SpeakerTalk

Christopher Lord · Junior Contributor 10+ Joined: 11 Mar 2007 Posts: 10

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

proffski

speakerguru

Christopher Lord · Junior Contributor 10+ Joined: 11 Mar 2007 Posts: 10

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.

Christopher Lord · Junior Contributor 10+ Joined: 11 Mar 2007 Posts: 10

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.

speakerguru · Posted: Tue Oct 16, 2007 10:13 am Post subject:

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.

ColinR · Posted: Tue Oct 16, 2007 2:46 pm Post subject:

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.
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Christopher Lord · Junior Contributor 10+ Joined: 11 Mar 2007 Posts: 10

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?

speakerguru · Posted: Wed Oct 17, 2007 8:35 am Post subject:

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)

ColinR · Posted: Wed Oct 17, 2007 1:52 pm Post subject:

Christopher Lord · Junior Contributor 10+ Joined: 11 Mar 2007 Posts: 10

Thanks, this is good to know. What's the procedure for testing a capacitor without taking it out of the circuit board?

proffski · Posted: Wed Oct 17, 2007 10:08 pm Post subject:

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

Christopher Lord · Junior Contributor 10+ Joined: 11 Mar 2007 Posts: 10

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.

proffski · Posted: Wed Oct 17, 2007 10:50 pm Post subject:

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

speakerguru · Posted: Thu Oct 18, 2007 8:32 am Post subject:

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?