OA-4 was the low budget version of the 1960’s great success the OA-5. The difference between these models is primarily that the OA-4 has a smaller cabinet and one tweeter less, giving a slightly degraded frequency range compared to OA-5. It also had a somewhat less robust electronic design.
The model lived very much on the success of the OA-5 and was an alternative for all who couldn’t afford the larger model. OA-4 looked just like the OA-5 but was smaller but the Swedish loudspeaker guru, Stig Carlsson, had made the design!
OA-4 was, like the other models from the sixties, equipped with one of the cheap but very good sounding eight inch full range units, Philips 9710 in the eight ohms version. The tweeters were also like the other models from the sixties the Peerless MT 20 HFC. Unlike the OA-5 it used only three inwards firing tweeters instead of four. The filters were of good quality and made with air cored inductors and quality capacitors.
There are no real bass reflex tubes in the speaker only two round holes cut out under the box that are damped with damping material. The downside of the simplified design without port tube is something that the OA-4 has inherited from its big brother, the OA-5. This is a cheap solution to save costs in the production. Apart from this the speaker is well designed with a well braced box, quality components and a thorough design. This results in a good sound reproduction for its time.
Still, the OA-4 was one of the models Stig Carlsson avoided to talk about when he describes his different designs.
RESTORATION
The important thing with 1960’s models V-1, OA-4, OA-5 and OA-6 is to avoid touching the screws holding the damping basket behind the full range unit. This acoustic damping device has been impedance adjusted at the Sonab factory in order to give each individual speaker a correct bass reproduction and the correct transient response. It should therefore NOT be changed.
You should however dismount the full range unit and check it according to RESTORATION AND UPGRADING. One problem with these models is how to mount gold plated terminals neatly on the lower backside when there are vertical damping braces on the inside of the cabinet walls? Normally we recommend using a circular cutter and using countersunk round terminals. With use of this method you loose the stabilising effect of internal bracing in the lower backside of the speaker. This effect is however acceptable. An alternate method is to mount the terminals off-centre on the lower backside. This does not look quite as nice but you do not need to cut the braces.
Another problem is how to access to the crossover and internal wiring due to the fact that the cabinets are glued together. There are two ways to access the inside, both unsatisfactory. One way is to remove the acoustic damping device under the mid-bass and poke about that way. When using this method the factory tuned bass response is lost and this can not be reset without the use of measuring equipment. The other way is to saw the baffle or bottom open and risk ruining much of the speaker. Also there are great difficulties in restoring the cabinet to airtight original condition. Our advice is therefore: Let the crossover and internal wiring remain in the original way, except for the capacitor that needs to be replaced by now. Note: On some models the baffle is screwed into the cabinet, in that case the above mentioned problems do not apply.
Apart from this OA5 Typ 2(Mk II) is restored according to Restoration and Upgrading.
TECHNICAL DATA SONAB OA-4 TYPE 2
According to the SONAB manual and/or “HiFiHandboken”
Volume: 36 litres
Measures: 24 x 49 x 43cm (W x H x D)
Weight: 9 Kg
Principle: Omni-directional. Bass reflex. (Editors comment: Well. The models lack a real bass reflex tunnel. There are only two round holes in the bottom with glass wool inside, almost a like Dynaudio’s bass vented systems. Due to production technical reasons, was Sonab’s excuse, greediness we say. Stig Carlsson never used this solution in any of the following models.)
Impedance: Approx. 7 ohm
Frequency range: 44-18.000 Hz
Frequency response: 48-15.000 Hz +/- 5 dB on the floor against a wall
Cross over frequency: 3500 Hz (6/12dB)
Full range unit: 1 22 cm 8 ohms Philips 9710 full range unit.
Tweeter: 3 pc 5 cm, Peerless MT20HFC. One 16 and two 8 ohms.
Finishes: Rosewood, walnut, teak, oak and white lacquer.
Prices (in SEK)
1973: 630:- per pair
2nd hand: 100:- up to 1500:-
Fully restored in good condition: May cost up to 2.800:-
Fully upgraded: Up to 4.800:-.
Martin Dellstig - Co-Editor
This page was last edited 2008-03-03 22:48:58 by Per