Isophon BS35 loudspeakers


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.


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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.


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.



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.


13 thoughts on “Isophon BS35 loudspeakers”

  1. Pingback: Isophon BS35 speakerset complete | Kaizer Power Electronics

  2. Wooo, nice speakers! The drive units must have been pretty expensive back in 1969.

  3. If you look at the first catalogue page on this site, it says the price for the set for one speaker is 160 Deutsche Mark. Calculated to 1969 rates DM US dollars and then using a US inflation calculator, we end out with todays price of 250$ US for a set, just for the speakers and filters.

  4. Hi Mads,

    I am very happy to see what you have done with speaker units you received from me…and very glad that you like the sound from the Isophon speakers. After I shipped the Isophon units to you, I have to admit that it is difficult to live with my Acoustic Research AR3a only. I have bought another pair of Isophon BS35 for my old setup, and they sound just wonderful together wit my Leak St. 20 🙂

    Best regards

  5. Hi Kim

    I noticed that I had some traffic from a vintage audio forum not long ago and saw that you had posted this article. I still enjoy my set with the same EL34 amplifier.

    I also succeeded in finding a pair of JBL 4333 speakers and currently have one of two 50 Watt 6P45S mono blocks built.

    Great that you found yourself a new set, now that you missed it so much, because I would not have been ready to sell it back 🙂

    Kind regards

  6. Hi Kim

    I noticed that I had some traffic from a vintage audio forum not long ago and saw that you had posted this article. I still enjoy my set with the same EL34 amplifier.

    I also succeeded in finding a pair of JBL 4333 speakers and currently have one of two 50 Watt 6P45S mono blocks built.

    Great that you found yourself a new set, now that you missed it so much, because I would not have been ready to sell it back 🙂

    Kind regards

  7. Hi Mads,
    A pair of JBL4333 speakers is quite another game when you are about to choose amplifier(‘s) 😀 …15″ JBL drivers like LE15a and similar loves current, if you want the attack and punch that 15″ drivers from JBL is capable of. Most tubeamplifiers can’t deliver sufficiant current in order to keep control of the relatively heavy swinging system in a 15″ woofer driver. ….on the other hand…as you know at low level the tubes has better soundstage…if it possible to keep the fabulous JBL4333 at low level:-)

    Best regards

  8. Hi Kim

    Our current living room is not so big, so my EL34 amplifier does a decent job at playing the room up loud. That is why I am building the 50W mono blocks you can see here on my site. So that the EL34 can return to full duty on the BS35 🙂

    I am not sure what transistor amplifiers I would like or even look at, that is what I love about tube amplifiers, it is fairly easy to build high performance and high quality yourself.

    Kind regards

  9. mk,

    The JBL drivers you’ve gotten hold of (like the LE15 mentioned above) are typically quite efficient with edge wound voice coils; this was JBL’s particular forte (along with horn loading for mid and high frequency drivers) which is one of the reasons JBL drivers were quite commonly used by smart sound designers for those large and loud PA systems used by rock music acts (the Grateful Dead were the archetypes of this approach, and the last I heard they only used JBL high efficiency woofers). So, if properly matched to the air, the new JBLs won’t need much input power for more than sufficient (more or less) undistorted volume. You might be able to continue to stay in the low power tube amp world.

    That impedance matching is quite the difficult business, and since DIY folk can get into the game (it’s done by the mounting / enclosure / placement / stuffing / …), there’s a lot of information and misinformation and myth and mythstake and tin foil hat thought. Beware, and be sure to turn your BS detector up to 11!

    Electrical filter theory is well developed (at least to a first and second (perhaps third) approximation) and when it was formally applied to acoustic impedance matching at the troublesome lower frequencies, in the context of loudspeaker drivers (the usual electro dynamic sort, not electrostatic or planar or Heil or plasma or horns or …), it became possible to get a lot closer to rational enclosure designs. The work was done most thoroughly first by Thiele in Australia, and then more accessibly by Small in Australia and then the US. Which is why the process is referred to as Thiele-Small analysis. Wikipedia is pretty good on this, to a first approximation, anyway.

    You might want to check out the Elliot Sound Pages and his article on the QB5 alignment, based on some academic computationally aided work done in Central Europe (in Germany? or maybe Prague?). The problem is to maximize the impedance match between driver and air, without exceeding electrical power handling capacity of the voice coil, without exceeding material limits (glue softens (and then melts) generally before voice coils melt), while coping sensibly with changes in electrical properties (inductive and resistive impedance) of the voice coil as a result of increased heating, without exceeding mechanical excursion limits, while gaining improved frequency response at the low end, etc Just pouring on more power, like the high SPL car audio people do to compensate for the terrible conditions into which they are trying to squeeze in and operate speaker systems, is not a panacea for increased excursion requires quite compliant suspensions (which increase distortion), and huge excursions (which increase distortion by operating close to the magnetic edges of the magnet field motor), and heavy duty voice coils which increase distortion because they mass a veritable ton more than is needed otherwise (and all this is expensive, even if perhaps not quite so expensive as the prices charged!). Quite a juggling act.

    Rod Elliot’s report on the new (that is, new a decade or two after Thiele and Small) variation suggests a way to do a better job than before of balancing all these. That it involves some interesting electrical modification of the signal sent to the amplifier handling the woofer is just a bonus.

    You might be able to exploit some of this in your next loudspeaker system design.

    Best wishes and keep up the good work.

  10. Pingback: Isophon BS35 Speakers Progress Update #2

  11. Pingback: Isophon BS35 speakerset complete

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