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Old 4th October 2010, 03:08 PM   #1541
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Bleeder resistors are not intended to turn off amps quickly. You need different circuits for that. If you try to use bleeder resistors of a sufficiently low resistance to turn off the amp quickly, you are wasting an awful lot of power producing heat.

I have bleeder resistors in my amp, directly on the bulk caps, for the simple reason that my power supply is in a separate box from my amp (the power LED is in the amp box), and I don't want to have to worry about A) someone other than me turning on the power supply with the amp unplugged and leaving a voltage on the caps, and B) voltage regeneration on the caps with the amp unplugged due to no load.

With 35V rails and 15,000uF per rail, it would take my 2k2 bleeders 33 seconds to remove the voltage from the caps. I had 1k5 resistors there from when I was using 25V rails, and they got way too hot when I swapped transformers.
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Old 4th October 2010, 03:38 PM   #1542
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Quote:
My second amplifier had 3*6800uF per supply rail.
It reproduced undistorted music for about 2minutes after unplugging from the mains. Then turned itself off silently.
Exactly! This amp will go for about 10-20 seconds playing music depending on the volume, after the mains is unplugged. It also turns off pretty much silently with only a small pop from the bass driver, causing no harm at all (apart from maybe tweeters in a bi amp system).
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Old 4th October 2010, 06:14 PM   #1543
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Quote:
Originally Posted by AndrewT View Post
Why do I need a bleeder?
To bring the amplifier case to working temperature faster.
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Old 4th October 2010, 06:30 PM   #1544
AndrewT is offline AndrewT  Scotland
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Quote:
Originally Posted by pacificblue View Post
To bring the amplifier case to working temperature faster.

Adding a bleeder or two will increase the power dissipation.
This will lead to a higher internal operating temperature.
This may take the same time to reach quiescent condition or it may take longer.
It cannot be done in less time (=faster).
Yes, the temperatures may change faster (dT/dt), but the time will not be faster.
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Old 4th October 2010, 07:58 PM   #1545
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I was only joking, Andrew.
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Old 5th October 2010, 07:20 PM   #1546
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Quote:
Originally Posted by AndrewT View Post
My second amplifier had 3*6800uF per supply rail.
It reproduced undistorted music for about 2minutes after unplugging from the mains. Then turned itself off silently.

Why do I need a bleeder?
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Old 5th October 2010, 07:40 PM   #1547
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Originally Posted by regiregi22 View Post
I guess if his amp dosnt oscillate or distort as the caps discharge there is no need. I think bleeders are only useful as a safety precaution when working on the internals
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Last edited by Tripmaster; 5th October 2010 at 07:42 PM.
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Old 5th October 2010, 08:49 PM   #1548
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Sure, I wasn't expecting anymore from them

Thanks guys for your instructive answers
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Old 9th October 2010, 06:34 PM   #1549
JDawg is offline JDawg  Canada
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Here's some pics of my LM3875 gainclone that I just finished. This was my first time building one so the wiring and placement of parts might not be perfect. It works and sounds great though and I had a lot of fun building this amp.
Click the image to open in full size.

Click the image to open in full size.Click the image to open in full size.Click the image to open in full size.Click the image to open in full size.
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Old 9th October 2010, 06:47 PM   #1550
Atilla is offline Atilla  Norway
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Wow, this is nice. Impressive casework.

Where are the VU meters from, they look really shiny.
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