Since I mentioned it a few posts back, it's a safe assumption that I do.
Read, John.
To me, it seems to be a kindergarten lecturing sometimes. Maybe I am wrong.
I would expect 'SOTA' solutions in this thread, not elementary formula.
PMA, I am trying to give practical advice to designers, both amateur and professional. Often problem areas that I may have overlooked in my first 5 years of electronic design after finishing college (a long time ago). Perhaps, you should contribute more, and condemn less. I would certainly appreciate it, as you are a valuable resource, if you are willing to share, as I do. For the record, I rarely need to use coupling capacitors, my Parasound JC-1, JC-2, and JC-3 (just introduced) don't have them.
Now, let us 'creep forward' to understand what happens when you use a smallish cap. like .033uF, as in our last example, with a 1 meg resistor.
Now, the 1 meg resistor makes its own noise, and this must be shunted out by the coupling cap, if possible. If not, then the FULL 1 meg noise will be amplified by the next stage. Determining how well the .033 uF cap does this, takes a bit of figuring. (more later)
Now, the 1 meg resistor makes its own noise, and this must be shunted out by the coupling cap, if possible. If not, then the FULL 1 meg noise will be amplified by the next stage. Determining how well the .033 uF cap does this, takes a bit of figuring. (more later)
My tests were putting the cap into an RC coupling circuit at the input to my preamp and tapping them with a pencil looking at the output on a scope or listening through headphones or speakers. I've experienced, but did not quantify, the opposite effect- sound generation by electrostriction. Anyone can do these tests for themselves.
Over the years, people have gifted me with all sorts of "fancy" caps to try; I admittedly haven't kept systematic records since they either performed poorly, performed no better than conventional cheap caps, or were so out of spec from the get-go that it wasn't even worth trying them; they just get chucked into my "dodgy parts" box and gather dust. Since this is something that I personally don't find terribly interesting or important, I'm reluctant to name names without having systematic, well-documented records in case anyone's lawyer is feeling frisky. Sorry to disappoint. I will and have named names of conventional inexpensive caps that I've found to work as well as anything else out there.
My previous example using SY's criterion of 1 meg and .033uf did NOT come to any real noise problems, IF the cap was REALLY an interstage cap. Perhaps 5nV/rt Hz at 100 Hz, not too bad, but I have thought of another interesting example, one that many here might just try, if they were so inclined. (more later)
Although thermal noise might not be such a big problem with high impedances in interstage coupling, high impedances make the circuits much more susceptible to noise pickup of different kinds.
The capacitors role in audio circuits
Capacitors in audio electronics have very different roles. Maybe we should split these roles into specific groups before we discuss what types of capacitor to use and not use for audio.
Coupling capacitors main function in to block DC, i.e. they form HP filters with very low -3dB frequency. Coupling capacitors have no audio across them, only DC.
De-coupling capacitors have the opposite function, to short out frequencies way above the audio band. These capacitors have all audio frequencies across them, including DC.
Equalizing capacitors works within the audio band - and therefore might be the most critical capacitors.
And since the DC power for audio electronics usually comes from AC, power supply capacitors also play an important role.
There is no specific type of capacitors that is the ideal choice for all these very different tasks, are there?
So my question: What is the ideal capacitor for each of these roles?
Capacitors in audio electronics have very different roles. Maybe we should split these roles into specific groups before we discuss what types of capacitor to use and not use for audio.
Coupling capacitors main function in to block DC, i.e. they form HP filters with very low -3dB frequency. Coupling capacitors have no audio across them, only DC.
De-coupling capacitors have the opposite function, to short out frequencies way above the audio band. These capacitors have all audio frequencies across them, including DC.
Equalizing capacitors works within the audio band - and therefore might be the most critical capacitors.
And since the DC power for audio electronics usually comes from AC, power supply capacitors also play an important role.
There is no specific type of capacitors that is the ideal choice for all these very different tasks, are there?
So my question: What is the ideal capacitor for each of these roles?
I would answer: design your SOTA circuits the way you do not need any coupling capacitors.
I think a better approach would be to sum L+R below 50 Hz, since most of the warp frequencies are out of phase. However, that would probably eliminate one of the audiophile reasons to stick to LP's
But in high-gain design you need to deal with DC. So if SOTA for you is using servos, you still have caps in the audio path -- although as a separate feedback path.
On the other hand, servos can easily work with caps around 1 uF, where very good quality caps are both available and rather cheap.
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