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Old 27th February 2014, 11:40 PM   #1241
1audio is offline 1audio  United States
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This is closer to correct I think. Its still complex and marginally stable. I'll study the Crown schematic. If you have a QSC schematic I would like to look at it as well.
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Old 28th February 2014, 08:44 AM   #1242
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Here is one of QSC's schematics.

Very cool stuff too. Not a bridge, but bridgeable.

Very cool is how all output power transistors have all their collectors grounded, since the amp's output is also grounded, that makes for a very unique and interesting configuration.

The speaker has one end to ground, but the other end doesn't come straight from the amp's output, since that output is at ground, it comes from a center tap from the psu, but that psu doesn't even have a center tap, with a single winding transformer, so that tap is created otherwise.

The flying rails plus the grounded output make this very unusual. And when it's bridged, very interesting as well. Most of QSC's amps use this, and I saw the most powerful models using this plus a class H or G mode.
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Old 28th February 2014, 08:48 AM   #1243
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There is one other type of arrangement that I found also quite interesting, although not used much and not in commercial amps: it's a sort of bridge, with a class A amp, plus a plain class B on the other side that is used to drive the class A's rails to higher levels as needed when power increases.

This allows a more powerful class A amp without the drawbacks of very high dissipation and running hot when idling.

Those QSC amps are widely used and commercialized, so are the grounded bridges from crown and others, but that mix of class A and B in a bridge arrangement hasn't caught on. It may not be so practical after all. I think it was invented by a japanese. Very clever though.
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Old 28th February 2014, 08:57 AM   #1244
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There is one thing about the QSC flying rail topo that I think could be also quite interesting.

If I understand it right, their topo would not allow any DC out on the speakers in case of output failure from the amp. Since all the currents go through the filter caps, in case of output short, I think the most that can come out to the speaker is a big "klok"and that's it.

And I noticed that there are no soa/vi limiter protection. The protection is thermal, with a ptc sensor, and a simple muting / power on delay for the speakers, because I believe with this arrangement of filter caps driving the speaker, there must be a big "klok" at power up and perhaps a somewhat smaller one when powering off.

Interesting stuff to simulate.
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Old 4th March 2014, 07:34 PM   #1245
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TIAN probe issues.

I still have not made this work. The example that ships in LTSpice, "LoopGain2" syntax is not correct. I believe it is the "@" it is objecting to.

Does anyone have a WORKING example of this second method and some explanations of what I am looking at? It was intuitive I could do an AC simulation and look across the input diff pair, but what looks good there does not work in Trans. I can just d a trans at the zero gain point, but that does net tell me about all the other issues.

I really need some instruction here. I am attempting to insert it at the input to the VAS as I usually do TMS. But to get going, I will adapt to wherever.
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Old 4th March 2014, 07:59 PM   #1246
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It works for me, I've used it on all my amps. The @ operator takes curves from different steps of the simulation to make a single trace, so you may have inadvertently disabled the .step directive or something. You might check if your own copy of loopgain2.asc has been corrupted.
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Old 4th March 2014, 08:42 PM   #1247
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That is a step in the right direction. I could find nothing on what @ does!
.step is there. I get a "invalid token" in the big long string.

In the definition of the two sources, {u(-prb)} What is the "u"? prb is what I am stepping, I have that.

The help pages are really sad.
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Old 4th March 2014, 08:45 PM   #1248
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prb is a parameter that turns the probe on or off.

u is the parameter that needs to be stepped. The loopgain.asc should have this stepping directive on it - maybe I was right about the schematic corruption? Delete the file and then update LTSpice?
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Old 4th March 2014, 09:17 PM   #1249
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Quote:
Originally Posted by tvrgeek View Post
The help pages are really sad.
I believe SPICE goes back to the 70's. For anything like this with a long history (PCB software too), the "top dogs" have simply been with the technology as it developed, and took its limitations in stride because they already understood the mechanics and the software is just a convenience for them. They are so used to all these little quirks that they don't feel any motivation to fix them. Their attitude is, they have gotten past the learning curve and it's someone else's problem. There is also a conflict of interest because making software to make their job easy means they will have to compete with everyone they've made it easy for.

So if you want to understand the simulator, you probably need to go read the documentation of the original SPICE program. It's all online somewhere for free, and it's supposed to be really good. After you catch enough glimpses about how SPICE works underneath the interface, you begin to see why LTSpice has its limitations.

Ultimately SPICE was made to imitate electronics, not to actually model the physics. You can model a circuit using theoretical physics but it's too computationally intensive and takes too long. The industry has found that SPICE has the optimal level of simplification. So really, the SPICE engine itself is an evolving compromise driven by the needs of industry every time it's updated or rewritten.

This doesn't make it alright to be badly documented, but it helps to see why things have turned out as they have, which could help to do something about it.
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Old 4th March 2014, 09:27 PM   #1250
tvrgeek is offline tvrgeek  United States
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Added the line into the plot def file. I was trying to put the .func in the schematic. Guess it can't.
Darn if I can get it to plot this. If I type the string in, yes it does.
Using just the "help" I can't find a way to add a trace to "simply plot" as the demo says. Every permutation of syntax always complain "no number"

The HELP assumes you already know how, and so does the demo.
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