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Old 31st August 2012, 10:04 PM   #11
AndrewT is online now AndrewT  Scotland
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You can use BJTs in a capaciptor multiplier and by adjusting the resistor values trim the output voltage to a fixed %age of the input voltage.
eg add a 10k in parallel to C9.
Change R9 to 1k.
The voltage on C9 is now 10/11ths of the input voltage.
The output voltage will be ~600mV below that 10/11ths voltage on C9.
Fopr BJTs I would omit R10.
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Old 31st August 2012, 10:28 PM   #12
juma is offline juma  Germany
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Quote:
Originally Posted by rhythmsandy View Post
nice... method but need to source the IRF..
You can use any HexMosfets with similar Pd_max and Vgs_on (about 4V)
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Old 1st September 2012, 03:11 AM   #13
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Quote:
Originally Posted by rhythmsandy View Post
nice... method but need to source the IRF..
The ripple filters won't get AS hot as the power chip but they will need substantial heatsinks. And yes, they could share with the amplifier.

G
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Old 1st September 2012, 10:56 AM   #14
AndrewT is online now AndrewT  Scotland
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If the ripple attenuators are dropping 4V and the average current draw is 100mA then the average power dissipation is 0.4W.
The sinks, if any, can be very small, maybe 20C/W.
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Old 1st September 2012, 11:54 AM   #15
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Quote:
Originally Posted by AndrewT View Post
If the ripple attenuators are dropping 4V and the average current draw is 100mA then the average power dissipation is 0.4W.
The sinks, if any, can be very small, maybe 20C/W.
This IC can supply 8 amps RMS. Think your 'gonna need a bigger heatsink
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Old 1st September 2012, 12:00 PM   #16
AndrewT is online now AndrewT  Scotland
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Average current draw when playing audio tracks.

Not peak current draw on extreme transients that might last as long as a few tens of microseconds.
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Old 1st September 2012, 12:12 PM   #17
Mooly is offline Mooly  United Kingdom
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Quote:
Originally Posted by AndrewT View Post
Average current draw when playing audio tracks.

Not peak current draw on extreme transients that might last as long as a few tens of microseconds.
I really don't follow your logic on this one...

It's like saying the heatsinks on a 100watt rms class B amp can be kept really small because the average power on music is low.
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Old 1st September 2012, 12:34 PM   #18
AndrewT is online now AndrewT  Scotland
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No,
a 100W amplifier that has a quiescent current of just 20mA and +-50Vdc supplies will have a Pq of 2W. That alone would need a tiny heatsink.
Now play some music at -20dB ref maximum power, average level. The output stage will have an average dissipation of maybe 5W to 10W. The heatsink must be able to keep the amp cool for this.
If very compressed music is played at -10dB average level then the sink will have to dissipate maybe 10W to 20W and still play properly without overheating. In the low Pq amplifier , the music playing dissipation determines the sink size.

Now compare that to a high quiescent current amplifier say 200mA with Pq = 20W.
The sink must be bigger.
Now consider music playing. The dissipation hardly changes even for -10dB average levels. The same sink that copes with Pq also copes with music reproduction. In the High Pq amplifier the music playing dissipation can be very similar to the Pq dissipation.

Let's return to the cap multiplier.
The output voltage tracks the input voltage.
The Vdrop of the multiplier averages 4V (for this particular case).
The quiescent draw of the chip amp is ~ 40mA
When playing music on +-25Vdc supply rails the average current may rise to approximately 100mA (5W dissipation for 0.5W of music output) . In this situation the cap multiplier average dissipation is 4V times 100mA. Increase the output to 5W (-10dB) average then the current may go up a further 50% to 150mA. Or reduce -30dB and it drops to ~80mA.If the music is turned down to -30dB the average.
For all these audio signal situations the cap multiplier sees an average current draw of 40mA to 150mA.
The sink does not need to be big.

Even at 150mA for a 5W of average music power the dissipation is only 0.6W. A 20C/W sink will rise by ~ 20 *0.6*1.5 ~18C.

That is not a problem.
If a high current device is being used then the Rth j-a could well be low enough that it can be used to power a chipamp without any heatsink.
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Last edited by AndrewT; 1st September 2012 at 12:44 PM.
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Old 1st September 2012, 12:53 PM   #19
Mooly is offline Mooly  United Kingdom
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Hi Andrew,

I think we're just going to have to differ on this one. Even testing for a minute or so at high powers is going to cause a lot of heat dissipation. And that has to be a valid condition for a amp built as say a 50 or 100wrms design.
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Old 1st September 2012, 12:58 PM   #20
AndrewT is online now AndrewT  Scotland
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What size of cap multiplier sink is required for continuous power testing of the amplifier?

+- 50W into 8r0 is equivalent to 2.5Aac into the test load.
the supply rail sees a duty cycle of ~50%.
The cap multiplier now sees 4V & 1.25Aac, or about 5W during maximum power testing.

Do you/we design the amplifier heatsink and the cap multiplier heatsink for maximum power testing on a continuous duty?
I don't !!!!
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