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Old 13th November 2003, 07:18 PM   #1
Did it Himself
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Default V/I Limiting

Is this really such a bad thing sound-wise?

From what I can gather, the drive to the output device is shunted away
as
the current through the output 'sense' resistor is increased. It does not
appear to be a switching action, so how does it generate all these nasty
spiky waveforms I have been reading about?

Also, why is it called V/I limiting when as far as I can see it's just
current limiting?

Is the 'bad sound quality' of these schemes really just because the limit
has been set too low?
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Old 13th November 2003, 09:05 PM   #2
Steven is offline Steven  Netherlands
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You did not show a circuit, so I can only answer in general.

As long as the current limiter is not activated, it should not affect sound quality. But the moment the current limiter trips, a strong non-linear feedback loop is created around the output transistor and the limiter shunting away the input signal. This feedback loop is often not very stable, also because of interaction with a complex load. So the output signal is not just topped off nicely as with voltage clipping, but is more or less oscillating around the current limit. This is VERY audible.
To stay within the safe operating area (SOAR) of the output stage often V/I limiting is used. With V/I limiting the maximum output current is made dependent on the actual voltage across the output transistor. A large voltage across the transistor asks for a low current limit to keep the dissipation (VxI) within limits. This is normally accomplished by using level shift resistors from the current limiting transistors to the ground or the supply lines. Although this is good for the SOAR, it makes the feedback loop even more unstable. Also the addition of capacitors in this loop to slow down the initial limiting action can affect the stability of the current limiting action, although it helps to allow for high peak currents.
V/I limiters make sense for resistive loads, but complex loudspeaker loads often create such phase shifts between voltage and current that when the current is high the output voltage is low and consequently the voltage across the output transistor is high. In that case the V/I limiter may trip very early, unexpectedly early. But this may save your output stage; reactive loads increase the power dissipation in the output stage considerably.

Steven
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Old 13th November 2003, 09:54 PM   #3
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Steven,

Hello again! Sorry about the assumption and not posting a schematic, but I've only ever seen 2 schemes for this kind of thing. Here is an example of what I was talking about

Click the image to open in full size.

This to me looks like current limiting. However, I know of a slight variation of this circuit with a couple of caps and an extra diode in there that the manufacturer calls a V/I limiter. I go with your description of what a V/I limiter is and I cannot see how this circuit is one of those.

In any case, your excellent description has made me think not to bother anyway, and rely on rugged MOSFETS, rail fuses and gate zeners.

Thanks
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Old 13th November 2003, 10:06 PM   #4
Steven is offline Steven  Netherlands
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Hi Richie00boy,

You're right; this is just current limiting, nothing more. Gate zeners also act as current limiters, but such a current limiter will probably not oscillate.

Steven
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Old 13th November 2003, 10:12 PM   #5
sam9 is offline sam9  United States
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There is a write up of this in the Self amplifier book. If the values of the resistors are chosen correctly there shouldn't be any sonic effect. The difficulty is that a direct calculation of the values is subject to uncertainties. A SPICE simulation followed by constructing a circuit is the easiest way to choose the values. You could just build a circuit and substitute resistors but that is time consuming and carries the risk of destroyed output devices.
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Old 14th November 2003, 06:26 AM   #6
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Btw, I just worked out a cross-coupled version of a VI limiter circuit that looks like it should be able to provide protection against overbias and thermal runaway. If it works as well as I hope it will, it might make the output stage almost bulletproof, assuming the requisite amount of heat sinking and no resistor or connection failures.
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Old 14th November 2003, 09:42 AM   #7
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Take a look at the current-limiting section of the LC Audio Millenium amplifier. It doesn't 'creep' in on transients: it is either activated or not. Excessive voltage across the emitter resistors triggers a timer that removes drive to the o/p stage.
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Old 14th November 2003, 01:47 PM   #8
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Have you any more details on this? I looked at the LC Audio website, but couldn't see anything about current limiting.

Thanks
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Old 14th November 2003, 05:00 PM   #9
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It's available here http://www.soundlabsgroup.com.au/lca..._technical.htm
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Old 15th November 2003, 05:14 PM   #10
Steven is offline Steven  Netherlands
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Quote:
Originally posted by thoriated
Btw, I just worked out a cross-coupled version of a VI limiter circuit that looks like it should be able to provide protection against overbias and thermal runaway. If it works as well as I hope it will, it might make the output stage almost bulletproof, assuming the requisite amount of heat sinking and no resistor or connection failures.
Could you reveal some details already? Looks interesting! Does it try to measure/calculate the actual power in the output transistors? Is there a time element included?

Steven
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