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| Probing full bridge amplifiers with oscope |
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| Hyper_Spectral:
Full bridge amplifiers are proliferating rapidly in the car audio market, just 5 years ago the idea of full bridge was off-putting but now they make more power in a smaller package with the same relatively low distortion (remember when everyone hated Class D?) I'm hoping to get some clarification on how to probe a full bridge multi-channel amplifier (ch = 2/4/6/...) that has only 3 outputs for 2 channels. I.e. for a 2 channel it would be: Ch1 Ch 2 + -/+ - 1) how would one probe this amplifier? I've attempted probing it normally, and with the scope on Ch 1's + and Ch 2's -, basically bridged mode, without success. Am I correct in assuming that in this case I would need to use the amplifiers actual input ground (12vDC ground) for the scopes ground instead of the negative rail? Obviously I would then only get half of the waveform, which is fine for detecting clipping, and I'd need a 2ch scope to get the whole waveform? 2) Does this mean the center shared -/+ has the full sine wave on it, with each half 180 degrees out of phase? Could I just probe this while referencing the input ground? |
| BrianHG:
Does your scope have an ChannelA -(minus) ChannelB function? For digital scopes, this might be in the math functions. Or, ChannelA + ChannelB with an inversion function for ChannelB? Set the scope inputs to AC coupled. GND to the chassis GND. And, place 1 probe on the speaker + and the other on the speaker -. The other way would be to purchase a differential probe and just tie that probe's inputs to the speaker's + and - terminals. https://www.eevblog.com/product/hvp70/ Note that these techniques will image on your scope exactly what the actual speaker is receiving in voltage. |
| MagicSmoker:
--- Quote from: Hyper_Spectral on July 17, 2019, 06:14:55 pm ---... I'm hoping to get some clarification on how to probe a full bridge multi-channel amplifier (ch = 2/4/6/...) that has only 3 outputs for 2 channels. I.e. for a 2 channel it would be: Ch1 Ch 2 + -/+ - --- End quote --- Hmm, that looks unusual. How, exactly, do you connect a speaker to the above (specifically, is that +/- terminal used)? Are speakers coming with center-tapped voice coils now? --- Quote from: Hyper_Spectral on July 17, 2019, 06:14:55 pm ---1) how would one probe this amplifier? I've attempted probing it normally, and with the scope on Ch 1's + and Ch 2's -, basically bridged mode, without success. Am I correct in assuming that in this case I would need to use the amplifiers actual input ground (12vDC ground) for the scopes ground instead of the negative rail? Obviously I would then only get half of the waveform, which is fine for detecting clipping, and I'd need a 2ch scope to get the whole waveform? --- End quote --- For a typical bridged output - which is floating with respect to "ground" - you need to use a floating or differential measurement technique. That could be with an isolated scope (which few people own), a differential probe (much better) or, if your scope has at least two channels and supports basic math functions, you connect the probe tips from two channels to each side of the bridge, respectively, then subtract one channel from the other (the probe grounds can be connected to power ground, if available, or left floating). There are numerous other tricks that can be done so don't consider the above an exhaustive list. --- Quote from: Hyper_Spectral on July 17, 2019, 06:14:55 pm ---2) Does this mean the center shared -/+ has the full sine wave on it, with each half 180 degrees out of phase? Could I just probe this while referencing the input ground? --- End quote --- That's the proverbial $64000 question. In a typical bridged amplifier you have two half bridges which are fed signals that are inverted with respect to each other so when the load is connected between them it sees double the voltage, which quadruples the maximum power (assuming there is sufficient current available). This almost seems like a push-pull output stage, but that is really only used in tube amps and power supplies, so very puzzling. Got a make/model number for the amp so we can see how it is supposed to be used? |
| Hyper_Spectral:
--- Quote from: BrianHG on July 17, 2019, 07:52:20 pm ---Does your scope have an ChannelA -(minus) ChannelB function? For digital scopes, this might be in the math functions. Or, ChannelA + ChannelB with an inversion function for ChannelB? Set the scope inputs to AC coupled. GND to the chassis GND. And, place 1 probe on the speaker + and the other on the speaker -. The other way would be to purchase a differential probe and just tie that probe's inputs to the speaker's + and - terminals. https://www.eevblog.com/product/hvp70/ Note that these techniques will image on your scope exactly what the actual speaker is receiving in voltage. --- End quote --- The scope I'm using is a dso201 or similar, basically a cheap sub 50khz battery powered mini oscope. That will probably help both of you understand what I'm doing and that it's fine to ground the scope wherever. --- Quote from: MagicSmoker on July 17, 2019, 07:53:25 pm --- Hmm, that looks unusual. How, exactly, do you connect a speaker to the above (specifically, is that +/- terminal used)? Are speakers coming with center-tapped voice coils now? For a typical bridged output - which is floating with respect to "ground" - you need to use a floating or differential measurement technique. That could be with an isolated scope (which few people own), a differential probe (much better) or, if your scope has at least two channels and supports basic math functions, you connect the probe tips from two channels to each side of the bridge, respectively, then subtract one channel from the other (the probe grounds can be connected to power ground, if available, or left floating). There are numerous other tricks that can be done so don't consider the above an exhaustive list. That's the proverbial $64000 question. In a typical bridged amplifier you have two half bridges which are fed signals that are inverted with respect to each other so when the load is connected between them it sees double the voltage, which quadruples the maximum power (assuming there is sufficient current available). This almost seems like a push-pull output stage, but that is really only used in tube amps and power supplies, so very puzzling. Got a make/model number for the amp so we can see how it is supposed to be used? --- End quote --- See above for the type of scope I'm using, it's an isolated ground and single channel (or at least I only have one probe). I'm unsure of what a center tapped voice coil is, but I'm confident that's not what we're using. I assume it's some sort of voice coil that uses power on the center slug inside the voice coil? lol Here's a picture of the actual amp in question, although there are many "Brazilian" (i.e. full bridge) designs like this coming onto the market as of lately. Ch 1's negative is shared with Ch 2's positive. Not sure how they do channel separation like this unless something cancels out on the central output they share. |
| BrianHG:
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. |
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