Author Topic: Amplifier is toast -why?  (Read 9592 times)

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Offline michaelrTopic starter

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Amplifier is toast -why?
« on: December 02, 2017, 06:42:29 am »
My recently purchased audio amplifier is now toast.  Why?  Did I mess up; or is this just a bad amplifier?

While conducting a test run on my new audio amplifier (used in "mint condition"), using a signal generator as input, the smell of smoke filled the air.   This was on the third run, using purely resistive loads: 8 Ohm, 4 Ohm, and finally 2 Ohm.    Power measured was significantly lower than specification, at full gain/power:  all power leds lit up, but the clipping led did not.  My resistor, a heater element, did not heat up noticeably. The amplifier maintained 100% gain for a minute or two, prior to failing.  The amplifier's 10 Amp fuse did not blow (marked 8 Amps on the rear panel).   Visual inspection of the amp revealed at least 3 burned out resistors on one channel.  Trying out the second channel, I found it no longer puts out any signal/power (verified with oscilloscope).

Setup
The signal generator output was via BNC/alligator clip leads.  The amplifier Channel A audio input was via an 1/4" to 3.5mm jack adapter, hooked up to about 4' of 3.5mm TRS cable.  The alligator clips were clipped on to the TRS cable connector, with negative to sheath & positive to tip.  Output from the amplifier was via two wires, from an old extension cord, attached to Channel A binding posts, which was then clipped to heating element via alligator clips. 

Load: 2 Ohms resistive (heater element)
Input: 1.54 Vp-p, 400 Hz, sinusoidal
Measured Power @ 100% Gain: 4.0 Amps (amp clamp), 8.3 Volts (DMM)

Equipment Specs

Signal Generator:
     Output Impedance: 50 Ohms
     Frequency Range: 0-25 MHz

Amplifier:
     Make: GTD Audio
     Model: Q3000 "2U Professional Power Amplifier"
     Channels: 2 (non-bridgeable)
     Frequency Response: 10Hz-50KHz @ 1.5 dB
     Input Sensitivity: 0.77V
     RMS Power @ 2 Ohms: 1,500 Watts
     Link: Manufacturer Product/Specification Page with Pictures

DMM (Volts): GB Instruments GDT-11
DMM (Amps):  Radioshack 22-172 Clamp Amp

Thanks!
« Last Edit: December 03, 2017, 05:06:57 pm by michaelr »
 

Offline Audioguru

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Re: Amplifier is toast -why?
« Reply #1 on: December 02, 2017, 04:38:29 pm »
Some amplifiers are rated at "music power" which is never a continuous tone like you used. If the parts do survive then the power supply voltage sags after a few hundred milliseconds and reduces the output power.

But maybe your new amplifier had a part that would have failed with a music input anyway.
 
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Offline David Hess

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Re: Amplifier is toast -why?
« Reply #2 on: December 02, 2017, 04:44:55 pm »
A continuous sine wave produces much higher power dissipation in a power amplifier than typical audio.  This applies to the speakers as well; continuous high frequency sine waves are a good way to blow out a tweeter.
 
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Offline michaelrTopic starter

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Re: Amplifier is toast -why?
« Reply #3 on: December 02, 2017, 06:28:36 pm »
Some amplifiers are rated at "music power" which is never a continuous tone like you used. If the parts do survive then the power supply voltage sags after a few hundred milliseconds and reduces the output power.

But maybe your new amplifier had a part that would have failed with a music input anyway.

A continuous sine wave produces much higher power dissipation in a power amplifier than typical audio.  This applies to the speakers as well; continuous high frequency sine waves are a good way to blow out a tweeter.

Okay.  Well that narrows it down quite a bit!  Thanks.

The signal being the culprit may also fit the results of a previous "no load" test I had ran just prior.  Using a sinusoidal, 60Hz, 1.736Vp-p input (DSG), I measured an output of 9.9V AC (DMM), prior to the clipping led lighting up; and I measured 23.0V AC (DMM) at 100% Gain.  Reducing the input voltage to 1.54Vp-p (matching input sensitivity?) reduced the output voltage to 9.9V AC (DMM) or about 15Vp-p (Oscilloscope) @ 100% Gain.

If I may ask a couple of follow up questions...

How would one go about determining the maximum power an amplifier can handle, for a particular wave form?  Would a "no load" voltage test, varying input voltage and gain, have provided any clues?

