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Old 11th June 2003, 04:49 PM   #1
Nuuk is offline Nuuk  United Kingdom
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Default What's the problem - amp or speaker?

This afternoon, I disconnected my Mordaunt Pageant speakers from the GC's and connected up my full-range open baffle speakers.

When I powered up, one channel produced a short humming noise that died away after about 2-3 seconds but when the music started playing it was distorted on that speaker!

I connected my meter to the speaker terminals of the amp and noticed that at the moment of connecting the power, the meter reads 330mV but quickly drops to 0mV in a few seconds. Both amps do the same.

So why do I have a problem with the humming noise on one channel and not the other. Is it the speaker or the amp? I don't want to connect the other speaker up and risk damaging it!

The Mordaunt Shorts have been absolutely fine but of course they have a crossover between their drivers and the amps.

Any suggestions chaps?
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Old 11th June 2003, 05:28 PM   #2
raindog is offline raindog  Romania
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what are the impedances of the two sets of speakers? and you may also want to try the open bafle set on another amp
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Old 11th June 2003, 06:47 PM   #3
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what are the impedances of the two sets of speakers? and you may also want to try the open bafle set on another amp
Hi raindog, the speakers are 8 ohms impedance and worked well for a couple of weeks with the GC's prior to changing from the 18K/22K resistors on pins 7 and 8 to just a 216K from the non-inverting input to ground.

They also worked OK with my Arcam A60's!
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Old 11th June 2003, 06:58 PM   #4
mikelm is offline mikelm  England
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if the dc offset is OK the only other thing it could be is oscillation.
If you don't have a scope I guess you have to see if the amps are getting hot. To avoid turn on transient danger you could power up first & then connect the speakers. If there is no oscillation then try connecting swapped speakers and see if the fault goes with the speaker or the amps.

watch out though just in case the oscillation begins when you connect the speakers !

are the leads OK

hope these ideas help

mike
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Old 11th June 2003, 08:58 PM   #5
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Hi Mike, well the DC offset seems OK and the amps are just warm after running for a couple of hours. The second channel is fine, I have switched the power on and off half a dozen times!

I think that the speakers may be a bit fragile. I wasn't sure if it was OK to connect the speakers with the power on. In my main system I use Velleman speaker protection circuits but was hoping to avoid them as I am of the opinion that they do affect the sound.

I do get a very slight click when I power on with the Mordaunt Short speakers but I guess the difference is, the full-range units don't have any protection from a crossover. It's something that I need to sort out before connecting my Goodmans 201's as they will cost £120 to repair!
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Old 12th June 2003, 12:00 AM   #6
raindog is offline raindog  Romania
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NEVER connect anything to an amp while it's working, especially speakers or you risk damaging both the amp and the speakers( and i'm speaking from experience). if your amp seems to work fine whith two different sets of speakers there's probally something wrong with your full-range set
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Old 12th June 2003, 08:15 AM   #7
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NEVER connect anything to an amp while it's working
Thanks raindog, that's what I have always believed.

As you say, it looks more like a problem with the drivers. When I used these drivers the first time, I played about one and a half tracks before I realised that there was a problem on one channel. The supplier replaced the driver but it does look as if they are a bit 'fragile'.

On neither occasion when they failed, could I find a reason why, or a fault with the amps!
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Old 14th June 2003, 12:03 PM   #8
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Well, to answer my own question, it would seem to be the amp that is faulty!

Further investigation found that the DC offset rises to over 4 volts when I power down! I can't say why but have done the following:

1 Checked the PSU, tried three different ones and always the same problem.

2 Checked all the conections in the amp, PSU, umbilical leads and found no problems.

3 Checked all resistors and they are fine.

4 Rebuilt that amp with a new LM3875 and new decoupling caps. Result - same problem!

When measuring the rail voltages, I notice that when I remove the power, the negative rail decreases more slowly than the positive rail such that when they drop below 10 volts there is a couple of volts difference. I guess that is what is causing the DC offset which is still a few hundred mV after a couple of hours!!!

About the only thing that I can think to try now is removing the bypass caps which I will do after a suitable break! Anybody got any other ideas?
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Old 14th June 2003, 01:45 PM   #9
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Quote:
Originally posted by raindog
NEVER connect anything to an amp while it's working, especially speakers or you risk damaging both the amp and the speakers( and i'm speaking from experience). if your amp seems to work fine whith two different sets of speakers there's probally something wrong with your full-range set

There is no problem with connecting speakers to the amp while it's working. After all that's what some protection circuits are doing Even if you short the outputs accidentally, there is no problem with LM3875 as it has short circuit protection built in. Also, you can connect and disconnect interconnects, providing the volume is completely down.
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Old 14th June 2003, 04:13 PM   #10
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Thanks for that Peter, I did wonder also what was the difference of the relays in my loudspeaker protection modules switching in and out compared to unplugging the speakers. It's just I have always switched equipment off before plugging/unplugging with the exception of USB on the PC of course.

I have made the decision to use the loudspeaker protection modules with the GC's. It may take a little away from the sound but the cost of the speakers has to be the deciding factor!

NOW I have not yet had time to connect up this 'problem' amp to a speaker but the DC offset measurements are 'normal' again.

And I am writing an eleventh commandment which states 'Always test the test equipment'. I found that the 10 ohm resistor that I had across the speaker terminals to measure the DC offset, had gone open circuit!!!

It's a bit worrying why that happened (resistor was 0.25 watt) but replacing it with a new one solved the measurement problem.

Now I have to connect it to a speaker and see if the buzzing noise has gone.
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