If you get interested in using a transformer model, I have a scalable one for LT-Spice. It's also easy to measure a transformer and model it, if you have a variac. Check the link below.
http://www.diyaudio.com/forums/power-supplies/216409-power-supply-resevoir-size-205.html#post3560607
I will post the .asc files that have the latest version, if you actually decide to use it. (I would have just posted them here but I'm not at my main computer.)
At the least, you should probably include some resistance and inductance in series with your voltage source.
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Not at all. The supplies must be totally independent. They have no link with each other whatsoever, or it wouldn't work.
Since the only thing a bridged bridge has in common is the ground, each bridge provides a full swing, and in the bridged bridge, they are in opposition, doubling the swing again.
Good, so this can clear up a few things for others dropping in.
I found an illustration, posting...
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Please note that I said "effectively", not directly.
Thank you for the illustration; this is what I had ended up expecting to see so it's good to have that confirmed/clarified. I believe that this shows that the supplies are effectively connected in series, by the interconnection of the grounds. (e.g. trace the current loop for a current through the load - it passes through both supplies).
Anyway, back to your problem. I don't see what the problem is. Your ground symbol (the little downward pointing triangle) is in all the places that I would expect it to be and don't see any issue in running the sim in Spice (the amplifier doesn't appear to be stable but that's a different issue). Are you concerned about using the 70 V "battery" (immediately to the right of C19) to define the rail voltage and supply power, and if so why?
For a variac, you mean a model of it?
I'd be interested in trying this out. I tried setting something up but couldn't figure out how to even get the equivalent of ac mains. Since I only need a single secondary type, this should work.
I suppose those extra coils and resistors around the "ideal" transformer are there to make it less ideal and more realistic.
I wanted to simulate the whole thing, from the mains, with the transformer, the rectifier bridge and caps.
Cool, and that will benefit others who want to dabble as well.
Something to learn about spice in the process.
I loaded the schematic into LTSpice IV version 4.19i. A transient simulation took a long time and gave the impression of an unstable amplifier. A DC operating point simulation gives 229.725 uV output across the load, V+ = 34.9145 V and V-=-34.9151 V. (V+) - (V-) gives 69.8296 V, the "missing" 0.1704 V accounted for by the 681.813 mA quiescent current flowing in the supply series resistance of 0.25 ohm.
Unstable is possible, although the high side was stable by itself, I added the low side to it and tried bridging it.
Wierd that you get rails about right and I didn't. I am also using ltspice, and it should be the latest, as it does its updates, although there may be difference, as I am on a mac.
It's all about how you use it. I have used it countless times for opamp circuits with great results. Active EQ circuits, active crossovers, multisection tone controls, a re-engineering of the Carver Holographic Generator, even for passive speaker crossovers, where you need to know relative driver efficiencies, the source impedance and the impedance of the driver at the frequency you want to cross it over at. I don't know how to do Laplace Transform math very well, so I rely on Spice to verify my circuits before building them. It makes me a very capable audio engineer. And a good enough version is FREE.
Correct me if I am wrong, but the drive signals to the output stage of the grounded side of each grounded bridge are at the "nominal" signal amplitude, while the drive signals to the output stage of the non-grounded side of each bridge (whose output drives the loudspeaker) are at twice the "nominal" signal level (since they are nominal drive signals riding on top of the power supply, which itself is floating on a signal of nominal value).
If, for example, the "nominal" value of the signal on the right floating bridge is 40V peak, then its power supply has a signal component on it of 40V peak. That same power supply with the 40V peak signal on it feeds the other side of the bridge, which supplies the speaker with 80V peak, so the drive to that side's output transistors needs to be 80V peak.
Snce the other floating bridge supplies 80V peak of opposite polarity to the other side of the speaker, the speaker sees 160V peak.
So, bottom line is that there need somehow to be 80V peak signals out of phase driving those parts of the output stages connected directly to the loudspeaker. This appears to mean that we need a VAS on each side that can supply 80V peak, and which must be supplied with a non-floating low-current supply of perhaps +/- 90V.
Also, a little unclear on the drive system for the part of the output stage that drives the grounded side, as in the illustration is pretty abstract there, putting signals that appear to be absolute driving emitters that go to ground. Of course, its early in the morning here.
Cheers,
Bob
Well, I'm certainly not the authority on this stuff, you are
But I thought both sides were driving equally, with the same signal amplitude on either side.
I didn't see it that way, but I suppose you may be right.
From what I gathered from crown amps schematics, the vas stages are on the same rails as the outputs. So they provide full swing.
I suppose the key is the phase opposition.