I am still shocked at the difference in power delivered versus rating.  The amplifier fried at 2-4% of the RMS power rating, per my measurements and calculations.  Maybe an audio amplifier is not what I was looking for.  Is there equipment more suitable for amplifying these types of wave forms, that won't break the bank?

Thank You!


« Last Edit: December 02, 2017, 06:35:36 pm by michaelr »
 

Offline IanB

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Re: Amplifier is toast -why?
« Reply #4 on: December 02, 2017, 06:42:48 pm »
RMS Power @ 2 Ohms: 1,500 Watts

Huh?  :o

Was that a PA amp, or something designed for performance or concert halls? And 1,500 W per channel, or in total? And is the amp big, weighing 100 lb, and with cooling fans?

Of all the technical info you have provided, the one thing noticeably absent is any information about the amplifier. Make and model. Full specs. A picture to give context.

Because 1,500 W continuous output is a huge amount of power. Unimaginable for any consumer device. If you nominally assume an efficiency of 70%, then the input power from the mains would have to be over 2000 W. Not likely to be powered from a 120 V supply.

So a lot of details are missing here.
 
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Offline michaelrTopic starter

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Re: Amplifier is toast -why?
« Reply #5 on: December 02, 2017, 07:01:45 pm »
RMS Power @ 2 Ohms: 1,500 Watts

Huh?  :o

Was that a PA amp, or something designed for performance or concert halls? And 1,500 W per channel, or in total? And is the amp big, weighing 100 lb, and with cooling fans?

Of all the technical info you have provided, the one thing noticeably absent is any information about the amplifier. Make and model. Full specs. A picture to give context.

Because 1,500 W continuous output is a huge amount of power. Unimaginable for any consumer device. If you nominally assume an efficiency of 70%, then the input power from the mains would have to be over 2000 W. Not likely to be powered from a 120 V supply.

So a lot of details are missing here.

Good point! The amplifier is a GTD Audio "Professional Power Amplifier", Model Q3000.  It weighs 24 lbs and has dual cooling fans.  All I know is that they claim "1,500 Watts @ 2 Ohms RMS." A prior email to their support representative, asking for more details, was unresponsive.  Over rated perhaps?


https://gtdaudio.net/gtd-audio-2x250-watts-professional-stereo-power-amplifier-q-3000.html
« Last Edit: December 02, 2017, 07:24:55 pm by michaelr »
 

Offline IanB

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Re: Amplifier is toast -why?
« Reply #6 on: December 02, 2017, 07:35:33 pm »
There seem to be a few extra zeros added in that product spec for decoration. The title seems reasonable though: "2 x 150 watts".

That would then be:
- 150 W per channel into 2 ohms
- 75 W per channel into 4 ohms
- 40 W per channel into 8 ohms

Maximum output 300 W.

And I would suggest the maximum output is not continuous, but for short periods only.
 
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Offline janoc

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Re: Amplifier is toast -why?
« Reply #7 on: December 02, 2017, 07:35:53 pm »
Well, given that the page has in the title:

"GTD Audio 2x150 Watts Professional stereo Power Amplifier Q-3000" (emphasis mine), you are off by an order of magnitude or so. That 1.5kW is most likely only short term peak power. That should be written in the documentation for that thing.

Linear (i.e. not class D) power amps that are actually capable of delivering 1.5kW of output (e.g. for concert halls and such) are about 2-3x the size of what you have there.
 
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Offline Twoflower

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Re: Amplifier is toast -why?
« Reply #8 on: December 02, 2017, 07:51:04 pm »
The 115V power jack says 8A. So it can't deliver the 1.5kW continuous without having a power source build in. Even with 100% efficiency.

The numbers seem to be randomly chosen: The URL says 2x250W, the Product page 2x150W, the printing on the thing 3000W and what "400 Watts @ 8 Ohms  RMS ( 1500W + 150W  Stereo )" means is totally unknown to me. :wtf:
 
 
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Offline dmills

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Re: Amplifier is toast -why?
« Reply #9 on: December 02, 2017, 09:57:30 pm »
Rule of thumb: Any audio kit that has "PROFESSIONAL" printed on the front is unlikely to be anything of the sort (Much the same goes for test equipment!).

An old Crown Macrotec is what you actually want for this, they get much closer to spec but you will hate the shipping cost!

