Sonab OA5 mkII

Troels Gravesen july 2009

One of my neighbours' daughter threw a party and cranked up the volume of daddy's stereo and fried the voice coil on one of the Philips 9710 drivers in these vintage Sonab OA5 speakers. Poul Erik had bought these speakers at the local second hand market - he was there before me - and had come to like the vivid presentation of these high-efficiency, easy driven speakers of the Seventies. So, he googled "Philips 9710" and the connection was made. From eBay Poul Erik bought a pair of 9710s from Germany and one of the drivers was suitable for restoration. The other had had some surround repair - too much white PVA glue - and a resulting Fs of some 90 Hz. Too bad, but that's how goes buying on eBay from second hand dealers knowing nothing about loudspeakers.

The Sonab range of speakers from the Seventies are speakers I've long been looking for in order to find out what the 9710 could do in an omnidirectional set-up, and this OA5-mkII did call for a few surprises.

First of all I assume this is a Sonab OA5 mkII from reading the excellent website on these speakers:
The technical data suggests the following:

  • Volume: 47 liter
  • Dimensions: 24 x 61 x 43 cm
  • Weight: 10 kgs
  • Principle: Omni-directional bas-reflex loaded speaker (although it does not have port tubes, see later)
  • Impedance: 7 ohms
  • Frequency range: 37-18,000 Hz
  • Crossover frequency: 2,700 Hz
  • Bass/mid: Philips 9710
  • Treble units: Peerless MT 20 HFC

This is all fully in accordance with my findings, except for the 37 Hz bass extension. The speakers have no Sonab badges and taking them apart made me wonder whether this is really a commercial product or some cleaver cloning of the same. I believe this is a true Sonab product from the cabinet work, but the 9710 basket and the crossover PCB suggests amateur work.

The principles of these speakers can be found on the carlssonplanet website and I won't get into details with this, but the cabinet calls for some comments. Take a look a the photos below. What is this? A 47 liter cabinet with the 9710 driver placed in a metal mesh basket with some heavy glassfiber material between driver and mesh, making a low-pass filter that can be adjusted due to the basket being suspended on three screws that allows you to increase or decrease the acoustic resistance of the fiber material. An acoustic vent, sort of. Something similar can be found on some Lowther drivers. Next we have a cabinet divided into two compartments by two heavy fiberglass sheets. The lower compartment is open to the outside by two 54 mm holes at the bottom, no tubes here. In reality I guess we can call this a two-step aperiodic system with progressively diminishing acoustic resistance towards lower frequences. What is sure is this: I have never seen a more valve-amp friendly impedance than this, only varying 7 to 10 ohms from 20 to 1000 Hz! Take that! Consequently phase angles are modest, not to say almost flat.
Overall sensitivity is around 90 dB/2.8 volts taken from the midrange with a response rising some 10 dB towards 3-5 kHz. No surprise, the 9710 only has a modest 0.5 mH coil in series, not nearly enough to tame the terribly 9710 treble peaking. However, these speakers are not - fortunately - meant to be listened to on-axis and a rising frequency response is expected from an omni-directional speaker to balance the overall in-room sound.

From setting up the speakers first time it becomes immediately clear that these speakers have a bright presentation. Very bright indeed with way too much treble. There will be a few recordings that will stay on the shelf for good. No matter how the speakers are placed in the room, this stays the final verdict. I badly miss a pre-amp with tone controls to tame the treble region as would have been the case back in the Seventies. If I had to clone the OA5, I'd probably use a 2nd/3rd order filter to tame the treble range. Based on modelling this would also allow the drivers to be connected with same polarity, which may not be such a big deal due to the placement of the tweeters.
The tweeters were not the problem here. The troublemaker is the 9710 treble peaking and only a higher order filter can tame this. Having four tweeters allows a resonably low point of crossover and there's no point in not bringing down the 9710 treble peaking. However, these speakers were for loan, not for tweaking.

sonab_oa6-ii_18-2 sonab_oa6-ii_18-5

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.

sonab_oa6-ii_18-9 sonab_oa6-ii_18-10

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

sonab_oa6-ii_18-12 sonab_oa6-ii_18-1

Right: Damping material dividing cabinet into two sections.

sonab_oa6-ii_18-7 sonab_oa6-ii_18-8

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.

sonab_oa6-ii_18-13 sonab_oa6-ii_18-16

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

sonab_oa6-ii_18-15 sonab_oa6-ii_18

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.

sonab_oa6-ii_18-17 sonab_oa6-ii_18-11

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.


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.

sonab-ao5-ii_1mtfb_-22.2db sonab-ao5-ii_1mtfb_side_-22.2db

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.

sonap-both_mls_imp step_resp_1mtside

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.

sonab-ao5-ii_mls_against-wall_1 sonab-ao5-ii_mls_against-wall_2

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 2? 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.

Оригинальный источник: Sonab OA5 mkII

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