Today I was testing an 150W pioneer amp using a 5m cable , 1.4uH, 660pF, .066 ohm. I was applied a 1khz sinewave. Passing 20Vpp appears oscilation about 100Khz. Lowering applied signal, Below 20pp it dissapeared. i tryed a zobel network but didn't worked. What it might be?
The amp is a pioner VSX605S receiver.
The amp is a pioner VSX605S receiver.
20Vpp at the output. The input was a 1KHz 1Vpp sinewave from a rohde-schwartz audio analizer.
It was caused probably by the output cable, I just want to know why and how to kill it.
The test was made under my professor's supervision.
I was thinking it had might be caused by an out of phase feedback, turning it from a negative feedback to a positive feedback.
Without the 5 meters cable it had run up to 120Vpp.
It was caused probably by the output cable, I just want to know why and how to kill it.
The test was made under my professor's supervision.
I was thinking it had might be caused by an out of phase feedback, turning it from a negative feedback to a positive feedback.
Without the 5 meters cable it had run up to 120Vpp.
No problem.....
The use of instruments is not the problem, I have access to an digital osciloscope and an audio analizer (FFT, waveform, THD, etc.)
But even with it, as I'm only an student, I couldn't dicover, and I ascked for help becouse I think my professor is testing me!
The power BJTs are 2SA1302 and 2SC3286. They run in AB class.
I have to put this capacitor in the output transistor itself or in the darlington "driver"? Rod Eliott once told me that , he didn't know why, but the oscilation happens to appear only in the "negative" pair, and to use this miller cap in the driver, can I try this without damage of the receiver?
The use of instruments is not the problem, I have access to an digital osciloscope and an audio analizer (FFT, waveform, THD, etc.)
But even with it, as I'm only an student, I couldn't dicover, and I ascked for help becouse I think my professor is testing me!
The power BJTs are 2SA1302 and 2SC3286. They run in AB class.
I have to put this capacitor in the output transistor itself or in the darlington "driver"? Rod Eliott once told me that , he didn't know why, but the oscilation happens to appear only in the "negative" pair, and to use this miller cap in the driver, can I try this without damage of the receiver?
It strongly depends on the circuit - sometimes it helps to put cap to the output transistor, sometimes to the driver. You can have a look at my www - I had the similar problem - see "push-pull class A follower", which was in fact operated in AB class. Rod Elliott is right, very often the osccillations appear only at one half of the waveform. I hope you will not destroy the output transistors by small caps, does the amp have an overcurrent protection? You may start with smaller value, say 100pF.
An often ignored problem with regard to amplifier stability is the presence of a non-minimum phase, (RHP), zero generated by multiple signal paths from input to output.
An RHP zero has the magnitude response of an LHP zero, (i.e: it 'breaks up'), but the phase response of an LHP pole, (i.e: phase response 'breaks down', or tends to - 90 degrees in the limit).
This undesirable zero can be moved to the LHP by connecting a small resistor in series with the miller compensation capacitor(s), or moved to infinity by placing a capacitor in parallel with each of the often-used series resistors to the output stage.
This may, (OR NOT
) be the cause of your problems.....but give the later a try....a time constant of the order of 200nS for each parallel combination should suffice......
An RHP zero has the magnitude response of an LHP zero, (i.e: it 'breaks up'), but the phase response of an LHP pole, (i.e: phase response 'breaks down', or tends to - 90 degrees in the limit).
This undesirable zero can be moved to the LHP by connecting a small resistor in series with the miller compensation capacitor(s), or moved to infinity by placing a capacitor in parallel with each of the often-used series resistors to the output stage.
This may, (OR NOT
) be the cause of your problems.....but give the later a try....a time constant of the order of 200nS for each parallel combination should suffice...... Now that's a hell of a thorough answer - glad to see we have some properly trained EEs in the forum.
mikek said:An often ignored problem with regard to amplifier stability is the presence of a non-minimum phase, (RHP), zero generated by multiple signal paths from input to output.
An RHP zero has the magnitude response of an LHP zero, (i.e: it 'breaks up'), but the phase response of an LHP pole, (i.e: phase response 'breaks down', or tends to - 90 degrees in the limit).
This undesirable zero can be moved to the LHP by connecting a small resistor in series with the miller compensation capacitor(s), or moved to infinity by placing a capacitor in parallel with each of the often-used series resistors to the output stage.
This may, (OR NOT
I didn't want to start a new thread beacuse i have allso oscillation problem with my apmplifier. Here is the thing: I have made 4 amplifiers, 2 for bas and 2 for mid-high in same box. Amplifier for mid-high worked fine but other two had some distortion when i aplyed more input signal. At some quit level it did not distort. There was allso some strage sound comming from speaker when i pushed the membrane of speaker. Then i realized that i did not solder the zobel for those 2 problematic amplifiers. It worked fine with zobel, 15ohm with 100nf. Amplifiers now work ok but will they work ok for now on? I am worried about these oscillatons, before i have made amplfiers with no zobel and they were ok. I have used the same type of schematic.
The secon thing is that when i have disconected one channel of those oscilaton amplifiers, then the other channel worked without oscilation!?
Same thing is when channel swaped, one off and other on, but when both are on then they both oscilate!!
The secon thing is that when i have disconected one channel of those oscilaton amplifiers, then the other channel worked without oscilation!?
Same thing is when channel swaped, one off and other on, but when both are on then they both oscilate!!
Amplifier Oscillation
It is likely that connecting the 5M output cable added enough output capacitance so that the amp's phase margin went negative at a high frequency, but one where the the loop gain is still positive. A small resistor may work, but does not provide a frequency dependent impedance. A better approach would be to insert between the amp and cable a parallel combination of 5 uH and 2.5 ohms. At audio frequencies the inductor looks effectively like a short. At ultrasonic frequencies, it looks like a higher impedance and isolates the amp's output from the load, preserving a positive phase margin. The 2.5 ohm resistor serves as a damping element.
Hope this helps.
It is likely that connecting the 5M output cable added enough output capacitance so that the amp's phase margin went negative at a high frequency, but one where the the loop gain is still positive. A small resistor may work, but does not provide a frequency dependent impedance. A better approach would be to insert between the amp and cable a parallel combination of 5 uH and 2.5 ohms. At audio frequencies the inductor looks effectively like a short. At ultrasonic frequencies, it looks like a higher impedance and isolates the amp's output from the load, preserving a positive phase margin. The 2.5 ohm resistor serves as a damping element.
Hope this helps.
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