EEVblog Electronics Community Forum
Electronics => Beginners => Topic started by: michaelr on December 02, 2017, 06:42:29 am
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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 (https://gtdaudio.net/gtd-audio-2x250-watts-professional-stereo-power-amplifier-q-3000.html)
DMM (Volts): GB Instruments GDT-11
DMM (Amps): Radioshack 22-172 Clamp Amp
Thanks!
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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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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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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!
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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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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/images/cache/Q3000_OK.500.jpg)
https://gtdaudio.net/gtd-audio-2x250-watts-professional-stereo-power-amplifier-q-3000.html (https://gtdaudio.net/gtd-audio-2x250-watts-professional-stereo-power-amplifier-q-3000.html)
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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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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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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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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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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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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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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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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!
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& 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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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!
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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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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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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...
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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
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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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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.
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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!
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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.
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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)
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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The 1500W are PMPO at full moon >:D For a proper power measurement connect a dummy load with the matching resistance, attach a scope and feed the amp with a 1kHz sine wave, Turn up the volume until you start seeing distortion on the scope. Turn down a little bit, just before the distortion starts. Then read the voltage from the scope: P = U^2/R
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Seen many of these cheap shitty rackmount amps. I bet it's only got a pair of TIP2955/3055's in it per channel.. maybe two pairs if youre lucky.
In short, it blew up because it's a cheap piece of shit. Bin it, or gut it for parts.
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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.
I would tend to agree, although the amp did not fail immediately. It was about a minute or two, until it went south, with full power setting across a 2 Ohm load. I blew three internal resistors, for sure.
I've attached a couple pics, but I'll apologize for the quality up front. As soon as I get a new amp, or repair this one, I'll get a better camera and update the photos attached. Viewing the image with the circuit board on the right, the visible damaged resistors are on the lower right hand section of the board. What else could have fried? I don't know yet, but am open to suggestions as to where to look. Again, I apologize about the quality. It is the best I could do right now. ;)
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I've attached a couple pics, but I'll apologize for the quality up front. As soon as I get a new amp, or repair this one, I'll get a better camera and update the photos attached.
There's nothing wrong with the camera. You just need to make sure the pictures are in focus.
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Michael If you really want to repair that 2x15
00W Amp you should check all transistors in that region plus replacing the burned resistors. The good thing is that you have blown one channel. The other can be used to read the resistor values.
Yes the cam is a bit aged (I couldn't resist to check the embedded meta data). For a fixfocus cam like you have only light will help a bit.
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Wow, a single pair of TO-264 devices per channel. That's about 80W RMS into 8 ohms (safely) at most. They are probably 2SA1943/2SC5200 - or rather, cheap nasty copies of them.
Pretty much what I expected.
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Using a magnifying glass and good light, I've tallied up some of the major components, with their markings, if anyone may find it useful/interesting. Might as well make this a good crash course on amplifiers...emphasis on the crash part!
White blocks [4EA] - BPR56, 0.47 Ohms J, KOA595
o Non-inductive Ceramic Cement Resistor
Large black Box[1EA] - GHong MB, 5A 250 VAC, 5A 30VDC, OTX14F3 L-DC24V-2A
o Relay?
Large black chips mounted to heat sink[4EA] - Toshiba TTA1943. 408 [2EA] & TTC5200. 408 [2EA]
o TTA1943: Transistor Silicon PNP Epitaxial Type - 100-W high-fidelity audio frequency amplifier output
stage
o TTC5200: Bipolar Transistor - Amp Trans 15A 150W 230V BJT NPN PWR
Small black chips mounted on heat sink [2EA] - KSE340
o Bipolar Transistors - BJT NPN Epitaxial
Medium sized chips just in front of heat sink [4EA] - A1837 [2EA] & C4973 [2EA]
o Mosfets?
Large 4 pin black chip near capacitors [1EA] RRD GBJ 2510
o Glass Passivated Bridge Rectifier
Based on the large transistors, would this suggest each channel could put out 250W ideally? With losses that might put it near it's 880W PSU (110V x 8 Amp fuse).
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If the PNP output transistor is only rated for 100W, then that is your theoretical limit. You do not ADD the two ratings together the PNP and NPN transistors are the top and bottom half of the output pair. The maximum you can expect from it is the LOWER power rating of the two devices. And that is the theoretical ideal, not what you should expect in the Real World.
That is a cheap 50-75 W domestic amplifier dressed up to look "professional". The people who wrote the description of that gadget have a dart-board on their office wall. When they threw the dart, it hit "1,500 Watts" and they all had a good laugh. :-DD
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If the PNP output transistor is only rated for 100W, then that is your theoretical limit. You do not ADD the two ratings together the PNP and NPN transistors are the top and bottom half of the output pair. The maximum you can expect from it is the LOWER power rating of the two devices. And that is the theoretical ideal, not what you should expect in the Real World.
