Late in 2017 I eagerly started reading the latest book by Douglas Self entitled "Electronics for Vinyl". In this book Douglas provides a clear and detailed description of the low noise/high accuracy design of phono pre-amplifiers. The design philosophy includes the following two concepts:

• use low value resistors to minimize the thermal noise;

• use two or three resistors in parallel to achieve the high precision and non-standard resistor values required for RIAA equalization. RIAA equalization is the specification for a filter in a phono pre-amplifier used during the playback of vinyl records. The specification was established by the Recording Industry Association of America (RIAA) to allow the recording of sound to better match the mechanical limitations of cutting a groove in a vinyl record.

On page 41 in a section on Resistor Value Distributions, I read "...what is the actual distribution of resistor values like? It is not easy to find out..." and "...measuring thousands of resistors with an accurate DVM is not a pastime that appeals to all of us. Any nugget of information in this area is therefore very welcome."

This sparked my interest - I could build a machine to automatically measure a whole box of resistor values.

1000 resistors in a box!

During Christmas break 2017, in the time between family gatherings, I designed the Resistor Testing Machine shown on this page. Parts started arriving at my house in January 2018 and the machine was ready for its maiden run on February 19th, 2018.

Over the following few days I used the machine to measure three resistor values, 220 Ω, 1 kΩ and 10 kΩ. A text file with the value of each resistor tested is available by clicking the link associated with the value. If the Resistor Testing Machine measures a bad value, a blank line is written in the text file.

All resistors were metal film Vishay MRS16 series ±1% tolerance. The results were very interesting:

• the measured average of the 220 Ω resistors was almost spot on at 219.9 Ω (-0.05%).  The measured average was a little low for the 1 kΩ resistors (995 Ω or -0.51%) and 10 kΩ resistors (9969 Ω or -0.31%) but well within tolerance;

• the measured standard deviation was much tighter at 0.18%, 0.13% and 0.12% for the 220 Ω, 1 kΩ and 10 kΩ values respectively;

• the measured resistor values very closely followed a normal distribution;

• the measured average of 211, 10 kΩ resistors reported by Hugo Kroeze and referenced in Douglas Self's book was 0.05% low which is the same percentage error in average value I measured for the 220 Ω resistors. The average value for the 1 kΩ resistors and 10 kΩ resistors I measured is even lower than the measured average of 100, 1 k resistor measured by Doug Self which had a percentage error of -0.23%. The standard deviation I measured was between the values reported by Kroeze and Self.

All values for the Resistor Testing Machine were measured using a recently purchased Rigol DM3058E 5½ digit benchtop digital meter.  The accuracy specification for this instrument in 4-wire resistance measurement mode is 0.03+0.005 (%reading + %range) on the 200 Ω range and 0.02+0.003 (%reading + %range) on the 2 kΩ and 20 kΩ ranges.

I would be happy to measure more resistor values if you are interested in supplying the resistors. The Resistor Testing Machine was designed to use Tape & Box (TB) packaging but I would consider testing with other packaging options if the resistors can be feed automatically through the Resistor Testing Machine. 1000 values take about one and a half hours to complete. Please use the contact page if you in interested in testing more resistor values or have any questions.