Most PA kit is designed for 1/8th duty cycle which is ample for music, and the voltage headroom is very welcome, but you generally need to be a little careful using it for CW tone, some cope better then others.

Regards, Dan.
 
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Offline vk6zgo

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Re: Amplifier is toast -why?
« Reply #10 on: December 02, 2017, 10:13:27 pm »
Rule of thumb: Any audio kit that has "PROFESSIONAL" printed on the front is unlikely to be anything of the sort (Much the same goes for test equipment!).t

& for TV monitors, cameras, etc----"Professional" was always regarded as several levels below "Broadcast".
Towards the end of my association with such things,  their performance was often ( just) to Broadcast standards, but their construction was always "El Cheapo".
 
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Online Zero999

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Re: Amplifier is toast -why?
« Reply #11 on: December 02, 2017, 10:30:25 pm »
The specification just doesn't add up.

Yes, it's true that I wouldn't expect it to be able to deliver the full rated power continuously, but a properly designed amplifier (certainly one this size) should have decent enough overload protection, not to blow up like this. I think you should return it and get a full refund.
 
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Offline abraxa

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Re: Amplifier is toast -why?
« Reply #12 on: December 02, 2017, 10:52:13 pm »
Quote
Is there equipment more suitable for amplifying these types of wave forms, that won't break the bank?

I got the impression you were feeding it the sine wave to test if it works - not because that's your primary use case. What are you wanting to do with such an amp then?
 
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Offline michaelrTopic starter

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Re: Amplifier is toast -why?
« Reply #13 on: December 02, 2017, 11:06:44 pm »
Quote
Is there equipment more suitable for amplifying these types of wave forms, that won't break the bank?

I got the impression you were feeding it the sine wave to test if it works - not because that's your primary use case. What are you wanting to do with such an amp then?

Yes, I was testing to see if it works; but I also intended to use it as a signal generator amplifier.  Although, it would have been a nice perk that it could have been used as an audio amplifier also, if the need ever arose!
 

Offline dmills

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Re: Amplifier is toast -why?
« Reply #14 on: December 02, 2017, 11:28:27 pm »
& for TV monitors, cameras, etc----"Professional" was always regarded as several levels below "Broadcast".
Yep, Sony even included it in the names of their monitors, the BVM (Broadcast video monitor) series were better then the PVM (Professional video monitor) series. BVM was where you found the grade 1 picture tubes.

It is however only a rule of thumb in the Pro AV game, but is a pretty reliable one.

Regards, Dan.
 
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Offline michaelrTopic starter

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Re: Amplifier is toast -why?
« Reply #15 on: December 02, 2017, 11:46:43 pm »
The specification just doesn't add up.

Yes, it's true that I wouldn't expect it to be able to deliver the full rated power continuously, but a properly designed amplifier (certainly one this size) should have decent enough overload protection, not to blow up like this. I think you should return it and get a full refund.

I'm definitely considering it; and appreciate the confirmation!
 

Offline Richard Crowley

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Re: Amplifier is toast -why?
« Reply #16 on: December 03, 2017, 02:10:45 am »
Note also that measuring 400Hz with a RadioShack clamp-on ammeter is highly questionable.  Most gear indented for measuring mains power is designed only for commercial mains frequencies (50-60Hz).  Most general-purpose test gear is accurate only up to a very low frequency.  If you want to measure even audio frequency signals (not to mention ultrasonic of RF), you must use test gear designed for that purpose.  It is quite possible that you were actually pumping out significantly more power as your Radio Shack clamp-on meter was probably lying to you.

People have been complaining for at least 50 years about audio power amplifier specifications and how the marketing people inflate them. Expecting a cheap power amplifier like that to output thousands of watts is ludicrous.

 
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Offline mfratus2001

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Re: Amplifier is toast -why?
« Reply #17 on: December 03, 2017, 02:35:30 am »
Let's start at the beginning. This is YOUR new amp, but this is not A new amp (used in "mint condition").

The 10 Amp fuse in the 8 Amp fuse holder should have told you something... whoever had it before you already blew the fuse, I'm guessing.
That damaged or stressed some components, and so on. So apparently somebody already ran that test.

Yes, you should have returned it, but now it is either repair this under-$100 amp, or trash it.

I have a similar amp, with similar wild claims. There were a number of bad solder connections, and I know because it was flaky and I took the cover off to see what was up.

