Helios

Helios is a stereo audio effect that adds 300B SET-style harmonic warmth to a signal. It's an asymmetric soft-saturating waveshaper — a memoryless nonlinearity — voiced specifically to produce the kind of even-dominant harmonic ladder a 300B vacuum tube in single-ended operation generates.

It's deliberately not a tube simulation. Helios doesn't model anode load lines, output-transformer saturation, or the dynamic compression of a real output stage. What it does is take the audible character — the harmonic ladder, the soft saturation, the gentle asymmetry — and deliver it through a curve that's clean, fast, and predictable.

The product surface is intentionally narrow: Drive, Mix, Auto Gain, Even Only and Bypass. The voicing is fixed; the curve shape is locked; the asymmetry is baked in. "Do one thing well" — in this case, the 300B.

New in v1.1: the Even Only switch removes the odd half of the curve, so you can hear the even-order ladder on its own — see §05. Helios now also runs on Linux.

Helios is meant to be heard within a few seconds of inserting it. The fastest way to hear what it does:

• Insert Helios on a stereo bus or track.
• Set Drive to around 3–6 dB. You'll hear a subtle thickening — the 2nd harmonic doing most of the work.
• Set Mix to 0 dB (full wet). Mix lower (more negative) to blend in the dry signal; it starts at −12 dB.
• Leave Auto Gain at its default — it compensates for the level changes Drive introduces, so the controls do what you expect.
• Push Drive higher to feel more saturation, lower to back it off. Watch the harmonics display to see the ladder build.

The default state is a transparent character pass-through with subtle warmth. The signature only really blooms with Drive turned up.

Above the knobs is an 8-bar display showing the harmonic content of the output signal. The bars are labeled 1f through 8f — multiples of the fundamental — with the amplitude on the vertical axis in dBFS.

The bars are color-coded:

• Even harmonics (2f, 4f, 6f, 8f) are drawn in amber — these are the harmonics the design centers on.
• Odd harmonics (3f, 5f, 7f) are drawn in gray — present but not dominant by design.
• 1f (the fundamental) is the tallest bar in any normal program material — always gray.

An important note: the display is probe-driven. Helios feeds a known test signal through the curve to measure the harmonic response, then renders the result. This is deliberate — it shows you the curve's character, not a live FFT of the current music. Practical implications:

• The display is decoupled from your actual input signal level. The bars show what the curve produces at a reference level, not what's currently in your audio stream.
• At high Drive settings, the display may show more activity than the music itself contains — the probe excites the curve harder than typical program material.
• Use the display to read the ladder shape — relative levels of 2f, 4f, 6f, 8f. Don't read absolute output spectrum from it.

With Even Only on, the probe runs through the even-only curve, so the gray odd bars (3f, 5f, 7f) fall to the bottom of the scale.

For a true live output spectrum, run Bitscope after Helios for bit-depth and true-peak information, or use any standard spectrum analyzer plugin downstream.

Any curve can be split into an even part and an odd part about the origin, f = E + O. The even part produces the even harmonics (2f, 4f, 6f …); the odd part produces the odd ones (3f, 5f, 7f …) and also the slight gain compression of the fundamental. Helios's 300B voicing carries both — a dominant even ladder with a small odd companion, as a real single-ended stage does.

Even Only keeps the even part untouched and replaces the odd part with its straight-line slope through the origin. What remains is the even ladder plus a clean, uncompressed fundamental. It is an A/B tool: hear the 300B's even-order warmth with and without its odd companion, one switch, everything else identical.

What it does, measured

Single 1 kHz tone at −12 dBFS peak, 48 kHz, Mix at 0 dB, Auto Gain 0%, levels relative to the fundamental:

Three things to know before you judge it

• The fundamental is a little louder. The odd terms also compress the fundamental; without them it is not compressed. At +20 dB Drive Even Only is about 2.5 dB louder at the fundamental (under 0.15 dB at +12 dB Drive and below). With Auto Gain at its default this is compensated automatically. With Auto Gain at 0%, level-match by hand before you compare.
• "Even only" describes Helios, not your music. Helios no longer adds odd-order products of its own, but a real instrument's note already has harmonics at f and 2f, and the even curve's sum product of those lands at 3f. On music you will still see energy at odd multiples of a note. To check the claim, use a single test tone or a two-tone IM test, not program material.
• The output safety clip is odd. Helios's final soft-clip (see Signal Flow) begins about −1.7 dBFS and adds 3f and 5f when it works. Keep output peaks below that when you measure Even Only.

For measurements, use coherent sampling (FFT length equal to the sample rate, integer-Hz tones) to see the ≈ −230 dB floor; a windowed FFT — the harmonics display, or REW at default settings — shows window leakage at that depth instead. That is expected.

