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Posted
25 minutes ago, 21stcenturyquaker said:

 

How can I test if the speaker is ok?

Obviously I don't want to repeat the whole disaster over again if the speaker is ultimately responsible.  As I said previously, there didn't visually appear to be damage to the crossover.

 

 

It seems to me that you should be able to put a resistance meter across the spkr's terminals (remove the links, if the spkrs are set up for bi-wiring and measure each set, separately).

 

You should get a reading of something - it's only if you read a short that the spkr is damaged.

 

Posted (edited)
On 12/06/2023 at 10:44 AM, pete_mac said:


I couldn’t see this question answered anywhere… but amplifiers spit out AC. DC is the enemy of you speakers. 

 

If I may ask another silly Q, how does this operate in practice, let's say for reproducing a sole musical note at 15kHz (so we can avoid consideration of the wave superposition complexities). 

 

Is it like the neutral and active lines in our house, such that one amp output terminal remains at fixed potential while the other terminal reverses polarity 30 thousand times per second? Or do the terminals just switch polarity between themselves with each taking a turn 15 thousand times per second? Or is the polarity reversal much slower but with the audio signal encoded as an amplitude modulation that is independent of polarity?

 

I'm here to learn, as always, thanks!

 

 

Edited by tripitaka
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Posted
11 hours ago, 21stcenturyquaker said:

2 pairs of speakers have been the consequences so far

Yeah, I feel your pain, I try and fit protection kits to all vintage amps I sell, not worth the hassle should they fail, try and explain failed transistors to a guy who has fried his ALtecs/JBL's...

 

10 hours ago, 21stcenturyquaker said:

Obviously I'd like it repaired and fully serviced - recapped

Don't know the amp, is it 30-40 years old? Would only think of recapping at 20-25years old (depending on use/environment, ie, don't rush the recap.

Posted
7 hours ago, tripitaka said:

If I may ask another silly Q, how does this operate in practice, let's say for reproducing a sole musical note at 15kHz (so we can avoid consideration of the wave superposition complexities). 

 

Is it like the neutral and active lines in our house, such that one amp output terminal remains at fixed potential while the other terminal reverses polarity 30 thousand times per second? Or do the terminals just switch polarity between themselves with each taking a turn 15 thousand times per second? Or is the polarity reversal much slower but with the audio signal encoded as an amplitude modulation that is independent of polarity?

 

I'm here to learn, as always, thanks!

 

 

I do not get where you are coming from if you know what you know you know the answers to your own questions.

Posted
8 hours ago, tripitaka said:

Is it like the neutral and active lines in our house, such that one amp output terminal remains at fixed potential while the other terminal reverses polarity 30 thousand times per second? Or do the terminals just switch polarity between themselves with each taking a turn 15 thousand times per second? Or is the polarity reversal much slower but with the audio signal encoded as an amplitude modulation that is independent of polarity?

 

 

Voltages are potential differences.  They are relative to some reference.  If neither side of the speaker wires is connected to ground, then they are simply relative to each other (and floating from ground).  It makes no difference to the speaker.  For a pure tone, all it sees is current flowing one direction through it's voice coil for half a cycle, the the other direction for the next half cycle.  The amount of current follows a sinusoidal waveform.  For a 1kHz tone, it does this 1000 times a second.

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Posted

HI-FI being what it is Logic and Reality are not common bedfellows when it comes to Passive components. In the past there was a fad for direct wiring TT.s direct to the amps circuit boards and Spk cables direct to the Amps output. Look at all the components and connections  i have bypassed was the joyous cry. This fad was short lived due to the thin and bright sound that results.

Not a logical outcome? no a very logical one all these are taken into account by choice of resistors, capacitors and whatever else is done in the voiceing of the item all items in the signal path are accounted for.

Bypassing the fuses in my Spk degrades the freq balance. I have not tried expensive fuses (and fuses are not just in Spk but in Amps don't forget) as have never seen them for Spks.

The Age Old method for testing drive units is put a 1.5 volt battery across Spk terminals if the drive unit moves forward it not only works but is in phase.

Posted
31 minutes ago, aussievintage said:

 

 

Voltages are potential differences.  They are relative to some reference.  If neither side of the speaker wires is connected to ground, then they are simply relative to each other (and floating from ground).  It makes no difference to the speaker.  For a pure tone, all it sees is current flowing one direction through it's voice coil for half a cycle, the the other direction for the next half cycle.  The amount of current follows a sinusoidal waveform.  For a 1kHz tone, it does this 1000 times a second.

Perfect, thank you very much!  That was my (albeit terribly expressed) middle option in my original Q. 🙂

Posted
9 hours ago, Richard Tremain said:

I do not get where you are coming from if you know what you know you know the answers to your own questions.