That is a cheap 50-75 W domestic amplifier dressed up to look "professional". The people who wrote the description of that gadget have a dart-board on their office wall. When they threw the dart, it hit "1,500 Watts" and they all had a good laugh. :-DD
Those dirty rascals! Well, now their board is on EEVBlog. Ha!
Clarification: The TTA1943 is recommended for 100 Watt output stage, but it does have a collector power dissipation of 150W, at room temperature.
http://docs-europe.electrocomponents.com/webdocs/143d/0900766b8143d8f9.pdf (http://docs-europe.electrocomponents.com/webdocs/143d/0900766b8143d8f9.pdf)
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Medium sized chips just in front of heat sink [4EA] - A1837 [2EA] & C4973 [2EA]
o Mosfets?
A1837 = 2SA1837 (or equivalent). The "2S" numbers are owned by one company so the prefix is often dropped or substituted when making a compatible part.
C4973 = 2SC4973: same idea.
Worth knowing!
https://en.wikipedia.org/wiki/JIS_semiconductor_designation
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Michael If you really want to repair that 2x1500W Amp you should check all transistors in that region plus replacing the burned resistors. The good thing is that you have blown one channel. The other can be used to read the resistor values.
Yes the cam is a bit aged (I couldn't resist to check the embedded meta data). For a fixfocus cam like you have only light will help a bit.
The manual for my Kodak EasyShare 3.0 Mp camera was copyrighted in 2005.... You got me, even though I have no clue how you figured that out. It is time for an upgrade!
The resistors that match up to the burnt out ones, on the sister channel, are 10 & 12 Ohms, most likely carbon film based on their appearances. I'll check out the transistors next. Fixing this heap will boost my confidence a bit; and give me more experience. It's my beater now. :)
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RMS Power @ 2 Ohms: 1,500 Watts
Audio power amplifiers are rated in peak power, not average. Audio generally has a peak-to-rms crest factor of 10:1, and it needs the peak power to reproduce audio waveforms, but the continuous average power is much less. Clip indicators tell you you're exceeding max peak power, not average. You need to be very careful when reproducing plain sinusoidals due to their 1:1 crest factor, which makes it very easy to overpower any audio amplifier. It won't clip, although many amplifiers have thermal shutdown protection as well that might kick in. (And, no, I seriously doubt it specs RMS Power of 1500W...)
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Just to compare. A vintage power mixer I'm working on now has four pairs of 2SC3182 and 2SA1265 complementary power transistors rated at 70 watts each.But the total rated output power mono is 300w rms at 2 ohms ,200w rms at 4 ohms and 100w rms at 8 ohms .That's on a heat sink 16 inches X 4 inches X 2 inches.
Your unit has only 2 pairs rated at 100w and is supposed to output ,I'm guessing, 1500 watts per channel at 2 ohms?
I would hunt down the person that sold that to you and make him eat it.
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Medium sized chips just in front of heat sink [4EA] - A1837 [2EA] & C4973 [2EA]
o Mosfets?
A1837 = 2SA1837 (or equivalent). The "2S" numbers are owned by one company so the prefix is often dropped or substituted when making a compatible part.
C4973 = 2SC4973: same idea.
Worth knowing!
https://en.wikipedia.org/wiki/JIS_semiconductor_designation
Got it. So, 2SA1837 is a PNP transistor; and 2SC4793 an NPN transistor. That will help with testing. Thanks!
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The 1500W are PMPO at full moon >:D For a proper power measurement connect a dummy load with the matching resistance, attach a scope and feed the amp with a 1kHz sine wave, Turn up the volume until you start seeing distortion on the scope. Turn down a little bit, just before the distortion starts. Then read the voltage from the scope: P = U^2/R
Simple enough. I will do that as soon as I get my hands on a working amp.
Would this power level be considered safe for prolonged used; and could this technique be used at other frequencies?
Thanks!
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The usual reason for a burnup like that is that the b-e junction of the output transistor has gone open circuit. The (unheatsinked) driver transistor and its emitter resistor then have to handle the full output current.
When repairing an output stage you need to be thorough about replacing every damaged component. Otherwise you will have a repeat failure. Also, after the repair you need to test with a current limited supply (see dim bulb tester) otherwise if you have made just one mistake you can blow the whole lot again.
For any kind of professional use, an amp that can't stand full power output for a reasonable time is a liability. If a dropped mic or howl round blows the amp and shuts down the show, is that any use? :palm:
As I've said, the main reservation I would have is about using 2-ohm loads. Unless it's high end kit which specifically states that is permissible, you are risking a blow doing that. The majority of top brands do NOT recommend 2-ohm loads anyway.
In any case, with the possible exception of bass guitar it's rare to need more than 50Wpc RMS into decent PA speakers. These are more efficient than typical domestic speakers, and that sort of input will be more than loud enough for a small indoor venue.
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Would this power level be considered safe for prolonged used; and could this technique be used at other frequencies?
That depends on the amp's design and thermal management. When you perform the power measurement, also check the temperature of the power supply, and the driver and output transistors. The power is specified for 1kHz usually, but you can also check other frequencies if you like.
