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Old 10th December 2012, 05:25 PM   #1
iggy111 is online now iggy111  Croatia
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Default Few questions about BrianGT chipamp

Got some problems and questions about my recently built BrianGT lm3886 chipamp, didn't want to hijack several threads, so sorry for the long post right away...

1. Transformer humming - when I power up my amp I hear this relatively loud humming from the toroid which wasn't there a few days back. It hums for like 5sec, then quiets down for a second, and repeats that cycle over and over. Sometimes during playback it goes silent for some time, but will start humming all over again.

It's a 220V / 2x28V toroid, though my mains is 230V so the amp sees 2x40V or even a little more. Working on regs to bring it down to 2x35V or less.

2. When building carlosFM snubbered regulators - do I have to change the 100nf cap before the regs to 470nf as in his scheme or its not crucial? Also, is there a big difference in replacing the C1/C3 100uF PanasonicFCs with 1000-2200uF as he states or can I keep these?

3. Dual-mono/monoblocks or keep it stereo? How much of a difference is there going from stereo amp to monoblocks? I guess shorting the length of speaker cables should be beneficial by itself, but is it worth the cost of another psu and case, or is it better to spend that money on better caps, say nichicon FG, Elna Silmic II..? Though I'm not saying I won't do both...
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Old 10th December 2012, 05:29 PM   #2
AndrewT is offline AndrewT  Scotland
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Quote:
Originally Posted by iggy111 View Post
Got some problems ......................1. Transformer humming - when I power up my amp I hear this relatively loud humming from the toroid which wasn't there a few days back. It hums for like 5sec, then quiets down for a second, and repeats that cycle over and over. Sometimes during playback it goes silent for some time, but will start humming all over again..................
I would stop using the amplifier and properly recheck all the transformer connections and ensure the transformer is working properly !
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Old 12th December 2012, 11:44 AM   #3
iggy111 is online now iggy111  Croatia
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I checked all my connections and everything is fine. I can still hear the humming when the trafo is out of the amp and connected to the mains only.
A friend of mine heard it and said its not uncommon, that it could be a cheaper trafo made from metal plates and not a solid core. Any comments on this?
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Old 12th December 2012, 11:54 AM   #4
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Dc on mains is a possibility.
Mains DC and Transformers
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Old 12th December 2012, 12:18 PM   #5
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Yes, possibly DC on mains. For example a neighbour could be using something with half wave rectification that is putting some DC on your mains. This might explain the intermittent nature of it.
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Old 15th December 2012, 10:29 PM   #6
iggy111 is online now iggy111  Croatia
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How do I check that, just stick my multimeter into mains outlet, set to dc? Also is there a way to block that dc?
I noticed that trafo doesn't hum every day in the same cycle, which could prove your theory.
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Old 16th December 2012, 12:57 AM   #7
JMFahey is offline JMFahey  Argentina
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Dear Iggy, please stick nothing in your wall socket, except your amp power plug.
The small buzzing you hear *sometimes* is no big deal, not worth the risk.
And after all, you might neither find what's the cause nor be able to correct it, so .....
I hear you talking
Quote:
How do I check that, just stick my multimeter into mains outlet, set to dc?
and my blood freezes.
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Old 16th December 2012, 12:12 PM   #8
AndrewT is offline AndrewT  Scotland
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Originally Posted by iggy111 View Post
How do I check that, just stick my multimeter into mains outlet, set to dc?...........................
If this is an indication of your safety awareness when working with mains electricity, then I suggest you stop Playing at amp building.
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Old 16th December 2012, 07:01 PM   #9
iggy111 is online now iggy111  Croatia
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Ok, I made a bad joke about that one... I didn't stick anything in my wall socket, and being a newbie I try not to do anything before getting some answers here or from a local expert.
Anyway - thanks for caring
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Old 16th December 2012, 07:43 PM   #10
AndrewT is offline AndrewT  Scotland
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OK, Now that we have your safety awareness clarified.

Here is something I do. But be aware it does leave mains exposed and does require DMM probes being used to measure Mains Voltage.

Run the mains cable from the plug top to an insulated terminal strip. The type with the protective towers surrounding the securing screws.

Attach a 1r0 resistor across two spare terminals of that 5way strip. Insert a shorting link from the Neutral to one side of the 1r0.
Take the three wire flex from the L terminal and the Earth terminal and the other end terminal of the 1r0 to your equipment.

Wrap the whole terminal strip in insulating tape.
Screw the terminal strip to a backing board using the spare fixing hole/s between the terminals.

Now you can power up the equipment with the 1r0 in the Neutral lead.
The Probes of a DMM can be used to "punch through" the insulating tape down onto the securing screw head. You can measure Mains voltage. You can measure Volts Drop across the 1r0.
From these two measurements you can derive other consumption information.

When you withdraw the probes, you are left with small holes in the insulating tape and these are mm removed from the top of the securing screws. You cannot accidentally electrocute yourself if you are careless. Nor can anyone else who wanders in supervised.
But don't leave it plugged in and leave it unattended, just in case an unsupervised person decides to investigate.

DO NOT use any meter set to DCV to measure Mains voltage.
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Last edited by AndrewT; 16th December 2012 at 07:47 PM.
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