Source is a Technics 270C and a 205CII Low output.
This has 2mV nominal output voltage, it should use the MC-Hi setting.
https://audio-database.com/TechnicsPanasonic/etc/epc-205c-iil.html
Amplifier a brand new Yamaha A-S501. I compared settings at 0dB volume control setting.
Is that the setting where you normally listen to music?
In MM - No hum, lots of hiss. No sound from Optical with Sonos connect powered up / powered off
In MC High - No hum, Lots of hiss (same as MM)
This does not sound right. Also, what has the Sonos to do with this? The Zen Phono has no optical output?
Swapped it out for my old 540p and the hiss is absent at FV,
Is "FV" the volume set to maximum? You do not normally listen at that volume setting?
Looking at the service manual, the Yamaha A-S501 has 29dB gain in the power Amplifier. That alone would suffice for an "integrated Amplifier" with 100W/8Ohm output.
It uses an electronic volume control, actually described as "7.1ch Sound Processor for High-Quality Audio with Built-in Micro-step Volume" from ROHM Semi - the BD34704KS2 which also integrates tone controls. This volume control integrates a number of gain circuits and can produce as much as 32dB gain. It seems the A-S501 makes 24dB Gain available.
There is an additional "Loudness" Amplifier circuit with a mix of gain and attenuation that is kinda complex to decypher, but it should net out to bout unity gain.
So total gain from the input to the speakers will be 29dB + 24dB = 53dB (or 446 times) and maximum output (before clipping) will be around 20V. Thus with the volume at maximum the full output will be reached with 20V/446 = 45mV.
The output from the Zen Phono in MC High (48dB Gain) will be 500mV for the EPC-205IIL at 5cm/S and for the "legal maximum" of 25cm/S (which is the vinyl equivalent of "0dBFS" in digital audio) it will be 2.5V or 35dB higher than the level needed to produce full output from the Amplifier.
In other words, the gain of the A-S501 with the volume maxed is so high, It needs to be turned down by 35dB to make sure a loud LP played with the Technics EPC-205IIL and the Zen Phono set to MC HO will not overload the Amplifier (and the system will play with absolute maximum undistorted Volume possible).
In other words, the Amplifier cannot be used sensibly with Vinyl setup and the volume set higher than 35dB below maximum. If the volume control is turned up higher the amplifier will be overloaded with signal and without signal, noise from the Vinyl setup will be boosted significantly above the maximum levels one may encounter when the amplifier is set to absolute maximum usable volume.
It is not reasonable to expect to hear no noise when turning up the gain by 35dB above the usable maximum.
Why Yamaha have elected to implemented such extremely high gain, I guess they feel that IC they use has it, so why not make it available. Where this setting would be on the volume dial is I do not know. THe best option would be to send a -20dBFS 200Hz digital signal into the Amplifier (digital input) and measure the AC voltage on the speaker terminals using a multimeter. The correct setting would be with around 1.6V on the Speaker Terminals. **
When the volume is set correctly (35dB below maximum or lower) I expect no noise to be audible and if the needle is dropped on the LP the volume should be extremely high.
Thor
** Edit - The Volume control is a 10K linear pot into an 8-Bit ADC. This would more or less match 1 ADC code with one 0.5dB step. This would mean that the volume control needs setting to 70 ADC codes below maximum or around 70% of maximum (note the range is 270 degrees travel, not 360...). If we map this position on a clockface the setting would be 2 O'Clock appx.
This "no higher than 2 O'Clock" suggestion is based on a number of reasonable assumptions, but by no means reliable or verified.