In order to get a feel for the signal levels suitable for the HD500, I've made a few measurements of the gains available from the different inputs to the various outputs. I've also measured the gain of all the FX. A small Excel spreadsheet containing the data can be found here (unfortunately, I can only upload a PDF version of the data, which I've done here). A brief summary is shown below:
All measurements were made at 1kHz with Master Volume = max, S/PDIF gain = 0dB, and Input2 = Variax (although this didn't make any difference because I had Inputs 1 & 2 panned hard Left/Right).
- Guitar input pad = -5.2dB compared to normal.
- 1/4" (line) output has 6.1dB more signal than 1/4" (amp).
- Aux & Guitar (normal) have the same gain structure (except Guitar has the tuner and programmable input impedance).
- Guitar (normal) input to 1/4"(amp) output has 4.9dB of loss with no FX, no Amp, and Mixer = 0dB.
- Compared to 1/4" (amp) output, the XLR output is 9.4dB less while the phone output is 15.4dB more.
- Compared to the Aux/Guitar(normal) input, Mic (min gain) has 6.2dB less gain while Mic (max gain) has 38dB more gain.
- Aux/Guitar(normal) input clips at 0dBFS (measured at S/PDIF) with 8.3Vpp
- Mic (min gain) input clips at -1.1dBFS (measured at S/PDIF) with 14Vpp
- Mic (max gain) input clips at -1.1dBFS (measured at S/PDIF) with 91mVpp
- CD/MP3 input clips at various levels near 20Vpp, depending on which output is used (XLR, 1/4", or Phones)
By far most of the FX have 0dB of gain, but there are several exceptions (e.g., Graphic EQ = 2.9dB, Tape Echo = -5dB, and others). FX were measured with Mix=0% (to eliminate comb filtering effects) and Gain=0dB (usually). The PDF files contain the whole list.
Using some of this data you can see the clip level is essentially set by digital clipping, although the mic input stage only gets you to within 1.1dB of fullscale (close enough to call it fullscale in my opinion). The output stages will not get close to their clip levels (unless you use the CD/MP3 input which bypasses the digital engine). That is, it's sufficient to look at the S/PDIF signal to keep your signal below fullscale. You don't have to worry about clipping the input stages. You also don't have to worry about clipping the analog output stages.
I hope you find this useful in optimizing the setup of your equipment.