Brian, Don't trigger it with AC. use a very small dc source, and ONLY for a Nano second (don't want to break anything) .
Are you ALSO using a LONG decay ? and yes - trigger will make the scope show under 0, that is not the point, the point is that a DC voltage at the input of the amp will swing the output +, ierregardless of the input polarity at instintainous swing, NOT a waveform, you are missing the total point of this, if you are using a waveform to do the test. (the reason is that the waveform WILL trigger sooner or later, the point is to trigger with DC, AND have both polaritys of the input go + at the output.
I'm NOT saying that the amp only does +, what I'm getting at is the output of any "burst" will start on a + swing, and everytime it will do that.
Are you ALSO using a LONG decay ? and yes - trigger will make the scope show under 0, that is not the point, the point is that a DC voltage at the input of the amp will swing the output +, ierregardless of the input polarity at instintainous swing, NOT a waveform, you are missing the total point of this, if you are using a waveform to do the test. (the reason is that the waveform WILL trigger sooner or later, the point is to trigger with DC, AND have both polaritys of the input go + at the output.
I'm NOT saying that the amp only does +, what I'm getting at is the output of any "burst" will start on a + swing, and everytime it will do that.
Polarity?
About our friend from Manitoba, I'm sorry but you seem to be reading too much into how dual supply amplifiers work. No matter how you trigger a scope, if you put a positive going signal into an amplifier, the output will go in one direction, usually positive. If you put in a negative going signal, the signal will go in the opposite direction.
Hopwever, if you use a source like a battery and use a triggered scope you can be fooled. For example, if you set the scope to be triggered on the positive going slope. Since most power amps are AC coupled, that is, they will not amplify DC, if you apply a negative going pulse, the amp's output goes negative. However, this will not trigger the scope and even though the output goes negative, the voltage tapers off towards zero as the AC coupling cap charges. Then you remove the DC source, the amp's output leaps positive, triggerring the scope and you get the illusion that the amplifier's output went positive though you applied a negative input.
When you applied a positive going pulse, the output went positive triggering your scope. When the pulse was removed, yiu should have seen a negative going pulse. As a result, when you apply the pulse, whether it be positive or negative, you only see a positive output because that is what your instrument is set to detect.
A circuit that behaves this way is called a "differentiator". It happens when the frequency applied to a circuit is lower than the low end cut off frequency of a circuit.
Applying a DC source to determine amplifier polarity can only work if the amplifier's frequency response goes all the way to DC. Something that is very rare as the amplifier becomes thermally unstable. To prevent DC offset drift with temperature, amplifier designers, like myself, AC couple the amplifier.
As well, you mention that the amp's ground is offset from earth ground. That we connect the instrument to earth ground and not amp ground. Howzat again?? I always connect the scope's ground lead to the amplifier's ground. There is NO offset when measuring the amplifier. As well, to pass CSA and UL, the circuit's ground, at some point in the unit must be bonded to earth ground. We generally do this in only one place to prevent ground loops but these are definitely connected, if for no other reason, for EMI suppression.
The true way to test polarity of your signals is to use a tone burst generator and trigger your scope from the leading edge of the tone burst. All tone burst generators provide a sync output for your scope. I use an Audio Precision Model One myself for this.
If I apply a signal to one of my amplifiers with the beginning of the first cycle of the burst positive going, and the scope triggering on the burst gate, the output will be positive going. If I invert the signal polarity, where the beginning of the first cycle is negative going, the output of the amplifier will indeed be negative going.
And, if on a standard half bridge, split supply amplifier, if you connect the negative terminal of the speaker to earth ground, it will work fine. However, if you do this with one of my single supply, full bridge Class-D amplifiers, you will blow the amplifier. However, this has nothing to do with signal ploarity but that each side of the speaker is floating at a level of 55V DC because I am only usng a single power supply. However, the ground reference of the amplifier is still at earth ground.
It appears to me that a31_ford, has an incomplete understanding of the theory of power amplifier design and while it is admirable to see his deep thoughts in the area, he has made things out to be more complex than they really are.
About our friend from Manitoba, I'm sorry but you seem to be reading too much into how dual supply amplifiers work. No matter how you trigger a scope, if you put a positive going signal into an amplifier, the output will go in one direction, usually positive. If you put in a negative going signal, the signal will go in the opposite direction.
Hopwever, if you use a source like a battery and use a triggered scope you can be fooled. For example, if you set the scope to be triggered on the positive going slope. Since most power amps are AC coupled, that is, they will not amplify DC, if you apply a negative going pulse, the amp's output goes negative. However, this will not trigger the scope and even though the output goes negative, the voltage tapers off towards zero as the AC coupling cap charges. Then you remove the DC source, the amp's output leaps positive, triggerring the scope and you get the illusion that the amplifier's output went positive though you applied a negative input.
When you applied a positive going pulse, the output went positive triggering your scope. When the pulse was removed, yiu should have seen a negative going pulse. As a result, when you apply the pulse, whether it be positive or negative, you only see a positive output because that is what your instrument is set to detect.
A circuit that behaves this way is called a "differentiator". It happens when the frequency applied to a circuit is lower than the low end cut off frequency of a circuit.
Applying a DC source to determine amplifier polarity can only work if the amplifier's frequency response goes all the way to DC. Something that is very rare as the amplifier becomes thermally unstable. To prevent DC offset drift with temperature, amplifier designers, like myself, AC couple the amplifier.
As well, you mention that the amp's ground is offset from earth ground. That we connect the instrument to earth ground and not amp ground. Howzat again?? I always connect the scope's ground lead to the amplifier's ground. There is NO offset when measuring the amplifier. As well, to pass CSA and UL, the circuit's ground, at some point in the unit must be bonded to earth ground. We generally do this in only one place to prevent ground loops but these are definitely connected, if for no other reason, for EMI suppression.
The true way to test polarity of your signals is to use a tone burst generator and trigger your scope from the leading edge of the tone burst. All tone burst generators provide a sync output for your scope. I use an Audio Precision Model One myself for this.
If I apply a signal to one of my amplifiers with the beginning of the first cycle of the burst positive going, and the scope triggering on the burst gate, the output will be positive going. If I invert the signal polarity, where the beginning of the first cycle is negative going, the output of the amplifier will indeed be negative going.
And, if on a standard half bridge, split supply amplifier, if you connect the negative terminal of the speaker to earth ground, it will work fine. However, if you do this with one of my single supply, full bridge Class-D amplifiers, you will blow the amplifier. However, this has nothing to do with signal ploarity but that each side of the speaker is floating at a level of 55V DC because I am only usng a single power supply. However, the ground reference of the amplifier is still at earth ground.
It appears to me that a31_ford, has an incomplete understanding of the theory of power amplifier design and while it is admirable to see his deep thoughts in the area, he has made things out to be more complex than they really are.
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