Open Baffle Speakers...No Bass??? Try this...

[Popcorn]

WOW!!! That array is enough to blow anybody's socks away. Was there a need for any notch filters for your speakers? Any shout or beaming needed to be tamed?
Would the an array of BG20 give comparable performance?

[Paul_Hynes]

The arrays can go very loud if required as they have an efficiency of around 105 dB for 1 watt and they can theoretically handle 560 watts each side. The benefit of this capability relate to dynamic range and low distortion. The peak levels of a full concert orchestra are delivered with little compression. Rock music has a powerful delivery too. In fact all types of music work well on these lines with effortless delivery.

I use the drive units full range, with no filters or frequency correction, as I prefer to keep the coherent sound I get this way. Anything I have done to the line that has caused frequency dependant phase shifts has had a bad effect on the musical presentation. Hence no additional large and small drive units for the frequency extremes as the required crossovers would cause phase anomalies. I also wire all the drive units in each line in parallel to avoid driving drive units through other inductive voice coils. The amp has been designed to drive the line’s 0.75 ohm nominal impedance.

The line nicely compensates for drive unit low frequency roll off because of mutual coupling between the drive units and the bass goes very low with a natural presentation. No need for sub woofers. Line arrays have very little room interaction too so they present a nice stable sound field. Combing effects at higher frequencies are not obvious in the seating area so there is no requirement for notch filters in practice. We are so used to combing effects in real life that we can generally ignore this type of effect. Even with a single point source combing effects are caused by acoustic reflections around listening rooms and we have learned to ignore them. There is no perceived shout or beaming with the lines.

The BG20 is a much lower cost drive unit and the drive unit parameters are different. The B200 has a Qts more suitable for open baffle operation, a larger magnet and a cast chassis. I’m not sure but the BG20 may have a pressed steel chassis. A pair of BG13 lines were built prior to building the B200 lines. They were actually very good considering the low cost but they definitely required a sub woofer to fill the bass end out. I did not want to have to use a sub woofer so I decided to see what I could get out of the B200. The B200 is a much better driver sonically, than the BG13, which you would expect from the cost difference. I am very, very happy with the bass with the B200 line. The BG20 should give a good bass response in the line as the specification shows it goes down as far as the B200. It should also give a good account of itself, as it is the bigger brother of the BG13. If the budget is limited it is worth trying.

Before I purchased the all the drive units for the B200 arrays I acquired two B200 drive units and fitted them to a 2’x4’ open baffle so I could assess the drive unit’s capabilities. I let them run for a month in the family AV system so they would get chance to run in somewhat before I made a final choice. They were a bit rough and ready at first but had settled nicely after a month with a very good all round performance although they were a little light in the bass end just operated as a pair. This is not a problem in the line array as mutual coupling reinforces the bass well.

I would suggest that anyone contemplating building line arrays should get a pair of their proposed drive units for trial first and if you can live with the drive units on open baffles after a month of running it should be safe to continue with the line construction. Remember not to push a single pair of drive units too hard especially if they have a roll surround with long throw capability. The power handling and efficiency will be much better in the line.

Regards
Paul

[Popcorn]

Paul,
Thanks for your detailed explanation. Your array design looks like another interesting DIY project. However, driving the line's 0.75 ohm nominal impedance would seem like the biggest challenge here. Can an amp designed for 8 ohms speakers handle this? What sort of amp is needed?

What is the dimension of your line's baffle? How is it supported behind? I can see that you have used aluminium extrusions.

Thanks very much for the insight.

[Paul_Hynes]

Most normal amplifiers will not like to see the 0.75 ohm load. Interestingly, in my system, I do not require any voltage gain in the power amp because of the low impedance load doesn’t require large voltage swings to generate high sound pressure levels. I just use a source follower mosfet in class A.. Both my phono stage and my Altmann DAC can provide more than enough signal swing for the levels I use when listening. I use an LDR volume control between the two signal sources and the mosfet. The forward path is so simple and amazingly transparent because of the simplicity. The power amp circuit is :-

http://i700.photobucket.com/albums/ww7/paulhynes/SE1.jpg

The baffle is 2 metres high and 400mm wide. The drivers are offset from centre. There is another ladder frame behind the baffle arranged at 90 degrees as shown in :-

http://i700.photobucket.com/albums/ww7/paulhynes/Picture039.jpg

The frames are constructed from aluminium extrusions used for constructing exhibition stands. I had heavy-duty spikes turned from brass bar to fit the ends of the extrusion so a firm coupling to the floor is possible. The centre of gravity is a little too forward with all the heavy drive units on the front face, so I have wired the back frame to the wall behind via a heavy duty steel cable to prevent any accidents if anyone walks into the frames while dancing to the music. This is necessary because the whole assembly is very heavy and it could hurt someone if it fell on them. I keep thinking I should re-design the frame for more stability but this year has been a busy one. Maybe I will get around to this next year.

Regards
Paul

[Popcorn]

Paul,
Very interesting. The power amp circuit is very simple. Is that all that's between the source and the speakers besides the LDRs? What voltage are you using to power this circuit?

This definitely looks like a "must try" for me and I will be looking for more of your personal advice when I embark on it.

Thank you very much for the education.

P/S. Paul, I found this http://www.audiocircle.com/index.php?topic=86222 and it provided detailed info on how to build the SE1. My questions are answered. Thanks.

[Paul_Hynes]

I wanted to keep the signal path as clear of amplification stages as possible and the “Less is More” approach has worked very well in this instance. The high efficiency of the line arrays and the low impedance load allows removal of all voltage amplification stages in the line preamp and the power amp. Obviously the LDR volume control needs buffering from the low impedance loudspeaker load and the single ended heavy duty IXYS depletion mosfet in the signal path does this admirably configured as a source follower. The SE1 circuit was designed to allow DC coupling to the loudspeaker with minimal DC offset. The mosfets should be mounted close together on the heatsink so they thermally track thus minimising offset drifts.

Incidentally, the SE1 will deliver around 8 watts into 8 ohms with +/- 12v supplies, so it can be used with high efficiency “normal” loudspeakers (over 100 db for 1 watt) probably without a voltage gain stage.

Into 0.75 ohms the power delivery is restricted by the available current in the class A stage to around 2.5 watts. This may seem rather low but I can assure you that, with an efficiency of around 105 dB per watt, and the fact that the near-field sensitivity only drops by 3 dB with doubling of distance, rather than the 6 dB with normal speakers, this will go louder than I require before distortion becomes a problem. I had intended to increase the power delivery using parallel mosfets to around 20 watts into 0.75 ohms, but I have not found this to be necessary in my installation.

Sondale’s description of how to build the SE1 on AudioCircle is very detailed and you can always ask questions about anything that is not clear. When I get some time I will be publishing the plans for the loudspeaker construction on my AudioCircle circle and setting up a “questions and answers thread” for the arrays and the SE1 and the more powerful CIRC1, which is a balanced “Circlotron” output stage with a very simple configuration.

Regards
Paul

[kp93300]

I am a fan of OB sound also.
 My first contact with Ob sound  is Alon 2  .
My present speaker  has OB mid and high but boxed woofer. The OB 5 from GR research.

I have place an order  for another speaker that is O.B full range. Each speaker has 2    12in woofers and a coaxial mid and high. This is V2 from GR research also.
regards
kp93300