if the coil is rated at 9 or 6 volts why do you need a current limiting resistor. maybe if your relay is 3 or 4.5 volts then youll need a voltage drop resistor when using 9 volts supply. but most relays for this application are rated at 9 or 6 volts. psu's are a must for relay switching.
Sir, di ba masisira na yung 6V rated relay kung 9V ang i-apply mong supply?
and from my experience in mechanical relay switching i noticed that the spike pops are more evident when you disengage the coils because the magnet sucks in the dc supply which is the source of the pop when it leaks to signal lines, when you disengage the coils the dc supply leaks tru the signal, if im not mistaken thats the purpose of the diode capacitor network they will try to suck in the dc leak upon releasing the coil then the capacitor discharges the dc voltage gradually to avoid spike pop from leaking to the signal lines 
that is why you need a current limit resistor to prevent too much 'sucking in' of the dc supply.
The diode is there to prevent the coil from swinging negative (during inductive kick of the coil once the switch is opened).
anyway, may naka gamit na ba nito? ginamit ko kasi dati yung relay switching sa wah, pero di ko matanggal yung pop kaya i dropped the idea. IIRC that was way back in 2005. almost same nung pinost ni ubersam pero yung LED and resistor ko sa coil ng relay naka series. so once i engage the coil, naka-on na din yung LED.
yung Alexan enclosure, matibay yan, i have been using that one eversince mag diy ako. madali pa butasan. GD124 yung item code nyan. meron din dun nung almost half size ng GD124, yung parang size nung japanese apartment looper ni keeley.
may nakabili na ba kay epektos? oki ba pricing?
eto yung ROSS comp ko na using Alexan's GD124
