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| Probing full bridge amplifiers with oscope |
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| magic:
Alternatively, connect scope ground to car chassis and probe any of the channel connectors to see what's coming out of them. It is particularly curious what happens on the shared, middle tap. |
| Hyper_Spectral:
--- Quote from: BrianHG on July 17, 2019, 08:33:12 pm ---Since your scope is battery powered, and you will be using 1 channel, just tie the probe GND to the speaker - and the probe tip to the speaker + on the amp. You will see the correct signal. If you do this, do not make any contact between any metal frame or connector GND on your scope to any any GND in your 12v system. Also do not do this if you have 2 channels on your scope as each speaker - output on the amplifier will have a different voltage signal burning out you probe's GND or even the scope itself. --- End quote --- Thanks Brian, the scopes case is plastic so I should be alright. The 12v ground of the amp is in parallel with the heatsink so I'll probably just use that. I'm going to be using sine waves with equal output on the left/right channels so tolerances aside they should be similar. The scope is only one channel so I'm good to go either way. If someone reads this in the future though, that's good knowledge. I'll report back with my results, which I'm sure will be positive. I too want to probe that central "shared output" to see what's on it. |
| MagicSmoker:
I downloaded the owner's manual for the specific amplifier and near as I can tell - I'm not too familiar with car audio stuff - is that the center +/- terminal is actually power ground and that channel 2's signal is permanently inverted, hence you need to connect the speaker "backwards" to channel 2 if you want to maintain correct phasing when using each channel separately, rather than bridged. So, no big mystery, really. Given the type of scope you have, I agree with BrianHG's suggestion that you simply connect your scope to channel 1 (+) and channel 2 (-). Note that most amps - especially Class D types - will not produce a reasonable looking output waveform unless a load is connected. If you want to test amps more than just this one time then get some wirewound resistors to use instead of speakers. I suggest (4) 2 Ohm resistors that way you can wire them up in series/parallel combinations to get 0.5, 2, 4 and 8 Ohms total. |
| Hyper_Spectral:
--- Quote from: MagicSmoker on July 17, 2019, 09:01:45 pm ---I downloaded the owner's manual for the specific amplifier and near as I can tell - I'm not too familiar with car audio stuff - is that the center +/- terminal is actually power ground and that channel 2's signal is permanently inverted, hence you need to connect the speaker "backwards" to channel 2 if you want to maintain correct phasing when using each channel separately, rather than bridged. So, no big mystery, really. Given the type of scope you have, I agree with BrianHG's suggestion that you simply connect your scope to channel 1 (+) and channel 2 (-). Note that most amps - especially Class D types - will not produce a reasonable looking output waveform unless a load is connected. If you want to test amps more than just this one time then get some wirewound resistors to use instead of speakers. I suggest (4) 2 Ohm resistors that way you can wire them up in series/parallel combinations to get 0.5, 2, 4 and 8 Ohms total. --- End quote --- Those would be some large resistors! 200w per channel out of this amp is a lot of power. I've never had a problem getting an output from a Class D amp with the scope until now (but they were all half bridge). I tried connecting the scope to Ch 1 + and ch 2 - and i had no output so either there was an issue with the dsp and the demand of the sine wave or there was no signal. Generally if I'm concerned that the psu rails will sag under load I'll just back the gain down after reaching the clip point. Most music is recorded at -5 db anyways and I use 0db tones. I'm hearing two different things now, connect the probe to the + output and then the scope ground to 12vDC ground, and one saying try it in bridge mode. I'll try both? |
| MagicSmoker:
--- Quote from: Hyper_Spectral on July 17, 2019, 09:14:59 pm ---Those would be some large resistors! 200w per channel out of this amp is a lot of power. I've never had a problem getting an output from a Class D amp with the scope until now (but they were all half bridge). I tried connecting the scope to Ch 1 + and ch 2 - and i had no output so either there was an issue with the dsp and the demand of the sine wave or there was no signal. --- End quote --- Class D amps have an LC or LCL output filter that averages the pulse-width-modulated (PWM'ed) square wave into the original audio waveform (well, close to it, anyway), but this filter needs a load present to actually do its job. 50W-100W chassis mount wirewound resistors are maybe $4 each from a reputable source like Mouser, Digikey, etc. Probably even cheaper than that if you get them from a dodgy source like ebay or Amazon. Also, they can be be grossly overloaded for short periods without harm - 10x rated power for 1-2 seconds is easily doable. --- Quote from: Hyper_Spectral on July 17, 2019, 09:14:59 pm ---I'm hearing two different things now, connect the probe to the + output and then the scope ground to 12vDC ground, and one saying try it in bridge mode. I'll try both? --- End quote --- Well, I don't know about connecting the ground clip of the scope to 12V ground - likely some kind of push-pull step up power supply is inside the amp to boost the nominal 12V from the car battery into +/-30 to +/-50V or so, and the ground reference on the power side might not (shouldn't, really) be connected to chassis ground on the 12V side. So, you could connect scope ground to the +/- terminal and touch the probe tip to Channel 1 (+) or Channel 2 (-) to see if each channel is working, then connect scope ground to, say, Channel 2 (-) and scope tip to Channel 1 (+) to see if bridged mode is working. Again, with some kind of load present to give the best chance of seeing a realistic output from the amp. |
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