I'm still not fully understanding how all this works once the grounded bridge is again bridged, but that's an interesting thing, that it actually can be.
Well, have you looked at crown's schematics? This might enlighten your lantern quite a bit.
This grounded bridge is a very clever trick. We're going to make a grounded bridge with oldies 3055s, with rails at 35V, making sure they'll stand the near 70V by testing them first. So assuming the psu is stiff enough and rails don't sag too much, we should get some 200W on 8ohms from that bridge. Now we can bridge that bridge again, and in a perfect world, that would mean something like 800W on that same load. Of course the amps would then see 2ohms on each side and the psu would sag much more, plus the outputs would be under quite some strain, but we can add pairs...
Well, I'm certainly not the authority on this stuff, you are
But I thought both sides were driving equally, with the same signal amplitude on either side.
I didn't see it that way, but I suppose you may be right.
From what I gathered from crown amps schematics, the vas stages are on the same rails as the outputs. So they provide full swing.
I suppose the key is the phase opposition.
I'm still not fully understanding how all this works once the grounded bridge is again bridged, but that's an interesting thing, that it actually can be.
Well, have you looked at crown's schematics? This might enlighten your lantern quite a bit.
This grounded bridge is a very clever trick. We're going to make a grounded bridge with oldies 3055s, with rails at 35V, making sure they'll stand the near 70V by testing them first. So assuming the psu is stiff enough and rails don't sag too much, we should get some 200W on 8ohms from that bridge. Now we can bridge that bridge again, and in a perfect world, that would mean something like 800W on that same load. Of course the amps would then see 2ohms on each side and the psu would sag much more, plus the outputs would be under quite some strain, but we can add pairs...
Thanks for the response.
I haven't looked at any Crown schematics in a long time, certainly not the grounded bridged one, so I should at some point when I get the chance.
Certainly is a neat idea (especially bridging two grounded bridges), even if the simplified diagrams don't show enough detail about how the output stages are actually driven. Note that the IPS and VAS associated with each output stage should in principle be able to run off of that output stage's rails (even when those rails are floating on top of signal). It just looks like the input to a couple of them may also have to be riding on those rails. I'll get my head around it one of these days
.Cheers,
Bob
yes keeping the Crown gnded bridge floating amp's input comfortably within the floating suppy of that amp may require positive feedback too - like diff amps that can accept cm V beyond their rails
I have some sims somewhere for headphone sized amps but they require the op amp models to correctly sim supply V effects - which Boyle and simple decendants used by everybody don't
I have some sims somewhere for headphone sized amps but they require the op amp models to correctly sim supply V effects - which Boyle and simple decendants used by everybody don't
They're available easily. But some are a little easier to read than others. Perhaps look at the MicroTech1000 or 12000.
Most of their amps are grounded bridges, and they're known for their performance and other things (such as lower weight for example)
The bridging of the bridge is one of the neatest things that got me interested.
And there is also the advantage of the lower vce0 requirements, the simpler power supplies, and we can get quite a punch from much lower rails, so we can use existing parts and get more power than we could with conventional topos.
simplified diagrams don't show enough detail about how the output stages are actually driven. Note that the IPS and VAS associated with each output stage should in principle be able to run off of that output stage's rails (even when those rails are floating on top of signal). It just looks like the input to a couple of them may also have to be riding on those rails. I'll get my head around it one of these days
That would be great. I'd like to understand more of those things and your insight would be very helpful to better grasp how this really works.
I think I have a fairly good understanding of it now, but nowhere near enough and not as much as I'd like. For one thing, I really would like to understand more about how they're driven. The high side input seems somewhat conventional, but what drives the low side still eludes me.
I've tried to simulate this, but so far I don't understand enough of it.
I read all your material with great interest. Nice work!
If you have good understanding on how the low side is driven, I'd like to know.
That divider network used in the crown amps seems simple enough, but how does this actually work??
The power supplies are without a center tap, so there are no middle point to grab on to, but that is needed, so a divider network can provide this, however I tried that in sims, unsuccessfully, so I'm missing something. The input diff amp on the low side gets a piece of the output (the real one, from the high side) through a divider, on its inverting input, and the positive input is that one getting an artificial middle point between the rails through a divider. When I tried that, it didn't work.
Grounded bridge
I had to build a model try to figure out what they are doing. I don't think its got a lot of value today unless its important that an output terminal be grounded. I have attached a simulation that should explain the concept. I think its more complex than necessary.
You can build a full bridge amp with a single floating supply which also has the benefit of little ground noise pollution from the high current power supply. I did that in the early 1980's in the first gen Spectral DMA 100. It worked great. In fact it would work with only the feedback resistors and connections to the gates of the output transistors between the driver and the output. No ground connections at all in the output supply or load. I added a cap bypass to ground from the output supply because it felt right but wasn't necessary.
