SpeakerTalk Forum Index SpeakerTalk
This forum has been set up to facilitate discussion of 1970s KEF speakers and drive units. The owner of the Forum has no connection with KEF Audio.
 
 FAQFAQ   SearchSearch   MemberlistMemberlist   UsergroupsUsergroups   RegisterRegister 
 ProfileProfile   Log in to check your private messagesLog in to check your private messages   Log inLog in 

Is it possible to test crossovers?

 
Post new topic   Reply to topic    SpeakerTalk Forum Index -> KEF Speakers from the 1970s
View previous topic :: View next topic  
Author Message
Christopher Lord
Junior Contributor 10+


Joined: 11 Mar 2007
Posts: 10

PostPosted: Mon Oct 15, 2007 1:18 pm    Post subject: Is it possible to test crossovers? Reply with quote

Hi.
Having now blown 2 pairs of T27s in a pair of second-hand Concertos, I suspect that the crossovers might be to blame. Can anyone let me know how I can test them without specialised equipment?
Thnks
Back to top
View user's profile Send private message
proffski
Über Contributor 1000+


Joined: 22 Aug 2003
Posts: 1297
Location: Tewkesbury UK

PostPosted: Mon Oct 15, 2007 8:29 pm    Post subject: Re: Is it possible to test crossovers? Reply with quote

Christopher Lord wrote:
Hi.
Having now blown 2 pairs of T27s in a pair of second-hand Concertos, I suspect that the crossovers might be to blame. Can anyone let me know how I can test them without specialised equipment?
Thnks


What wattage amplifier were you using?
Were there any tone controls, if so were any turned up?
What was the music?

The amplifier may have been clipping or the series capacitors to the tweeter may indeed have given up the ghost and playing at being resistors instead.
They will be the 5uF ones if I remember correctly.
Have you access to a basic multimeter?
_________________
I contend that for a nation to try to tax itself into prosperity is like a
man standing in a bucket and trying to lift himself up by the handle.
-Winston Churchill
Back to top
View user's profile Send private message Send e-mail
speakerguru
VIP Contributor 750+


Joined: 18 Nov 2005
Posts: 958
Location: Green Hut, Tovil

PostPosted: Tue Oct 16, 2007 8:33 am    Post subject: Re: Is it possible to test crossovers? Reply with quote

Christopher Lord wrote:
Can anyone let me know how I can test them without specialised equipment?
Thnks

Basically, no. That's like asking how to bake a cake without an oven. Can you solder?

You will need at very least a multimeter, preferably one that can measure capacitance. They are cheap as chips these days. Have a look on Ebay and the Maplin website.

Most likely cause for tweeter failure is a party.
Back to top
View user's profile Send private message
Christopher Lord
Junior Contributor 10+


Joined: 11 Mar 2007
Posts: 10

PostPosted: Tue Oct 16, 2007 8:58 am    Post subject: Reply with quote

Yes of course I have a multimeter, but I suppose you might need something like an oscillator or a test tone. This is the first time I've ever even thought about testing crossovers. But if I've simply blown them by having too powerful an amplifier then I feel stupid. It's a Pioneer A302R. I don't have the manual, but on the back is says 325 watts. But there is no specification of watts per channel and what makes me suspicious is that I bought some cheapo 8 ohm tweeters to replace the blown KEF tweeters, and I have noticed that one of them anyway seems to be trying to play in the bass range of the spectrum and therefore making a sort of booming sound. Does that sound like faulty crossover? But I'd like to know how I can test the crossovers before replacing them in case that's not the problem. Otherwise I risk installing yet another pair of T27s and blowing those too. Any thoughts welcome and thanks.
Back to top
View user's profile Send private message
Christopher Lord
Junior Contributor 10+


Joined: 11 Mar 2007
Posts: 10

PostPosted: Tue Oct 16, 2007 9:02 am    Post subject: Reply with quote

Oh yes sorry didn't answer: tone controls (which are active on this amp) were set to flat response, music was Sade I believe, ie well-defined electric jazzy recording with good dynamic range and well-separated instruments.
Back to top
View user's profile Send private message
speakerguru
VIP Contributor 750+


Joined: 18 Nov 2005
Posts: 958
Location: Green Hut, Tovil

PostPosted: Tue Oct 16, 2007 10:13 am    Post subject: Reply with quote

Good, you have a meter. Use it to check if any of the capacitors in series with both tweeters is short circuit. Replace as necessary. The tweeters will not have blown because of amplifier size unless you used it well into clipping. The 325W probably refers to the mains input power consumption at full output on both channels. My guess would be around 60-80W of audio output per channel.
I guess your PC has a sound card of some sort, in which case you can probably find a software test tone generator somewhere. I can recommend the one at http://www.esseraudio.com/ttg.htm but it's not free. If you get one then apply around 1 or 2 volts rms to the crossover input and the tweeter should only be heard as the frequency rises above 1kHz or so.
Good luck.
Back to top
View user's profile Send private message
ColinR
Über Contributor 1000+


Joined: 31 Jul 2004
Posts: 1175
Location: Staffordshire

PostPosted: Tue Oct 16, 2007 2:46 pm    Post subject: Reply with quote

Apart from changing the 5uF capacitors and getting another pair of T27, consider dumping the Pioneer A 302R for something with a lot more power.

From the freely available German instructions on the Pioneer website.

Power output 30 Watts per channel into 8 Ohms with 0.07% THD.

Power output 35 Watts per channel into 8 Ohms with 1% THD.

Power output 45 Watts per channel into 4 Ohms with 1% THD.

