Anyone know circuit theory better than me? (Parallel-bridging Firebird sail panel)
08-10-2009, 01:25 AM
#1
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Anyone know circuit theory better than me? (Parallel-bridging Firebird sail panel) Okay - the Firebird version of the Monsoon amp has two 4-ohm channels, each driving one 4-ohm voice coil on the sail panel woofer.
What happens if I wire those two channels in parallel into a single 2-ohm voice coil woofer? Seems to me that each channel individually is only pushing as much current as it would drive into a 4-ohm load...
What might go wrong?
1) DC offset between the two channels. But I could solve that if I put a cap in series with each channel before connecting them... Or, even better, if I was gonna use a subsonic filter anyway, that's gonna block DC as well - just put one on each channel individually before wiring them in parallel.
2) Different amplitude on each channel. In particular, I'm not sure whether one channel fades with the front speakers and one with the back. Thing is, if this were a potential problem, I'd think it would happen with the original implementation. The two voice coils of the DVC woofer should act as a mutual inductance between the two channels on the amp, so if one channel were driving and one weren't, the channel that's driving should be sourcing current into the other channel. I think I can conclude this isn't a problem.
3) Phase offset between the channels. I think this one's not a problem by the same argument as #2; if it happened and was an issue, it should be an issue with the original DVC setup, too.
Anything else I'm missing?
What happens if I wire those two channels in parallel into a single 2-ohm voice coil woofer? Seems to me that each channel individually is only pushing as much current as it would drive into a 4-ohm load...
What might go wrong?
1) DC offset between the two channels. But I could solve that if I put a cap in series with each channel before connecting them... Or, even better, if I was gonna use a subsonic filter anyway, that's gonna block DC as well - just put one on each channel individually before wiring them in parallel.
2) Different amplitude on each channel. In particular, I'm not sure whether one channel fades with the front speakers and one with the back. Thing is, if this were a potential problem, I'd think it would happen with the original implementation. The two voice coils of the DVC woofer should act as a mutual inductance between the two channels on the amp, so if one channel were driving and one weren't, the channel that's driving should be sourcing current into the other channel. I think I can conclude this isn't a problem.
3) Phase offset between the channels. I think this one's not a problem by the same argument as #2; if it happened and was an issue, it should be an issue with the original DVC setup, too.
Anything else I'm missing?
08-10-2009, 08:05 AM
#2
My first question would be "why on earth would you want to do that?"
If I understand correctly, you have an SVC 2-ohm speaker that you want to connect to both sail panel channels of the Monsoon amp? Not a good idea. You would be essentially shorting the two channels together or bridging (depending on how you made the connections) a non-bridgeable amp. And no matter how you connect the wires, you can't make a single 2-ohm load into the 4-ohm load that the amp requires.
If I understand correctly, you have an SVC 2-ohm speaker that you want to connect to both sail panel channels of the Monsoon amp? Not a good idea. You would be essentially shorting the two channels together or bridging (depending on how you made the connections) a non-bridgeable amp. And no matter how you connect the wires, you can't make a single 2-ohm load into the 4-ohm load that the amp requires.
08-10-2009, 03:49 PM
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My first answer would be "So I could push twice the current into such a speaker (as I could push into a 4-ohm single voice coil wired to a single channel)."
But really, I don't much care _whether_ it works - I don't own any such speaker, and I'm more likely than not to replace the Monsoon amp now. I do care a great deal about understanding _why_ it would/not work, and I'm not currently satisfied that I do.
What I'm describing is (almost) shorting the channels together in parallel - I'm proposing pushing twice the current, not twice the voltage. The DC offset problem's a deal-breaker, though, so I can't directly short the channels together - I'd need a cap in series with each one first. Again, a subsonic filter should block DC just as well as a simple cap.
As a side note, I don't understand what's special about a "bridgeable" amp. My best educated guess is that "bridgeable" in this context is always talking about twice-the-voltage bridging - wiring the two channels in series - and that "bridgeable" means that you DON'T have a common ground shared across all the channels.
Here's why. Suppose all the minus terminals on all the channels are tied to a common ground. If you wire two channels in series (bridging them), hoping to get twice the voltage, you'd tie the plus terminal of one channel to the minus terminal of the other channel. But all the minus terminals are tied together (to the common ground), so now you've shorted across one channel of your bridge (tied the plus terminal to its own minus terminal), and you either blow a fuse or release the magic smoke.
If I'm right about what "bridgeable" really says about an amp, it's completely irrelevant to what I'm proposing.
OR, I'm completely wrong, which is really okay! But if I am, I'd like a EE-level explanation of why.
(For my part, once I get some free time, I'll draw schematics of how I understand the 4-ohm DVC and 2-ohm SVC circuits would work in my model. What I really should do is try to draw a black box around the speaker and see if I can demonstrate that the Thevenin or Norton equivalents are the same.)
