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Old 8th February 2014, 10:45 AM   #1201
tvrgeek is offline tvrgeek  United States
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Guess that is the way to go. Use Excel to organize all my acquired models and let the standard library do it's thing. Just not as convenient.

Sharing. I hear you. The problem is most of them I have stripped the copyright comments ( when LTSpice cleans them up) so it is inappropriate to put them out there. Most came from OnSemi, Central, Fairchild THAT, Exicon etc, so they are available. I have not measured any myself. I did generate more complete values for one series of 1N series zieners based on parameters from two sites and some interpolation. A few diodes seemed to work too well and sure enough, the model had no capacitance. At least that is one you can gustimate from the data sheet. ( default is zero) For a few, I am adding the display text for voltage and current.

I guess most vendors expect us to just have a directory of their "contents of one" as we should know what we are looking for.
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Old 22nd February 2014, 03:14 PM   #1202
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I suppose this would be an appropriate forum for my spice question.

I was trying to simulate a grounded bridge amp, using a single power supply without a center tap, and I can't figure out how to make this work properly.

Using a single voltage source, set up as DC to provide the rails and have a separate ground reference seems impossible, or I just don't know how it can be done.

I tried that and it (ltspice) wants to put the minus side of the power source as the ground reference level, so this can't work as the ground must float freely between the rails.

How can this be done? If at all.
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Old 22nd February 2014, 05:39 PM   #1203
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Your amp needs a reference between the rails for audio ground right? For a single-rail supply this would always have to be done with a virtual ground, which can be as simple as two 1k resistors (resistive divider) across the rails where their centerpoint is the ground. Put the LTSpice ground here and all will be well.

I don't know how bridge amps usually do it, they may reference the negative rail for input ground if that is the intended configuration for the chip/circuit. You should look at where signal ground is in the app note/datasheet if there is one.
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Old 22nd February 2014, 06:27 PM   #1204
jcx is offline jcx  United States
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Quote:
I tried that and it (ltspice) wants to put the minus side of the power source as the ground reference level, so this can't work as the ground must float freely between the rails.
not true at all - Spice lets you put the little down pointing triangle symbol on any one circuit node you want
if you try putting it on more than one node they all become shorted together - the same behavior as with any like named nodes in a schematic
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Old 22nd February 2014, 06:49 PM   #1205
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Quote:
Originally Posted by keantoken View Post
Your amp needs a reference between the rails for audio ground right?
Yes, and it's provided by the low side of the grounded bridge, not by a center tap from a psu.

Quote:
Originally Posted by keantoken View Post
For a single-rail supply this would always have to be done with a virtual ground, which can be as simple as two 1k resistors (resistive divider) across the rails where their centerpoint is the ground. Put the LTSpice ground here and all will be well.
Can't do that! The ground is the output of the low side in the bridge and I do need a center tap for some references, but those aren't ground. And the psu in a grounded bridge is without a center tap. The low side grabs on to the ground, makes it the reference and makes the rails fly around it.

My spice problem is that using a single power source doesn't work because spice makes the negative end of that source as the ground reference, and that doesn't work because it's actually supposed to be left flying as the negative rail. The ground can't come from a power source, it's set by the low side amp.

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Originally Posted by keantoken View Post
I don't know how bridge amps usually do it, they may reference the negative rail for input ground if that is the intended configuration for the chip/circuit. You should look at where signal ground is in the app note/datasheet if there is one.
There isn't any, it's a discrete amp, all bjts. It's a bridge, but not the classic one, it's a grounded one, with one side's output bound to and setting the ground and thus the reference.

So spice is giving me an issue with that, if I want to use only one power source as it's supposed to be, then it won't let me.
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Old 22nd February 2014, 06:53 PM   #1206
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Originally Posted by jcx View Post
not true at all - Spice lets you put the little down pointing triangle symbol on any one circuit node you want
if you try putting it on more than one node they all become shorted together - the same behavior as with any like named nodes in a schematic
That is fine, and having a single power source should work, as long as it's not trying to force the ground on its negative end, and thus shorting that rail to the ground, which is set by the amp's low side output.

It does work if I put 2 power sources in series, but then there is a center tap.
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Old 22nd February 2014, 07:16 PM   #1207
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For bridged amp the output is basically centered by divider regardless of whether you explicitly add one - the output has to be between the rails to get full output voltage. So by adding a resistor divider (AKA virtual ground) as a reference you're not ruining anything. It's probable this divider is already on the schematic if you look for it (although it can be done with zeners or transistors instead of resistors).
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Old 22nd February 2014, 07:41 PM   #1208
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Originally Posted by keantoken View Post
For bridged amp the output is basically centered by divider regardless of whether you explicitly add one - the output has to be between the rails to get full output voltage. So by adding a resistor divider (AKA virtual ground) as a reference you're not ruining anything. It's probable this divider is already on the schematic if you look for it (although it can be done with zeners or transistors instead of resistors).
I'm referring to the grounded bridge as designed by crown a long time ago.

To make sure we're on the same wavelength, here's a link to their paper about it:

http://www.crownaudio.com/media/pdf/amps/grbgpapr.pdf

You can see on page 6 on that document how the psu is very simple, no center tap, totally floating, and it's the low side of the bridge that sets the ground reference and it forces the rails to float around that ground.

In the amps that I'm trying to bring together as such a bridge, it takes 2 power sources to make it work, as using one is giving the issue I describe. I can use that center tap as a reference, that isn't the ground but rather the middle point of the psu, or I can make up a divider for that, but that point is not the ground.

If using a single power source as it's meant to be, it won't work because of the negative end of that source being set as the reference to ground, which shorts half the amp, because the negative rail should swing as the positive does and the reference to ground is not set by the psu but by one side of the amp.

That's what I've been trying to solve, unsuccessfully so far. And I'm wondering if there really is a solution to this.
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Old 22nd February 2014, 07:58 PM   #1209
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As long as all the current flows through the load, not ground, then you only need a virtual ground as reference for the feedback network; this is unique to a bridge amp. It's hard to say how this could be done without seeing the schematic, though it seems you have good reasons not to share it.
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Old 22nd February 2014, 09:15 PM   #1210
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Originally Posted by spookydd View Post
I'm referring to the grounded bridge as designed by crown a long time ago.
The grounded bridge is just a bridge amplifier. It's just configured in a way such that one side of the speaker is referenced to signal ground. Its biggest advantage (over a "normal" bridged amplifier) is probably simpler biasing of the voltage gain stages and the feedback network.

Please note that the description of Class-H in that document you linked is wrong. (And that what they call Class-H, Europeans and Japanese tend to call Class-G). In their figure 9, transistor Q1 does not enter saturation before Q2 turns on (well, not if the amplifier is designed properly anyway). There is indeed additional signal distortion as they show in their figure 10, but this is caused by the crossover from diode to Q2, not by Q1 entering/leaving saturation.

Going back to your issue, it would be much easier to help you if you could post a schematic.
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