Hello,
These days I seem to wake up dreaming more about dB’s and dollar signs and less about acceleration.
Sitting here looking at manufacture data sheets for metal film resistors I see lots of mention of resistance non-linearity in parts per million per volt and parts per million per degree Celsius.
That ppm stuff means that resistors cause Harmonic Distortion. (Mostly 3rd and 5th Harmonics)
At lower frequencies the metal film resistors that we use do change temperature as the voltage across the resistor swings across zero to positive and again as the voltage swings across zero to negative. The resistors change temperature with the frequency of the applied voltage.
ppm dec equ % dBc
1 0.000001 0.0001 -120
10 0.000010 0.001 -100
100 0.000100 0.01 -80
Low (5) ppm resistors cost dollars each, High (200) ppm resistors cost pennies each.
In the realm of Jfets, 2nd harmonic is highest except when we tune to the sweet spot.
Testing just for fun.
I will let you know.
Thanks DT
These days I seem to wake up dreaming more about dB’s and dollar signs and less about acceleration.
Sitting here looking at manufacture data sheets for metal film resistors I see lots of mention of resistance non-linearity in parts per million per volt and parts per million per degree Celsius.
That ppm stuff means that resistors cause Harmonic Distortion. (Mostly 3rd and 5th Harmonics)
At lower frequencies the metal film resistors that we use do change temperature as the voltage across the resistor swings across zero to positive and again as the voltage swings across zero to negative. The resistors change temperature with the frequency of the applied voltage.
ppm dec equ % dBc
1 0.000001 0.0001 -120
10 0.000010 0.001 -100
100 0.000100 0.01 -80
Low (5) ppm resistors cost dollars each, High (200) ppm resistors cost pennies each.
In the realm of Jfets, 2nd harmonic is highest except when we tune to the sweet spot.
Testing just for fun.
I will let you know.
Thanks DT
It's a pretty cheap experiment to perform whatever test(s) you are planning, but do them twice:
Is the harmonic distortion the same for case 1 and case 2??
case 1: the resistor under test is an R ohm resistor whose tempco is X
case 2: the resistor under test is a series-parallel connection of qty=four of those same R ohm resistors, with the same tempco, from the same bag. This assemblage is another R ohm resistor.
Is the harmonic distortion the same for case 1 and case 2??
Temp rise or change is proportional to power dissipated. For small power dissipation, there is almost no temperature change. Power is signal squared, so the relationship is not linear. Also the time constant of power to temp rise must be considered. My bet that a thermal explanation for resistor distortion is unlikely for audio frequencies. Might become a small factor when resistor is operated at near its power rating limit. Will be interesting to see you testing results.
Mark,
I have done tests with a single resistor and a series parallel network of 4 and then 9 of same resistor.
If the single resistor is a high quality low ppm metal film it may test well on its own and the network may not show any or very little improvement. 5 ppm Vishay PTF resistors perform very well.
Single Vishay RN55 100 ppm resistors have more room to improve and perform much better in multiple resistor networks.
The Jfet bias resistors may have low power and low enough heat and voltage that they may not generate much distortion to start with, we will see.
The "One and Only" the mathematical models I have seen for TCR and VCR resistor distortion make 3rd and varying amounts of 5th Harmonic. As you point out there may be other mechanisms at play that makes 2nd harmonic distortion.
Thanks if you will share your insider knowledge.
I do this for fun and learning.
Thanks DT
as Papa is highly praising them ...... "Dales are gooooood! ......... value is written in numbers!"
investing energy in all aspects of actual construction, fiddling with parts as icing on the cake
considering how slow I'm prospering with construction, tnx God I'm never reach stage of fiddling with parts
investing energy in all aspects of actual construction, fiddling with parts as icing on the cake
considering how slow I'm prospering with construction, tnx God I'm never reach stage of fiddling with parts
Hello,
Heat transfer and dissipation is not instantaneous, it has some latency. Given the same watts input bigger heavier ½ Watt size resistors generate less distortion than everything being equal smaller lighter ¼ Watt resistor. It is a matter of BTU input, specific Heat, mass and surface area kind of thing. An example; naked metal resistors even having a very low TCR are very light and have high surface area. As a result they heat and cool rapidly. They have far worse Harmonic Distortion than typical thin metal film resistors with a ceramic core. At least that is what the AP APx555 on the bench tells me.
