Many audio circuit designers seem to ignore the fact that, with regard to noise, all resistors are the same EXCEPT when DC flows through them - which, of course, causes the "excess noise". Most applications of resistors in audio circuits see no DC, yet many audiophiles believe that an expensive resistor will result in lower noise. Of course, low excess noise resistors are useful in applications where there DC does flow - the plate load resistors in vacuum-tube circuits, for example. This article seems to underplay that fact.
The final graph is very dramatic, and a salutary reminder to do one's calculations. However to dissipate 0.3w in a 1K resistor would require 24v RMS, which is typical of a power amp rather than a low level circuit. In a low level circuit with output of 1v RMS and R2 =2k2 the stabilised resistance change would be 3ppm for the bulk resistor and 0.5ppm for thin film.
It is great to build very high quality audio amplifiers! But...that quality can be ruined by the speaker attached to the amp. So while this article is a welcome reminder about the importance of component selection, don't over-design the amp and under-design the speaker. And make sure the supply noise is not a contributor.
Oh, and the overall gain that is required will dictate what the noise performance of the amp stages should be.
Many applications simply do not require the ultra-low noise performance. The high performance is needed in the professional audio, recording, and measurement industries, and also for the audiophiles among us.
This article was informative in describing noise considerations beyond thermal and current noise, such as temperature-induced nonlinearity.
However, the bold print lead in is “Experiments have sought to show why some resistors are ‘noisier’ than others”, suggesting perhaps the actual measurements of thermal and excess noise are naïve or useless in their incomplete nature. But, the authors then devote a page to thermal resistor noise calculations (J.B. Johnson, “Thermal Agitation of Electricity in Conductors”, Physical Review, vol. 32, July, 1928, page 97), and restate that carbon resistors are noisier than thin film resistors, and that metal foil resistors are best, seemingly well known information, even known to many non PhD “experimenters”.
The description and caution that one should consider temperature-induced nonlinearity in power applications (and don’t forget inductance, especially if you see spontaneous RF oscillations, a good point) is all helpful information. Probably many designers have not taken temperature-induced nonlinearity into proper account, where there is enough power dissipation for temperature-induced nonlinearity to be a factor. (As an earlier post points out, temperature-induced nonlinearity is likely to be insignificant in many small signal applications.)
On the other hand, since our 2007 JCan experiment article (on measuring thermal and excess noise) began with the line “You have probably heard someone say that some resistors are “noisier” than others”, the lead in title to this article, in my opinion, felt like a bit of a snobbish remark towards those of us “experimenters” who go to the effort to actually measure parameters. It is unclear why that negative approach was needed for an otherwise informative aggregation article.
A reader has emailed the following comment:
On Page 36 of the Sept 27 EE Times story about "Selecting resistors for high-end audio applications", there is the following statement in the lower part of the left-hand column:
"The major objections to wirewound resistors are their inductance, which results in chopping of the signal peaks, and ----------."
That is the biggest pile of $%*$& that I have read in many years. If an inductor is a linear device, and all wirewound resistors that I have seen in 40+ years of electrical engineering have linear inductance, because they are not wound on iron cores, there is no way that the inductor will chop signal peaks, but leave the remaining portion of the signal alone/untouched. I fully agree that wirewound resistors can very easily be inductive as well as resistive, but there is no way they can "chop" signal peaks.
H. R. Hofmann,
Past President of the IEEE EMC Society,
President of Hofmann EMC Engineering
Mr Hofmann is right. They certainly ARE linear inductive.
A w.w. resistor load from a MOSFET source follower caused oscillations. Tank. Capiche ?
I would add as well as Vishay's very good parts the British Company Welwyn is their standard for many resistors of all kinds.
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