11-07-2015, 09:26 AM
If that input cap really was 5n6, then the LF -3dB point would be 130Hz at best. In reality, the input impedance of the IC will increase that. They say >750k, so taking that as a lowest worst-case value, we're at 170Hz. Perhaps 56n?
If you preferred to be more conventional, I see no reason why you shouldn't return the LS to the -ve rail once the polarity of C6 has been reversed.
I'm surprised at the lack of PSU decoupling. In fairness, the datasheet doesn't show it in any of the suggested applications, but there is one on the diagram on page 2. Often, they are omitted for clarity on the application examples. While it certainly seems to be behaving, I'd consider adding something. Perhaps 100n on the rear of the PCB, directly under the IC might be a good plan. Of course, there might have been some bulk decoupling on the PCB it plugs in to.
Having said that, there isn't much on the positive rail, assuming the internal schematic is accurate. The VAS (T9) is returned to ground. Apart from the upper output pair, there are just current sources. Perhaps it is quite tolerant in itself, but returning the LS to +ve with no decoupling seems odd - all the signals are returned to ground, so the LS should be as well. Any variation on the +ve caused by a lack of decoupling will superimpose themselves on to the output, causing measurable distortion.
Regarding heat sinking, a sine wave is a tough test for an amplifier. When playing music, unless it's modern stuff that's been heavily compressed, then there will be a Peak to Mean Ratio of 20dB at least. Of course, in power terms, 20dB is a factor of 100, so if the peaks are on the cusp of clipping, the average level will be 40mW. Try it - I suspect that your heat sink will be absolutely fine for audio use.
This could be the core of a really useful bench amp or signal tracer or something similar. Just sorry that I can't add any more info about the origins, but I could imagine it being used in a TV or mono record player or something similar - 4W peak is really quite generous for this when used with a typically efficient loudspeaker (a Hacker portable set goes very loud with its ~1W amp). Zin is probably high enough to work with a ceramic pickup and a 1M volume pot. Of course, there might have been a really quite elaborate pre-amp on the main PCB.
Any date codes that might give a clue?
If you preferred to be more conventional, I see no reason why you shouldn't return the LS to the -ve rail once the polarity of C6 has been reversed.
I'm surprised at the lack of PSU decoupling. In fairness, the datasheet doesn't show it in any of the suggested applications, but there is one on the diagram on page 2. Often, they are omitted for clarity on the application examples. While it certainly seems to be behaving, I'd consider adding something. Perhaps 100n on the rear of the PCB, directly under the IC might be a good plan. Of course, there might have been some bulk decoupling on the PCB it plugs in to.
Having said that, there isn't much on the positive rail, assuming the internal schematic is accurate. The VAS (T9) is returned to ground. Apart from the upper output pair, there are just current sources. Perhaps it is quite tolerant in itself, but returning the LS to +ve with no decoupling seems odd - all the signals are returned to ground, so the LS should be as well. Any variation on the +ve caused by a lack of decoupling will superimpose themselves on to the output, causing measurable distortion.
Regarding heat sinking, a sine wave is a tough test for an amplifier. When playing music, unless it's modern stuff that's been heavily compressed, then there will be a Peak to Mean Ratio of 20dB at least. Of course, in power terms, 20dB is a factor of 100, so if the peaks are on the cusp of clipping, the average level will be 40mW. Try it - I suspect that your heat sink will be absolutely fine for audio use.
This could be the core of a really useful bench amp or signal tracer or something similar. Just sorry that I can't add any more info about the origins, but I could imagine it being used in a TV or mono record player or something similar - 4W peak is really quite generous for this when used with a typically efficient loudspeaker (a Hacker portable set goes very loud with its ~1W amp). Zin is probably high enough to work with a ceramic pickup and a 1M volume pot. Of course, there might have been a really quite elaborate pre-amp on the main PCB.
Any date codes that might give a clue?