The main focus this time is to sort out if the Jfet bias resistor makes a difference or not.
Thanks DT
At some level it may not make any difference. Real world with the music playing there is no 192k FFT bin size and there are not 10 averages to separate the noise from the Harmonic distortions. -130 dBc harmonics may be lost in the real world noise floor.
Think of a single ended circuit with DC bias.
The One and Only,
Yes the focus is to measure the effect of the bias resistor in a single end JFET circuit.
In many or most resistor distortion measurement applications you see an AC generator a resistance bridge and an analyzer.
This time around there will be a filtered 24Volt DC power supply providing the bias current, 0.008 ish Amps. 0.008 * 0.008 *100R := 0.0064 Watts of dissipated electrical power, not a lot of heat but not 0 either.
In this case there be no 0 crossings but there will be relative heating and cooling cycles with the application of AC signal. We will see how things measure, DC blocking capacitors and all.
Thanks DT
When these are back in stock try it, you will be very pleased I suspect - https://diyaudiostore.com/collectio...s-dc-filter-p089zb-kit?variant=39297638465609
Resistor distortion due to dissipation has been documented here and in Linear Audio, member Ed Simon has done a lot of work on this.
If you have very low distortion equipment, resistor distortion may even dominate at mid and low frequencies (distortion due to dissipation is of course higher, in general, at lower frequencies).
Very real. Especially in feedback resistors that carry the full Vout, and determine the overall distortion.
Jan
Last edited:
What would be interesting is distortion measurement on a finished amp. One made with RN55 and another with "exotic" kind of resistors and see if there is any difference. It should probably be the same amp where resistors are swapped out. My guess is that resistor distortion is little compared to the contribution from the semiconductors (and capacitors in signal path).
You need to look at the system. How else can an amp have 0.05% distortion with a tube or transistor that has 10% on itself.
As an example, in an amp where the feedback resistors determine the gain, obviously any distortion in these resistor immediately translates into amplifier distortion.
If you really are interested, the attached may be useful.
Jan
In another thread I detailed about repairing a Sansui BA-60 amp that had high distortion in one channel. All the voltages checked out which had me stumped for a bit until I started thinking about the signal path. A 4.7k carbon comp resistor in the feedback path had drifted up to 5.2k. Replacing that resistor drastically dropped the THD to better than factory specs. I don't think the increase in resistance alone would cause the issue.
Hello All,
I looked at the TRI-AD wall wart plus filter again.
Typically when you speak of noise the reference is the random stuff that fades with averaging. Most of what we see in this plot is recurring switching spikes.
The dominant spike or peak is 60Hz.
The overall noise level between 20Hz and 22kHz is 74 or 75 uVrms, not too bad if it was random stuff.
For focusing on JFET and resistor Harmonic Distortion + noise makes me rethink using the wall wart.
I think that I will use a couple of 12V gel cells in series for the initial testing.
Thanks DT
see the attached screen shot:
I looked at the TRI-AD wall wart plus filter again.
Typically when you speak of noise the reference is the random stuff that fades with averaging. Most of what we see in this plot is recurring switching spikes.
The dominant spike or peak is 60Hz.
The overall noise level between 20Hz and 22kHz is 74 or 75 uVrms, not too bad if it was random stuff.
For focusing on JFET and resistor Harmonic Distortion + noise makes me rethink using the wall wart.
I think that I will use a couple of 12V gel cells in series for the initial testing.
Thanks DT
see the attached screen shot: