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Old 21st February 2009, 09:40 PM   #1
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Default Bleeder resistor!

Hello.I want to put Bleeder resistors in my LM3875 but, I have seen some different values like: 2.2k,3.3k,4.7k,10k.my power supply has 20000uf per rail. I just do not like turning off my amp and still listen to music for about 20 seconds.I did not have this problem before because I had a small power supply 10uf caps but, I wanted to "feel"the bass so I put more capacitance 20000uf per rail. Now I have more bass and more music(after turning amp off lol).
My question is Which one should I use?
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Old 21st February 2009, 10:15 PM   #2
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Lower value will bleed the power quicker, however it probably won't be quick enough for your tastes. I have 15,000uF per rail in my power supply. I used 1k5 bleeder resistors (calculated the power dissipation and used resisters rated at 4 times that value). It still takes a long time to stop playing music.

Consider this. With the values I used and 27V rails, after 22.5 seconds there is still 10 volts on each rail. If you want to have the music stop when you shut the power off, bleeder resistors are not the solution. You would need too low a value to bleed that much capacitance quickly, and it would waste a lot of power (as heat).
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Old 21st February 2009, 10:29 PM   #3
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you are right Redshift187,I have been playing with this issue but,I think the fastest way to cut the (Extra juice) to the speakers is using a relay but, I do not want to use a relay.I think I will leave it alone and just turn off the source first and then the amp.thank you .
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Old 22nd February 2009, 12:39 AM   #4
Segran is offline Segran  Sweden
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What's your reason for not wanting to use a relay? A relay gadget on the speaker output also protect you seakers in case of a catstrophic chip failure, preventing DC from fyring the drivers.

In my Speaker Protect board get dealyed switch on, immidiate switvh off when AC power disappears as well as protection for DC above 0,6v in milliseconds. I have also added A/B switching function to the Speaker protection board. Right now I'm adding Clipping Indicators as well to the same board.

But maybe that's beyound your scope, so why don't you add a simple Mute function using a FET on the input, controlled by the secondary AC from your transformer! A FET, couple of diodes, a couple of resistors and a cap would fo the trick!
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Old 22nd February 2009, 12:41 AM   #5
Segran is offline Segran  Sweden
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What's your reason for not wanting to use a relay? A relay gadget on the speaker output also protect you seakers in case of a catstrophic chip failure, preventing DC from fyring the drivers.

In my Speaker Protect board get dealyed switch on, immidiate switch off when AC power disappears as well as protection for DC above +/-0,6v in milliseconds. I have also added A/B switching function to the Speaker protection board. Right now I'm adding Clipping Indicators as well to the same board.

But maybe that's beyound your scope, so why don't you add a simple Mute function using a FET on the input, controlled by the secondary AC from your transformer! A FET, couple of diodes, a couple of resistors and a cap would fo the trick!
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Old 22nd February 2009, 02:47 AM   #6
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I suggest to try an active discharge circuit instead of bleeder resistors as in US Patent # 5523665.

- Richard
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Old 22nd February 2009, 05:04 AM   #7
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Quote:
Originally posted by Segran
What's your reason for not wanting to use a relay? A relay gadget on the speaker output also protect you seakers in case of a catstrophic chip failure, preventing DC from fyring the drivers.

In my Speaker Protect board get dealyed switch on, immidiate switvh off when AC power disappears as well as protection for DC above 0,6v in milliseconds. I have also added A/B switching function to the Speaker protection board. Right now I'm adding Clipping Indicators as well to the same board.

But maybe that's beyond your scope, so why don't you add a simple Mute function using a FET on the input, controlled by the secondary AC from your transformer! A FET, couple of diodes, a couple of resistors and a cap would fo the trick!

The problem with these options becomes apparent when you have your power supply in a separate enclosure.
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Old 22nd February 2009, 05:33 AM   #8
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I like making it "as simple and clean as possible" from input to the output. Also I do not like using fuses on the output or speaker dc blocker protection( I use fusses ONLY before the power transformers) one for the hot and one for the neutral.I know most people do not agree with me on these. But I see it this way.

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Old 24th February 2009, 10:57 AM   #9
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Another possibility would be to use the relay on the bleeder resistors. You can use resistors with low values, thus discharge very fast. During normal operation the resistors are disconnected, thus not wasting power and you can use a smaller power rating for them, because they only have to stand the discharge, nothing else.

E. g. to discharge 20.000 F in ~1 s, you need a 10 Ohm resistor. If you have 30 V rails, and the resistor is always connected, it would convert 90 W of electric energy into heat. Give it the usual 100 % of safety margin and you need a resistor rated at 180 W.

When it is only connected with the amplifier switched off, it only has to withstand the energy that is stored in the capacitors for the discharge time. 20.000 F * 30 V = 0,6 As -> 30 V * 0,6 As / 1 s = 18 W for only 1 second. A 10 W resistor could probably cope with that.
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Old 24th February 2009, 02:33 PM   #10
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The resistor would see about 90W peak, but only for a small amount of time. At 0.46 seconds the power would be down to 10W. The resistor would handle it but if the amp were being turned on and off a lot, I would suggest using two 20R 10W resistors in parallel. Same discharge time, twice the power handling.

How would you suggest to switch the resistors into the circuit? I like this idea (it bothers me slightly that my amp takes 30 seconds to shut off too).
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