My confusion also related to the thread topic - why do rail voltage levels not necessarily matter when playing music but fry speakers in a second if not encoding music?  I can now see that the answer might be that, in music, the electrical flow is always doing work (moving the assembly against a magnetic field) which would be seen as electrical resistance, whereas for fixed voltage once the cone has reached an extremity reflecting the voltage level then there is no more resistance and current flow would be uncontrolled.

Of course I may still have it all wrong.

This might even be the reason why square waves are supposed to be so hard on drivers?

Posted
22 hours ago, 21stcenturyquaker said:

How can I test if the speaker is ok?

With multimeter, measure resistance of individual drivers (disconnected from any crossover), Expect somewhere 2-16ohms, OL/Open Loop = open circuit is bad, burnt out voice coil. Can you source replacement voice coils rather than the whole driver? Sometimes cheaper to replace driver but be careful when buying from china

Posted
48 minutes ago, 21stcenturyquaker said:

The drivers are complete cactus: melted polypropylene.

 

WOW!  :o

 

Posted

Yes, it caused quite a scene at home as the family evacuated the room leaving Dad to shut down the disco - no smoke machine required 😄

 

Sad thing was I was listening to a new cd transport I'd brought home only an hour before - it now resides under a cover for the foreseeable future......😑

Posted

I haven't read all the replies in this thread but from the symptoms described I'd suggest the power amp is the culprit.  With 39 volts d.c. present on the left channel output it is almost certainly a fault in the amplifier that has causes the output to clamp up to one of the supply rails.  Direct current is something your speakers should definitely not be getting.

 

The woofers have smoked up and by the time you actually got to switching the amp off the voice coils in the woofers have become toast.  :(  This is why solid state power amplifiers should always incorporate either a protection circuit that constantly monitors the conditions on the output and automatically disconnects the speakers to protect then or some other methods in the design to prevent this from happening.  

 

AFAIK these amplifiers were offered as DIY kits but Hugh feel free to chime in and correct me if I'm wrong.  The basic power amp boards don't have any integral protection against such failures other than supply rail fuses which by the time they blow the carnage has already taken place. :(  A simple protection board wired between the amplifier output and the speaker terminals would almost have certainly saved your speakers from this type of failure.

 

Now before you go out and buy replacement woofer drivers for your blown speaker I highly suggest checking that the inductors and / or resistor components of the low pass filter in the speaker's crossover network haven't been damaged. 

 

To prevent any further damage to your power amp please do not switch it back on and certainly don't connect any speakers to it until it has been repaired.  I hope Hugh or someone else can repair your amp for you but if you can't get any joy locally PM me.

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Posted
34 minutes ago, Monkeyboi said:

AFAIK these amplifiers were offered as DIY kits

This unit was indeed put together by a diy-er and probably not a very good one: having previously opened the case some of the wire routing was poorly executed. 

 

With that and it's recent failure in mind I have decided not to pursue a repair but may list the amp for a knocked down price for anyone skilled/interested enough and who is prepared to take the risk involved in repairing it.

 

It is a pity though as the Naksa is a great performer for not a lot of money.

Posted
45 minutes ago, Monkeyboi said:

I highly suggest checking that the inductors and / or resistor components of the low pass filter in the speaker's crossover network haven't been damaged. 

Thanks for your input monkeyboi.

 

How do I do this? 

Posted
1 minute ago, 21stcenturyquaker said:

Thanks for your input monkeyboi.

 

How do I do this? 


Get to the crossover circuit in the speaker and Do a visual inspection 1st,  look for and  identify  things that look “cooked”  warped, color change or changed shape.   Then you need to pull it out of the circuit to measure if it ticks the above criteria.   However, crossover components including coils will take more punishment as the sizes of the coils are larger than a speaker coil.  Capacitors will have voltage ratings so if 39VDC was present,  they maybe effected. 

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Posted
3 minutes ago, 21stcenturyquaker said:

Thanks for your input monkeyboi.

 

How do I do this? 

 

First off you will probably have to probably remove the two blown woofers to access the crossover network,  Look for obvious signs of heat damage to the inductors (coils) and any resistors.  Generally cooked parts will show signs of discolouration, burning or heat fatigue but sometimes the damage is hidden from obvious casual visual inspection. Depending on how long the fault persisted before the power was switched off will depend on the possible extent of the damage (if any).

 

Whilst the resistance of inductors and resistors can be easily verified by the use of a simple multimeter a shorted turn in an inductor can only be readily verified with either a LCR (inductance capacitance resistance) meter or a shorted turns tester. 

Posted
10 hours ago, tripitaka said:

My confusion also related to the thread topic - why do rail voltage levels not necessarily matter when playing music but fry speakers in a second if not encoding music?  I can now see that the answer might be that, in music, the electrical flow is always doing work (moving the assembly against a magnetic field) which would be seen as electrical resistance, whereas for fixed voltage once the cone has reached an extremity reflecting the voltage level then there is no more resistance and current flow would be uncontrolled.

