The weight of a tube, without the hum.
Not a sampled snapshot. A compressor modelled from the ground up, with the needle, the glow and the grip of the real thing — and none of its noise floor.
Audio Unit · VST3 · CLAP · Standalone — macOS.
Hear it first
Same take, same level. The only difference is the compressor.
Audio demos coming shortly.
Every part of it was built, not photographed.
What you see is a lit 3D scene. Brushed steel, olive paint that has been through a few racks, a warm bulb behind the meter glass and two bottles whose filaments really do brighten with the signal. Nothing here is a photograph and nothing is a bitmap stretched to fit: every knob, every screw and every chipped edge is geometry with a material on it, lit once and rendered.
It is exported at your screen’s own resolution. The front is rendered on a 2500 × 1248 grid, and then every control is rendered again at every position it can hold — 601 images in all. At 100% one pixel of the render lands on one pixel of your display with nothing interpolated; from 50% to 125% the artwork is resampled at high quality rather than redrawn soft.
The meter is not a loop. The needle is driven by the gain reduction the engine is actually producing, with its own ballistics — 35 ms to rise, 180 ms to fall — and when the transport stops it walks back to rest on its own instead of freezing wherever it happened to be. The tubes follow the input level and fade with an 800 ms tail.
- 2500 × 1248 grid: at 100%, one render pixel per screen pixel
- 601 sprites — every control at every position it can take
- The needle follows the real gain reduction, and returns to rest by itself
- Auto Gain compensates Input inversely, so you judge tone and not loudness
- Dry/Wet on board for parallel work without a second bus
Inspired by the RCA BA-6A
The BA-6A was RCA’s variable-mu limiting amplifier for broadcast, built around a push-pull pair of 6SK7 tubes. Volvo takes its layout and its behaviour from that machine, including the three controls below, which do on screen what they do on the chassis. It is not a capture of any particular unit and does not claim to be one: after seventy years no two of those amplifiers measure alike.
Bal-Limit
Off · Single · Dual
Chooses how many recovery networks sit in the control line. Single lets go the same way every time. Dual adds the second one, which charges as the passage goes on: a short peak releases like Single, a sustained one takes seconds — lengthened to two seconds on sustained peaks
, as RCA put it. In Off nothing is compressed and the signal still runs through the tubes and the iron.
Pre-Curve
The 6A curve, or flat
The response the detector and the colour sit on. On 6A it is the machine’s own: +2.39 dB at 58.6 Hz, −6.00 dB at 20 Hz and a touch of air at 13.3 kHz. On Flat it is flat within ±0.02 dB from 20 Hz to 16 kHz, with a second-order high-pass at 5 Hz keeping the subsonics out.
Filter
Sidechain, 50 to 250 Hz
A high-pass on what the compressor listens to, never on what you hear: the audio path measures identical with it off and wide open. At 250 Hz a 30 Hz kick stops driving the gain altogether, while a voice at 1 kHz still compresses 7.18 of its 7.47 dB.
A limiting amplifier from 1950
Where the selector, the CHECK band on the meter and the two time figures Volvo reproduces all come from.
RCA introduced the BA-6A as a limiting amplifier for broadcast. Its job was to catch
peaks before the transmitter without the programme audibly pumping, and the catalogue
puts it plainly: prevents distortion and adjacent channel interference
.
Underneath is a nine-tube, three-stage variable-mu machine built to run unattended in
a broadcast room, on tube types any station already had on the shelf.
Nobody signed it. RCA credited the design to its engineers as a company, the way it credited everything in the BA series, and no individual name survives in the catalogue or in the schematic. What does survive is the drawing, published in Audio Engineering in November 1950, and everything that gives the unit its character is already in it: the push-pull pair of 6SK7s that makes the gain, the 6H6 that rectifies the signal to feed them their control voltage, and the switch that picks how many networks that voltage discharges through when the music drops.
RCA published exactly two time figures, and they are the two Volvo reproduces:
0.0006 seconds of attack and a recovery that in the double position
is lengthened to two seconds on sustained peaks
.
The control is called BALANCE-LIMIT because it does two jobs. Its three decks hang off the same shaft: one picks check resistors, another the recovery networks, and the third injects mains hum for the alignment. Move the pointer over the drawing to read it up close.
| Gain reduction | up to 40 dB |
|---|---|
| Frequency response | 20 Hz – 20 kHz |
| Noise | −80 dB ±5 |
| Gain | 30 dB |
| Input / output impedance | 600 Ω |
| Attack | 0.0006 s |
| Recovery in Double | 2 s on sustained peaks |
| Tube complement | 6SK7 ×2, 6J7 ×2, 6H6, OD3, 6V6-GT ×2, 5R4 |
The two time figures are RCA's; the rest comes from the spec sheet of the modern clone built today around the same circuit. There are no published measurements of a real BA-6A, and there is a reason: the interstage transformers degrade over the decades, so no two units measure alike. That is why Volvo does not claim to be a capture of any of them.
Specifications
- Formats
- Audio Unit, VST3, CLAP, Standalone
- Platform
- macOS 11 or later, Apple silicon and Intel
- Controls
- Input, Threshold, Make Up, Dry/Wet, Attack, Release, Pre-Curve, Filter, Auto Gain, Power, Bal-Limit
- Latency
- 64 samples, reported to the host
- Automation
- All parameters, with stable IDs across versions
- Activation
- Two machines per licence. Works offline once activated.
One licence. Two machines. Yours.
No subscription and no dongle. Activate from the desktop app or from your account — and release a machine yourself the day your computer dies.
$79once
Buy VolvoSecure checkout. VAT handled at the till. 14-day refund, no questions.