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Old 3rd November 2011, 02:16 PM   #21
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Default triple output stage

Quote:
Originally Posted by OnAudio View Post
Hi Alex,

This is how the full amplifier looks.

Thanks.
if this is a triple output stage where should vbias thermally coupled? in sloans book he said that it should be in the first pre driver transistor stage .does mounting all the pre driver transistors help prevent thermal runaway.or only vbias mounted at final stage is enough to prevent thermal runaway in EF configuration?im confused in this one.
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Old 3rd November 2011, 02:38 PM   #22
AndrewT is offline AndrewT  Scotland
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If the two stage output (driver and output device) is intended to power a 2ohms speaker then it ain't gonna work.
Try to estimate the peak output current when the 2ohms reactive speaker demands ~three times the current that a 2r0 resistor load would demand.

From there work back through the output and driver stages and try to estimate the driver base current peak that must be supplied by the VAS.
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Old 3rd November 2011, 02:59 PM   #23
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Quote:
Originally Posted by AndrewT View Post
If the two stage output (driver and output device) is intended to power a 2ohms speaker then it ain't gonna work.
Try to estimate the peak output current when the 2ohms reactive speaker demands ~three times the current that a 2r0 resistor load would demand.

From there work back through the output and driver stages and try to estimate the driver base current peak that must be supplied by the VAS.
Perhaps he is assuming a 4 ohm nominal load with minimum impedance of 2 ohms?
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Old 3rd November 2011, 03:01 PM   #24
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Quote:
Originally Posted by OnAudio View Post
Hi Alex,

This is how the full amplifier looks.
I think the 2200uF NFB capacitor is a total exaggeration, and could certainly be reduced to a value where even film types could be used, like 80uF or so. Don't you think so?

The input cap probably too, to even smaller values.
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Old 3rd November 2011, 03:01 PM   #25
AndrewT is offline AndrewT  Scotland
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The schematic shows +-40Vdc and states 200Watts into 2ohms.
If he meant 4ohm capable and 2r0 test loading then I would expect to see that somewhere.
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Old 3rd November 2011, 03:05 PM   #26
AndrewT is offline AndrewT  Scotland
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Quote:
Originally Posted by carlmart View Post
I think the 2200uF NFB capacitor is a total exaggeration, and could certainly be reduced .......Don't you think so?

The input cap probably too, to even smaller values.
C8 is silly large.
C6 is sensible, but near the upper end of what is more normally specified, but still well short of DC coupled.
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Last edited by AndrewT; 3rd November 2011 at 03:08 PM.
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Old 3rd November 2011, 03:16 PM   #27
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Default about vbias

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Originally Posted by 454Casull View Post
Perhaps he is assuming a 4 ohm nominal load with minimum impedance of 2 ohms?
im talking about the correct placement of the vbias so that output transistor will not result in thermal runaway.i know that v bias provides the correct base current for these pre driver stages.does mounting this predrivers on the same heatsink with the output devices prevents thermal runaway?should the predriver mounted separately on the main heatsink(output transistor)?

cheers,
joel
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Old 3rd November 2011, 05:21 PM   #28
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Quote:
Originally Posted by AndrewT View Post
If the two stage output (driver and output device) is intended to power a 2ohms speaker then it ain't gonna work.
Try to estimate the peak output current when the 2ohms reactive speaker demands ~three times the current that a 2r0 resistor load would demand.

From there work back through the output and driver stages and try to estimate the driver base current peak that must be supplied by the VAS.
Hi Andrew,

Thanks for pointing that out. Would this do ?

Thanks.
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Old 3rd November 2011, 08:20 PM   #29
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Quote:
Originally Posted by drowranger View Post
if this is a triple output stage where should vbias thermally coupled? in sloans book he said that it should be in the first pre driver transistor stage .does mounting all the pre driver transistors help prevent thermal runaway.or only vbias mounted at final stage is enough to prevent thermal runaway in EF configuration?im confused in this one.
Hi Drownranger,
Mounting the Vbe device onto the heat sinks with the predrivers is a place to start.You could also checkout the technique used by the Leach amp. If you have gone through Sloan 196-201, then checkout Cordell 290-313 and Self Chapter 15.
Thanks.
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Old 3rd November 2011, 08:21 PM   #30
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Quote:
Originally Posted by AndrewT View Post
If the two stage output (driver and output device) is intended to power a 2ohms speaker then it ain't gonna work.
Try to estimate the peak output current when the 2ohms reactive speaker demands ~three times the current that a 2r0 resistor load would demand.

From there work back through the output and driver stages and try to estimate the driver base current peak that must be supplied by the VAS.
I have skimmed again through Sloan pages 41 to 42, Burning amplifier, Cordell pages 9,10 and 117-210, Self Chapter 8 and I am still not sure whether a resistive load is not enough.
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