KEF C80 Bass issue

Hi everyone,
I have recently purchased a pair of KEF C80 in great shape.
They sound very nice and open but lack in the Bass section.
I am using a Sony CD player and Technics SL-MA1 turntable with Shure M105E cart, Yamaha C-4 Pre Amp, 2 X bridged mono Rotel RB-870BX Power amps (300w each at 8ohm) and good cables between the components and the speakers.

I tried many combinations with wiring (A & B on the speaker back) various distance from the wall, AMPs bridged and non bridged, different Pre Amp (Rotel RC-850) even an ADC sound shaper III...still very dry almost non existing Bass.

Room size is 11.5x15f and height is 7.2f

I checked the B139 drivers and they work fine, also showing the correct ohm on the tester. I looked at the huge crossover. Caps look in great shape.

I thought it was my room but I also have pair of KEF 104ab which I am using in the same room with excellent Bass....and the 104ab is only a two way speaker with a passive BD139 unit.

Any idea where I should look next to try and solve this mystery?

Thank you,

The C80 has two settings on the crossover. One is for placement in the middle of the room, the other for placement close to the wall and this one has less bass output. Make sure you are using the right one. There are little icons on the back near the speaker terminals which indicate which is which.

Thanks Raynald,
Tried both as well as different room location, no luck.

I removed the crossover in one of the speakers and started to trace it, comparing with a diagram posted on one of the threads here. I immediately noticed that some of the numbers on the caps and and resistors do not match the diagram. But I could be mistaken as I am not an expert in reading electronic diagrams.

I will post a photo of my XO with arrows pointing at such examples as well as the diagram and hopefully someone can tell me if my assumption is correct.

Yes, I am not the original owner of the speakers.

There is a forum on this site that can also be very helpful for Kef speakers:

http://www.hifiloudspeakers.info/

Some people who worked at Kef around the time your speakers were designed post there and are a great resource.

I have a pair of the C-80's that I purchased new. I really like them and they should not be lacking in bass. I recall that the different speaker termination settings are pretty subtle.

If one of the woofers got changed out it's possible the polarity of them got mixed up. It's something to double check if you have not done so.

My speakers were sold as a matched pair and have consecutive serial numbers. Supposidly they were measured at the factory. I think the wood grains match.

Is there a way to tell if yours are a matched pair?

I will be posting on hifiloudspeakes, great suggestion
The serial numbers do not match but since this is not a referance series I did nt expect them to be, I did check the polarity and it is fine, I also verified that the elements are the correct ones for the C80 (T33 SP1210, B160 TS1203 and B139 TS 1212) they measure fine as well.

I am attaching a photo of the XO and a diagram I found somewhere that should be of the C80 XO. I believe that the 4 Resistors in parallel (4x5ohm) are not correct as well as the 2x600uF in parallel. there could be more but I may just not be reading the diagram right.

You may find this fun:

http://www.kef.com/uploads/files/en/museum_pdf/80s/C80_Installation_Instructions.pdf

and this:

http://www.hifiloudspeakers.info/Anatomy/KEFDocuments/KEFSpeakers61to86/Gallery/index.htm


According to the installation instructions the C80 should have matched veneer. That will tell you if you have an original pair. Does the wood grain on each panel match the same panel on the other speaker? They should be successive slices of the same piece of wood.

I would agree if the speakers would cancel each others Bass but that is not the case. I inspected cable polarity (internaly in the boxes) and set them up according to KEF recommendation, including set them as facing each other, playing mono and reversing polarity in the back verifying that they do not cancel one other.

Besides, I would expect good Bass even from a single speaker, but even that isn't the case. I tried the same with my 104aB and even with one speaker playing I get much better Bass.

I am still waiting for a registration code from hifiloadspeakers.com before I can post there :scratch2:

Golanr said:

I will be posting on hifiloudspeakes, great suggestion
The serial numbers do not match but since this is not a referance series I did nt expect them to be, I did check the polarity and it is fine, I also verified that the elements are the correct ones for the C80 (T33 SP1210, B160 TS1203 and B139 TS 1212) they measure fine as well.

I am attaching a photo of the XO and a diagram I found somewhere that should be of the C80 XO. I believe that the 4 Resistors in parallel (4x5ohm) are not correct as well as the 2x600uF in parallel. there could be more but I may just not be reading the diagram right.

Click to expand...

The 2 x 600uf in parallel would yield a 300uf equivalent, and there are two specified in the schematic, and there is already 1 x 300uf. Why Kef would do that is unusual unless they needed a low loss component in there and the 300uf on its own wouldn't work. Or someone needed to replace that but didn't have the correct value.

That said, even with KEF's pretty decent crossover building, the NP electrolytics of that era were ok, but probably should be replaced by now. If you plan to do troubleshooting, then you should be trying to draw a schematic from the board and the component markings and compare to the schematic you posted to see if at least the circuit layout is correct, and then if the component values are similar. Too bad your posted schematic doesn't have component numbers (R1, C1, etc.)

If you have the right equipment, would be instructive to see what the resistor, inductor, and capacitor values are now some 25 years after these were made. Replacement with new good components will be an improvement.

Now, one thing to try before anything else.

The B139's in this speaker are 4 ohm version IIRC. I'd consider trying to run a full range signal directly into them without crossovers from one of your amps just to see if they will put out low bass and some loudness when driven. If the bass then comes back then you know the problem is likely in the crossover. Caution: do have some type of low frequency filter on them so they don't start "pumping" back and forth with any subsonic signals - they're not meant to handle that sort of thing.

mech986,
This is a great suggestion, I will test teh B139 direct and update the forum.
What I will do is reconnect the ADC Sound shaper reduce probably everything from 320hz and up to -12db and keep the rest at flat. I also have a low (subsonic) filter on the Yamaha C-4 Preamp which I will turn on as well (I do it anyway when using the TT)

Golan,

Are there different versions of the B139 driver? Perhaps someone put the wrong drivers in at one point.

