Bottlehead Forum
General Category => Technical topics => Topic started by: Jamier on June 22, 2019, 07:00:08 AM
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I’m working on an amp that, by design, has a “floating†ground.I’m thinking I should Earth ground the input shields since tying them to the circuit ground will probably not accomplish what I want ( noise rejection). Is this correct?
Jamie
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You should consider providing a whole lot more info about what you're working on. Connecting shield grounds to audio ground does tend to defeat the purpose of the shield in the first place.
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I was given a pair of K-501 boards by a friend. I built chassis for them with the intention of mounting the inputs off board. I have mounted RCAs on the chassis which require an input cable run of about 4 inches. The connection for the inputs,on the board, is about 4 inches from the PT. I concluded, possibly erroneously, that I should use shielded cable to connect the RCAs to the board. The circuit ground floats so I figured, possibly erroneously, that grounding the shield to the circuit ground would have no effect. At this point I intend to ground the shields at the ground on the IEC, but I do have concerns that they will pick up noise from the adjacent AC input.
Jamie
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At the IEC, connect the ground straight to the chassis. Wherever you have cable shields, connect them at the load end to the chassis. I would either connect the audio ground to the chassis right where that signal cable enters the board, or use a pair of anti-phase diodes to tie the audio ground to the chassis.
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PB, I don't have any 2A diodes but I have a crapload of 1A uf4007s. Can I parallel 2 of those in each direction, or is that a no go?
Jamie
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You want the diodes to be able to blow the fuse if the SHTF. I suspect a UF4007 would still blow a 1A fuse without trouble.
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PB, So should I parallel 2 uf4007s in each direction, or just use 1 in each direction?
Jamie
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You could try it both ways.
If there's a local electronics shop, the diodes don't have to be anything particularly special.
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What about a 2A fuse?
Jamie
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What is the current fuse value?
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In don't know. I'm away from it at the moment. Do you have formula for calculating the diode rating.
Jamie
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Yes, you look at the datasheet for the diode and read the number for forward current.
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Peak or average? I figure your going with average, right? So if I do have a 2A fuse, I probably need to use 4A diodes in anti-parallel to insure that the 2A fuse will fail if the SHTF?
Jamie
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I would stick with average. You don't want the diode to blow with the fuse!
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So, am I correct? Or?
Jamie
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At least a 2A rated diode, sure.
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PB, If I use the anti-phase diode approach, can I connect the audio ground to the chassis at the line end or should it also be grounded from the load end, like the shields? What would yield the best result? I'm concerned that, just like the shields, there is a possibility of picking up noise from the AC input, even with the diodes in place. Connecting at the line end is easier, mechanically, but it's worth the extra work to do it from the load side if I am running the risk of greater noise pickup.
Jamie
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Less asking more doing! These are really low stakes experiments that take very little time to carry out!