Sonab OA5 mkII
Copyright 2009 © Troels Gravesen 

CROSS-OVER    CABINET    MEASURE-MENTS    READERS' COMMENTS

I guess these speakers are high WAF, modest size and exquisite veneering.
The speakers came with a black stocking around the 9710 driver. Removed here for the photo shoot.

The metal mesh basket holding a ring of fiberglass material. See next photo.

Right: Damping material dividing cabinet into two sections.

Base plate covering two bottom holes of 54 mm diameter. Apparently no feet was ever attached to these speakers.
Some small rubber or felt pads would do well in stabilising the speaker on the floor. A DIN socket is seen at the end
of the base plate for connecting to the amplifier. I removed this a added a pair of heavy screw connectors.

Left: Cabinet interior. Right: Two sheets of fiberglass placed as shown on drawing below.

Left: Two renovated 9710 drivers, i.e. cone suspension added a thin layer of flexible coating.
Right: I rarely have to sew making loudspeakers, but this calls for a stocking of front grille cloth
as the drivers pointing upwards are true dust collectors.

Right: Crossover is simplisity itself. 0.5 mH to the bass drivers and a 2nd order filter for the tweeters consisting of 6.8 uF cap and 0.5 mH coil.
I feel certain the crossovers are original as few diy'ers would use wax for coating the coils. The film capacitor (polyester) will probably last forever,
so no reason to change this with some PP that may not be much better. The coil in series with the 9710 is from pretty thin wire, but we're not
dealing with heavy loads of amperes here, so not much would be gained from changing this one either.

Sonab OA5 mkII cabinet

Cabs are made from thin 12 mm particle board veneered on both sides to make an overall thickness of 13 mm. Bottom and top are made from 16 mm particle board. A number of fillets are placed inside to strengthen the cabinet and damp resonances. Se photos above.

These cabs are lightweight! Some 10 kgs in total incl. drivers, crossover and damping material and we may expect a highly resonant system but this doesn't appear to be the case. The bass quality is anything but boomy. Not reaching deep, but short and fast, undoubtedly due to overall construction and not least the special use of damping material.

This is how the damping sheets are arranged in the cabinet.
The 9710 driver is placed to the left here.
Cross braces are placed symmetrically.

Sonab OA5 mkII crossover

OA5 mkII crossover design, simplicity itself. 1st order to the 9710 and 2nd order to the tweeters.
All tweeters are 8 ohms.

Crossover layout.
The PCB looks like the very first PCBs I made many years ago with a marker
directly on the copper side before etching. That's what made me wonder if
this speaker is a cleaver clone or the real thing.

Measurements

How on Earth do we measure an omnidirectional speaker spewing sound in all directions? Definitely not an easy task and what's shown below is with every possible reservation except for the impedance measurements. At least these can't be argued.

Left: Conventional gated measurement, 1 meter distance at 20 cm above top panel height.
Speaker placed like A (see below). Blue = minimum phase.
Right: Red = position A, Blue = position C.

Various speaker positions during measurements. Position B not shown here.

Here a measurement on-axis, 0.5 meter distance normalised for 2.8V/1 meter.
System sensitivity is around 90 dB/2.8V/1m valued from the response @ 500 Hz.

Left: Impedance of left and right speaker showing decent bass tuning, i.e. placement of damping material.
Right: Step response measured from position C (side).

The last four graphs are from various measurements with speaker against a wall with different distances to side wall.

Left: 9710 side against wall and 0.5 m to sidewall (blue) and 1.5 m to side wall (red). Right: Tweeters against wall and speaker 0.5 m to side wall and 1.0 m to side wall (red). With the 9710 not up against the wall and 1 meter to side wall makes a decent smooth, albeit seriously rising response. This is against most recommendations I can find that will suggest the 9710 being close to the wall, but this seems to consistently produce a serious dip in the ~130-200 Hz range, something that will add to the thinness of sound. It lacks weight as most music has a lot of energy in this range.
By the way: These speakers do not say much below 50 Hz where the 37Hz claimed low-end extention seems fairly optimistic. The heavy cabinet damping is partly responsible for this. The 9710 can do better, but not at the same time produce an impedance profile like seen here.

Final comments

Taking the Sonabs to our living room I tried all possible speaker placements. From our sofa, some 2½ meter to the speakers, I found the speakers placed flat against the rear wall to produce the best sound. Having the speakers standing out from the wall with either the 9710 or the tweeters against the wall just didn't work. Phase tracking between drivers is fairly good when I measure the response on-axis, i.e. like measuring a conventional speaker (data not shown), but despite treble coming from all over the place, having the drivers "on-line" appear to compromise driver integration, thus flat against the wall with a 20 cm distance between cabinet and wall.

These speakers are quite good at "making music in a room" - if you get my meaning. Forget about pin-point imaging and the like. Listening to the ever-lasting Jazz at the Pawnshop, all the usual details of rattling glasses and people speaking is pretty much gone. Overall the sound lacks bass and the middle and upper treble is dominant and prevents you from turning up the volume, which again will make the sound even slimmer as we do need a certain loudness level to energize the room. A drummer seriously hitting his cymbals... let's be honest about the treble coming from 5 drivers in different locations, it just isn't good from these speakers.

This may all sound a bit negative, but I still think omnidirectional speakers are interesting and I may well give it a shot some day. The later Sonab models with tilted midbas and quarter-circle of tweeters may sound differently and seems more like something that did address the issues discussed above. Stig Carlsson's persistent work on speaker and room integration is admirable to say the least but this OA5 wasn't the final answer to this most troublesome area.