But really, I don't much care _whether_ it works - I don't own any such speaker, and I'm more likely than not to replace the Monsoon amp now. I do care a great deal about understanding _why_ it would/not work, and I'm not currently satisfied that I do.
What I'm describing is (almost) shorting the channels together in parallel - I'm proposing pushing twice the current, not twice the voltage. The DC offset problem's a deal-breaker, though, so I can't directly short the channels together - I'd need a cap in series with each one first. Again, a subsonic filter should block DC just as well as a simple cap.
As a side note, I don't understand what's special about a "bridgeable" amp. My best educated guess is that "bridgeable" in this context is always talking about twice-the-voltage bridging - wiring the two channels in series - and that "bridgeable" means that you DON'T have a common ground shared across all the channels.
Here's why. Suppose all the minus terminals on all the channels are tied to a common ground. If you wire two channels in series (bridging them), hoping to get twice the voltage, you'd tie the plus terminal of one channel to the minus terminal of the other channel. But all the minus terminals are tied together (to the common ground), so now you've shorted across one channel of your bridge (tied the plus terminal to its own minus terminal), and you either blow a fuse or release the magic smoke.
If I'm right about what "bridgeable" really says about an amp, it's completely irrelevant to what I'm proposing.
OR, I'm completely wrong, which is really okay! But if I am, I'd like a EE-level explanation of why.
(For my part, once I get some free time, I'll draw schematics of how I understand the 4-ohm DVC and 2-ohm SVC circuits would work in my model. What I really should do is try to draw a black box around the speaker and see if I can demonstrate that the Thevenin or Norton equivalents are the same.)
08-10-2009, 06:34 PM
#4
It's the excess current that would kill the amp not the voltage. You say you only want to double the current without changing the voltage. The output section of the amp is voltage limited but wiring it to half the rated load would increase current flow to the point of letting the smoke out of the electronics.
09-03-2009, 01:11 AM
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I tried drawing it up, but got fed up with my schematic capture tools. How lazy I've become!
The idea's still pretty simple. Suppose one channel can safely source 4A. Into a 4-ohm load, then, it'd be 16 volts between the terminals on that channel. (And 16 volts x 4 amps == 64 VA. Not sure that's quite the same as 64W here, because I still think a speaker's a really inductive load, and I've got to worry about phase angles and such when I think about power dissipation. But anyway, let's pretend it's a purely resistive 4-ohm load.)
I've got two such channels. If I wire them in parallel into a 2-ohm load, the expectation was that each one would still swing 16 volts, and could still safely source 4A. The 2-ohm load is only seeing 16 volts, since I wired the channels in parallel, so I'm sourcing 8A total into it (16V / 2 ohm). Handwave handwave handwave, 4A come from each channel, and I'm operating everything within my current limits.
I'm at least halfway right, but that's not quite far enough. The goal was to get half my current out of one channel and half out of the other channel. And that part will work... so long as the internal resistances of the two channels are equal. But I see no reason to believe I could rely on that. If the internal resistances of the 2 channels are mismatched, then whichever one's got a smaller internal resistance ends up sourcing more than its half of the current, and we let the magic smoke out.
(My suspicion is that the internal resistances ARE designed to be equal, and that the difference between the T/A and Camaro versions of the Monsoon amp are simply that it comes bridged at the factory for the Camaro, where, not coincidentally, it's driving a 2-ohm SVC sub in the sail panel. But proving it would take a lot more work than I care to put into it.)
The idea's still pretty simple. Suppose one channel can safely source 4A. Into a 4-ohm load, then, it'd be 16 volts between the terminals on that channel. (And 16 volts x 4 amps == 64 VA. Not sure that's quite the same as 64W here, because I still think a speaker's a really inductive load, and I've got to worry about phase angles and such when I think about power dissipation. But anyway, let's pretend it's a purely resistive 4-ohm load.)
I've got two such channels. If I wire them in parallel into a 2-ohm load, the expectation was that each one would still swing 16 volts, and could still safely source 4A. The 2-ohm load is only seeing 16 volts, since I wired the channels in parallel, so I'm sourcing 8A total into it (16V / 2 ohm). Handwave handwave handwave, 4A come from each channel, and I'm operating everything within my current limits.
I'm at least halfway right, but that's not quite far enough. The goal was to get half my current out of one channel and half out of the other channel. And that part will work... so long as the internal resistances of the two channels are equal. But I see no reason to believe I could rely on that. If the internal resistances of the 2 channels are mismatched, then whichever one's got a smaller internal resistance ends up sourcing more than its half of the current, and we let the magic smoke out.
(My suspicion is that the internal resistances ARE designed to be equal, and that the difference between the T/A and Camaro versions of the Monsoon amp are simply that it comes bridged at the factory for the Camaro, where, not coincidentally, it's driving a 2-ohm SVC sub in the sail panel. But proving it would take a lot more work than I care to put into it.)
