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Old 17th August 2009, 08:50 PM   #1
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Question LED for dual mono Gainclone

Hi guys, quick question.

I have fitted a single LED to one of the rectifier boards (wire linked to the front panel) on a dual mono GC (Audiosector LM3875). Does it make a difference if I only fit one, or will it benefit from having one on each rectifier?

Many thanks,

Chris.
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Old 17th August 2009, 08:57 PM   #2
kaos is offline kaos  United States
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A purist would tell you to match both channels in every regard. An engineer will tell you that it doesn’t matter. Your amp, you choose.
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Old 17th August 2009, 09:05 PM   #3
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Both channels are matched in every other respect, but the engineer in me says one LED shouldn't make a difference. After all, it only draws a few milliamp's.

Just another quick question. The LED stays on for around 20 seconds after the amp is powered down. Is there any way I can prevent this (some sort of discharge circuit maybe)?

Chris.
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Old 17th August 2009, 09:11 PM   #4
jaencer is offline jaencer  United States
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ya use a bleeder resistor in the power supply. like a few Kohms right across the rails, close to the diode bridges. this will just eat power, but it is also for safety. It will also allow the amp to turn off when you turn the amp off, rather than the music continuing for a long time
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Old 17th August 2009, 09:13 PM   #5
jaencer is offline jaencer  United States
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then again i wouldnt worry about it. if you do that, make sure to use a nice big resistor, it'll get hot. and i dont know your rail voltages, so just use the appropriate resistor for like 100mA or so.
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Old 17th August 2009, 09:35 PM   #6
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Thanks for the reply. The rail voltage is around 27V. To be honest it's not much of a problem. I always reduce the volume to zero, then power down the source before I switch the amp off anyway. It's just the LED is a little annoying.

Chris
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Old 17th August 2009, 09:58 PM   #7
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Power the LED from the AC of the transformer secondary, via its own bridge rectifier and limiting resistor, with no smoothing capacitor. It will extinguish immediately the power goes off then.

You CAN drive an LED without full-wave bridge rectification (single anti-parallell diode across the LED) but they tend to have an irritating flicker that way.
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Old 18th August 2009, 08:29 AM   #8
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Thanks, I hadn't thought of doing that. Is it just a case of adding another bridge in parallel to the rectifier on one of the secondary windings?

I am running in dual mono, with 2 18-0-18, 120VA transformers. If I wanted to keep the channels equal, I would have to do the same with the other channel, athough the transformers will have enough headroom to cope with the (small) extra load.

The LED does flicker now and again anyway, which is distracting. I suppose I could just add a small frame transformer from an old transistor radio to power the LED. I have a bridge in my spares box which I could uses.

I may have just answered my own question, with a little help.

Thanks,

Chris
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Old 18th August 2009, 11:43 AM   #9
osscar is offline osscar  Latvia
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Hi,
im using in my 3875 gainclone two very nice (IMHO) LED`s for 220V AC which connected parallel with primary windings of transformers.
But these LED`s little pricey....

http://i623.photobucket.com/albums/t...r/DSC03596.jpg

http://www.apemswitches.be/1-26847-S...ails.php?id=80
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Old 18th August 2009, 01:25 PM   #10
sangram is online now sangram  India
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A single diode (half-wave rectification) with a 10uf cap and the appropriate resistor makes for a quick end to the light when you turn it off.
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