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Tweakers' Asylum Tweaks for systems, rooms and Do It Yourself (DIY) help. FAQ. |
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Tweakers' Asylum Tweaks for systems, rooms and Do It Yourself (DIY) help. FAQ. |
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In Reply to: One more Q - are the effects the same? posted by Christopher Witmer on April 28, 2007 at 05:40:42:
The choke tweak is elegant in its simplicity. The transformer tweak needs some additional work, the R-C network as described, to make it behave. Once loaded, it may be a little better than the choke tweak, depending on the properties of the individual choke and transformer being compared."Characteristic impedance" is the relationship of current to voltage of a traveling electrical wave. Electrical disturbances move at the speed of light, so they have very long wavelengths at audio frequencies. We can consider cables as having negligible length and lumped values of resistance, capacitance, and inductance as long as we limit ourselves to the audio band.
Unfortunately, audio systems produce audio-band artifacts when exposed to radio-frequency noise. The Hammond choke tweak and several other esoteric measures improve audio-band performance by reducing the magnitude of RF noise in the environment and on the AC line. The frequency range of noise that can affect the audio system extends to the UHF band and beyond. This is the range where noise wavelengths are comparable in size to the wires being used, and the concept of characteristic impedance becomes important in understanding the noise behavior. Simply put, a low-loss cable with C capacitance per unit length and L inductance per unit length has a characteristic impedance of Z = SQRT(L/C). This is not the same as the small amount of resistance in typical wires.
Cables, such as power cords, interconnect and speaker cables, and segments of AC power wiring within the walls of your dwelling, have resonant frequencies because the traveling noise waves are reflected from the ends. Engineers use the concept of "matched impedances" to calculate how much energy is passed along and how much is reflected. If a cable is terminated in a resistance exactly equal to its characteristic impedance, all of the energy is absorbed in the resistance and none is reflected. Such a cable will not resonate.
Most cables do resonate because they are not terminated in matched impedances. This resonance can create strong RF tones out of weak background noise, in the same way an organ pipe creates a defined tone that can be painfully loud from the soft white noise of the inlet air. Your audio system is made of cables that can resonate, and is bathed in the resonant RF tones of your AC wiring environment.
The R-C loading of the transformer secondary is there to keep the transformer winding from resonating due to its inductance and capacitance. Since it is necessary but not of a critical value, I reason that it might as well be designed to match the impedance of the AC power cable without drawing too much power. It will only actually load the power cable if the transformer core allows the cable resonances to get through. The core may not, in which case the design is superfluous. I made capacitance and inductance measurements on a piece of plastic wiring cable I have, and calculated the characteristic impedance to be 120 ohms.
Some damping is better than no damping, so an exact match is not required. I've found that it is also useful to employ resistors and capacitors that are not made with steel parts. Take a magnet along if you can browse through parts bins.
To do the varnish dipping properly, you would have to heat the varnish and deal with the flammable vapors. This sounds like another Darwin Award project.
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Follow Ups
- One post for your latest questions. - Al Sekela 11:46:58 04/28/07 (2)
- Little Grasshopper is thus enlightened - Christopher Witmer 13:59:25 04/28/07 (1)
- EM field theory has been a tough subject - Al Sekela 17:34:48 04/28/07 (0)
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