End to end, what happens to a sample as it passes through Helios:

• Input — stereo, 32- or 64-bit float (host-negotiated).
• Drive gain — linear input gain set by the Drive control.
• Oversample up — minimum-phase polyphase FIR. The curve always runs at 176.4 kHz (44.1 kHz family) or 192 kHz (48 kHz family): 4× at 44.1 / 48 kHz, 2× at 88.2 / 96 kHz, and no oversampling at 176.4 / 192 kHz. That gives the curve room to work without aliasing.
• Asymmetric soft-saturating waveshaper — the heart of Helios. A memoryless nonlinearity, voiced to produce the 300B-flavored even-dominant ladder. With Even Only on, its odd part is replaced by a straight line (§05).
• DC blocker — a gentle ~5 Hz high-pass that removes the static DC offset the asymmetric curve introduces.
• Oversample down — minimum-phase polyphase FIR, back to host sample rate.
• Mix — blend with the delay-matched dry signal.
• Auto-gain compensation — matches the K-weighted loudness of the result to the dry input, scaled by the Auto Gain amount.
• Safety soft-clip — transparent below about −1.7 dBFS, a smooth knee above it, and a hard ceiling at −0.3 dBFS, so heavy Drive can never push the output over full scale.
• Output — stereo, same precision as input.

All internal maths is 64-bit double precision. Reported latency: 4 samples at 44.1 and 48 kHz (under 0.1 ms), 2 at 88.2 kHz, 1 at 96 kHz and 0 at 176.4 and 192 kHz. Plugin Delay Compensation handles this transparently; Helios is safe to drop into monitoring and live tracking chains.

One design choice worth flagging: the oversampler is minimum-phase, not linear-phase. A linear-phase oversampler would introduce pre-ringing that smears transient attack — an octave-up ghost preceding each kick or snare hit. Minimum-phase oversampling preserves the attack at the cost of small phase shifts at high frequencies, inaudible on real material. The transient handling matters more for warmth-on-music than phase linearity does.

Master bus / glue

Drive 1–3 dB, Mix 0 dB. Subtle thickening across the whole mix — the 2nd harmonic adds weight, the soft saturation acts as gentle compression. Try it on already-mixed material; if the harmonics display shows the 2nd in roughly the −40 to −30 dB range below the fundamental, you're sitting in 300B territory.

Bass tracks

Drive 4–8 dB, Mix 0 dB. Bass benefits enormously from 2nd-harmonic generation — the octave above the fundamental adds perceived weight on systems that don't reproduce the fundamental cleanly. A small-speaker bass trick that goes back decades.

Vocals

Sparingly. Drive 2–4 dB, Mix −3 to −6 dB. Helios adds body and analog warmth to a vocal without sounding processed. Mix lower if it starts to sound thickened.

Hi-fi playback chain

Drop Helios on the master output of a playback chain (in JRiver, MetaHost, or similar) for a single-ended-tube listening character without owning a tube amp. Drive 2–3 dB, Mix 0 dB, Auto Gain at its default. The effect is meant to be subtle here — analog character on digital playback.

Even vs. even-plus-odd A/B

Any of the settings above, then toggle Even Only with Auto Gain at 100%. The only difference between the two positions is the odd companion and the gentle compression of the fundamental that comes with it. Neither is "correct" — pick by ear per source.

Drum bus

Drive 3–6 dB, Mix −3 to 0 dB. Helios adds glue and presence to drum buses similarly to how a hardware tape machine or single-ended tube preamp would. The transient handling of minimum-phase oversampling matters here — kicks and snares stay punchy.

Helios is designed to live alongside the rest of the Pataki Audio catalog. A typical signal chain in an active-speaker setup might look like:

• Janus — linear-phase crossover splitting the bus by driver band.
• Voicing — Baxandall-style tonal balancing per band or on the sum.
• Helios — single-ended tube warmth on the master sum.
• MetaHost — routes all of the above across the multichannel speaker bus.
• Bitscope — diagnostic readout on the output of any stage you want to verify.

Helios pairs particularly well with future Chronos releases for "vintage gear" coloration: Helios handles the nonlinearity (memoryless saturation), Chronos handles the linear coloration (load an IR of any analog hardware you want to color the chain with). Together they cover the two halves of making digital sound like vintage analog.

Why does the harmonics display show more activity than my output spectrum has?

The display is probe-driven — it analyzes a known test signal through the curve, not your live audio. The probe excites the curve harder than typical music does, so the display can show more "ladder" than the music itself contains. Read it for shape (relative levels of the even harmonics), not for absolute output spectrum. For live spectrum, use Bitscope or any analyzer downstream.

I don't hear anything at Drive 0 dB.

Correct. Drive at 0 dB sits near the bottom of the saturation curve where it approximates unity gain — Helios is intentionally transparent there. Start at 3–6 dB to hear character. The plugin is designed to be heard when you turn it up, not to color everything by default.

Helios changes loudness when I turn Drive up. Isn't Auto Gain supposed to fix that?

Auto Gain compensates for the average level shift the curve introduces. Residual loudness change at high Drive is partly from saturation also compressing peaks — that's part of the character, not a bug. If the loudness change is bothersome, either keep Drive moderate or lower Mix to taste.

Why isn't there an asymmetry knob? Every other tube plugin has bias controls.

