So the issue of crackling speakers comes up quite a bit, and indeed I’ve had my share of crackles which I’ve had to resolve one way or another.
As we all know, the most common culprits are:
Slow SD card
SD card file structure not optimal for fast file access
Level too high in config
Low frequencies in font files overloading the speaker as it tries to resolve frequencies outside its working range
Less common causes include:
Bad Proffie amplifier
Issue with 5 volt Proffie booster
Correct me if I’m wrong, but I think it’s fair to say that by design, the Proffieboard does tend to push things a little audio wise, putting it close enough to the edge that a small thing like slightly low battery voltage can tip it over into Cracklesville. Only this morning I had one that crackled with the battery at 3.35 volts, but was clean at 4.5 volts. I suspect the SD card is a little slow too (I’m just charging it to do more tests as I write this) which obviously results in tiny things combining to cause problems.
But one thing that I just stumbled on this morning was the Prof’s comment on another audio crackle thread about speaker ohms:
And I’m curious to know more.
Speakers are commonly rated at 4 ohms, but I routinely check the resistance once I’ve soldered the board as a means of checking that the connections are sound and the speaker in spec.
More often than not, I’ve found the resistance to be slightly lower than 4 ohms, often around 3.6 ohms. I’d never thought much of it as I know these things are not absolutely precise. But in light of the Prof’s comments above, it now occurs to me that in a system which is already pushing things a little, could this reduction of a few tenths of an ohm be enough to leave the audio teetering on the edge? Bear in mind that as a percentage, a 0.4 ohm reduction on a 4 ohm speaker is 10 percent out of spec.
All of which begs the question, is there any merit in adopting as best practice adding a 0.5 or 1 ohm resistor to the speaker positive line to ensure that the speaker impedance is exactly at or just above 4 ohms?
Yes, some chassis are pretty marginal in terms of where they place the speaker, and some speakers have more protruding diaphrams than others, so that’s definitely another cause to watch for.
I’ve also tried 8 ohm speakers in the past, and as you might expect they aren’t as loud, but looking back, I don’t remember ever having crackle issues with an 8 ohm. The Prof once posted that it’s almost impossible to overdrive an 8 ohm speaker with a Proffie amp, which again suggests that there’s something to be said for ensuring that speaker impedance doesn’t dip below 4 ohms.
Next time I have a crackly speaker, I’m gonna calculate the resistor value needed and fit one before I make any other changes! Will be interesting to see what happens.
This does not belong in the “less common” causes.
Speaker problems are very common.
The 4 ohms are at some specific frequency I think.
The short answer is: maybe
A lot of speakers are not really suited to handle the power we push into them, and a resistor can be a nice way to protect the speaker and avoid some distortion. You also loose some volume of course, but if it sounds like crap at high volumes, does it matter? It depends on your preference, would you rather have better sound or higher volume?