The low resistor values were chosen for better noise performance, guaranteed. Unfortunately that also means the output of the LF356 will be loaded by 1.1 kΩ, which will lead to higher distortion, so there's a tradeoff there.
The reason for the multiple 20 dB gain stages is pretty simple. The LF356 has 5 MHz (typical) gain-bandwidth product, so an amp with 40 dB of gain (100x) would have 50 kHz bandwidth (typical). That's not enough for good audio performance, especially as the loop gain will approach unity near the end of the audio band with such a setup.
"Typical" in this context means the based on a measurement of a large number of devices, one can expect a random sample to have 5 MHz GBP. There's no mention of the statistical distribution in the data sheet and the GBP is not measured in production. If it was, there'd be numbers either for minimum, maximum, or both. So the designer plugged his engineering brain in and broke the gain into multiple stages. With 10x gain per stage each stage will have 500 kHz bandwidth (typical) and the amp will have ~26 dB of loop gain available at 20 kHz.
The four 20 dB gain stages are two for the left channel and two for the right. So each path will have 40 dB of gain, which is pretty reasonable for a phono amp.
Tom
The reason for the multiple 20 dB gain stages is pretty simple. The LF356 has 5 MHz (typical) gain-bandwidth product, so an amp with 40 dB of gain (100x) would have 50 kHz bandwidth (typical). That's not enough for good audio performance, especially as the loop gain will approach unity near the end of the audio band with such a setup.
"Typical" in this context means the based on a measurement of a large number of devices, one can expect a random sample to have 5 MHz GBP. There's no mention of the statistical distribution in the data sheet and the GBP is not measured in production. If it was, there'd be numbers either for minimum, maximum, or both. So the designer plugged his engineering brain in and broke the gain into multiple stages. With 10x gain per stage each stage will have 500 kHz bandwidth (typical) and the amp will have ~26 dB of loop gain available at 20 kHz.
The four 20 dB gain stages are two for the left channel and two for the right. So each path will have 40 dB of gain, which is pretty reasonable for a phono amp.
Tom
Oh. Four stages per channel. They can't all be 20 dB then. That would mean 80 dB total gain, which is way too much. 35-45 dB gain at 1 kHz for an MC input is pretty normal as far as I understand it.
Maybe they're 10 dB... If only we had a schematic...
Anyway. I'm willing to bet that they're all the same gain, that the blue capacitors are bypass caps, and that the ceramic discs are some form of compensation. Then maybe followed by a passive RIAA filter with a buffer at the output. That'd be my guess without seeing a schematic.
Tom
Maybe they're 10 dB... If only we had a schematic...
Anyway. I'm willing to bet that they're all the same gain, that the blue capacitors are bypass caps, and that the ceramic discs are some form of compensation. Then maybe followed by a passive RIAA filter with a buffer at the output. That'd be my guess without seeing a schematic.
Tom