UK Vintage Radio Repair and Restoration Discussion Forum - View Single Post - Looking for help please with 1964 Trio W-41 stereo valve amp

PaulR · 5th Dec 2017, 1:17 pm

Others will be much more expert on this than me but dredging up A level physics from many years ago a basic explanation of impedance and resistance would be that resistance is the measure of the resistance of a material to the flow of a direct current. This is caused by the ease or otherwise of the flow of electrons through that material caused by the atomic make up of the material itself and can be measured by an ohm meter.

Impedance relates to alternating current. If alternating current is passed through a coil, say, then the wire from which the coil is made up will have a resistance, but the current will also set up a magnetic field which will resist the flow of current. The amount of this "resistance" or reactance is dependant on the frequency of the current as well as the physical make up of the coil (number of turns etc). The combination of the two makes up the impedance of the coil. There is a similar explanation for capacitors and circuits that have both. I seem to remember that the square root of -1 was involved in the calculation! You need a special impedance meter to measure it.

I hope that is right!

Edit.

Regarding impedance matching I imagine a signal as pulses of water flowing down a pipe. If two pipes of the same diameter are joined the maximum amount of water will flow through and the "signal" will remain. If the second pipe is narrower then the signal will still remain but less water will flow and some of the signal may be reflected back up the pipe to interfere with the signal coming down it. If the second pipe is much wider than the first one the water will flow but the pulses or signal will be less apparent. To me that is similar to an electric signal in a wire. To get the most signal and best power transfer the the impedances must be equal. You will still get a good signal but less power if the load impedance is higher that that of the source ad you won't get a good signal if the impedance of the load is less than that of the source. It may not be the perfect analogy but it works for me!

5th Dec 2017, 1:17 pm	#88
PaulR Dekatron Join Date: Oct 2003 Location: Southport Lancashire, UK. Posts: 3,236	Re: Looking for help please with 1964 Trio W-41 stereo valve amp Others will be much more expert on this than me but dredging up A level physics from many years ago a basic explanation of impedance and resistance would be that resistance is the measure of the resistance of a material to the flow of a direct current. This is caused by the ease or otherwise of the flow of electrons through that material caused by the atomic make up of the material itself and can be measured by an ohm meter. Impedance relates to alternating current. If alternating current is passed through a coil, say, then the wire from which the coil is made up will have a resistance, but the current will also set up a magnetic field which will resist the flow of current. The amount of this "resistance" or reactance is dependant on the frequency of the current as well as the physical make up of the coil (number of turns etc). The combination of the two makes up the impedance of the coil. There is a similar explanation for capacitors and circuits that have both. I seem to remember that the square root of -1 was involved in the calculation! You need a special impedance meter to measure it. I hope that is right! Edit. Regarding impedance matching I imagine a signal as pulses of water flowing down a pipe. If two pipes of the same diameter are joined the maximum amount of water will flow through and the "signal" will remain. If the second pipe is narrower then the signal will still remain but less water will flow and some of the signal may be reflected back up the pipe to interfere with the signal coming down it. If the second pipe is much wider than the first one the water will flow but the pulses or signal will be less apparent. To me that is similar to an electric signal in a wire. To get the most signal and best power transfer the the impedances must be equal. You will still get a good signal but less power if the load impedance is higher that that of the source ad you won't get a good signal if the impedance of the load is less than that of the source. It may not be the perfect analogy but it works for me! __________________ Paul Last edited by PaulR; 5th Dec 2017 at 1:42 pm.