Of course I may still have it all wrong.

This might even be the reason why square waves are supposed to be so hard on drivers?

The simplified answer is the rail voltage is the absolute maximum voltage the amp can provide to the speaker.  With normal music at sane volumes the voltage will be significantly less and the AC nature of music signal means the peaks are short term and there fore have less impact.   But if the music is so loud that that the peaks reach rail voltage and are extended in time then speaker damage can result.  This is "clipping".

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Posted
6 hours ago, 21stcenturyquaker said:

The drivers are complete cactus: melted polypropylene.

Can you post pics?

Posted (edited)
16 hours ago, RoHo said:

The simplified answer is the rail voltage is the absolute maximum voltage the amp can provide to the speaker.  With normal music at sane volumes the voltage will be significantly less and the AC nature of music signal means the peaks are short term and there fore have less impact.   But if the music is so loud that that the peaks reach rail voltage and are extended in time then speaker damage can result.  This is "clipping".

 

Not to dispute what you say, but 'AC nature' (edit: by which I specifically mean the periodic reversal of current direction, thanks @aussievintage) and 'sinusoidal nature' aren't the same thing.

 

According to your argument (which incidentally I also believed until reading this thread) you could happily feed a sinusoidal DC* (see edit below) wave into a speaker and it would be fine (though you wouldn't get music, of course). 

 

Yet, as a result of reading this thread, I now believe the speakers would blow, since there would be recurring periods of zero resistance ('infinite' current flow) that would burn the coils.

 

As usual, I claim no expertise in anything.  All we need now is a volunteer to test this hehe

 

Edit: 

* @aussievintage prefers this to be called an "AC waveform with a DC offset". I have no objection to this, so long as it is recognised that the current is never changing direction, which is the key point.

 

 

🙂

Edited by tripitaka

Posted
Just now, tripitaka said:

Not to dispute what you say, but 

'AC nature' and 'sinusoidal nature' aren't the same thing.

 

According to your argument (which incidentally I also believed until reading this thread) you could happily feed a sinusoidal DC wave into a speaker and it would be fine (though you wouldn't get music, of course). 

 

That is a misunderstanding of basic electricity.  You can't have "sinusoidal DC".  You can have Sinusoidal AC with a DC component added to it.  btw.  if you fed it to a speaker you would burn it out due to the DC component (not the AC)

 

As for AC being sinusoidal - well it can have other waveforms, such as a square wave, which can be mathematically represented as a series of summed  sinusoidal waveforms, so in the end...

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Posted
16 minutes ago, aussievintage said:

 

That is a misunderstanding of basic electricity.  You can't have "sinusoidal DC".  You can have Sinusoidal AC with a DC component added to it.  btw.  if you fed it to a speaker you would burn it out due to the DC component (not the AC)

 

As for AC being sinusoidal - well it can have other waveforms, such as a square wave, which can be mathematically represented as a series of summed  sinusoidal waveforms, so in the end...

Im always ready to be taught basic electricity, thanks.  So may I ask what would call the wave if I set up a bunch of car batteries to deliver 50V DC and then placed in front of it a variable device which I rapidly switched so as to yield 0 -> 50v in a recurring (let's say sinusoidal just for laughs) pattern?

Posted
2 minutes ago, tripitaka said:

Im always ready to be taught basic electricity, thanks.  So may I ask what would call the wave if I set up a bunch of car batteries to deliver 50V DC and then placed in front of it a variable device which I rapidly switched so as to yield 0 -> 50v in a recurring (let's say sinusoidal just for laughs) pattern?

 

That's a 50 volt peak to peak ac wave form with 25 volt dc offset added to it.

Posted (edited)
7 minutes ago, tripitaka said:

Im always ready to be taught basic electricity, thanks.  So may I ask what would call the wave if I set up a bunch of car batteries to deliver 50V DC and then placed in front of it a variable device which I rapidly switched so as to yield 0 -> 50v in a recurring (let's say sinusoidal just for laughs) pattern?

 

If it was a switch (say a relay contact) it would be interrupted d.c. not an alternating current.  In the case of a speaker, if the d.c. component was very small then the speaker could still function but with a forward or reverse biasing of the cone positioning with respect to its at rest (no voltage) condition.

 

Edited by Monkeyboi
Posted
18 minutes ago, Monkeyboi said:

 

If it was a switch (say a relay contact) it would be interrupted d.c. not an alternating current.  In the case of a speaker, if the d.c. component was very small then the speaker could still function but with a forward or reverse biasing of the cone positioning with respect to its at rest (no voltage) condition.

 

That is my (new-found) understanding, so I find some relief in that🙏

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