I use it now for a monitor amp, testing speakers, and all-around background music in the shop.
 
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Offline michaelrTopic starter

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Re: Amplifier is toast -why?
« Reply #18 on: December 03, 2017, 04:13:17 am »
mfratus2001:  You are quite correct.  Good observation.  It is possible that the properly rated fuse may have prevented the meltdown.  Although, I  wonder, as I had only read 4.0 Amps on the ammeter.  If the amplifier was consuming the other 4 Amps, then it may very well have helped.  I guess we'll never know. ;)

You know, I had a bad feeling in my gut about this purchase.  After reflecting on the situation, I doubt I can return it, due to the seller's return policy; and am likely unable to apply the warranty, as it used.  I may be stuck with it. 

Fixing the amp would make a decent project, for a beginner.  It would be nice if all it needed was three burnt out resistors!

At the same time, I will likely need another amplifier anyway, if all I can pull with this one is something south of 32 Watts...
 

Offline michaelrTopic starter

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Re: Amplifier is toast -why?
« Reply #19 on: December 03, 2017, 04:36:01 am »
Note also that measuring 400Hz with a RadioShack clamp-on ammeter is highly questionable.  Most gear indented for measuring mains power is designed only for commercial mains frequencies (50-60Hz).  Most general-purpose test gear is accurate only up to a very low frequency.  If you want to measure even audio frequency signals (not to mention ultrasonic of RF), you must use test gear designed for that purpose.  It is quite possible that you were actually pumping out significantly more power as your Radio Shack clamp-on meter was probably lying to you.

People have been complaining for at least 50 years about audio power amplifier specifications and how the marketing people inflate them. Expecting a cheap power amplifier like that to output thousands of watts is ludicrous.

I did wonder about the test instrument's accuracy, at that frequency.  The ammeter is a Radioshack Model 22-172: Digital Clamp-on AC/DC Multimeter.  It can measure frequency, from 40Hz to 10Mhz.  Although, I have not verified the ammeter accuracy, at that frequency.  I'll put doing that on the to do list and report back.

Let's just say this is my first amplifier purchase; and I didn't have much time to get oriented, as I never really had a need for them, until just now.  I'm learning  ;D
« Last Edit: December 03, 2017, 05:11:42 am by michaelr »
 

Offline janoc

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Re: Amplifier is toast -why?
« Reply #20 on: December 03, 2017, 01:07:12 pm »
mfratus2001:  You are quite correct.  Good observation.  It is possible that the properly rated fuse may have prevented the meltdown. 

For sure not. The fuse reacts way slower than a power transistor blows up, especially if it was a peak current that blew it (supplied from the capacitor bank and not directly mains supply). That fuse is there to protect you from fire, not the amplifier from blowing up.

His point was that someone has been obviously messing with the device because the fuse has been replaced and thus it was in who knows what state, not that a smaller fuse would have saved you.

 
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Offline IanMacdonald

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Re: Amplifier is toast -why?
« Reply #21 on: December 03, 2017, 02:08:34 pm »
Fuses are safety devices. They are not there to protect the electronics, but to protect you. The better grade professional amps will have overload protection, the cheap ones rarely do. A sudden short at high power WILL blow the output transistors if there is no current limiting/overload protection. Not might. Will.

Most amps of that kind of power rating (if genuine?) have bridge outputs. A common mistake on bridge outputs is to ground one side of the speaker output via test equipment.

You can easily determine the likely power rating from the PSU voltages and whether it's bridge or not.  There is no need for a clamp meter provided your test load has a dependable resistance.

It's rare to find an amp that won't take its full rated sinewave power on both channels with 4 ohm loads for a few minutes at least. Except, not many budget PA amps will tolerate a 2 ohm load.

On the repair bench it was normal to run a full power test after a repair. A minute or two on sinewave, then maybe 15 with heavy metal at just below clipping. Blowups were rare. If it won't stand this it's a potential liability - no-one wants to have the PA fail in a performance.  :-[

In fact, the greatest heating load on the output stage occurs at half max RMS power with a regulated supply, and around a third power with an unregulated supply. (Think about it in terms of a series dropper resistor and you see why)

I'd always advise against 2-ohm setups anyway - It's very hard on the equipment and the high cable losses mean that you gain very little in loudness.

