Sonab OA-5 Type 1 (Mk I)

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Sonab OA-5 is without a doubt the most sold speaker in Sweden ever. The whole Sweden suddenly wanted to have a pair of that time most well sounding and simply beautiful speaker. The standing matchbox design matched the design fashion in the 1960’s and the omni-directional sound fitted the new modern human, unplanned floating around in the sofa, on the floor and in the bed. OA-5 Type I was equipped with 1 pc of the cheap but very well sounding 8 inch full range speaker in an 8 ohm version, together with 4pcs omni-directional tweeters directed inbound of the same model that made such a success in the coalscuttle and OA-6 Type 1, the Peerless 5cm MT20HFC. The filters were designed with air cored inductors and age resistant plastic film capacitors. However the OA-5 Type I suffer similarly to the SONAB OA-4 Type I from a huge problem: the upper side of the baffle with all the speaker elements is lowered 6 cm below the speaker cabinet walls. This was together with the non-existing bass reflex tunnel, probably due to a bad sneaky attempt to minimize the production cost. The speaker position below the cabinet walls produces an unacceptable sound problem in the mid-range and upper range. The Carlssonplanet editor has in vain tried to restore several pairs of OA-4 and OA-5 Type 1, keeping the original design. In spite of every experiment with various solutions, the sound has never reached normal Carlsson level, even though the speaker elements and filters should provide – and also do provide in the Type 2 versions SONAB OA-4 Type 2 and SONAB OA5 Type 2!

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RESTORATION HINTS
The important part with the 1960’s models V1, OA-4 and OA-5 of both Type I and Type II is not to touch the screws holding the damping basket behind the full-range speaker. This damping basket has been calibrated regarding impedance at the Sonab factory, in order to give each individual speaker a correct bass sound and a correct transient response and should therefore not be changed.
The full-range speaker should however be removed and managed. The speaker may otherwise be restored according to the RESTORATION AND UPGRADING. But the only way to get an acceptable sound quality in the model is to raise the baffle to the correct level. This allows the tweeters to distribute their sound freely, as was the initial intention with the design. I addition the mid-range register is cleaned-up. This restoration work is however rather extensive and complex and may include many problems. The original baffles are often glued to the cabinet, and the underlying enforcement support and the small space makes it difficult to remove, resulting in a damaged fibre board. If you do this large modification, you should absolutely invest in the new Peerless polypropylene tweeters. For further information see RESTORATION AND UPGRADING. However there are more problems to it: How to attach gold plated connectors in the lower end of the speaker? It is also glued and may only be removed by brute force. It is in addition enforced with vertical damping stays in the middle inner part of the cabinet walls. You may use a circular-saw and mount the countersinked connectors to the left and to the right on the rear wall (as the Y-mounting anyhow will make them right- and left hand speakers). The terminal binding plate may be bought from HiFiKit www.hifikit.se. Another problem is how to reach to the inductors and the internal cabling for replacement when the cabinets are glued together. The only solution is to remove the damping basket behind the full-range speaker and go that way. But this also means that the factory calibrated bass calibration is damaged, and is not possible to restore without measuring equipment. The other solution is to open the baffle with a saw with the risk of destroying much of the cabinet giving large problems to restore the cabinet in original shape. If you on the other hand decide to raise the baffle you should also replace the internal cabling etc. It is however easier to replace the capacitor for the tweeter filter, usually soldered standing between the two front tweeters! The alternative is a somewhat fun but huge restoration of a speaker that completed still is one of the lower performing 1960-models. It could also be a simple restoration according to the RESTORATION AND UPGRADING instructions, giving a somewhat less good sounding speaker, but still in original form. The Y-mounting of the tweeters on the OA-5 is recommended by Carlssonplanet och Stiftelsen Stig Carlsson, for instructions refer to News letter 5. If you also want detailed instructions including figures on the Y-mounting, click here. If you get any problem, contact CarlssonPlanet!

TECHNICAL DATA Sonab OA-5 Type 1

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According to Sonabs handbook and/or ”Hifihandboken” (NOTE! All data below refer to OA-5 Type 2, which may have had a slightly larger volume, due to its raised baffle)
Box volume: 47 litres
Measures: B x W x D: 24 x 61 x 43 cm
Weight: 10 Kg
Principle: Omni-directional, bass reflex type. It is not a traditional bass reflex box due to the fact that there are no port tubes, just two tuned ports in the bottom of the cabinet similar to Dynaudios “Variovent” or the ScanSpeak Flowresistance.
Impedance: Approx 7 Ohms Frequency range: (DIN 45500): 37-18.000 Hz
Frequency response: 42-15.000 Hz ± 4dB on the floor against the wall
Crossover frequency: 2700 Hz
Mid-bass: 1pc 22 cm 8 Ohms Philips 9710 full-range speaker
Tweeters: 4 pcs 5 cm 8 Ohms Peerless MT20HFC
Finishes: Rosewood, walnut, teak, oak, natural pine and white lacquer. Also available in red, blue or green stain.
Prices: 1973: 745 SEK per pair (Type 2).
Second hand: 100 up to 1.200 SEK.
Restored: If well restored up to 2.500 SEK. They can well be worth this if one is not too critical regarding the musicians’ lateral placement in the sound image. If the baffle is raised and better polypropylene tweeters (Peerless 62CT13) are mounted, new filter cabling, etc. and the speakers otherwise in good condition, the prices may even reach up to 4.500 SEK.

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Martin Dellstig – Co-Editor

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