Sonab OA-4 was the low budget version of the 1960’s great success OA-5. The difference is primarily 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 solution.
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 and the Swedish loudspeaker guru had made the design! OA-4 Mk I was equipped with one of the cheap but very good sounding 8 inch wideband element Philips 9710 in 8 ohms version, together with three tweeters, positioned in a inbound/omni polar direction. The tweeters were identical to the tweeters in the “Coalbox” and OA-6 Mk I, the Peerless 2-inch MT 20 HFC. The filters where based on air winded coils and age resistant plastic film capacitor.
However the OA-4 Type 1 suffer as the OA-5 Type 1 do from the same kind of unnecessary problem: the top baffle with all the elements is countersunk around 6 cm from the top edge of the loudspeaker walls. This will of course prevent the high range and mid range sound to spread freely in the room. This and the lack of a real bass reflex port, was a failed effort to save costs.
This positioning of the speakers only caused unnecessary and unacceptable sound problems in the mid range and high range. The bass sound is not in level with the real bass reflex solutions.
Restoring the OA-4 Type 1 but keeping the original solution with countersunk baffle is not really worth the effort. The speakers with this design fail to provide the normal Carlsson quality level, which should be possible with the used elements and indeed is shown to be possible with the OA-4 Type 2 and OA-5 Type 2!
RESTORATION
The important thing with 1960’s models V-1, OA-4 and OA-5 both Type 1 and 2 is to avoid touching the screws holding the damp basket behind the wide range element. This damp basket 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 wide range element and make the modifications according to RESTORATION AND UPGRADING. The only solution to get an acceptable sound reproduction from the Mk I models is to raise the baffle to the right level. This allows the tweeters to freely spread their sound, all according to the basic idea with the design. It also clears up the mid range.
However this restoration work is rather extensive and tricky and may cause many problems. The original baffle is glued to the cabinet (sometimes screwed), and the inside bracing with limited space, makes it very difficult to remove without the mediocre chipboard to crack.
Should the baffles brake, making new baffles is a possible solution to this problem. Another problem is how to mount gold plated terminals in a good way in the glued cabinet with vertical braces on the inside of the cabinet walls. I have brutally used the circular cutter and used a recess plate connector. This also decreases the braces stabilising effect in the rear part of the speaker, but I think this is something you can live with.
Another problem is how to reach the coils and the cabling for replacement purpose. Since the cabinets are glued, there are only two alternatives and both are bad:
One is to dismount the damp basket below the wide range speaker and go that way. However this will also destroy the factory adjusted bass calibration, which isn’t possible to restore without measuring equipment. The other alternative is to use a saw to remove the whole baffle with a great risk of destroying too much of the cabinet and also cause great problems to restore the cabinet to original look. However if you decide to raise the baffle then it is an opportunity to change the cabling a.s.o.
It is much easier to replace the high frequency dividing capacitor usually standing on top of the baffle between the two front tweeters.
The alternatives are:
• A giant restoration and upgrading as described above on a low performance loudspeaker from the 1960’s series.
• Simple restorations according to RESTORATION AND UPGRADING whereby the original less well sounding attributes are restored.
On the other hand you will be the proud owner of a pair of original Carlsson speakers.
TECHNICAL DATA Sonab OA-4 Type 1
According to the Sonab instructions manual and/or HiFiHandboken
Volume: 36 litres
Measure B x H x D: 24 x 46 x 43cm
Weight: 9 Kg
Principle: Omni-directional. Bass reflex. (Editors comment: Well. The model lacks a real bass reflex tunnel. There are only two round holes in the bottom with glass wool inside, almost a variant of Dynaudio’s bass vented systems. Due to production technical reasons, was Sonab’s excuse. Or greediness? Stig Carlsson never used this solution in any of the following models.)
Impedance: approx. 7 ohm
Frequency range: 44-18.000 Hz
Frequency range: 48-15.000 Hz +/- 5 dB on the floor against a wall
Cross over frequency: 3500 Hz
Bass/mid range element: 1 pc 22 cm 8 ohms Philips 9710 fullrange element.
Tweeter: 2 pc 5 cm 8 ohms och 1 pc 16 ohms Peerless MT 20 HFC
Finishes: Rosewood, walnut, teak, oak and white laquer.
Price 1973: 630:- SKR per pair
Second hand price today: 100:- up to 1500:- SKR
Fully restored pair in good condition may cost up to 2.400:- SKR, if the tweeters are replaced with the Peerless CT62 up to 4.500:- SKR.
However it is doubtful if it is economically justified to replace the tweeters if the baffle isn’t lifted up at the same time.
No comments yet.