09-10-2016, 12:35 PM
It's true that measuring audio power is a complex matter.
The main advantage of sine wave testing is that it's easy to do and standardised. But it doesn't tell you much about how the amp will behave with real audio. A sine wave has a low PMR (peak to mean ratio) compared to music (which might be 20dB or more - a factor of 100 in terms of power), so with the usual unregulated PSU, the instantaneous peaks might well reach higher levels than the "RMS" power output suggests. That's an advantage in reality.
I put "RMS" in quotes, as "RMS power" is a meaningless marketing term that came about because "continuous average sine wave power" is a bit of a mouthful
Instantaneous or short-term peak power is a figure of interest, and there are ways to measure it (IHF, etc). By comparing that with the continuous power, that gives an indication of the "stiffness" of the power supply. It can make a subjective difference to the sound quality, so it's something good designers pay attention to.
In this case, I'm not 100% clear of what Andy is seeing, but I'd start by checking out the PSU capacitors, the fuses and the bridge (always start with the power supply!). There's no output capacitors, but others might cause problems (e.g. the bootstrap caps - C709/710).
Silicon transistors are usually pretty good when not abused, and failures tend to be CE shorts, or a loss of Hfe. Leakage can happen as well. Some plastic packages can suffer from moisture ingress...
Those sub-£20 eBay kits can be pretty good for basic testing (and they do just about every other type of component as well). I rarely feel the need for more exhaustive testers because nothing tests a transistor like the circuit it's working in, so usually, you can tell from the surrounding conditions if the transistors is doing the right thing or not. Not always, but usually. Occasionally I might build a basic "jig" for testing more esoteric things, but one of the aims of good circuit design is to get away from depending on certain parameters of the transistors - especially if it's going into production
The main advantage of sine wave testing is that it's easy to do and standardised. But it doesn't tell you much about how the amp will behave with real audio. A sine wave has a low PMR (peak to mean ratio) compared to music (which might be 20dB or more - a factor of 100 in terms of power), so with the usual unregulated PSU, the instantaneous peaks might well reach higher levels than the "RMS" power output suggests. That's an advantage in reality.
I put "RMS" in quotes, as "RMS power" is a meaningless marketing term that came about because "continuous average sine wave power" is a bit of a mouthful
Instantaneous or short-term peak power is a figure of interest, and there are ways to measure it (IHF, etc). By comparing that with the continuous power, that gives an indication of the "stiffness" of the power supply. It can make a subjective difference to the sound quality, so it's something good designers pay attention to.
In this case, I'm not 100% clear of what Andy is seeing, but I'd start by checking out the PSU capacitors, the fuses and the bridge (always start with the power supply!). There's no output capacitors, but others might cause problems (e.g. the bootstrap caps - C709/710).
Silicon transistors are usually pretty good when not abused, and failures tend to be CE shorts, or a loss of Hfe. Leakage can happen as well. Some plastic packages can suffer from moisture ingress...
Those sub-£20 eBay kits can be pretty good for basic testing (and they do just about every other type of component as well). I rarely feel the need for more exhaustive testers because nothing tests a transistor like the circuit it's working in, so usually, you can tell from the surrounding conditions if the transistors is doing the right thing or not. Not always, but usually. Occasionally I might build a basic "jig" for testing more esoteric things, but one of the aims of good circuit design is to get away from depending on certain parameters of the transistors - especially if it's going into production
