IGS Audio Rubber Bands is a dual-channel stereo passive equalizer in the Pultec style, paired with a two-stage tube output amplifier called the Organic Amp — a Polish interpretation of the EQP-1 topology. The passive filter block delivers the characteristic interactions between simultaneous boost and cut at the same frequencies that define classic 1960s passive equalization. The Organic Amp, built around four 6N1P-EW tubes, recovers level lost across the passive network and contributes its own tube harmonic character. Components are hand-matched to 1% tolerance; input transformers are Carnhill, output transformers are Edcor; relay bypass per unit. Format is 2U with an external 34 W supply.
Pultec Topology — Simultaneous Boost and Cut
The defining feature of a passive EQP-1-style filter is the ability to simultaneously boost and cut the same LF frequency — the result is a response curve that adds density to the low end without muddying the midrange. Boosting and cutting 60 Hz at the same time does not produce a simple net result: the interaction of first-order filters generates a curve that no active shelving EQ can replicate. This is the same mechanism responsible for the Pultec's reputation on kick drum and electric bass. In the Rubber Bands, the Bass section (20/30/60/100/180/240 Hz) handles both Boost and Cut continuously; the High Boost section (5/6/8/10/12/16 kHz, Q 0.5–10) offers sweepable bandwidth alongside the frequency selector; and the High Cut (6/10/20 kHz) completes the EQP-1A control set. The 1% component tolerance ensures both stereo channels track identically — a level of matching impossible without manual selection.
Organic Amp — 6N1P Tubes and Active Stage Character
The Organic Amp is not a neutral buffer. Four 6N1P-EW tubes in a two-stage configuration contribute their own harmonic distortion (THD 0.25%, THD+N 0.33%), which at subtle EQ settings layers on top of the correction and adds density, warmth, and three-dimensionality. Optional replacement tubes — 6BC8 or 6BC7 — shift the harmonic balance of the Organic Amp stage without touching the passive filter, allowing personalization of the amplifier's color. Relay bypass enables instant A/B comparison between the equalized and unprocessed signal with a single switch.
KEY FEATURES
- Dual-channel stereo passive Pultec-style EQ (EQP-1 topology) + Organic Amp tube stage
- Passive filter: simultaneous LF Boost and Cut — classic Pultec filter interaction
- Bass Boost/Cut: 20, 30, 60, 100, 180, 240 Hz (continuously variable, ±15 dB)
- High Boost: 5, 6, 8, 10, 12, 16 kHz (Q 0.5–10, continuously variable)
- High Cut: 6, 10, 20 kHz (continuously variable)
- All frequency selectors stepped switches
- Organic Amp: 4 × 6N1P-EW tubes (optional: 6BC8 or 6BC7)
- Component tolerance 1% — matched stereo channel tracking
- Transformers: Carnhill input (10 kΩ) + Edcor output (600 Ω)
- Relay hardwire bypass
- Input impedance: 10 kΩ (balanced)
- Output impedance: 600 Ω (balanced)
- Max input: +18 dBu; max output: +22 dBu
- External PSU, 115/230 V AC, 34 W
- Format: 2U, 19"; packaging: 70 × 50 × 30 cm; shipping weight: approx. 5 kg
- Made in: Poland
TECHNICAL SPECIFICATIONS
- Type: stereo passive Pultec-style EQ + Organic Amp tube stage
- Input impedance: 10 kΩ, Carnhill transformer (balanced)
- Output impedance: 600 Ω, Edcor transformer (balanced)
- Operating level: +4 dBu
- Max input level: +18 dBu
- Max output level: +22 dBu
- Dynamic range: 95 dB(A) at 1 kHz, +18 dBu
- Frequency response: 20 Hz – 20 kHz, ±0.3 dB
- SNR: 81 dB(A)
- THD: 0.25% (20 Hz – 20 kHz)
- THD+N: 0.33% (20 Hz – 20 kHz)
- IMD (SMPTE): 0.32% (20 Hz – 20 kHz)
- Crosstalk: 80 dB (20 Hz – 20 kHz)
- Q range: 0.5–4 (continuously variable)
- EQ range: ±15 dB
- Tubes: 4 × 6N1P-EW (optional: 6BC8 or 6BC7)
- Power: external PSU, 115/230 V AC, 50–60 Hz, 34 W
- Format: 2U, 19", depth 230 mm
- Shipping weight: approx. 5 kg
- Made in: Poland
