The Alter 500 is a single-channel FET compressor in API 500 format, built by IGS Audio on the UREI 1176 topology from the 1970s. A field-effect transistor as the gain-control element gives the Alter the characteristic speed and aggression of attack not achievable with opto or VCA designs. The input is electronically balanced via a modern low-noise op-amp; the output runs through a Carnhill transformer at 600 ohms. The signal path uses two discrete amplification stages built on 990-type circuits (discrete, API-compatible), with the output control placed between them. Gain reduction metering runs across a 20- or 30-point LED bar. Relay bypass on the front panel. Gain up to 36–40 dB, THD up to 0.2%, SNR above 99 dB, frequency response ±0.1 dB across 20 Hz – 20 kHz.
Six ratio positions and SLAM mode – what they add in practice
The original UREI 1176 provided four ratio positions: 4:1, 8:1, 12:1, and 20:1. The Alter 500 extends this range with two additional positions: 1:1 (linear amplifier – signal passes through the FET circuit colour without compression) and 2:1 (light compression on dynamic material). Six ratio positions total cover the range from transparent tonal shaping through standard tracking compression to aggressive limiting. SLAM mode activates the equivalent of the 1176's "all-buttons-in" trick – simultaneously engaging all ratio buttons on the original hardware produced a non-standard, heavily coloured mode particularly valued on drums and bass. On the Alter 500, SLAM is an independent switch that operates regardless of the ratio position, allowing this character to be engaged at any point in the session without changing ratio.
Attack, release and gain staging with stepped controls
Attack time is selected across three positions: 0.02 ms (Fast – full transient control on drums and percussion), 0.2 ms (Mid), and 0.8 ms (Slow – lets transients pass, compresses the sustain). Release time covers 50 ms (Fast), 600 ms (Mid), and 1.1–1.5 s (Slow). Stepped input and output controls ensure full gain-staging recall between sessions. Maximum gain reduction is 20 dB. The signal routing Input → FET (GR) → Amp Stage 1 → Output → Amp Stage 2 → Carnhill mirrors the 1176 topology directly: the Output control is not a post-amplification attenuator but an active element that sets the operating point of the second gain stage, which is why its position affects both level and compression character.
KEY FEATURES
- FET compressor 1176-style, API 500 format, mono
- Two discrete gain stages based on 990-type circuits (API-compatible)
- Input: modern low-noise op-amp (balanced)
- Output: Carnhill transformer, 600 Ω
- 6 ratio positions: 1:1 / 2:1 / 4:1 / 8:1 / 12:1 / 20:1
- SLAM mode – all-buttons-in equivalent, independent of ratio switch
- Attack (3 positions): 0.02 / 0.2 / 0.8 ms
- Release (3 positions): 50 ms / 600 ms / 1.1–1.5 s
- Stepped input and output controls
- GR metering: LED bar (20–30 points)
- Relay bypass
- 3-year manufacturer warranty
TECHNICAL SPECIFICATIONS
- Type: FET compressor/limiter (1176-style), API 500, mono
- Gain control element: FET (field-effect transistor)
- Amplification stages: 2× discrete, 990-type circuit
- Input: balanced, modern low-noise op-amp
- Output: Carnhill, 600 Ω
- Input impedance: 1 kΩ
- Output impedance: 600 Ω (Carnhill)
- Ratio: 1:1 / 2:1 / 4:1 / 8:1 / 12:1 / 20:1
- Attack: 0.02 / 0.2 / 0.8 ms
- Release: 50 ms / 600 ms / 1.1–1.5 s
- Maximum gain: 36–40 dB
- Maximum gain reduction: 20 dB
- Maximum input level: +20 dBu
- Maximum output level: +22 dBm
- THD: up to 0.2%
- Frequency response: ±0.1 dB, 20 Hz – 20 kHz
- SNR: above 99 dB
- Power supply: ±16 V
- Dimensions (H×W×D): 132×37×158 mm
- Weight: 700 g