Response to article

Hi Troels,
Firstly, thanks for your web-site. I have really enjoyed reading your articles over the past few years. The first thing I do each day as I logon to my PC is to check and see if there are any additions to your web-site. You have provided a great resource for the DIY community. Secondly, your article on the Sonab speakers is very interesting. I am hoping you continue to look into omnidirectional speakers - the audio community seems very divided on their merits, however it seems that good omnidirectional speakers have some very passionate supporters. I found these reviews of the Shahinian Arc speakers, which may interest you. I think the Arcs use SEAS  CA22RNY and 27TBF/G drivers. The slope of the Arc front panel is consistent with your comments about the later versions of the Sonabs. Anyway, thanks for the great work. Regards, Campbell A.

(Response from Johan/Norway, suggesting this OA5 mkII is indeed a kit version, which explains the missing badge. Furthermore there should have been a mesh support on top of the speaker and the 9710 basket for the damping material was made from plastic in the original. Thanks Johan for your comments.)

- jeg er også glad for det at du har prøvd ut Sonab OA 5 mk II, og som Carlssonentusiast vil jeg bemerke følgende. De svake punktene gjengivelsen henger sammen med to ting: foreldede elementer og manglende avstemming av kassen. Både Philipsbassen og Peerlesstweeterene har sine svakheter, og de kan med fordel byttes slik dt er anbefalt på Carlssonplanet og Faktiskt. Man kan også sette inn et bassrefleksrør slik enkelte entusiaster har gjort, med godt resultat. Det ser ut som du har fått en kit-modell av OA 5 mk 2, altså et hjemmebygg. Det går frem av dempekurven, som er av plast i orginalen og av den manglende runde pinnen som skal stikke opp mellom diskaneten på orginalen. Hvis det mangler emblemer på fronten også så er det nok temmelig sikkert et kit som noen har laget. Jeg er av den overbevisning at du har tenkt disse tankene, og jeg har et håp om at du skal prøve ut en modernisert verson av OA 5 mk II. Igjen takk for mye intressant lesning! Mvh Johan.

Hi!
It’s fun to read when somebody really looks into to something. In your piece of the Sonab OA-5 there are a few things that strike me as odd. I think, as you already suspect, that these are not the same as original factory-built units. Some things look correct, others don’t.
Correct looking:
The box (the four side walls). The damping-material and it’s position inside the box.
Incorrect looking: The support for the net between the tweeters is not there (see picture 1 below). The baffle it self looks strange but perhaps it is just repainted. The crossover looks like some I have seen on kits for the “Pop-boxen” except for the capacitor which is much newer than that. (Pop-boxen = DIY-version of OA-5).
This damping basket looks like nothing I have seen from Sonab, either it looks like picture two below or it’s made of plastic on the ones I’ve seen. The bottom of the speaker is not as it normally is, perhaps the chipboard under the speaker has been replaced. The DIN-plug is also not original.

Perhaps this is a speaker that has been refurbished with home-made parts?

Listening:
When you talk about your listening impressions they don’t sound to me as the numerous pairs I have had at home. They are normally not so bright, rather the opposite! If you are standing up close to the speaker they are of coarse very bright since you then also get the direct radiating sound from the speaker (9710). The tweeters are often “hissing” producing terrible “sssszzzssz” noise and the bass is not too bad. With good tweeters they are nice with Dire Straits and similar music. If the tweeters are in working order but hissing you can fix them somewhat, see: http://www.carlssonplanet.com/renovation.php?lang=sv&. Scroll down to: “Coata gamla diskanter” and read on.
So my question is really if you have tuned the damping-basket before listening / measuring? There is a method to tune the basket for the correct damping using a tone generator, a volt-meter and an amp. If the damping basket and its material is too tight you would get poor bass. If you check out the download area of CarlssonPlanet you’ll find three documents that might be of interest if you have not already seen them: http://www.carlssonplanet.com/downloads/index.php?act=viewall. OA-5K Broschyr. Popboxen – original article from 1964. Carlsson Ortho-Acoustic loudspeakers: Design and Performance principles. Some are in Swedish but I think you’ll manage. I hope you will find something of interest in these ramblings because I found your study interesting! When somebody does something like this about something that interests me I’m glad about it. So I try to respond with some input of my own. Would you like a copy of how the damping is to be adjusted? Can the damping basket be adjusted on your speakers?
Thanks a lot for the interesting reading, Martin.

My comments in bold:

Hej
Tack för din intressanta genomgång av Sonab OA5.Tyvärr är  det nog ingen kit(byggsats)utan ett DIY. Flera detaljer talar mot, t,ex
- Pinnen mellan diskanterna saknas
- Det ska vara en tygpåse (original fjernet af mig da den var i dårlig stand)
- Det ska vara tätningslist under basen (original fjernet af mig og erstattet med gummiliste)
- Basen ska var fäst med gummibussningar
- Det ska vara fästbyglar till diskanterna
- Baffeln ska vara försänkt flera cm
- Delningsfiltret ser inte rätt ut
- Kondensatorn är för modern (filmkondensatorer fandtes længe før Sonab, så ??)
- Jodå,det ska finnas ett Sonab-märke
- Dämpkorgen har fel metallnät
 
Detta fann jag vid en snabb genomgång.Kan precisera om du vill. Jag har några bilder från kitet till Typ 1 om du är intresserad?
mvh, Rolf

Hej, det här är vad jag har om OA5 kit.Obs att det är typ1,men den stora skillnaden är baffelns läge och ett högre galler.Bilderna är scannade ur en tidning-TfA nr 18 1968. Konstigt att inte någon enda människa tycks ha någon byggbeskrivning i original  kvar.Det såldes ju massor....mvh Rolf

Click image to download 4 kit files (zip file, 3MB). Pics 2 and 4 from left taken from carlssonplanet website on OA5 mkII.