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KLH Model 20- lack of bass response

G

geedub52

New Member
Hi, folks. I'm new here, but have read a lot of good info on the site about KLH 20 speakers and rebuilding them.

I rescued mine a few months ago from a garbage pile by the street. The cabinets were in beautiful condition, but they were incredibly mildewy smelling. I had to replace the grills, grill cloth, and insulation; and had to use vinegar and a mildew eliminator both inside and outside the cabinets. While doing that, I also recapped and resealed them.

So, now I have beautiful speakers without the stench, but unfortunately, there is no bass from the woofers. They work- they nice, but just no bass.

I have resealed the cloth surrounds using the sealant from Vintage-AR Parts and Service from ebay. The boxes are tight- I resealed the edges around the tweeters and woofers using HVAC putty rather than foam.

I also added a 4 ohm resister in the speaker wire to increase impedance to 8 ohms overall, since my amp would not handle 4 ohms.

Maybe the vinegar/mildew eliminator damaged the speakers? I don't know anything about rebuilding speakers- should I send them to a specialist for repair/rebuild?

Any thoughts or suggestions about how to get some bass out of these speakers would be very much appreciated.
 
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Couple of thoughts. It's possible one of the woofers was wired in reverse phase so that would have a cancellation effect on the bass. Have you tried running the speakers independently? That might give an indication whether or not the woofers are operating correctly. Another thought. Remove the 4 ohm series resistors. They should never be wired in series in a woofer circuit. Re-check your crossover wiring and make sure the polarities are correct.
 
Thanks for the help, Homebrew. You got me on the right path.

I was reluctant to pull out the drivers to try and spot a problem, so I reversed the wires at the amp for one of the speakers, and sure enough, the bass improved. Not dramatic, but noticeably better. Despite my careful cutting and labeling of the wires when cleaned the inside of the box replaced the capacitors, something must have gone wonky.

In addition, I've readjusted my expectations for these speakers. I like the sound, but mid and high frequencies dominate the bass, unlike my more modern speakers (ELAC Debut 2.0 B6.2 and and JBL 2600 speakers). That said, on some heavy bass recordings I have (Keiko Matsui for example), the bass comes through well enough- not as strongly as the other speakers but definitely sufficient.

So, with the corrected polarity, boosting bass with the amp, occasional support with my subwoofer, and adjusted expectations, I think I'm in pretty good shape now.
 
Reversing the wires at the amp may have improved the bass, but it threw the tweeters out of phase. The only way to correctly phase your speakers is to do it at the woofer terminals. Otherwise you've just traded one problem for another.
 
Been using a pair of 17s in the work room for years (8 ohm cousin of the 20) don't expect very deep bass out of them. Smaller woofer and cabinet than the 5, 6, and 23 group that are the more highly regarded KLH speakers.

Did not see any mention of the switch on the back. The 20 should have the three way switch that many KLH speakers have labeled NORMAL, DECREASE, INCREASE. They gave three settings for tweeter level. Mine stay in the NORMAL position.
 
You will need to re-seal the woofer surrounds. As well as everywhere an air leak may occur. There are many posts about this.
 
To definitively check for a sealed cabinet, with the speakers assembled, gently push the woofer cone in by hand, The cone should return to its rest position fairly slowly, not immediately. A fairly slow return indicates the trapped air in the cabinet is resisting (controlling) cone movement which the way an acoustic suspension speaker should be.

There will be a small, controlled leak, usually around the spider/VC/dust cap area ,designed into the speaker. If the cabinet is not otherwise sealed, bass response and, actually, a major part of the frequency response, will suffer.

Doug
 
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