Isophon BS35 loudspeakers

Introduction

I started building a valve amplifier and thought to myself that I needed some proper Hi-Fi speakers from about the same age as the valve amplifiers was refined to the art it is today. I got recommended a restored set of Isophon speaker units from 1969 – 1976 that I needed to build a enclosure for.

 

Safety

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Considerations

Should you let old be old or use new knowledge or better components that the original, the answer to that question is, replace old components with newer where it is necessary.

Enclosure layout and size. I followed the recommended enclosure volume for the bass unit and kept the original front layout with the bass in the tweeters on top, bass in middle and middle at the bottom.

Adding the a newer additional tweeter to help the original in the band of 15 – 20 kHz.

Adding the possibility to adjust tweeter and middle tone with – 2 to 3dB steps through switch-able series resistors.

 

Construction

Here is the schematic of the wiring for the speaker.

The speaker set is originally the Isophon BS35 from 1969

The speaker set uses the following units, bass PLS245 60 4, middle-tone HM1318 and tweeters F713 and KK10/8

The Isophon catalogue pages from 1969

The Isophon catalogue pages from 1976

24th December 2010

My father and I began the construction of the enclosure on Christmas day

5th February 2011

Over the months since Christmas time was spend on filling out any gaps or holes and sand it down, this was done 3 times to even out pieces that was not totally aligned.

15th April 2011

The enclosures was painted by a professional painter company. The finish really does show how careful one have to be with woodworking, all the small dents and errors can be seen easily. This is what makes these speakers mine and unique!

With the units mounted in the enclosure it really sets them off and beautiful and simple, old speakers with a modern touch.

Here is a shot of the internal wiring, filters and resonance damping material on the walls.

Conclusion

Building this speaker set took many more hours than I expected, one does not simply put a wooden box together and make it look good. It takes skill and patience to get a good result in wood working when it is painted professionally.

The sound of the speaker set is absolutely amazing, the music suddenly opens up and you can hear each instruments more clearly for themselves.

I have for many years only had speaker sets for my computer with satellites and a sub-woofer. What I discovered was that what I used to listen to was over driven bass, muddy vocals and instruments that all blended together. It is a joy without comparison to anything I ever had to listen to.

I am now forever a person that will only own stereo speakers with individual speakers and filters for tweeter, middle and bass. Full-tone speaker units does no longer exist in my world, they tear music down and reduces it to garbage.

 

Demonstration