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Would this power level be considered safe for prolonged used; and could this technique be used at other frequencies?
Most audio amplifiers (except the real professional ones) won't stand that power for longer than a minute or so. The heat sink and the power supply (transformer) usually isn't designed for this. Once I've read a rule of thumb: an audio power amplifiers supply / heatsink is designed for about 70% of its nominal sinewave power. Appears to be true for quality gear, but I've seen worse. It can be overloaded (100% of its nominal power) for a short period time. Prolonged use will overheat various components until shutdown by magic smoke. That's the usual deal with "normal" audio power amplifiers. If you run the amplifier with a light load (higher impedance than 8 Ohm, open load), it'll stand longer. Increased frequency at high output voltage levels may cause the ouput R/C damping to burn (especially the R). Audio amplifiers nearly always have a R/C from output to ground for frequency stability added.
So if you need an rugged amplifier for lab use, either get a professional one or really know what you do when using a consumer audio amp.
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Most audio amplifiers (except the real professional ones) won't stand that power for longer than a minute or so. The heat sink and the power supply (transformer) usually isn't designed for this. Once I've read a rule of thumb: an audio power amplifiers supply / heatsink is designed for about 70% of its nominal sinewave power.
Actually a myth, and easily busted on the testbench. Under sinewave conditions the max heating effect on the output transistors occurs at some point below max output.
They are effectively dropper resistors, and when there is no voltage drop then no heat is produced. Of course that never arises with a sinewave output. In fact, the voltage across each transistor follows a cosine law, being the supply minus the output voltage. Therefore the heating effect follows a squared cosine law across the half cycle. (V2/R)
Thus for small reductions in output power, the increase in transistor heating follows a square law. Reducing the output moderately will cause the output stage to run considerably hotter than at full sinewave power.
Only when you reduce the output to less than 50% or so is the current sufficiently lower that the total heat produced starts to fall.
It's possible that the mains transformer could overheat under continuous full output, but I've never seen one burn out.
The thing that will kill a domestic grade amp on stage, is hooking too many paralleled speakers to it. A 2-ohm load is a definite no-no. 4 ohms, stretching things. 8 ohms, OK. With a very low impedance load and loud but not max volume the thing will get roasting hot.
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If you are planning on repairing that (could be a good educational exercise, even if the amp is crap as such), I would also look very carefully into the protection circuitry on that board (if there is any!).
You may want to make sure that there is at least over-temp and over-current protection for the end stage, also possibly some zeners to clamp the drive voltage, etc. That could save you from having to fix it again later.
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Most audio amplifiers (except the real professional ones) won't stand that power for longer than a minute or so. The heat sink and the power supply (transformer) usually isn't designed for this. Once I've read a rule of thumb: an audio power amplifiers supply / heatsink is designed for about 70% of its nominal sinewave power.
Actually a myth, and easily busted on the testbench. Under sinewave conditions the max heating effect on the output transistors occurs at some point below max output.
The first graph below from Bob Cordell's Designing Audio Power Amplifiers shows the relationship between maximum power output and maximum power dissipation.
The second graph shows the load line for both a resistive and typical reactive speaker load drawn against the safe operating area specification of the transistor. Keep in mind that the safe operating area including secondary breakdown decreases as the junction temperature rises. So for reliable operation, the transistors need to be derated based on worst cast operating temperature which will occur at about 1/3rd of maximum output power.
If a power amplifier is designed for lowest distortion, then often multiple output devices are used in parallel to prevent excessive Ft and Gm droop in which case sufficient power derating may already built in. There is also some advantage to using multiple lower power output devices in parallel to achieve a lower case-to-heat sink thermal resistance. Consumer level designs are unlikely to do either of these things for reasons of cost so are unlikely to handle a 2 ohm or maybe even 4 ohm load at high continuous output power. Even an 8 ohm load may be pushing it unless additional cooling is used.
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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?
What should be and what is are often not the same thing. I've seen more than a few audio amplifiers that were far from bulletproof. Pushing a sine wave at full power is brutal service and while the amp *should* shut down in time, quite a few of them won't. It's not uncommon for the output transistors to sacrifice themselves to protect the fuse.
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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".
Those are 2 different things tho. One is producent (or people reviewing hardware) dividing their catalog into "tiers" like
- cameras for filming your vacation and your kid
- cameras for "professionals" (as in "people doing it for money", not neccesairly experts) and more serious amateurs
- specialistic cameras (high speed, sport cameras, etc)
- broadcast/movie/other highest tier devices
The other is "let's put some big numbers and nice sounding words like "PROFESSIONAL" on the case so somebody clueless might think they are getting a bargain buying it".
Other reason is so one amateur can brag to another, same reason why some devices brag about random features on the case of device, like cameras having big embossed 3CCD, or writing zoom numbers and number of megapixels on camera case
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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!
A signal generator amplifier? What frequencies are you talking about?