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Multi-Way Conventional loudspeakers with crossovers

Is my KEF CS5 crossover faulty?
Is my KEF CS5 crossover faulty?
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Old 26th November 2001, 05:28 AM   #1
paulot is offline paulot
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Hello,

I have a pair of 15-year old KEF CS5 speakers (T33A HF, B220G LF and a 13" x 7 " passive radiator). When I put an ohmeter to the leads, I get an open circuit. Strangely, the speakers sound fine... I get the same reading using two different ohmeters (digital and analogue).

Are my dividing networks faulty? If so, how do I diagnose which capacitors should be replaced?

Thanks.

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Old 26th November 2001, 05:50 AM   #2
walker is offline walker  Australia
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Paulot, it sounds as ie KEF are using a high pass input filter in these speakers. This has been done before to protect ported enclosures, (passive drivers operate on a similar principle). If this is the case, they should measure open circuit to DC. If you could do an impedence/frequency plot you would get a better idea of your speakers impedence.

Regards WALKER
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Old 26th November 2001, 05:54 AM   #3
paulot is offline paulot
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Thanks Walker. If I understand you correctly, the "open" condition of the speaker as measured by an ohmeter is normal? Does this mean that under load, the speakers have an actual impedance of around 8 ohms?
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Old 26th November 2001, 06:02 AM   #4
walker is offline walker  Australia
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Paulot, I believe so. I have repaired many speakers in the past and have on occasionally come across this low frequency protection. Basically; a capacitor in series with the input leads. Ported drivers have little protection from low frequency polling, (the voice coil hitting the magnet poles, NOT GOOD). KEF speakers that I have come across have always been well made and this type of protection would not surprise me with speakers of this quality.

Regards WALKER
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Old 26th November 2001, 09:16 AM   #5
Geoff is offline Geoff  United Kingdom
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Paulot

Walker is right regarding the CS5 crossover, there is no dc path through either the lowpass or the highpass section. email me if you would like a copy of the schematic.

Geoff
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Old 26th November 2001, 03:30 PM   #6
AndrewJ is offline AndrewJ
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Paulot, The CS5 did indeed use a series capacitor in the xover to the bass unit. It's function was in fact more involved than simply providing d.c. protection. Just as a vented enclosure has higher potential efficiency or bandwidth than a closed box of the same volume, assisted alignements where a passive filter (in this case the series cap)is combined with the loudspeaker, have an even higher potential efficiency. Beneficial by-products are d.c. protection and reduced excursion at frequencies below the tuning frequency. The negative is the alignement is higher order (5th order instead of 4th) and therefore worsened transient response.
The capacitor value is carefully chosen, as it interacts with the motional impedance of the loudspeaker to achieve the correct alignement. It is not merely a large value capacitor!
Many KEF speakers were in fact designed this way.
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Old 27th November 2001, 01:05 AM   #7
paulot is offline paulot
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Walker, Geoff and AndrewJ,

Thank you so much for your replies. This is my first post on this forum and I am quite overwhelmed by the level of expertise here.

Regards.
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Old 27th November 2001, 03:06 AM   #8
walker is offline walker  Australia
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AndrewJ, I would like to better understand the principles that you discussed. I can't say that I understand how the capacitor adds efficiency or reduces transient response.
Are you aware of any sites that may help me better understand this?

Paulot, all information is from forum users memories and understanding. The depth of understanding in this site too has surprised me, there are some passionate researchers here, (you could be mistaken by thinking it was a hobby). We do however all have misunderstandings and biases, (well ok, maybe I don't) so it is often best to verify what is written here by further research.


Regards WALKER

Never forget that the best improvement you can make to any HIFI system is the cheapest and only as far away as your next purchased recording.
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Old 27th November 2001, 03:22 PM   #9
AndrewJ is offline AndrewJ
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Walker, there is a common misconception about speaker efficiency, that assumes that if one puts a drive unit into a vented box, the efficiency will be greater than in a closed box. This is not true!. Speaker efficiency is defined in the piston range, at a frequency well above the cutoff frequency. The efficiency in this region is governed entirely by moving mass, Bl product, drive unit cone area and the amount of voice coil in the magnetic gap. It has nothing to do with the box. To increase efficiency, just add more Bl or area or reduce mass etc. Where the box comes into play is determining the sphape of the low frequency response. There is an optimum choice of the driver parameters that will give the desired low frequency response in any given enclosure. If I then add more Bl, sure I will get more midband efficiency, but my absolute bass level will actually go down as the system becomes overdamped. It turns out that the drive unit parameters suited for a vented enclosure with a butterworth low frequency alignement require higher Bl and lower mass than for a closed box butterworth alignemnt of the same -3dB frequency, hence the higher efficiency. Now it becomes clear why an assisted alignement is more efficient. The driver parameters required to get a fifth order butterworth response (4th from the venting plus 1st from the capacitor)involve higher Bl and lower mass. Efficiency doesn't come free. We normally have to pay for it by larger magnets.
The transient response is derived from the fourier transform of the frequency response and vice versa. The higher the order of the alignement,the more phase shift is introduced around the cutoff frequency and hence the longer the impulse response. Speaker design is a blend of compromises. More bass in the passband is traded for less below and worsened transient response. For some applications this is a good trade off. It is possible to obtain an even higher efficiency figure with a second order passive filter attatched to the driver (capacitor plus inductor) but the trade offs become increasingly severe.
As a side note, it should be realised that the true efficiency increase of a vented box is not as high as it first appears. Efficiency is referenced to the nominal impedance or voice coil resistance. This is merely a calculation convenience. If one examines the impedance of the speaker at the -3dB frequency, the magnitude is a maximum for a closed box, but a minimum for the vented box. Part of the reason we have more output from the vented box is that we are dragging more current out of the amplifier!!
Hope this helps your understanding.
Regards Andrew
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Old 28th November 2001, 02:32 AM   #10
walker is offline walker  Australia
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AndrewJ, cheers, thanks for taking the time. I agree with you about effeciency of a system, and transient responce the rest seems to make sense too.

Hope I'm never too old to learn.
Regards WALKER

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