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Old 19th June 2010, 09:59 AM   #31
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A CRT based TV can generate strong fields up to a few metres away.
Badly layed out power amp bridge rectifiers cause strong magnetic fields.
Switched mode psus often have air gapped storage inductors that radiate strongly

Valve amps with C core chokes/output transformers with an airgap are also sources
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Old 19th June 2010, 10:18 AM   #32
marce is offline marce  United Kingdom
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Yes, its EMC, you design products and lay out PCB's to mitigate these problems. There is a whole range of info and advice on combatting EMC, from Henry Otts books and web site, to the EMC compliance club in the UK, FCC CE etc.
So what I am asking is what effect would magnetism (be it Ferro-, para- dia-) in some components have in a correectly designed and layed out circuit, taking into account EMC design principles.
The h field will induce current in copper tracks as well as iron, maxwells stuff.
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Old 22nd June 2010, 02:32 PM   #33
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Most commercial hifi is still using single layer pcbs with jumper links, not exactly designing for EMC compliance
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Old 22nd June 2010, 06:02 PM   #34
marce is offline marce  United Kingdom
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Yes, its a shame as double sided and 4 layer PCB,s can be sourced very cheaply these days. Yet theoreticaly all equipement sold in Europe should be CE tested and in the US FCC compliant, Hmmm.
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Old 22nd June 2010, 06:11 PM   #35
marce is offline marce  United Kingdom
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Back on topic!
I am doing a SMD class D amp, and quite like the Vishay RCL series thick film. These will handle 0.5W (0612) and 1W (1218) and are very compact for the power handling, plus being terminated on the long side have less overall parasitic inductance.
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Old 15th October 2011, 03:11 PM   #36
Butcher is offline Butcher  New Zealand
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"cupped my ear near it, they were absolutely silent"


haha, thats funny, and I'm not even drunk! HA
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Old 17th October 2011, 09:33 PM   #37
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One way to figure out if they are carbon or metal film resistors is their temperature coefficient. In general, the resistance of metal film resistor will increase by increasing temperature (i.e PTC or positive temperature coefficient), while carbon film resistors will see their resistance drop by increasing temperature (i.e. NTC).

Some, it is a very simple test: measure the resistance of the resistor while you heat it up (keep a solder iron very close to it, the value will change in seconds....). Carbon film will drop fast.


So copy-paste info text from www:

Temperature coefficient

The value of a resistor is dependant on the length, cross sectional area and resistivity of the resistive material it is made from. The quoted value of a resistor however is actually given as "So many ohms at a particular temperature". This is because the temperature of the resistor also affects its value.

The change in resistance due to a change in temperature is normally quite small over a particular temperature range. This is because the manufacturer has chosen a material having a resistivity not greatly influenced by temperature. That is, the material (and so the resistor) has a low TEMPERATURE COEFFICIENT. In other words, there is only a small change in value per C. This change in value is normally quoted in parts per million (ppm) so a typical resistor would have, as part of its specification a quoted temperature coefficient such as;

Temperature coefficient: 50ppm/C

Meaning that the change in value due to a temperature change of 1C will not be more than 50Ω for every 1MΩ of the resistor's value (or 0.05Ω for every 1KΩ of its value).

The temperature coefficient quoted above would be typical of a metal film resistor. Carbon film types have temperature coefficients typically around 200 to 500ppm/C
The change in value of a resistor with changing temperature is not very dependent on changes in the dimensions of the component as it expands or contracts due to temperature changes. It is due mainly to a change in the resistivity of the material caused by the activity of the atoms of which the material is made.

Last edited by AmpliFire; 17th October 2011 at 09:36 PM.
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