Originally Posted by AVSBama
You really just don't have a clue about capacitors. Why do spout about what you know so little about. Don't you realize this make all that you say suspect.
My, this thread certainly has been a busy one.
I looked at Mortyboy's LTspice analysis of the ripple current and voltage pumping in the 1600's output supply capacitors and was impressed with his effective and knowledgeable use of some of LTspice's more arcane proprietary elements as well as the general elegance of the simulation in capturing the most critical behavior without adding undue complexity. Did Mortyboy actually claim somewhere in this thread that the Peavey amp would fail prematurely or was he just raising a red flag over possible issues?
By Mortyboy's linked analysis (if memory serves), with compressed club music driving the amp to rated power into 4 ohms, the caps would run very close to the rated current and they would run a little over double that current into two ohms. This was the reason for raising the red flag, something I find very reasonable rather than taunt-worthy (don't know which behavior is more objectionable - the bully-gang taunting or the ensuing crybaby tantrums).
Anyway, again if memory serves, these capacitors are rated for 2000 hours life when operated at the rated voltage (87V), ripple current (about 4A?) and rated still air temperature (85 deg C). Two thousand hours is about 50 weeks of 8 hours per day, or roughly 1 year of professional use. Assuming the amplifier has a claimed product life of 5 years (is that correct?), can the capacitors be expected to last that long under worst case conditions?
If used sanely with respect to their ratings, aluminum electrolytic capacitors wear out due to a slow loss of electrolyte via partial pressure driven diffusion through their end seal. It is my understanding that this is strictly a function of capacitor core temperature. Ripple current does not directly reduce life - its effect is to raise core temperature, which then reduces life. If the extra heat can be removed, ripple current may safely exceed the listed rating by a factor of double or more.
The typical assumption is that capacitor life roughly doubles for every 10 deg C reduction in core temperature and that rated normal core temperature rise for 85 deg C types is typically 10 deg C (i.e., 95 deg C core). A one year life must be doubled about two and a half times to get to the 5+ years of life listed for the product, thus operating capacitor core temperature must be reduced by about 25 degrees from the data sheet assumption, that is, to about 70 deg C.
The biggest unknown with this product is the heat removal factor due to the fan and air baffle system. Life will be limited by the capacitor with the least cooling. However, even moderate airflow can double effective heat removal so let's take that as a rough guess for the 1600 (instrumenting the capacitor with a thermocouple in its core would be the best way to avoid guessing).
According to Mortyboy's analysis ripple current will be slightly over double the listed rating with a two ohm load at full power. That means ESR losses will go up by a factor of four or five (yes, ESR goes down in the short term when core temperature goes up, but it also rises over life, so let's assume it's a wash on average). However, heat removal is twice the data sheet assumption as well, so core temperature rise may be roughly double+, that is, about 20 to 25 degrees C. Subtract this from the 70 deg C limit required to meet product life and ambient air temperature must be limited to 45+ deg C worst case. This is 110 to 120 degrees Fahrenheit, which is quite hot, even for an enclosed equipment rack without air conditioning.
In the very worst case, perhaps one out of several thousand users might manage to wear out the capacitors within the 5 year product life, but, IMO, even if this really were to occur, it would be completely buried in the noise from all other weird causes of failure (typically customer misuse).
It looks to me like Sickneedhelp walked right down the center of the tightrope between cost and reliability (at least with regard to the output electrolytic capacitors).
Regards -- analogspiceman
Disclaimer: bear in mind that this was a very rough "back-of-the-envelope" engineering check done strictly for my own amusement. YMMV