Component Rating Tester (for Audio Circuits) [ongoing...] (#666.012)
Important:
This is still in progress. I’m updating as I learn more theory, progress in design/manufacturing/testing, etc.
2024 12 04
Background - Why does this exist?
While shopping for amp heads, I went down a modding rabbit hole and people modding the tone-shaping circuitry in their amps with different value components (specifically resistors and capacitors). All this, was simply to change the frequency of the high-pass or low-pass filtering - because that’s all that these combinations of parts do. For example, a capacitor before a split to [1] resistor to ground and [2] main output, will result in the following high pass filter. Image below from LabCenter.com.
However, these Youtube modders changed component values based on what people in fora (forums) told them (examples: 1, 2, 3, 4), or what they heard around the very reliable audiophile watercooler. This serves no purpose other than change for the sake of speculative change, and all without testing. They never test, but always rely on what memory they had of their sound before they changed tubes, changed resistor, changed cables… So, to test, you’d ideally model the circuit in a program to simulate it (too advanced for me and this purpose) or calculate the filtering manually (too “on paper” for my taste), but what if you could just quick swap different components ratings to hear changes live, in the same sitting, without soldering 12 times?
Why not just get a potentiometer?
I purposefully didn’t want a variable resistor/capacitor as the linearity and logarithmic(ness?) of it is variable (pun very much intended), it’s impossible to recall exactly the same position on the dial, and [1] I wanted reliability across components and their tolerances and [2] ability to recall an exact rating instead of fiddling around with a potentiometer and a scope until I got the right resistance/capacitance, and only then being able to check how that rating would sound. That, and variable resistors are, relatively speaking, far more expensive than regular, even “pro-audio”-grade, capacitors as the ones I ordered.
Explaining my reasoning for component choices:
I chose thin film resistors for their cost:performance and apparent self low noise (EETimes), because I specifically need the field recording mics on which I will test this to have low self noise.
And, I got (mainly) metalized polypropylene capacitors because they have plently of benefits that are important in audio (ElectroTechnik)
Before I go fiddling with >400Volt circuitry in an amp…
I am not yet comfortable inside an amp… mainly because I don’t have one anymore 😂 (I’m saving up for an Engl Fireball + DIY cab; Stay tuned.)
So, this tester will be trialed on my super sensitive DIY field recording mic, the EMR 5024. I will also further explore why I got vastly superior THD with merely changing the capacitor rating (see update “2024 05 02” on that blog post).
I have modified the microphones’ circuit diagram (Thanks, circuit-diagram.org) to show the variable nature of the resistor and capacitor for this project, and included the Neutrik XLR pin numbers for easier assembly.
Further on electrical safety:
While the capacitors and resistors chosen ARE rated for relatively high current and could be used in an amp, the Lorlin DS-1 (1-pole 12-position Rotating Switch) I chose for this is rated for a max of 5 Amps at 300V, and the contact probe solution that I will custom-make uses a connector with a max rating of 2 Amps at 50 VDC. It might be fine for most parts of a tube amp in the tone shaping section, but I’m still learning, so best not risk frizzing up my hair more than it already is. See homepage for proof/poof of hair.
Parts List
I tried to choose an as wide a range as possible of resistors and capacitors, that are of useful values and would affect signals inside our audible range.
12x Capacitors: 33 pF, 100 pF, 220 pF, 470 pF, 1 nF (1000 pF), 2.2 nF, 4.7 nF, 10 nF, 22 nF, 47 nF, 100 nF (0.1 µF), 470 nF (0.47 µF)
12x Resistors: 470 ohms, 1 kΩ, 2.2 kΩ, 4.7 kΩ, 10 kΩ, 22 kΩ, 33 kΩ, 47 kΩ, 100 kΩ, 220 kΩ, 470 kΩ, 1 MΩ
Metal chassis
1-pole, 12-position Rotating Switch (this Lorlin DS-1 is super cool, it allows limiting how many positions you have and comes in surface-mount or solder joint versions)
Either two crockodile clip cables for each polarity, or install a plug/play-able port like I did (though I just did it for fun)
B.O.M.:
I happen to use Reichelt.de for these kinds of purchases, and have made a public BOM for each version. (prices/availability from time of writing)
the Resistor version project HERE (€16.38)
and, the Capacitor version of the project HERE (€19.12)
