Head-amp control
The head amp is the analogue input stage: what the microphone actually sees, before anything is converted or processed. On a rig with a REAC stagebox those controls are on the box, at the far end of a cable, and the mixer drives them over the same link that carries the audio.
Where the controls are
On the Processing chip, at the top of the channel strip, as a head-amp card:
The head-amp block: gain, trim, pan, phantom, pad and polarity, each shown only where the bound device offers it.
| Control | What it does |
|---|---|
| Gain / sensitivity | The analogue input sensitivity. On a head-amp channel this is the box's sensitivity control — there is deliberately no second, duplicate control. |
| Phantom +48 V | Condenser microphone power. |
| Pad −20 dB | A 20 dB attenuator ahead of the preamp, applied by the box. Engaging it does not change the sensitivity value; it shifts the whole range down by 20 dB. |
| Polarity | Inverts the signal's polarity. A native sign flip in the mixer's own processing. |
| Trim | A fine digital adjustment after conversion, composing with the gain. Not part of the head amp proper. |
The same head-amp identity — gain, phantom, polarity — also lives on the Source layer chip, where it belongs to the source rather than to whichever channel is currently processing it. Rename or re-gain a source there and it follows the source when you patch it elsewhere.
Capability gating
The head-amp controls a channel offers depend on what is actually patched to it. Phantom, pad and sensitivity are real only on a source whose device declares a head amp — a REAC stagebox input, or another adapter that advertises one. On a channel fed by an application or a plain sound card they are absent, not decorative.
Re-patch a channel and the controls follow within a moment: the server re-advertises the channel's capabilities to every connected screen, so a tablet that was already open shows the change without a reload.
On a REAC stagebox
Phantom, pad and sensitivity are sent to the box and applied there. Two consequences worth knowing on a live stage:
- A change is a physical change at the box. Phantom power on a channel with a dynamic microphone plugged in, or a ribbon, is exactly as bad an idea here as on any other desk. Mute the channel before you switch phantom.
- The mixer holds the state. The recorded head-amp settings are re-applied to the box, so a box that was power-cycled or a transport that was restarted comes back with the settings the desk believes in rather than whatever the box booted with.
Sensitivity is an analogue step on the box, not a digital gain — which is why it appears in dBu and why the range shifts when the pad is engaged.
Trusting what you see
A soft meter is not proof that phantom power is present at the microphone. If a condenser is silent, confirm at the box — an indicator on the box itself, or a microphone you know works — before concluding the mixer is at fault. The control reports what was sent.
The spare microphone
A microphone dies mid-song. There is a spare hanging beside it, already plugged into a different port. SPARE on the channel's tile moves the channel onto it in one press.
Preparing one
The swap only exists on channels you prepared, and preparing one is a soundcheck job:
- Plug the spare microphone into its own port and patch nothing to it.
- Set that port's own preamp — gain, pad and phantom — for the spare microphone, not for the primary. A ribbon and a condenser want opposite things, and the desk will not guess.
- Declare it as the channel's spare with the show.
A channel with a spare declared shows a SPARE button on its tile in the fader bay. Channels without one show no button, so there is nothing to press by mistake.
Swapping
Press SPARE. The channel re-points to the spare port immediately. The button lights and stays lit while the channel is standing on its spare.
Press it again to go back. The desk remembered the primary, so the reverse is the same one gesture — it is a latch, not a one-way door. A dead microphone is usually a temporarily dead microphone.
Reloading, or another surface opening mid-swap, shows the same thing: the swap is desk state, not a fact about the screen you pressed it on.
Two things that deliberately do not follow
The preamp does not follow. Gain, pad and phantom belong to the port, and the spare's port already carries what its own microphone needs. Copying the primary's settings across would be the desk guessing about hardware it cannot see, and +48 V guessed onto a ribbon is damage rather than a wrong level.
What you see after the swap is the spare port's head-amp state, which is exactly the point: an unprepared spare is visible at a glance instead of silently wrong.
The alignment does not follow either. The channel's alignment delay described where the dead microphone was standing. The spare hangs somewhere else, so the number is stale the moment the swap lands, and the desk clears it rather than keeping a lie. Re-align the spare when there is a quiet moment — see alignment.
Direct paths
A source can take a direct path to an output without consuming a channel — the post-preamp route a Roland desk offers. Direct paths are opened and closed on the Source layer chip, alongside the head-amp controls.
If the controls are not there
- The channel has nothing patched, or its source declares no head amp. Check the patchbay.
- On a REAC rig, the box may not have established. See the stagebox is not establishing.