I had to build a model try to figure out what they are doing. I don't think its got a lot of value today unless its important that an output terminal be grounded. I have attached a simulation that should explain the concept. I think its more complex than necessary.
You can build a full bridge amp with a single floating supply which also has the benefit of little ground noise pollution from the high current power supply. I did that in the early 1980's in the first gen Spectral DMA 100. It worked great. In fact it would work with only the feedback resistors and connections to the gates of the output transistors between the driver and the output. No ground connections at all in the output supply or load. I added a cap bypass to ground from the output supply because it felt right but wasn't necessary.
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I think this particular simulation isn't of the grounded bridge. It is indeed bridged, but not grounded. Neither output is hooked to ground, plus there are 2 separate psus, which isn't the case in a grounded bridge. Both sides of the bridge work off a single psu.
I'm playing with it, maybe it will help me learn something...
Your right, I got pieces wrong. Two big issues are where the bridge is grounded and what is the return path of the current from amp to amp. It seems it needs to be much more complex. Both supplies will need a center tap to work it seems and where the center tap of amp 2 connects back to the center tap of amp 1 is the key to the puzzle.
The one piece from the description I did get is that the second amp derives its input from the output of the first.
The Crown marketing material is really misleading and merges unrelated concepts and completely fails to to explain the grounded bridge. Is there a schematic of a crown amp that can be reversed available?
The one piece from the description I did get is that the second amp derives its input from the output of the first.
The Crown marketing material is really misleading and merges unrelated concepts and completely fails to to explain the grounded bridge. Is there a schematic of a crown amp that can be reversed available?
The grounded bridge doesn't need a center tap, and both sides work off the same power supply. Actually I don't think it can work properly with separate supplies, when the low side is forcing the swing on the psu connected to it, this swing must be applied to the high side, so that obviates a unique psu.
Quite true. So far, what I gather is that the inverting input on the low side gets a fraction of the output signal, which comes from the high side (the output from the low side is the grounded one). That fraction of the output signal is made with a resistive divider. And then the non inverting input gets a fixed center tap from the rail, made also with a resistive divider.
Yes, they can be misleading, but that's nothing new in marketing. They can twist the truth a little to suit their needs, just like the media give a "spin" to the truth to make the news and the worst ones are the politicians, not to mention lawyers...
Sure there are schematics, I have tons of them and they're quite easy to find.
I'm not sure we're allowed to post service manuals on forums though. This can be heavy too. Although the schematic alone can't be too heavy.
I've seen commercial amps schematics posted before, so perhaps it's ok to do so. I'll look around for a readable one...
Here is the sch of the microtech1000.
I had to compress some more so the file size was within the forum's limits.
Hopefully still readable enough.
I'd love a good simulation of this, but a mock up with opamps would still be nice to better understand how it works.
What I don't quite understand yet is what fraction of the amp's output is needed to inject in the low side's input. There is a relation that I don't understand yet.
The flying rails concept is also used in some other topos that aren't bridges. QSC for example has amps with such topos. But the bridge goes further than the simple flying rail amp.
I had to compress some more so the file size was within the forum's limits.
Hopefully still readable enough.
I'd love a good simulation of this, but a mock up with opamps would still be nice to better understand how it works.
What I don't quite understand yet is what fraction of the amp's output is needed to inject in the low side's input. There is a relation that I don't understand yet.
The flying rails concept is also used in some other topos that aren't bridges. QSC for example has amps with such topos. But the bridge goes further than the simple flying rail amp.
By the way, there is a simulation that I would like to do. It's a flying rails amp such as the QSC. Those are quite interesting, with the complementary output stages with all the collectors at ground, so the heatsinks are at ground, and the transistors don't need to be isolated. Nice for heatsinking for efficiently.
Those are also using a single secondary without a center tap for the psu, just like the grounded bridge, and the speaker goes to ground and an "artificial" center tap from the psu where the filter caps join up. They use more caps of lower voltage than in a grounded bridge, but with the simple transformer, it's less costly with simpler transformers.
That use of the filter caps reminds me of the old class b amps with the single rail psu and a big output cap.
Those are also using a single secondary without a center tap for the psu, just like the grounded bridge, and the speaker goes to ground and an "artificial" center tap from the psu where the filter caps join up. They use more caps of lower voltage than in a grounded bridge, but with the simple transformer, it's less costly with simpler transformers.
That use of the filter caps reminds me of the old class b amps with the single rail psu and a big output cap.