Hi Phil. I was looking in the other direction (everything up). Some days I can hear the hiss; other days not. (Must depend on the degree of my allergy congestion?)
I take your point about "fade to hiss".
But I wouldn't worry about the rest of the recording chain, as it seems the DP headphone circuit isn't as quiet as the rest of the DP circuitry.
For what it's worth my relative measurements (Test Gear 0 dB = .775v) of the monitor circuit after a factory reset show:
With DP-24 ON and Test Gear attached to DP-24 Left Monitor Out RCA jack:
- DP-24 All Faders/Monitor Knob Full Off: -90 dB (the stated S/N spec in the owner manual)
- DP-24 All Faders/Monitor Knob Full On: -90 dB
With DP-24 ON and Test Gear attached to DP-24 Headphone stereo 1/4" phone jack using a stereo-to-mono 1/4" patch plug:
- DP-24 All Faders/Monitor Knob Full Off: -82 dB
- DP-24 All Faders/Monitor Knob Full On: -82 dB
(Consider all the above "very likely", since mine isn't the most expensive test gear.)
My pseudo-mastering room's ambient noise floor is 27 dB (about the level of a very quiet whisper; approaching the noise floor in professional recording studios).
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For others who may read this and be less familiar with these things:
The 8 dB difference in the noise floor of the monitor out circuit v the monitor headphone circuit could be the source of the barely audible hiss when monitoring with really good headphones. TASCAM could have made the headphone circuitry as quiet as the rest of my DP-24's circuitry, but apparently chose not to do that.
However,
- The noise floor (input/record/playback signal-to-noise ratio of the throughput chain) for my DP-24 is -90 dB (per specs) - that's really good for almost all situations. My DP-24's circuitry will be 90 dB quieter (lower) than the music I process with it.
- As Phil said earlier in this thread, the very low hiss isn't going into the recording itself; it's only in the headphone circuit.
- When played over speakers, any noise in the recording lower than a quiet whisper (27 dB) will be masked (hidden) by the ambient room noise of my mastering room. This masking would be even greater in the typical home listening environment that will have higher ambient noise levels.
- When listened to using very good headphones, if there is noise in the recording below -82dB, it could become evident in a complete fade out at the end of a song when listened to using playback equipment with a noise floor lower than -82 dB.
IMO, though, even if the headphone circuit's very low level (-82 dB) hiss masks lower noise in one or more of the mic or instrument inputs that could end up in the recording itself, for all practical purposes, it's not worth worrying about. The dynamic range of virtually any music will mask the noise if it were making it onto the recording, and in most cases, a "fade-to-noise" wouldn't be noticeable until the very last millisecond of the song, if at all, at -82 dB or less.