Looks like the right one for the speaker, TS1212 4ohm. It was made for the C80 only.

Also connecting it directly without the XO did not show any improvement. I checked it again while it was outside, it is showing 3.7ohm and it is not rubbing. His twin is the same. The same amp is used for the 104aB and the Bass is amazing so I do not think the problem is there.

I'm confident you already have this:

http://www.kef.com/html/en/explore/about_kef/museum/1980s/C10_C20_C30_C40_C40D_C42_/index.html

http://www.kef.com/html/en/explore/about_kef/museum/1970s/Reference_Series_104/index.html

What I'm going to suggest is try another amp with this speaker. I think for some reason the Rotel doesn't like the 4 ohm resistive load of the C80. The sensitivity of both speakers should be roughly the same given the similarity in concept.

The 104ab is far more sensitive than the C80. The C80 sensivity rating is in the upper 80's and the 104ab is 96 db. I didn't realize the 104ab B139 was a passive radiator. The C80 is a sealed box. That's a considerable difference in sensitivity rating.

poortom said:

The 104ab is far more sensitive than the C80. The C80 sensivity rating is in the upper 80's and the 104ab is 96 db. I didn't realize the 104ab B139 was a passive radiator. The C80 is a sealed box. That's a considerable difference in sensitivity rating.

Click to expand...

Knew this would catch a few. The KEF 104, designed in 1973 and updated with the aB crossover in 1977 was rated differently than the now standard 1Watt @ 1 meter convention used since the 80's. Here's the spec from the links above:

Sensitivity: 12.5 watts for 96dB at 1m and 400Hz in anechoic conditions

That's 12.5 watts at a nominal 8 ohms for 96db output at 1 meter. I checked all my KEF literature and that's the manufacturer's rating system at the time, as were all the other in the line (KEF Corelli, Calinda, Cantata, R105, R103). For comparison, the R105 with 12 inch woofer, needed only 8 watts to produce 96db, but its true 1W/1m sensitivity was only 86db/w/m. The tiny R101, similar to the LS3/5a, was rated at an inefficient 81db/w/m.

I finally found a review from Canadian Stereo guide Spring 1977 which states 92db/w/m at 400hz sensitivity for the KEF 104aB.

For comparison, the C80 is rated at Sensitivity: 87/89*dB for a pink noise input of 2.83V (*dependent on contour control) which is a typical rating for an 8 ohm speaker. However, this is a 4 ohm resistive speaker so the input voltage for 1 watt output should be 2 volts (V = sqrt of [1watt x 4 ohms]). So, I'm guessing the C80 efficiency should be around 84-85db/w/m at 4 ohms with 2 volts in.

If the same amp were putting out 2.83V into each, then the sensitivities are closer at 92 vs. 89 db, but that means the 104 will still sound twice as loud (3db).

So maybe that is the real answer here, the C80's may just be that much more inefficient, and need a lot more power to sound a) as loud, and b) as bassy.

So, to the OP, maybe you have to ignore the actual volume settings, and push both up to a particular sound level and see what they do sound like musically-wise, when that loud, instead of worrying about why they sound so less loud with the same amp setting (assuming they are working properly).

Very comprehensive and interesting, but the issue is not about how loud they sound. At any volume, and yes I did play with the volume as when testing between the two, the C80 has a very dull Bass. The mid on the other hand is very pronounced, it is virtualy over taking the sound coming out of the C80.

I'll try a different amp, I have a Yamaha and a Denon I will try them with.

Thanks for your insight, Mech986. It's interesting that the mid sounds pronounced on your C-80's Golanr. I a-b compared my C-80's with my AR-10pi speakers a while ago with a yamaha amp. I thought the AR's sounded much more forward than the C-80's.

BTW, The apparent loudness between the speakers was very similar. I've always thought the mid bass on the C-80 is a bit on the pronounced side. I do think the low bass on the C-80 rolls off fairly rapidly especially compared to my AR's. I recall reading a comment about the C-80 a while ago on a forum where someone thought the B139 needs a little bigger enclosure than the C80 for it to extend down more.

My mid range drivers are also TS-1203's I have a spare on hand that I checked.

Mech986, as far as I recall 2 caps in parallel add their capacitance which means the 2x600 are acctualy 1200, I verified this by reading on the properties of capacitors. I wonder what the impact is in this case.

Did not have time to try the C80 on a different amp, will try this weekend.

Golanr said:

Mech986, as far as I recall 2 caps in parallel add their capacitance which means the 2x600 are acctualy 1200, I verified this by reading on the properties of capacitors. I wonder what the impact is in this case.

Did not have time to try the C80 on a different amp, will try this weekend.

Click to expand...

You are quite right, my mistake! You are certain they are 600ufd?? If so, on looking back at the schematic, there is no 1200uf cap specified anywhere, if the schematic is correct. So if this is on the woofer side, and I think it is, I think its there for one of the two 300 ufd caps. Would be interesting to try to sub a new Non-polar 300uf cap for each of those two points and see what happens to your sound.

If it was a late production change, you could always refit or recreate the paralleled caps. Maybe the resistor change was part of that. Hmm, the only way to know is to find other examples of the crossover board to compare to. Or consider giving KEF America a call and see if they can come up with a crossover schematic. Years ago they were very helpful to me, but I don't know if today they are around or could help you with that. Might be worth a try though. If there were production revisions based on serial number, they would be the only source for it.

In the meantime, you could consider using these caps:
http://www.parts-express.com/wizard...ce=&srchCat=161&srchMfg=&srchPromo=&srchAttr=