When you get things fixed don't turn the amplifier up more than halfway as it will clip and knacker another set of T27s.
_________________
This post or any other information supplied to this website or any other by myself is not available for any form of commercial purpose i.e. to hi-fi magazines or as sales and marketing material for sleezeBay or Audiodogging pimps and the like.
Back to top
View user's profile Send private message
Christopher Lord
Junior Contributor 10+


Joined: 11 Mar 2007
Posts: 10

PostPosted: Tue Oct 16, 2007 11:49 pm    Post subject: Reply with quote

Colin thanks that sounds like good advice, but I don't understand what the figures mean. Are you saying that it's the distortion from the amplifier that is blowing the tweeters?
Back to top
View user's profile Send private message
speakerguru
VIP Contributor 750+


Joined: 18 Nov 2005
Posts: 958
Location: Green Hut, Tovil

PostPosted: Wed Oct 17, 2007 8:35 am    Post subject: Reply with quote

When a power amplifier clips the "squared off" waveform contains much more high frequencies than the original undistorted music. Tweeters only work on the basis that regular program material has a high peak to mean ratio and a frequency probability density which falls off at high frequencies. As soon as you go into clipping, all bets are off.

I have a KEFTOPICS issue which covers this in detail. I will try to get it posted on this site (if Terry would contact me please)
Back to top
View user's profile Send private message
ColinR
Über Contributor 1000+


Joined: 31 Jul 2004
Posts: 1175
Location: Staffordshire

PostPosted: Wed Oct 17, 2007 1:52 pm    Post subject: Reply with quote

Quote:
I don't understand what the figures mean. Are you saying that it's the distortion from the amplifier that is blowing the tweeters?


With my amplifiers rated at 60, 102, 140 & 216 Watts rms per channel an acceptable total distortion figure at full power is 0.02%.

Malcolm Jones once suggested with my birds I could expect a small fire with certain KEF designs. Yes I agree, but I'll never blow a tweeter with a clipping amplifier.

So to answer your question, yes, along with components of doubtful history. The Pioneer A 302R should be at best regarded as 20 Watt per channel amplifer and treated as such.
_________________
This post or any other information supplied to this website or any other by myself is not available for any form of commercial purpose i.e. to hi-fi magazines or as sales and marketing material for sleezeBay or Audiodogging pimps and the like.
Back to top
View user's profile Send private message
Christopher Lord
Junior Contributor 10+


Joined: 11 Mar 2007
Posts: 10

PostPosted: Wed Oct 17, 2007 6:23 pm    Post subject: Reply with quote

Thanks, this is good to know. What's the procedure for testing a capacitor without taking it out of the circuit board?
Back to top
View user's profile Send private message
proffski
Über Contributor 1000+


Joined: 22 Aug 2003
Posts: 1297
Location: Tewkesbury UK

PostPosted: Wed Oct 17, 2007 10:08 pm    Post subject: Reply with quote

I'd agree with the two Gurus, the capacitors may well be kaput and yes the amplifier quite likely was pushed into clipping.
_________________
I contend that for a nation to try to tax itself into prosperity is like a
man standing in a bucket and trying to lift himself up by the handle.
-Winston Churchill
Back to top
View user's profile Send private message Send e-mail
Christopher Lord
Junior Contributor 10+


Joined: 11 Mar 2007
Posts: 10

PostPosted: Wed Oct 17, 2007 10:29 pm    Post subject: Reply with quote

Yes all this fits the facts. What is so frustrating is that the Concertos sound wonderful with the proper tweeters in, but I have only heard them like that when I first installed the replacements. Then I turned the amplifier up and one after another they died, and it is true that there is a degradation of the sound at high volumes - which seems to mean that in fact the amplifier's volume control should not cover its actual range, but only about half of it. On the other hand I have another set of 8 ohm speakers hooked up and they haven't blown. So this brings me back to the crossover question.
Back to top
View user's profile Send private message
proffski
Über Contributor 1000+


Joined: 22 Aug 2003
Posts: 1297
Location: Tewkesbury UK

PostPosted: Wed Oct 17, 2007 10:50 pm    Post subject: Reply with quote

Replacing four capacitors will only cost a few bob, unlike the tweeters!
Wilmslow still do the excellent Concert Crossover Upgrade Smile , but cheap it is not... Sad
_________________
I contend that for a nation to try to tax itself into prosperity is like a
man standing in a bucket and trying to lift himself up by the handle.
-Winston Churchill
Back to top
View user's profile Send private message Send e-mail
speakerguru
VIP Contributor 750+


Joined: 18 Nov 2005
Posts: 958
Location: Green Hut, Tovil

PostPosted: Thu Oct 18, 2007 8:32 am    Post subject: Reply with quote

A short is a short, on or off the board. (Disconnect the amplifier and drive units before testing) The capacitors can only cause the tweeters to blow if they are short circuit. If they were open circuit you would not hear anything from the tweeters.
The capacitors are old; why not just replace them anyway? There's not that many components in a Concerto XO. Just test them all. How long can it take?
Back to top
View user's profile Send private message
Display posts from previous:   
Post new topic   Reply to topic    SpeakerTalk Forum Index -> KEF Speakers from the 1970s All times are GMT + 1 Hour
Page 1 of 1

 
Jump to:  
You cannot post new topics in this forum
You cannot reply to topics in this forum
You cannot edit your posts in this forum
You cannot delete your posts in this forum
You cannot vote in polls in this forum


Powered by phpBB © 2001, 2005 phpBB Group