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500 Units 2 4 6+
Discount -10% -15% -20%
SKU 349 Categories ,

Stereo Buss Operation

Create Sophisticated EQ Curves!

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.

Standard Cut and Boost 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 developed the propitiatory design of an Active Transformer to solve the problems of the common passive transformers.

Passive transformers are susceptible to electromagnetic interference (50Hz Hum, RF Smog) and the frequency response is dependent on load and source impedance, negatively influencing the overall performance of analogue studio equipment.

The HRK Active Transformer solves the problems of the passive transformer while preserving and even enhancing the positive trades of the transformer output stage, including the harmonics content and the tonal response.

The HRK Active Transformer delivers the sound that we love, including the harmonics content and the tonal response, and removes all the hassles associated with passive transformers.

The Active Transformer produces a subtle “Air” effect and provides extended LF performance.

The Active Transfomer saturation depends on the output level. Higher output signal levels will saturate the Active Transfomer more, like in the classic studio equipment.

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.

Play Video
Play Video
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EQ550P Specification
Input Type / Input ImpedanceBalanced Electronic20k
Output Type / Output ImpedanceBalanced HRK Active TransformerLess than 10R
Initial Gain0dBAll controls set to flat response
Max Output Level+25dBu@1%THDBalanced Signal
Power Consumption180mA max. 
FormatAPI 500 Format 
Phase ReverseNo0 deg.
Static Noise-89dBu@10Hz-22kHzAll controls set to flat response
Thermionic ValveNOS Dual TriodeHigh Quality NOS Selected Tube

ALL HRK Products Are Brand New - Assembled and Fully Tested

All HRK Products are tested with the industry best audio measuring equipment

Audio Precision System

All Items are dispatched from Poland in the European Union within 1-3 working days.

Country / Region Shipping Cost Customs Fee VAT
All 27 European Union Countries Delivery Time: 2-7 Days – 14.99USD for orders below 349USD- FREE shipping for orders over 349USD NO Customs Fees – Yes, for personal orders- Not charged with the EU VAT number
United Kingdom Delivery Time: 3-7 Days – 14.99USD for orders below 349USD- FREE shipping for orders over 299USD NO Customs Fees No VAT charge. HMRC collects the VAT.
USA, Canada Delivery Time: 5-10 Days – 14.99USD for orders below 349USD- FREE shipping for orders over 349USD NO Customs Fees for orders up to 800USD – charged by the USA Customs Office No VAT charge.
Australia Delivery Time: 5-14 Days – 19.99USD for orders below 449USD- FREE shipping for orders over 449USD No info available – refer to local regulations. No VAT charge.
Switzerland Delivery Time: 3-7 Days – 14.99USD for orders below 349USD- FREE shipping for orders over 349USD No info available – refer to local regulations. No VAT charge.
Norway Delivery Time: 3-7 Days – 14.99USD for orders below 349USD- FREE shipping for orders over 349USD No info available – refer to local regulations. No VAT charge.
Worldwide Destination Delivery Time: 3-14 Days – 29.99USD Refer to local regulations. No VAT charge.

EU Customers

The EU Customers will not pay any extra customs fees.

Business customers with valid EU VAT number will not pay the VAT tax.

Please enter your EU VAT Number at the checkout, and the VAT will be deducted from the Total Payment. The VAT number is automatically checked in the VIES database.

For personal orders from the EU, the VAT will be added at the checkout.

UK Custmers

We ship directly to United Kingdom.

UK VAT tax is collected by HRMC

Non-EU Customers

Customers from outside the EU purchase the items with no VAT.

Please select the destination country, and the VAT will be removed automatically.

If the VAT is not removed, please email customer service via the contact form, and a custom invoice will be issued.

USA and Canada

Orders from the USA below 800USD are FREE from customs tax.

We ship directly to USA and Canada

Payment Options:

You can pay for your order with any Credit Card and Apple Pay!

7 Days Return Policy

Our return policy is a part of the general Terms & Conditions, which you must acknowledge at the checkout. We stand for fair rules and a win-win approach.
We understand that our customers may wish to return the product, and we provide such an option.
However, because we are a small company, we can’t afford to run a grade B outlet shop like big distributors and providing the 30 days return period. We can’t also scrap every returned unit.
We could charge our customers more to offset the cost of the returns, and this is what major distributors do. Or we can keep our prices low and hope that our customers will appreciate our policy.
This is why we provide the 7 days test period after receiving the package with the ordered items. If you wish to return the item, we will accept the return request, providing that the item is not damaged.

If the item is damaged during the transport, we will do our best to help you and provide the replacement product.

The Impact of Switched-Mode Power Supplies used in 500 Racks on Analog Audio Circuitry

Switched-mode power supplies (SMPS) are ubiquitous in modern electronics due to their efficiency and compact size. However, they can cause issues with analog audio circuitry. The primary reason is that SMPS operates by rapidly switching on and off, typically at high frequencies. This operation can generate noise and harmonics across a wide frequency spectrum, which can infiltrate sensitive analog audio circuits and degrade audio performance. This issue is often termed as “switching noise.”

The ripple voltage, which is the residual periodic variation of the DC voltage, can also be a source of concern. In an audio system, this ripple voltage can couple into the audio path, leading to audible noise or hum.

The Solution: HRK Advanced On-Board Active SMPSU Filter

The problems caused by SMPSU can be mitigated by implementing an additional active PSU filter. This filter works by suppressing the switching noise and reducing the ripple voltage. It does so by providing a low impedance path at the switching frequency, which effectively ‘shorts out’ the noise, preventing it from entering the audio circuitry.

Moreover, it can improve the Power Supply Rejection Ratio (PSRR), a measure of how well an audio device rejects noise coming from the switched-mode power supply. A higher PSRR means the circuit is less susceptible to power supply noise, thereby improving the overall sound quality of the analog audio system.

Therefore, HRK Active PSU filter is an effective solution to ensure the integrity and quality of audio in analog systems powered by switched-mode power supplies.