Locked by design. Helios's identity is the 300B voicing, with a specific asymmetry baked in. Exposing bias as a parameter would turn Helios into "a generic asymmetric waveshaper that happens to default to 300B" rather than "a 300B warmth plugin." Different product. If you want generic, every DAW has Saturn, Decapitator, etc. Helios does one thing on purpose.

Will there be more voicings — EL84, EL34, console-style?

Possibly in a future release if there's demand. Helios ships with a single 300B voicing on purpose — focused product, do one thing well first. Even Only is not a second voicing: it is the same curve with its odd part removed.

Why is there some odd-harmonic content in the display? I read this was an even-only design.

The default voicing deliberately keeps a small odd companion. An earlier internal design targeted strictly even-only via analytic-signal squaring through a Hilbert transform. That delivered spectrally clean output but the wrong sound — real 300B SETs have a small amount of odd-harmonic content (the 3rd at roughly −45 to −50 dB below the 2nd in the canonical 300B datasheet curves), and removing it removed the recognizable single-ended character. The honest framing: Helios is an asymmetric SET-style waveshaper, not a mathematically pure even-harmonic generator. If you want the even ladder alone, switch on Even Only (§05) — it removes the odd part from the same curve, so you can decide by ear.

Even Only is on, but my analyzer still shows energy at 3×, 5× the note.

On music that is expected: a note's own 1st and 2nd harmonics combine in the even curve into a product at 3×. Helios adds no odd-order products of its own; check with a single sine. Also keep output peaks below about −1.7 dBFS, where the safety clip starts adding 3f and 5f, and use a coherent FFT to see the true floor.

Even Only sounds louder.

At high Drive it is — the odd terms are what gently compress the fundamental, so without them the fundamental is up to about 2.5 dB louder at +20 dB Drive. Leave Auto Gain at 100% for a level-matched A/B.

My host shows Helios's Bypass and the host's bypass behaving as one.

Correct since v1.1.1: Helios's Bypass is the host bypass parameter, so the two can no longer disagree. Earlier versions exposed a second, host-owned "Bypass"; automation drawn on that one has no target any more — redraw it on Bypass.

Is the latency low enough for live monitoring or tracking?

Yes. 4 samples at 48 kHz is under 0.1 ms — well below any human perception threshold for monitoring — and at 176.4 / 192 kHz Helios reports no latency at all. Helios is safe to use in live and tracking chains.

How do I install it on Linux?

Unzip and copy the Helios.vst3 folder to ~/.vst3/ (or /usr/lib/vst3/ for all users), then rescan plugins in your host. Use the x64 download on a PC (glibc 2.35 or newer — Ubuntu 22.04+, Debian 12+, Fedora 36+ or a current Arch) and the ARM64 download on 64-bit ARM Linux such as a Raspberry Pi 4 or 5 running 64-bit Raspberry Pi OS (glibc 2.28 or newer). Neither needs a separate C++ runtime. On Windows, copy Helios.vst3 to C:\Program Files\Common Files\VST3\.

Does Helios introduce any DC offset?

The asymmetric curve produces a static DC offset by design — that asymmetry is what generates the even-harmonic content. A DC blocker on the wet path removes it before the signal hits Mix. You should not see DC at the output under any setting.

Helios began life as an even-harmonic generator using analytic-signal squaring — Hilbert-transform the input, square the analytic signal, take the real part. The math is elegant: theoretically clean even harmonics with no difference-frequency intermodulation, no DC, no odd harmonics at all. An earlier internal design (call it v2.5) was built and measured on that basis. It worked, in the sense that the spectrum was clean.

It didn't sound right. Through several listening sessions on real music, it became clear that real tube warmth — the sound you actually want when you reach for "tube warmth" — comes from the shape of the input-output transfer curve, not from the spectral purity of the harmonic content. The cleanliness was the wrong target. Real 300B SETs have measurable odd content, output-transformer interaction, dynamic compression — and removing all of that, however cleanly, removed the recognizable character.

The shipping v1.0 abandoned analytic-signal squaring entirely and rebuilt around an asymmetric soft-saturating waveshaper. The voicing was iterated by ear against canonical 300B references — the WE 300B datasheet, Stereophile measurement archives, diyAudio simulations — until the harmonic ladder lined up roughly with the 300B reference (2nd dominant, 4th at −35 to −40 dB below 2nd, 6th below −65 dB, with the small odd companion every real single-ended design carries) and the result sounded like a 300B on real material.

The acceptance criterion that mattered: sounds like the 300B, not just has the ratios. The numbers bound the voicing; the ear decided when it was right.

v1.1 came back to the even-only question from the other side. Instead of a separate even-only generator, Even Only splits the shipping curve into its even and odd parts and drops only the odd part's nonlinear remainder. The even ladder is therefore bit-identical to the 300B voicing — the switch changes one variable and nothing else — which makes it an honest A/B rather than a second sound. With the switch off, Helios runs exactly the v1.0 path, verified bit-identical by test.