HTH.
« Last Edit: December 03, 2017, 02:29:30 pm by IanMacdonald »
 

Offline michaelrTopic starter

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Re: Amplifier is toast -why?
« Reply #22 on: December 03, 2017, 04:36:25 pm »
Note also that measuring 400Hz with a RadioShack clamp-on ammeter is highly questionable.  Most gear indented for measuring mains power is designed only for commercial mains frequencies (50-60Hz).  Most general-purpose test gear is accurate only up to a very low frequency.  If you want to measure even audio frequency signals (not to mention ultrasonic of RF), you must use test gear designed for that purpose.  It is quite possible that you were actually pumping out significantly more power as your Radio Shack clamp-on meter was probably lying to you.

People have been complaining for at least 50 years about audio power amplifier specifications and how the marketing people inflate them. Expecting a cheap power amplifier like that to output thousands of watts is ludicrous.

I did wonder about the test instrument's accuracy, at that frequency.  The ammeter is a Radioshack Model 22-172: Digital Clamp-on AC/DC Multimeter.  It can measure frequency, from 40Hz to 10Mhz.  Although, I have not verified the ammeter accuracy, at that frequency.  I'll put doing that on the to do list and report back.

Let's just say this is my first amplifier purchase; and I didn't have much time to get oriented, as I never really had a need for them, until just now.  I'm learning  ;D

Ammeter and DMM Accuracy @ 400 Hz Determined:
It appears the measurements I took with clamp amp and DMM are reasonably accurate. 

The DMM RMS voltage reading @ 400 Hz was verified with a signal generator.  Generating a 10Vp-p (~3.5V RMS) sine wave input, it read 3.35V @ 60 Hz and 3.3V @ 400 Hz.  The DMM manual is no longer in my possession, but the meter's precision is 0.1V; and the readings were within 10% of a perfect sine wave.  While I was at it, I found the DMM's 60 Hz reading held in the range of 9Hz-300Hz and remained within 5% up to about 800Hz.

As for the ammeter, I no longer can take a direct current reading @ 400Hz, as my amplifier is toast and my signal generator is wimpy (hence the amplifier).  Although, I do have readings from during the test of Voltage (8.3 Volts) and Resistance (2.0 Ohms).  Ohm's Law gives 4.15 Amps.  My clamp amp read 4.0A during this same period.  So that ammeter was reading to within 10%, of the actual current value, as well.

These readings also are anecdotally confirmed by the 2 Ohm heater element not heating up.  1.5 kW, even 150W, should have gotten red hot!
« Last Edit: December 03, 2017, 05:17:40 pm by michaelr »
 

Offline janoc

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Re: Amplifier is toast -why?
« Reply #23 on: December 03, 2017, 06:45:06 pm »
These readings also are anecdotally confirmed by the 2 Ohm heater element not heating up.  1.5 kW, even 150W, should have gotten red hot!

If the amp was grossly overloaded, it likely blew up much faster than a resistor would have managed to heat up significantly. That heater element does not get hot instantaneously, even when powered with 1.5kW!  On the other hand, all a transistor needs to die is exceeding the maximum ratings for some milliseconds, depending on type. So your analogies are waay off here.
« Last Edit: December 03, 2017, 06:47:33 pm by janoc »
 
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Offline David Hess

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Re: Amplifier is toast -why?
« Reply #24 on: December 03, 2017, 07:19:00 pm »
How would one go about determining the maximum power an amplifier can handle, for a particular wave form?  Would a "no load" voltage test, varying input voltage and gain, have provided any clues?

For a class-B or class-AB amplifier, the worst case power dissipation occurs at 1/3rds of the maximum output power.  At least in the US, amplifiers are suppose to be able to sustain this for 1 hour into an 8 ohm load. (1)

Quote
I am still shocked at the difference in power delivered versus rating.  The amplifier fried at 2-4% of the RMS power rating, per my measurements and calculations.  Maybe an audio amplifier is not what I was looking for.  Is there equipment more suitable for amplifying these types of wave forms, that won't break the bank?

Well designed power amplifiers do not have problems like this.  They either protect themselves or are sufficiently power derated to handle the worst case circumstances which is not difficult to do but it adds expense.

(1) Federal Trade Commission (FTC), “Power Output Claims for Amplifiers Utilized in
Home Entertainment Products,” CFR 16, Part 432, 1974.
 
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