Kef 107 bass rolloff issue

[guitarplayr82]

I have a pair of Kef 107s and am experiencing a very significant (15-20 db) bass rolloff right at the crossover point in both speakers. The bass is definitely still there and reacts to different settings on the kube, however it is very quiet. This leads me to think there may be an issue with the woofers in both speakers, or maybe an issue with the kube. I am wondering what steps I can take to diagnose this or if maybe re-foaming the speakers could solve this problem?

 

[SaSi]

Hi there,
the foams are either there or disintegrated. If they are solid, then they should function ok. In any case, the foams have very little effect in dampening the woofers. It's the spider that takes care of that.

Not sure what you are referring to by crossover frequency. Can you be more specific? The coupled cavity causes a resonance in the 180Hz region and the crossover is tuned to reduce that, and make it flat.

The combination of the woofers and midrange (B110) response should be flat merged together.

You can take the KUBE out of the equation and see how they fare without it. IIRC, it should give you a slightly reduced response in the low end and a higher rolloff frequency.

The first point I would check would be the capacitors in the crossover. I would unsolder one lead and measure them with an LCR meter. They should not match their rated value necessarily but be close around it.

 

[guitarplayr82]

Hi there,
the foams are either there or disintegrated. If they are solid, then they should function ok. In any case, the foams have very little effect in dampening the woofers. It's the spider that takes care of that.

Not sure what you are referring to by crossover frequency. Can you be more specific? The coupled cavity causes a resonance in the 180Hz region and the crossover is tuned to reduce that, and make it flat.

The combination of the woofers and midrange (B110) response should be flat merged together.

You can take the KUBE out of the equation and see how they fare without it. IIRC, it should give you a slightly reduced response in the low end and a higher rolloff frequency.

The first point I would check would be the capacitors in the crossover. I would unsolder one lead and measure them with an LCR meter. They should not match their rated value necessarily but be close around it.

SaSi, thanks for your response.

I will check the caps - recapping my vintage gear always brings back the bass so maybe it's a similar problem here...

Here are the frequency responses with and without the Kube. These are measured from about 9' exactly centered with an Umik 1 mic, REW software in a lightly treated living room. The individual Left-Right speaker measurements (not attached) produce the same curves.

The pronounced bass rolloff seems to follow the crossover frequency between midrange driver and low drivers. The Kube at maximum Contour provides bass boost as well as a flattening around 2khz as I had read it should, but clearly the bass is still a problem.

Also, there seems to be a natural "house curve" from 700hz - 20khz, but I am running the audio flat directly from a Benchmark DA into Adcom power amp. It's possible this is room effect, but I am wondering about the tweeters. Any thoughts on this?

Two more quick questions as I am new to this forum, if you don't mind.
How do I get at the woofers? Also, is there somewhere online I can find a parts list or schematic? I would like to preemptively order new caps. These units are from the 80s and received very little use so I do suspect the caps should be replaced.

Thanks for the help!

An externally hosted image should be here but it was not working when we last tested it.

An externally hosted image should be here but it was not working when we last tested it.

 

[sreten]

Hi,

Check the bass drivers first, and refoam if necessary.
Its by far the most likely, and well known problem.

Age is the real surround killer, not usage.
FWIW your capacitors are very probably fine.

rgds, sreten.

 

[Fenalaar]

Hmm - can you make a sketch of how the entire system is wired? Esp. with regards to the Kube.

I've had 107/2s, and the way these are made, is that the XO has impedance correction and xover slopes. All level adjustments and filters for unlinearities are made in the Kube. The old 107 model has an even bigger range of adjustments than the newer /2. The attached pic shows Stereophiles measurement of the original R107 Kubes curves. 10dB is easily within the adjustment envelope of the Kube...

Oh, by the way - your attachments didn't work...

Johan-Kr

 

[Zep Dude]

Hi, I'm the original poster. My assistant posted under his username, but I realized I should join the forum anyway. Thanks for your responses.

The signal flow is:
Benchmark DA (volume controlled output) > Kube input > Kube output > Adcom GFA 545 power amp > Kef 107s

We ordered the foam replacement kit as a first step -they probably could use it as these were purchased in the 80's.

Here are the frequency response curves. What I'm wondering about is the bass rollof and also the high frequency rolloff. It does follow a "house curve" but not by design (there is no EQ in the circuit).

Any thoughts on this?

 

[Fenalaar]

Here are the frequency response curves. What I'm wondering about is the bass rollof and also the high frequency rolloff. It does follow a "house curve" but not by design (there is no EQ in the circuit).

Any thoughts on this?

Hmm - the attached graph is Stereophiles measurement of the original 107.

Your measurement without Kube isn't that far off, so I think the Kube is the culprit, and not giving a proper bass boost. The suspension is likely to be just fine...

Can you try to do a frequency sweep of the Kube, and post the response curve?

If the Kube is defective, I'd swap it for a MiniDSP nanoDigi between your front nd and your DAC, and do the curves there...

Johan-Kr

 

[Zep Dude]

Well, it took a while but we resolved the problem. Upon opening the cabinets, all of the bass drivers were almost completely without foam on both the middle and edges. It had completely deteriorated, so those poor drivers just couldn't push enough air to generate any real bass -like drums with holes in them.

Disassembly and refoaming was a bit of a process and we couldn't find any instructional videos specifically for the Kef 107's so we had to figure things out on the fly. I created a video of the process from start to finish and will post it at some point.

The 107's now sound amazing. The bottom end is just where it should be (liking the default Kube settings with lowest cutoff frequency, 12 o'clock Contour and Q Factor) and the high end is now just where it should be -clear and open without being hyped.

Thanks for your suggestions.

Angelo Montrone
MajesticMusic.com

 

[sreten]

Hi,

FWIW open the image in a new window to see it.
Thankfully the replacement modern foam does
not deteriorate the way the older stuff does.

rgds, sreten.