I was trying to figure out the requirements for the speaker protect of a BTL amp. My basic question is: can I take an ordinary "single ended stereo" speaker protect module and use it for one BTL amp channel?
TomChr offers his Guardian-686 speaker protect module with superb documentation, and we know that it's designed for his BTL amp, but I was trying to understand from first principles: can I get the same result (keep aside reliability and sound quality) if I use any standard stereo single-ended speaker protect module?
This sort of speaker protect module has the following connections:
What I am proposing to do:
Is this going to work?
TomChr offers his Guardian-686 speaker protect module with superb documentation, and we know that it's designed for his BTL amp, but I was trying to understand from first principles: can I get the same result (keep aside reliability and sound quality) if I use any standard stereo single-ended speaker protect module?
This sort of speaker protect module has the following connections:
- two lines for power (which we will ignore...)
- L-speaker in "live"
- L-speaker in "ground"
- R-speaker in "live"
- R-speaker in "ground"
- L-speaker out "live"
- L-speaker out "ground"
- R-speaker out "live"
- R-speaker out "ground"
What I am proposing to do:
- I would connect the two amp outputs from the BTL amp to the L-speaker-in "live" and R-speaker-in "live"
- I would connect the L-speaker-out "live" and R-speaker-out "live" connections to the red and black binding posts of one speaker output of the amp chassis
- I would connect the L-speaker in "ground" and R-speaker in "ground" to the same place where I'm connecting the amplifier's power ground. This means, given my knowledge, the point between the smoothing capacitors of the power supply.
Is this going to work?
How do you intend to drive the BTL amplifier?
What kind of input signal(s)?
Symmetrical?
Balanced?
Will the BTL amp have an on-board inverter circuit included, or do you intend to rely on a pair of inverted signals from an external source?
Do you have any details regarding the speaker protection module, that is intended to be used for this purpose? Schematic? Photos?
Regarding your main question .... I would cautiously assume that if the input signal is well behaved and fully symmetrical, with respect to analog ground, then your strategy should work.
I am not sure though, how this strategy would work, if the input signal was to be taken from a fully "floating" symmetrical input signal source, such as for example: an interstate transformer, one that has absolutely no signal reference to BTL amplifier ground.
What kind of input signal(s)?
Symmetrical?
Balanced?
Will the BTL amp have an on-board inverter circuit included, or do you intend to rely on a pair of inverted signals from an external source?
Do you have any details regarding the speaker protection module, that is intended to be used for this purpose? Schematic? Photos?
Regarding your main question .... I would cautiously assume that if the input signal is well behaved and fully symmetrical, with respect to analog ground, then your strategy should work.
I am not sure though, how this strategy would work, if the input signal was to be taken from a fully "floating" symmetrical input signal source, such as for example: an interstate transformer, one that has absolutely no signal reference to BTL amplifier ground.
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I would say unconditionally yes - if the inverter isn't properly balanced then there may be an offset voltage on the speakers doing them no good already...
You could imagine a protection circuit that measures the difference between the two outputs for its purposes and shuts down both amps together. This could be a few less components, but wouldn't detect if both amps were developing similar DC offsets as they aged, which isn't particularly far-fetched prospect.
Having standard protection on each amp allows them to be used singly or bridged as you like. Seems the right thing to me.
You could imagine a protection circuit that measures the difference between the two outputs for its purposes and shuts down both amps together. This could be a few less components, but wouldn't detect if both amps were developing similar DC offsets as they aged, which isn't particularly far-fetched prospect.
Having standard protection on each amp allows them to be used singly or bridged as you like. Seems the right thing to me.
I'd never expected that the input would impact my question. But here goes.
I am not thinking of a specific amp yet. Therefore, it's possible that the amp will be designed with a balanced input from the start, and will later have a BTL architecture, like the Neurochrome 686. Or it could be two single-ended amps I pick up and the put an unbalanced-to-balanced converter module upstream of them and drive them in opposition to each other.
Exactly what I said. I'd be eager to learn how this impacts my speaker protect?
No, actually. I am almost deliberately trying to keep away from a specific design, so that I can understand the impact of various things at a generic way.
So, basically, there needs to be a ground reference shared between the speaker protect and amp? Or the amp and source?
Thanks.
Yes, I agree that the problem of each half of the BTL developing DC independently is a different one, and I guess my approach of a separate circuit for each half-amp will catch those situations.
It seems to me that the input considerations are pretty much of a red herring. You are looking for a DC difference between the two output terminals. That is what will damage the speakers. For the case where each individual output is referenced to common (ground) then using two conventional protection circuits that sense that offset will work. For the case where the two outputs are the same, but not DC zero with respect to common then it doesn't work. There are circuits that sense the difference between the positive and negative outputs where the negative side of the two bridge tied outputs are common to each other but not necessarily circuit ground that can work for either bridge tied or conventional output. The key here is that the power for the protection circuit is not necessarily at the same potential as the low side of the output.
Imo, if you intend to use two independent speaker protect sections for each of the separate amplifiers that are to constitute the bridge, then each such amplifier is 'single ended', and the respective speaker protect module would measure the output of such amplifier as referenced to ground.
Now, after you bridge, you could/would basically provide a symmetrical signal to the two SE inputs (with no ground reference), a signal which "could" be floating (as in true symmetrical, as from an interstage transformer), but the two speaker protect modules shall remain referenced to ground anyway. Does that constitute a problem? I do not think so, but one needs to remember that the speaker protection module will, in essence, establish a reference to ground, even if not initially intended.
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