- Inspired by Famous Valve Passive EQ
- Adds Harmonics and analog warmth
- enhances the sound stage and content separation
- Pro-Grade NOS Dual Triode
- perfect for tracking and stereo bus processing
- Intuitive dial-Until-You-Like operation
- Designed and built for stereo use
- Hybrid Tube Class A Gain Stage
- Two Bands Shelf EQ Operation
- Separate Control for Boost and Cut
- 3 Frequency Settings for HF Band
- 3 Frequency Settings for LF Band
- Coupled with Custom HRK Active Transformer
- On-Board Voltage 500 Rack PSU Filters
- Direct Sale - Save on Distributors Fee!

-
EQ550P | CLASS A TUBE Passive EqualiserThe EQ550P is a Passive Tube-based 2 Band Shelf Equaliser inspired by the famous Pultec design.
Each Band has a separate Boost and Cut Control which allows creating the sophisticated EQ curves.
Each Boost or Cut control has three different and independent frequency settings which give a total of 9 setting combinations per Band.
The Passive filter section changes the EQ curve by attenuating the signal. The Tube gain stage restores the signal to the nominal level adding the combination of harmonics and transients.
The EQ550P is very musical and easy to use. It can process any music material, instruments, vocal or recordings, producing amazing results.
The Valve - Discrete Transistors Hybrid circuit is the result of a long development process. It produces real Tube harmonics and reaches tone while maintaining the noise on an extremely low level.
Stereo Buss Operation

- Our improved production process ensures stereo matching between units
- If you purchase two units in one order, both units are matched for stereo operation, bus processing and mastering.
The unique feature of Passive Equalizer is the ability to Cut and Boost at the same time.
The EQ550P can create sophisticated EQ curves which cannot be achieved by the active shelf equalizers.
The ability to select three different frequencies independently for Cut and Boost provides countless possibilities in creating custom EQ curves.
The plots below show a few examples of such EQ curves.
Like any other equalizer, the EQ550P can be used to cut or boost the shelf frequencies.
The plots below show the maximum cut and boost for each band and frequency settings.
HRK Active Transformer

Bart Hrk has developed a proprietary design for an Active Transformer, aimed at addressing the limitations of traditional passive transformers.
Traditional passive transformers are known to be vulnerable to electromagnetic interference, such as 50/60Hz hum and HF RF smog. Additionally, their frequency response is contingent upon load and source impedance, which adversely affects the overall performance of analog studio equipment.
The HRK Active Transformer is a game changer that not only resolves the issues associated with passive transformers but also retains and amplifies the desired characteristics of the transformer output stage. This includes the enrichment of harmonic content and the enhancement of tonal response.
What sets the HRK Active Transformer apart is its ability to deliver the rich, beloved sound that audio enthusiasts seek, including the harmonic content and tonal response, while simultaneously eliminating the drawbacks of passive transformers.
Furthermore, the Active Transformer introduces a delicate “Air” effect and boasts an extended low-frequency (LF) performance, adding depth and clarity to the sound.
Another remarkable feature of the Active Transformer is its dynamic saturation, which is dependent on the output level. As the output signal level increases, the Active Transformer exhibits greater saturation, reminiscent of the warm and rich sound produced by classic studio equipment.
In summary, the HRK Active Transformer is an innovative solution that offers superior sound quality, eliminates electromagnetic interference, and enhances harmonic content and tonal response. It is an invaluable asset for professionals and audiophiles seeking to elevate their audio experience.
EQ550P Demo Videos
The demo video presenting the older version of the EQ550P with a Silver front panels.
The current Black front panel version is delivers the same performance as the units presented in the video.
All videos are server hosted to insure the best quality of sound without the YouTube compression or normalization.
EQ550P Specification | ||
Input Type / Input Impedance | Balanced Electronic | 20k |
Output Type / Output Impedance | Balanced HRK Active Transformer | Less than 10R |
Initial Gain | 0dB | All controls set to flat response |
Max Output Level | +25dBu@1%THD | Balanced Signal |
Power Consumption | 180mA max. | |
Format | API 500 Format | |
Phase Reverse | No | 0 deg. |
Static Noise | -89dBu@10Hz-22kHz | All controls set to flat response |
Thermionic Valve | NOS Dual Triode | High Quality NOS Selected Tube |