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Old 14th September 2002, 12:37 PM   #1
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Default 2 ohm stable Symemetrical amp

Hello. Could anybody tell me how to make this amp stable in 2 ohm's without adding more O/P transistors?

The schematic belongs to Anthony E. Holton at www.aussieamplifiers.com

Link to Schematic:
http://www.aussieamplifiers.com/sym-sc1.gif


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Old 14th September 2002, 02:14 PM   #2
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What makes you think it is unstable into 2 ohms?
What do you mean by 2-ohms - resistive load?

I would be very suspicious of any amp that is unstable into any resistive load. Instability can be caused by many sources of feedback, not just the deliberate feedback loop. It can also be caused by inadvertent effects due to grounding and RF pick-up. When loading is small supply line currents are big and the latter forms of feedback become more troublesome. I would check these sources first. Otherwise, simple ways to increase the phase margin of the feedback loop are to increase the value of C4 or the value of R11 - the latter will increase the amp's gain of course.

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Old 14th September 2002, 03:01 PM   #3
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The reason I asked was becasue I have read the with lower OHM's you should add more O/P Transistors.
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Old 14th September 2002, 04:14 PM   #4
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Quote:
Originally posted by halvardf
The reason I asked was becasue I have read the with lower OHM's you should add more O/P Transistors.
Yes, that's true but you can also lower the power supply voltage in order to make the power dissipation lower. But the amount of output transistors is based of how much power each transistor can deliver and dissipate. It's also determined by the SOA (Safe Operating Area) of the transistor. Datasheets of the particular device tells you that.
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Old 14th September 2002, 04:16 PM   #5
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<i><b>The reason I asked was becasue I have read the with lower OHM's you should add more O/P Transistors.</i></b>

That's not a stability issue, it's a current/thermal issue. As you reduce the load impedance for a given output voltage, you draw more current which means more power which means more heat, all else being equal.

A given transistor can only handle so much current. Paralleling multiple output devices spreads the total output current across multiple devices, reducing the amount of current any single device has to carry.

But it's not simply a matter of adding output devices. If you're going to be driving significantly lower impedance loads than the original design was intended, you're going to have to dissipate significantly more heat which means that in addition to adding more output devices, you're going to have to increase the amount of heatsinking.

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