A vocal is the one thing in a mix that everybody follows, so its level has to stay steady without losing life. A vari-mu compressor is a classic way to do that: the tube is the gain element, so the vocal gets leveled and thickened at the same time. This guide shows four ways to start with the Rockruepel COMP.TWO, our all-tube vari-mu compressor, using the vocal examples from the plugin page.
Quick answer: For gentle leveling, use light compression and let the tube add shine. For density, drive the input and blend the compressed signal in parallel with a very gentle release. For tamed peaks with some edge, use fast attack and fast release with the input driven. For a finishing touch on a vocal that is already compressed, push the peaks back only a few dB. Use the sidechain high-pass so low-end noise does not trigger the compressor, match the level so the compressed version is not simply louder, and trust your ears over any number.
Why use a vari-mu compressor on vocals?
In a vari-mu design there is no separate gain-reduction element: the tube itself does the amplifying and its gain changes with the signal. That is what gives the sound its thickness and its soft, smooth response. We explain the circuit in What Is a Vari-Mu Compressor? and compare it with VCA, FET and optical designs in Compressor Types Explained on Puremix.
On vocals, engineers use vari-mu for the same reason John Paterno gives in his variable-mu compression excerpt: it sounds thicker and still reacts relatively quickly. His test is a good one to borrow. While you compare settings, listen for whether the compressor is "pinching" the vocal or making it bigger.
COMP.TWO is a faithful emulation of the hardware compressor built by Oliver Gregor of Rockruepel, an all-tube vari-mu design with no transistor stages in the circuit. It also adds modern controls, such as a sidechain high-pass filter, a dry/wet control, Mid/Side processing and a visualizer.
Set it up before you touch the knobs
- Match the level. Turn on In/Out Link, which compensates the input with the output so the volume feels constant. A compressed vocal that is also louder will always sound better at first, so compare at the same level.
- Use the sidechain high-pass. The Sidechain High Pass filter keeps low-frequency noise from triggering the compression. Rich Keller does the same on his vocal compressor with a 100 Hz high-pass, so pops and foot stomps do not trigger it. See Rich Keller's vocal Distressor settings.
- Know the tube stage. Amp Only bypasses the compression circuit so you hear only the tubes and transformers. Transformers lets you choose between the simulation of the original transformers and a cleaner signal. This is useful for hearing how much of the sound is color and how much is compression.
- Use Mix for parallel. The Mix knob blends the dry and wet parts of the signal, so you can compress hard and bring it in under the original vocal.
- Consider oversampling. It can improve signal-to-noise and avoid phase distortion and aliasing.
- Use Bypass honestly. Press it often, at matched levels, to check that you are improving the vocal and not only changing it.
The manual does not publish numeric ranges for the controls, so the starting points below are described by direction (fast or slow, subtle or driven), as in the plugin's own examples. Set the exact amounts by ear and by watching the gain reduction in the visualizer.
Four starting points for vocals
1. Gentle leveling with shine
The aim is a vocal that stays steady while remaining musical. In the plugin's Female Voc. 1 example, COMP.TWO adds shine and controls the dynamics smoothly, and you can hear it hold the vocal steady where the singer starts to push on the words "at all". Start with light compression, raise the Input a little for the tube color, and stop as soon as the vocal sounds even.
2. Parallel density
When a vocal needs to sit in a dense mix without being squashed, drive the Input, compress, and then blend the result with the dry vocal using the Mix knob. The Female Voc. 2 example uses this approach to add subtle density and shine so the vocal keeps its place in the mix. The Man Voc. 1 example uses parallel compression with a very gentle release to smooth out the sound and make it feel less angular and peaky than the raw recording. This idea has a long history on vocals: see Michael Brauer's parallel vocal compressors.
3. Tame peaks and add edge
For a vocal with bursts of energy on certain words, use fast attack and fast release. In the Man Voc. 2 example, those settings tame the peaks, and driving the input adds an edge that suits the style. This is the most obvious setting of the four, so check it with the whole mix playing.
4. Finishing glue on a vocal that is already compressed
If the vocal was already compressed while tracking, use COMP.TWO lightly as a finishing step. In the Rap Voc. example, the saturation plus pushing the peaks back a few dB subtly thickens the vocal and makes the performance feel steadier and more engaging.
Check the vocal in the mix
Compression on a vocal can only be judged in context. Solo is useful for hearing the tube color, but the final decision belongs to the full mix: does the vocal stay in front without being pushed, and do the quiet words stay audible? If you want to train your ear for this, read how to hear compression on vocals. In the plugin's presets, the rockgluepel preset in the "Like You Do" example brings the vocal forward and adds density and tube warmth to the whole mix, which shows how the same plugin works on a bus as well. For more on that side, see Why Your Mix Doesn't Glue.
One compressor or several?
COMP.TWO includes a Console Remote, which lets you control every instance of the plugin from a central interface. That makes it practical to use more than one: mixing engineer Amir Mahrous says he always uses the compressor on vocal mixing and that this lets him run the plugin on several stems and tracks. Compression in stages, each doing a little, is also the logic behind Vocal Compression Chain: Why 3 Compressors Beat 1.
Troubleshooting
| Problem | Try this |
|---|---|
| The vocal sounds pinched or smaller | Reduce the amount of compression, slow the attack or blend it in parallel with the Mix knob |
| The vocal pumps or breathes unnaturally | Slow the release and check the gain reduction in the visualizer |
| Low-frequency noise moves the compressor | Raise the Sidechain High Pass |
| It sounds better but you are not sure why | Turn on In/Out Link and compare again at the same level |
| The sound is too colored | Lower the Input, or switch Transformers to the clean option |
Try it on your own vocal
You can start a 30-day free trial of COMP.TWO (you will need an iLok account) and read the user manual for every control. Fab Dupont, who leads Process.Audio, says about it on the plugin page: "Fast and easy to dial in, and a perfect match to my hardware unit."
FAQ
Is a vari-mu compressor good for vocals?
Yes. Vari-mu compressors are known for thickness and a smooth, soft response, which works well for leveling and adding density to a vocal. They are also used on the mix bus.
What settings should I use on COMP.TWO for vocals?
It depends on the vocal. Use light compression for leveling, a driven input with the Mix knob for parallel density, and fast attack and release to tame peaks. Judge by ear and with the gain reduction visualizer, not by a fixed number.
Should I use COMP.TWO in parallel on a vocal?
It is a good option when you want density and shine without losing the natural dynamics. Use the Mix knob and compare at matched levels.
What does the sidechain high-pass do?
It filters the signal that triggers the compressor, so low-frequency noise such as pops or stomps does not cause gain reduction.
Can I use more than one instance?
Yes. The Console Remote lets you see and control all the instances from a central interface.
Is there a free trial?
Yes, a 30-day free trial. You need an iLok account.
Want to hear how engineers use compression on vocals in real sessions? Our sister company Puremix has the details: watch the free Puremix videos or read the best tube compressor plugins of 2026. To learn compression in a structured way, take Foundations of Compression on Campus.
Disclosure: Puremix, Campus and Process.Audio are sister companies. COMP.TWO is a Process.Audio product. Details were checked against the plugin page and manual in November 2026.