. Move the sub 23 cm and see, where it gets you. Then you will probably still need to adjust the phase. If it helps, add a delay correction to the sub.
One thing that could also be helpful is to put something on the floor that absorbs, like a thick carpet. And maybe as well on the wall behind the speakers, so that the reflected out-of-phase waves do not cancel out the in-phase waves from the front.
I found an interesting phenomenon, touching the cones when playing, the cone motions on main baffle and center sub are sometimes different in timing and amplitude. (I kept attentions on those drum hits and bass plunks, and felt the cone motions simultaneously. )
The woofer on main baffle works up to higher frequency, so it feels 'faster' and reacts to more notes, quite a lot of transient response coming along with music. While the woofers on center sub work only in the bottom octave, so the motions are slower and react to less notes. (no low bass contents on many of them)
For most of the bass notes, their "major initial attack" seem quite coherent -- they push my fingers simultaneously (I might say almost, with the unknown 'resolution' of my touching feel). The woofer on main baffle pushes with sharper edges and shorter movement, while the one on center pushes gentler and with slightly longer strokes. In spite of somewhat different 'feels', I do sense the 'starting time' of their motions is almost the same.
But in some particular notes, I can obviously feel their initial attacks not at the same timing. The one on main baffle takes the lead all the time in such case. And when this happened, the motions of the center sub would often be even slower and softer start with even longer strokes, and also stop the motions in longer time. That is, the center sub delays its whole action and produces lower frequency with higher amplitudes.
Back to the listening position, my ears can not distinguish all these detail differences among all three baffles anymore. They are blent into a big sound. Sometimes there'd be very short period of low bass 'tails' coming behind the initial attacks, which are mostly a sense of vibration or blowing of air. Fortunately they don't make things muddy or blurred because they are very different 'sound', so I can tell which is which.
I can only guess it's the complex overtone structures in the low frequncy music notes.
As to that 23cm path length difference, I decided to forget it. For such low frequencies with so long wave lengths, it's just too small to be considered. I have other things to worry about...
PS. I can move everything in the house whenever I want. My major 'problem' is that the wife and kid are just too noisy. I can enjoy my time only after I drive them into the bedroom, but that's usually too late to play loud, or I'm simply too tired to do so. Sigh...
The woofer on main baffle works up to higher frequency, so it feels 'faster' and reacts to more notes, quite a lot of transient response coming along with music. While the woofers on center sub work only in the bottom octave, so the motions are slower and react to less notes. (no low bass contents on many of them)
For most of the bass notes, their "major initial attack" seem quite coherent -- they push my fingers simultaneously (I might say almost, with the unknown 'resolution' of my touching feel). The woofer on main baffle pushes with sharper edges and shorter movement, while the one on center pushes gentler and with slightly longer strokes. In spite of somewhat different 'feels', I do sense the 'starting time' of their motions is almost the same.
But in some particular notes, I can obviously feel their initial attacks not at the same timing. The one on main baffle takes the lead all the time in such case. And when this happened, the motions of the center sub would often be even slower and softer start with even longer strokes, and also stop the motions in longer time. That is, the center sub delays its whole action and produces lower frequency with higher amplitudes.
Back to the listening position, my ears can not distinguish all these detail differences among all three baffles anymore. They are blent into a big sound. Sometimes there'd be very short period of low bass 'tails' coming behind the initial attacks, which are mostly a sense of vibration or blowing of air. Fortunately they don't make things muddy or blurred because they are very different 'sound', so I can tell which is which.
I can only guess it's the complex overtone structures in the low frequncy music notes.
As to that 23cm path length difference, I decided to forget it. For such low frequencies with so long wave lengths, it's just too small to be considered. I have other things to worry about...
PS. I can move everything in the house whenever I want. My major 'problem' is that the wife and kid are just too noisy. I can enjoy my time only after I drive them into the bedroom, but that's usually too late to play loud, or I'm simply too tired to do so. Sigh...
Are you using T-Bass on the main speakers as well or only on the subwoofer?
Did you check with Graham Maynard about the T-Bass circuit in combination with the active filter of the plate amp? As far as I remember the T-Bass provides some boost to counter the natural roll-off at very low frequencies. With an active low-pass set to 40 Hz you probably render that useless.
Did you check with Graham Maynard about the T-Bass circuit in combination with the active filter of the plate amp? As far as I remember the T-Bass provides some boost to counter the natural roll-off at very low frequencies. With an active low-pass set to 40 Hz you probably render that useless.
I use T-bass on all three of them.
My previous xover setting on main baffle was also very low, while the T-bass still overwhelmed the whole thing -- drawing extra current on the very low frequency makes it very different.
Although the xover on plate amp is set at 40Hz, T-bass still works. (The level on plate amp is set very low) Of course I can skip the T-bass and turn up the volume to get the simular response, but they just "sound" different. I've also tried playing with the xover point on the plate amp, when it's set higher then a point, I saw a dip occured at around 80Hz -- that's another 'proof' of T-bass's work.
My previous xover setting on main baffle was also very low, while the T-bass still overwhelmed the whole thing -- drawing extra current on the very low frequency makes it very different.
Although the xover on plate amp is set at 40Hz, T-bass still works. (The level on plate amp is set very low) Of course I can skip the T-bass and turn up the volume to get the simular response, but they just "sound" different. I've also tried playing with the xover point on the plate amp, when it's set higher then a point, I saw a dip occured at around 80Hz -- that's another 'proof' of T-bass's work.
Yes, the T-Bass works. There is little else it can do. And it will still have its positive effect on the ring in. It is just such a pity to boost the bass with it and then attenuate it again with a cross-over.
Could the dip at 80 Hz be due to phases adding up wrongly due to the cross-over's influence?
Could the dip at 80 Hz be due to phases adding up wrongly due to the cross-over's influence?
I should've described it the other way around.
I need the sub to handle only 40Hz and down. The bottom octave filling is all I need. In such case, T-bass is still working good.
I don't attenuate anything I need.
Above 40Hz (upto about 160Hz), the woofers on main baffles are already more than enough.
I can recheck the 80Hz dip to see if it is the cancelling or the T-bass. Maybe later....
By the way, I didn't check the impedance of this center sub until just now. It drops below 1 Ohm in the 30~50Hz range!!
This is what it looks like:
(Note the unusual scale)
If I made the scale of vertial axis for impedance peaks of normal woofers (200 Ohm full scale), then it would look like this:
edit: I'll update here if the plate amp is blown.
I need the sub to handle only 40Hz and down. The bottom octave filling is all I need. In such case, T-bass is still working good.
I don't attenuate anything I need.
Above 40Hz (upto about 160Hz), the woofers on main baffles are already more than enough.
I can recheck the 80Hz dip to see if it is the cancelling or the T-bass. Maybe later....
By the way, I didn't check the impedance of this center sub until just now. It drops below 1 Ohm in the 30~50Hz range!!
This is what it looks like:
(Note the unusual scale)
If I made the scale of vertial axis for impedance peaks of normal woofers (200 Ohm full scale), then it would look like this:
edit: I'll update here if the plate amp is blown.
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