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Old 8th August 2008, 08:05 PM   #1
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Default DC block before or after power amp?

I have a tube mixer that spits out DC for a while when the tubes are warming up, and I've blown one set of speakers because the amp I was using didn't have any DC protection.

I'd like to build a standalone DC blocking box so that I can use any amp and not worry about my speakers, but I'm not sure what the pros and cons are of DC blocking before the amp so there's no DC going in, or after the amp so that even if the amp spits out DC, it doesn't go to the speakers.

It's not for super critical listening, but needs to sound good, so I'm a little wary about just running the input through back to back electrolytics. I'd like to keep it simple, though.

thoughts?

thanks,
spencer
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Old 8th August 2008, 11:29 PM   #2
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Default Re: DC block before or after power amp?

Quote:
Originally posted by slothlove

It's not for super critical listening, but needs to sound good, so I'm a little wary about just running the input through back to back electrolytics.
Why would you need to use back to back lytics? There are countless high quality film capacitors out there that you can use as DC blocking caps on the output of the tube mixer.

There are DC speaker protector kits available as well for the output of the amp.

http://store.qkits.com/moreinfo.cfm/K4700

http://store.qkits.com/moreinfo.cfm/K4701

http://sound.westhost.com/project33.htm
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Old 9th August 2008, 11:18 AM   #3
AndrewT is offline AndrewT  Scotland
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Hi,
use a film capacitor with a voltage rating higher than the voltages inside your tube mixer.
Fit the cap either at the output of the mixer or at the input of the power amp.
Connect the RCA terminal end of the cap to signal ground via a high resistance. Try values between 100k and 2M2.

For belt and braces you can also add on DC detection and automatic isolation (relay).
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