High-End Audio Components: Resistors
Resistors have two main issues when you want to design a low noise, low distorsion system : noise and linearity. Imagine also that you design a RIAA filter : tolerance and linearity are major concerns too.
Regarding linearity, it can become a big issue. Resistance value varies upon temperature and voltage. So your best designed RIAA filter can be a mess if you don’t select the right resistors in terms of technology and vendors. Trust me: a simple Monte-Carlo simulation (with SPICE) with medium range/price resistors can generate an awful grimace on your engineer face.
Regarding noise, let’ s say Johnson noise is produced by all resistances, intrinsic (like the base spreading resistance of a transistor) or extrinsic (like the PCB tracks resistance). By definition, resistors produce noise. The more the value, the more the noise!
At 25°C and 22 kHz bandwidth, a 47 Ohms resistor generates around -135 db ; a 47 kOhms potentiometer, around -100 db.
Consequently you’d better select the best technology for your resistors.
My choice is definitely Metal Film resistor. There is no linearity problem at normal level signals. And with 1% tolerance (at minimum), 100 ppm/°C and 1 ppm/V, this technology provides what we need for accurate filtering. Forget Metal-oxide mass-market resistors with typically 5% tolerance, 300 ppm/°C and high ppm/V.
The next question is the classical one : Thru-Hole (TH) or Surface-Mounted (MS) ?
When size and placement constraints are not an issue (that should never be an issue when designing high-end audio!), don’t hesitate : TH resistors are the best. Power dissipation (and rating) is better (remember the non-linearity issue in ppm/°C…) that SM. TH resistors have also a very low voltage coefficient in comparison of SM’s. The only drawback is some kind of parasitic inductance that SMs don’t have too much.
If you need to use SM resistors (but please check twice you really need to…), please use MELF SM resistors and forget Thick-Film resistor ! The MELF ladies have a voltage coefficient less than 1 ppm and noise about 0.05-0.4 uV/V only. In terms of size, I recommend 0805 (125 mW – better linearity and lower THD) or 0603 (100 mW). You can solder 0805 manually too.
