The EQM500 is a mono passive equalizer in 500-series format, built by Polish manufacturer IGS Audio on the circuit principles of the classic Pultec designs from the 1950s. All three bands – LOW, MID, and HIGH – are LC resonant circuits using UTM Industry inductors wound to 1% tolerance. Input and output are transformer-balanced via UTM transformers. Frequency selection runs through Grayhill stepped switches throughout, making every setting fully recallable without relying on memory or marked positions on a pot. Boost and cut range is 12 dB with a fixed Q of 0.7 – wide, musical curves suited to tonal shaping rather than surgical correction. The EQM500 is assembled as a single-slot 500-series module and can be paired with other IGS Audio or third-party 500-series units inside any standard lunchbox or rack frame.
Midrange control as the primary problem-solver
The MID band covers 12 frequency steps across 200 Hz – 7 kHz and operates in cut-only mode. That is a deliberate design decision: passive midrange EQ works most naturally when attenuating. It is the right tool for pulling out room resonances that survive acoustic treatment, resolving frequency conflicts between instruments stacked in the same range, and reducing vocal presence issues without touching the surrounding harmonic content. The 200 Hz – 7 kHz range concentrates the highest energy density in most audio material – where problems in a mix are most audible and where imprecise moves have the widest consequences on translation.
LOW and HIGH – passive boost that behaves like a passive boost
The LOW and HIGH bands each provide six selectable boost frequencies. Passive LC boost does not mirror the behaviour of a digital or active EQ at the same gain setting – the curve rises gently, widens as it increases, and rolls off naturally at the band edges. That asymmetric shape is what gives Pultec-style EQs their reputation for adding body to low end and air to the top without the hardness that appears with linear-phase or steep active filters. Hard-wire bypass enables clean A/B comparison with the full signal path active. At 20 Hz – 20 kHz ±0.1 dB the EQM500 introduces negligible coloration outside the frequencies being processed.
KEY FEATURES
- 3-band passive equalizer in 500-series format, mono
- LC resonant circuits with UTM Industry inductors, 1% tolerance
- Transformer-balanced I/O (UTM) throughout
- Grayhill stepped switches – fully recallable settings
- MID band: 12 frequency steps, 200 Hz – 7 kHz, cut only
- LOW and HIGH bands: 6 frequency steps each, boost only
- Boost/cut range: 12 dB
- Q = 0.7 – wide, musical correction curve
- Hard-wire bypass
- Frequency response: 20 Hz – 20 kHz ±0.1 dB
TECHNICAL SPECIFICATIONS
- Type: passive band equalizer (Pultec-style)
- Format: 500-series, single slot, mono
- Bands: 3 (LOW, MID, HIGH)
- MID frequencies: 12 steps, 200 Hz – 7 kHz (cut only)
- LOW / HIGH frequencies: 6 steps each (boost only)
- Correction range: ±12 dB
- Q: 0.7
- Inductors: UTM Industry, 1% tolerance
- Transformers: IGS Custom / UTM, input and output
- Switches: Grayhill (USA)
- Frequency response: 20 Hz – 20 kHz ±0.1 dB
- Bypass: hard-wire
