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Monday, January 11, 2021

KLH Model Twenty-One II -- Is My Speaker Dried Out?

A few years back Rogier PA1ZZ very kindly sent me a box of electronics parts.  Included was an FM table-top radio with a nice walnut case.  Thanks Rogier! 

I hadn't looked at the receiver in years, but this week I dusted it off and looked it up on the internet.  Turns out that it is kind of famous.  It was produced by the KLH company.  The K stood for Henry Kloss, one of the giants of Hi-Fi audio gear.  Henry appears in the picture below. 

I got the receiver working, but it sounds awful.  It sounds much better with an external speaker, which is disappointing because the internal speaker was the main attraction of this receiver. It even has a little badge on the front panel trumpeting(!) its "Acoustic Suspension Loudspeaker."

I'm wondering if the problem is in fact the speaker.  The cone looks intact, but it seems very dried out.  It has been more than 50 years...  What do you guys think?  Picture above.  Any other suggestions on what to do with this thing, or how to make it sound better? 

Some KLH history:  



KLH receiver with pillow


  1. Bill - There are a couple of possibilities here -

    1.) The speaker has been stored in a damp location and the speaker cone has absorbed moisture causing it to warp and drag on the magnet assembly.

    2.) The flexible mounting material for the cone has dried out -oxidized- which can cause the speaker to have a "rattle". It can also cause the cone to warp with a resulting drag on the magnet assembly.

    The mounting material - usually a type of foam rubber - is available, and can be replaced. If the cone is warped, you are looking at having the speaker re-coned. Do-able, but a pain in the butt for FYI the first couple of times you try it.

    Bruce - KK0S

  2. Early speakers had a fabric suspension.This was likely an early foray into rubber, likely a rubberized fabric. Today's are a dense 'micro'-foam for acoustic seal They are also a deep 'roll' in, out or both, to allow travel for bass response, where many were merely rippled. It's a 4-way compromise: travel and freedom, seal and voice-coil centering. Function or authenticity, what's your choice?

  3. Bill,
    Try using two index fingers on the left and right side of the center dome.
    Push in gently and see if there is resistance from the voice coil being bound up.

  4. The speaker cone doesn't seem at all restricted. And the material around the cone seems fine. So I'm not sure why it sounds so bad. I suppose it might sound better once I boxed it all up and sealed it. It is also supposed to have a "pillow" in the cabinet. I'm not kidding. I think I lost the pillow. But I don't think that sealing it all up and putting a pillow in there would make it sound better.

  5. Bill, that 'Acoustic Suspension' *means* a sealed enclosure, and a fiber+fabric cover to at least some of the (typically 3 adjacent) internal surfaces could be the 'pillow' effect, to dampen internal reflections and resonances. Standard practice for Hi-Fi since it became a 'fad' in the late '60s.This was leading-tech, Hi-Fi pioneer stuff.

  6. As Dex points out, acoustic suspension systems do usually include some sort of acoustic "stuffing" in the cabinet (Dacron pillow stuffing or fiberglass) to damp resonances. This can definitely improve the sound by eliminating "honkiness" and it also improves bass response somewhat by making the cabinet "look bigger" to the speaker. I'd strongly recommending using some such material in your cabinet.

    It's possible that the cone on your speaker has lost integrity over the years - the paper may have lost strength due to age or humidity/mildew and could be "breaking up" when in motion rather than behaving like a stiff-but-damped piston. A couple of years ago I bought a KLH 200 table radio (more recent than yours) and was quite unimpressed with the sound. My guess was that the original paper-cone drivers were bad (either of poor quality originally, or deteriorated).

    I found a modern speaker driver suitable for small acoustic-suspension cabinets which would fit (Dayton Audio 4" poly-cone PC105-8 from Parts Express) and replaced the old paper-cone drivers in the radio and outboard speaker cabinets. Huge improvement - the system sounds like a compact stereo rather than like a cheap clock radio.

    Another thing that can help (after dealing with any possible bad drivers) is to stiffen the cabinet walls - install some dowels side-to-side to brace them and limit resonant flexing.

  7. What do you guys think about just replacing the speaker? I'm not really interested in this as a "Hi-Fi" device -- heck, it is mono! Maybe just replace it with a good quality 4 inch speaker and be done with it? BTW, this receiver required a "pillow" in the cabinet to maintain the proper acoustics. Pete will laugh at me if he finds out that my HDR radios need pillows! See above.

  8. The Poly-cone would be a good choice and not expensive. The Visaton R10S 4" Full-Range Speaker 8 Ohm from Parts Express may also be a good choice that is closer to the original. Whatever speaker you chose, include the acoustic stuffing. You will be able to hear a difference.


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