Never as planned

Anyone seriously looking at distortion in resistors should take care to test at high frequencies. At 5 euros a pop (Farnell price), the Vishay MPM divider series are reassuringly expensive but I've just had to eliminate it from a preamplifier circuit as at 20dBu and 20kHz it produced 0.001% THD. That's a stupendous amount coming from a precision resistor.

Also out are reed relays. Those wonderful red Coto's are the choice for reliable switching at low currents and voltages, but at 30mA (and again at 20kHz) I found about 7uV of 3rd harmonic across the contact, which as expected came down a bit as the coil voltage was increased.

The irony (pardon the pun) is that I'm discovering all this whilst building a relay-switched gain control to replace an esoteric audiophile Japanese potentiometer that was plainly distorting. Mentioning this to the manufacturer (TKD) elicited exactly no response at all. Mind, you can get virtually zero distortion from even a cheap linear pot if you use it as the feedback divider in an inverting amplifier. Clearly doing the same with a logarithmic pot is a waste of time. Doing so with an expensive logarithmic pot is a great way of discovering just how many relays the same amount of moolah can buy.

Sunday 23 September 2012