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Notes & Errata High Visibility 6-Digit LED GPS Clock, December 2015 & January 2016: the parts list in the December issue should read: 9 BC547 (Q1-Q9) 10 BC337 (Q10-Q19) These type numbers were transposed in the original parts list but were correct in Fig.1 & Fig.2. Note also that Fig.1 should show R8 as 68Ω and the label on Fig.2 should read Q20-Q26 for the MPSA13 transistors (not Q20-Q16). The following additional notes also apply: (1) To calibrate the 32kHz crystal, set the XTAL menu option to between -512 (260ppm slower than default) and +511 (260ppm faster). This is adjusted automatically when a GPS module with a 1pps output is used. (2) When the alarm goes off, use a long (1s+) press of either pushbutton, or a second press of the Escape button on the remote to cancel it not the magnetic force alone as I had formerly believed. So the magnetic force of the voice coil is only responsible for moving the cone outward from the rest position, and it is the suspension “spring” which is responsible for moving it back to the rest position. It all seems rather obvious to me now but it seems odd that this is never explained in detail in most documentation I have read. The speaker’s spider is usually referred to as providing a centring action, which I assumed just meant bringing the cone to a central position when the speaker is not being used. I never realised how important a function this spring performed until now. If my new understanding of how the dynamic drive works is correct, and I believe it is, it shows that the linearity of the drive suspension is critical in terms of the distortion performance of the driver. No matter how linear the driving amplifier is, it is only responsible for half of the driving force applied to the cone – the suspension is responsible for the other half. I wonder, is it me being exceptionally dumb and missing the obvious, or are other people unaware of the exact details of how a moving coil driver 94  Silicon Chip altogether. A short/single press will simply activate the snooze function. (3) Maximum alarm duration has been extended to up to 15 minutes with a default of 10 seconds. (4) The unit can show the day of the week. Simply activate the date display function, then press the same button again. (5) A new menu item, “GPSLCK”, has been added to the options menu. If set to “IGNORE”, the unit will use GPS time even if the satellite fix is not perfect. This will allow the unit to work in marginal signal areas although time accuracy may not be quite as good. (6) A new brightness menu item, “CUR RD”, shows the minimum/ current/maximum raw LDR readings in 8-bit hexadecimal notation. The fourth digit decimal point lights when the data is going to be saved to flash memory and goes out once it’s saved. This can be used to troubleshoot the autodim function. works? Or have I got it all completely wrong? (P. T., Casula, NSW). • It is true that the suspension system does damp the cone motion and does provide some restoring force, so that the cone returns to its “centre” position in the absence of any voltage across the voice coil. However, the suspension is designed to be as linear as possible so that the damping and “restoring force” is equal for forward and backward excursions of the cone. It is not a “push-pull tug of war” as you postulate. In an ideal system, we could ignore the effect of the suspension on the basis that it is only there to “suspend” the voice coil over the magnet pole piece. In fact, what happens is that the driving amplifier is a very low impedance voltage source (or it should be) which has very tight control over the speaker excursion at any time. So if you apply a sinewave signal, there is no tendency for the coil to over-travel and its motion is a very good analogue of the input signal. Consider what would happen if there was a tendency for the voice coil to over-shoot (and there always is, in fact). This additional motion would induce an equivalent voltage in the voice coil and this extraneous signal would immediately be damped (think of it as being short-circuited) by the very low impedance of the driving amplifier. That is why it is most important, in a hifi system, to have very low resistance loudspeaker connecting leads. Otherwise the “damping factor” of the amplifier is reduced. The “electronic damping” works in all modes, forcing the cone to follow the signal voltage. If the cone tends to lead or lag the signal, there will be an error voltage which will be heavily damped by the amplifier’s very low impedance. The current that will flow in the voice coil, due to this damping of the error voltage, will force the cone back to the correct position. Another point to be considered is that the current in the voice coil is not necessarily in phase with the driving voltage; the load present by a loudspeaker is inherently inductive, with an impedance which rises with frequency. Therefore you cannot simply think that the current flowing in the voice coil following the peak of a sinewave is simply “declining in value towards the zero level” and that it “has the same polarity as it did during the first quarter of the wave and so should still be pushing the voice coil, not pulling it”. In fact, the current will always lag the voltage in a purely inductive load and that complicates the picture. Finally, consider that the air trapped in the speaker box also tends to modify the motion of the cone, damping it more for reverse excursions than for forward motion. Again, this is where the amplifier is supposed to maintain tight control over the motion of the cone and stop it responding to these external non-linearities. Ultimately, hifi amplifiers and loudspeakers do a remarkable job of turning electrical signals into analogous acoustic waveforms. Currawong valve amplifier current I have completed building the Currawong valve amplifier and have tested the unit for frequency response and also played some music from my iPod. The sound is very good. However, after installing the plexiglass cover I noticed that the 330Ω 5W resistors which I had soldered leaving the 5mm gap suggested were flush against the PCB. The problem is that siliconchip.com.au