Please look at the thread "simple amplifier?".
To summarize: I built a simple amp which runs just fine with low output-power, but when trying it with high output-power it blew. Where could be the problem?
To summarize: I built a simple amp which runs just fine with low output-power, but when trying it with high output-power it blew. Where could be the problem?
sajti said:
I just put a subwoofer with 8 Ohm to the amp. Input for amp came from subwoofer-output from an active crossover.
All other parts used are described in this thread: http://www.diyaudio.com/forums/showthread.php?threadid=15415&goto=newpost
Hi,
I checked the topic, but there are lot of discussion, and lot of schematics. Which version did You build?
Sajti
I checked the topic, but there are lot of discussion, and lot of schematics. Which version did You build?
Sajti
OK,
I found that. Looks nice, my only problem is the R9, R10, C3 network, which sets the bias for VAS stage (Q3). The most of the amlifiers I know use higher value capacitor for this application. 47-100uF is the most common.
As You wrote, this amplifier works properly with low power. How low this power? Did You masure it?
How big heatsink You use for the amplifier? Is it large enough?
Sajti
I found that. Looks nice, my only problem is the R9, R10, C3 network, which sets the bias for VAS stage (Q3). The most of the amlifiers I know use higher value capacitor for this application. 47-100uF is the most common.
As You wrote, this amplifier works properly with low power. How low this power? Did You masure it?
How big heatsink You use for the amplifier? Is it large enough?
Sajti
The amp was playing several times for an hour at 1-2Watt. Then when testing it on max. power output the amp blew in just 2 seconds...so there can´t be a heatsink-problem...
Did You checked the other channel? (I know this is not a good idea, to kill the other TIP142/147 too) Is it works without the loudspeaker? Even with high output?
This can be cooling problem, if the devices not properly attached to the heatsink. The high output level means high dissipation which can kill the transistors. But if there is no big difference between the temperature of the TIP142/147 and the temperature heatsink with low power, it looks OK.
Sajti
This can be cooling problem, if the devices not properly attached to the heatsink. The high output level means high dissipation which can kill the transistors. But if there is no big difference between the temperature of the TIP142/147 and the temperature heatsink with low power, it looks OK.
Sajti
It is no heatsink problem. The TIPs are not going to be hot without speakers. They even get only lukewarm when playing at low power with speakers connected. And they can´t blew in 2 seconds with mounted on a heatsink...
Could the TIPs be destroyed when only +45V rail collapses in duty? This perhaps could be appeared when playing at high power due to a loose contact, but I am not sure.
Or could the TIPs be blown by oscillations? What should I change in the circuit to prevent the amp from oscillations?
Could the TIPs be destroyed when only +45V rail collapses in duty? This perhaps could be appeared when playing at high power due to a loose contact, but I am not sure.
Or could the TIPs be blown by oscillations? What should I change in the circuit to prevent the amp from oscillations?
The +45 rail collapse causing distortion, but no device failure.
Oscillation can destroy the output transistors. But this type of failure resulted by high frequency, and high level oscillation. If You have oscilloscope You can check the output signal to find the oscillation.
Most of the time, for oscillation You don't need any input signal.
To avoid possible oscillation just insert some small value resistors (100ohms) in series with the base of the TIPs. Put this resistors as close to the transistors as possible!
Sajti
Oscillation can destroy the output transistors. But this type of failure resulted by high frequency, and high level oscillation. If You have oscilloscope You can check the output signal to find the oscillation.
Most of the time, for oscillation You don't need any input signal.
To avoid possible oscillation just insert some small value resistors (100ohms) in series with the base of the TIPs. Put this resistors as close to the transistors as possible!
Sajti
sajti said:
As far as I know are these oscillations appear often much to shortly to be recognized...
Are there any other things to change for prevention of amp-oscillation?
This oscillation appears if You connect the transistors with long wires. To protect them the most common solution to apply damping resistors, or use the shortest possible wires, or mount the output transistors to the pcb.
I don't remember exactly: Does Your amplifier works with high output level, and no load?
Sajti
I don't remember exactly: Does Your amplifier works with high output level, and no load?
Sajti
sajti said:
Yes, the amp works fine with high power but no load! I ran it with very much clipping connected to my oscilloscope. The Transistors are mounted the shortest way on the equipped board, the board itself measures just about 5cmX5cm - so there are no long wires, this also seems not to be the problem...
Hmmmmm...
My only advice to try the other channel with load, and increase the volume slowly.
Sajti
My only advice to try the other channel with load, and increase the volume slowly.
Sajti
djk said:
I will try...
What about putting an 47pF cap parallel to R8 (feedback) and/or putting a 10Ohm resistor with a parallel coil right on the output? I saw this design at other amplifiers to prevent oscillations...
The 47pF paralell with the feedback resistor helps to stabilize the closed loop gain. But if Your amplifier not stable with closed loop, You have to get output signal without any input.
To the output I recommend 10ohms instead of 100.
Sajti
To the output I recommend 10ohms instead of 100.
Sajti
"and putting a 10 Ohm resistor with a parallel coil right on the output?"
The damped output coil is mainly for driving capacitive loads, weird speaker cables, piezo tweeters, electrostaic speakers.
Darlington outputs tend to oscillate without a resistor in series with the base.
Good reading:
http://www.passdiy.com/pdf/a40.pdf
The circuit board is available for $6 from:
http://www.audioxpress.com/bksprods/pcbs/nelsonpass.htm
I have used this board with suitable high voltage transistors to construct a 400W amplifier.
The damped output coil is mainly for driving capacitive loads, weird speaker cables, piezo tweeters, electrostaic speakers.
Darlington outputs tend to oscillate without a resistor in series with the base.
Good reading:
http://www.passdiy.com/pdf/a40.pdf
The circuit board is available for $6 from:
http://www.audioxpress.com/bksprods/pcbs/nelsonpass.htm
I have used this board with suitable high voltage transistors to construct a 400W amplifier.
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