I've come across an old pair of KEF C25 speakers. Does anyone know the
impedance of these? If I try to read the DC resistance at the speaker
terminals, I get a very high reading (3 kilohms or more), which I
assume has to do with the crossover network, reading through a
capacitor or somesuch...

(William Viets) said:

I've come across an old pair of KEF C25 speakers. Does anyone know the
impedance of these? If I try to read the DC resistance at the speaker
terminals, I get a very high reading (3 kilohms or more), which I
assume has to do with the crossover network, reading through a
capacitor or somesuch...

Then they're either broken or you made some mistake.
The bass driver is connected via a coil, the tweeter via a cap.
You'll have to measure at least the coil + voice coil of the woofer.
It is said to be an "8 ohms" speaker.
KEF speakers usually have relatively flat impedance response, the
minimum impedance dip may lie somewhere at 6 ohms or so.

--
Sander deWaal
"SOA of a KT88? Sufficient."

On Sat, 04 Sep 2004 19:29:30 +0200, Sander deWaal
wrote:

(William Viets) said:

I've come across an old pair of KEF C25 speakers. Does anyone know the
impedance of these? If I try to read the DC resistance at the speaker
terminals, I get a very high reading (3 kilohms or more), which I
assume has to do with the crossover network, reading through a
capacitor or somesuch...

Then they're either broken or you made some mistake.
The bass driver is connected via a coil, the tweeter via a cap.
You'll have to measure at least the coil + voice coil of the woofer.
It is said to be an "8 ohms" speaker.
KEF speakers usually have relatively flat impedance response, the
minimum impedance dip may lie somewhere at 6 ohms or so.

Yes, I thought the same thing, untill I opened the cabinet and looked
at the crossover; its a relatively complex thing for a two-way
speaker, with 4 caps, 2 coils and 2 resistors on a circuit board.
There is definately a capacitor in series with the woofer, hence my
odd readings. Its a large 600 microfarad, which, if I did the math
right, is probably a high pass filter to keep frequencies below 60 or
70 hz out of the small woofer.

said:

Then they're either broken or you made some mistake.
The bass driver is connected via a coil, the tweeter via a cap.
You'll have to measure at least the coil + voice coil of the woofer.
It is said to be an "8 ohms" speaker.
KEF speakers usually have relatively flat impedance response, the
minimum impedance dip may lie somewhere at 6 ohms or so.

Yes, I thought the same thing, untill I opened the cabinet and looked
at the crossover; its a relatively complex thing for a two-way
speaker, with 4 caps, 2 coils and 2 resistors on a circuit board.
There is definately a capacitor in series with the woofer, hence my
odd readings. Its a large 600 microfarad, which, if I did the math
right, is probably a high pass filter to keep frequencies below 60 or
70 hz out of the small woofer.

In that case, you're right of course.
KEF loads are pretty benign in general, so you won't be far off to
call this a say 6 ohms speaker.
I still think it's an odd thing to do, though.
Who would put a (I may well hope bipolar) electrolytic to such abuse?
High AC currents and electrolytics......BRRRRRRR!

:-)


--
Sander deWaal
"SOA of a KT88? Sufficient."

IOn Sat, 04 Sep 2004 23:07:38 +0200, Sander deWaal
wrote:

said:

Then they're either broken or you made some mistake.
The bass driver is connected via a coil, the tweeter via a cap.
You'll have to measure at least the coil + voice coil of the woofer.
It is said to be an "8 ohms" speaker.
KEF speakers usually have relatively flat impedance response, the
minimum impedance dip may lie somewhere at 6 ohms or so.

Yes, I thought the same thing, untill I opened the cabinet and looked
at the crossover; its a relatively complex thing for a two-way
speaker, with 4 caps, 2 coils and 2 resistors on a circuit board.
There is definately a capacitor in series with the woofer, hence my
odd readings. Its a large 600 microfarad, which, if I did the math
right, is probably a high pass filter to keep frequencies below 60 or
70 hz out of the small woofer.

In that case, you're right of course.
KEF loads are pretty benign in general, so you won't be far off to
call this a say 6 ohms speaker.
I still think it's an odd thing to do, though.
Who would put a (I may well hope bipolar) electrolytic to such abuse?
High AC currents and electrolytics......BRRRRRRR!

:-)

I thought is was strange too, I don't think I've ever seen that in a
crossover. I'll be using them for the rear surrounds in a home theater
setup, so I don't think they'll be too stressed. (Most of the bass
goes to the subwoofer anyway.)

said:

I thought is was strange too, I don't think I've ever seen that in a
crossover. I'll be using them for the rear surrounds in a home theater
setup, so I don't think they'll be too stressed. (Most of the bass
goes to the subwoofer anyway.)

In that case, the filtering is done before the amplifying, so you
might as well skip the Caps altogether in this application.

--
Sander deWaal
"SOA of a KT88? Sufficient."

wrote in message


Yes, I thought the same thing, until I opened the cabinet and looked
at the crossover; its a relatively complex thing for a two-way
speaker, with 4 caps, 2 coils and 2 resistors on a circuit board.

More complex crossover networks are not unusual with higher quality
speakers. In essence the additional crossover components may improve the
smoothness of the crossover and decrease "lobing", an effect where a goodly
peak or dip in system response exists in certain off-axis areas of the room.
Lobing can have a secondary effect on the smoothness of the speaker's
response, as perceived at the listening location. The additional components
might also provide some equalization effects that relate to on-axis
response.

There is definitely a capacitor in series with the woofer, hence my
odd readings. Its a large 600 microfarad, which, if I did the math
right, is probably a high pass filter to keep frequencies below 60 or
70 hz out of the small woofer.

The math is probably not as obvious as it may initially seem. There's a way
to peak up the low end of some drivers a number of dB by putting a
relatively large capacitor in series with it. A friend of mine *discovered*
this effect about 15 years ago, and thought it might be patentable. His
patent search turned up prior art from the 1920s or 1930s.

The capacitor may also have the effect you suggested, as well. At
frequencies somewhat below the point were it increases bass response, it
provides some low end roll-off.

If you want to estimate the nominal impedance of this speaker, just measure
the ohms resistance of the woofer.

"Sander deWaal" wrote in message
...
said:

Then they're either broken or you made some mistake.
The bass driver is connected via a coil, the tweeter via a cap.
You'll have to measure at least the coil + voice coil of the woofer.
It is said to be an "8 ohms" speaker.
KEF speakers usually have relatively flat impedance response, the
minimum impedance dip may lie somewhere at 6 ohms or so.

Yes, I thought the same thing, untill I opened the cabinet and looked
at the crossover; its a relatively complex thing for a two-way
speaker, with 4 caps, 2 coils and 2 resistors on a circuit board.
There is definately a capacitor in series with the woofer, hence my
odd readings. Its a large 600 microfarad, which, if I did the math
right, is probably a high pass filter to keep frequencies below 60 or
70 hz out of the small woofer.

In that case, you're right of course.
KEF loads are pretty benign in general, so you won't be far off to
call this a say 6 ohms speaker.
I still think it's an odd thing to do, though.
Who would put a (I may well hope bipolar) electrolytic to such abuse?
High AC currents and electrolytics......BRRRRRRR!

:-)

My Kef Reference III's have it too.
The purpose is to protect the woofer from DC excursions.