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CP554 | Diode Bridge Compressor Introduction and Demo

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Video by Adam Steel. For more content visit

The Sound of Vintage Transformers

The extensive use of the transformers in the classic diode bridge compressor units designed by Rupert Neve, was dictated by technological limitations of the 60s and 70s.

Simply put, the transformers were the only available devices at the time which could do a specific function in the pro-audio circuitry. For example, matching the input and output impedance, level reduction or amplification, and transforming the signal from balanced to unbalanced or vice versa.

CP554 is designed and built with the use of modern electronic components. Although it follows the classic diode-bridge compressor design principles, it is not a copy nor a clone of any vintage unit. A large number of the transformers in the diode bridge is not a must‐have design requirement! Transformers can be replaced by modern electronic components delivering a better signal to noise ratio and better electromagnetic interference immunity.

In the default mode, the CP554 will provide just a mild amount of the harmonics produced by the diode-bridge circuitry, while being neutral and non‐destructive to the music material.

Why Diode Bridge Compressors are so Rare and Expensive?

There are plenty of studio grade compressors on the market. The most popular are VCA, FET and opto-compressors.

The diode bridge compressors are rather rare and the options are limited. The most obvious are made the AMS-Neve units based on the original Rupert Neve’s designs, the 2254, 2264 and 33609 stereo compressor.

Some manufacturers introduces the Neve’s clones based on the widely available original diagrams. It is important to point, that even the most accurate clone, won’t identical to original, due to the transformers available from Carnhill.

To use the diode-bridge element as the gain control block, the circuitry has to be designed in the specific way. The ideal symmetry of the signal is crucial. In 60s only transformer could deliver close signal symmetry.

The increased noise floor is the other problem of the classic diode-bridge compressors. Improper circuitry design will drastically increase the noise, rendering the compressor unusable for the studio application.

Building the 1:1 copy of the Neve’s compressor is easy, but it is not enough to make a good quality equipment. Only deep insight understanding of the diode-bridge compressor design is the key to success.

CP554 is not a clone! It is designed to fallow the principles of the diode-bridge circuitry and to obtain best possible performance.

The Vinatage Mode

Vintage Mode is a proprietary design by B. Hrk of analog processing circuitry which delivers the same type of transformer harmonics which are present in the classic diode bridge compressors.

Vintage Mode is a fully analog circuitry (No DSP) designed by applying the practical knowledge and understanding of a classic analog circuitry performance, response and behavior. The extensive knowledge of the laws of physics, how the transformers and vintage circuitry work, combined with experience in analog circuitry design, allows to accurately reproduce the same performance, character, vibe or mojo of vintage analog circuitry and transformers. Electronic engineering  is a science, not magic!

Vintage Mode circuitry ensures the same type of response of the signal as in the vintage transformers. More signal creates more harmonics. The overall experience is very similar to what any vintage unit would deliver, including the frequency and tonal response.

By default, the CP554 is neutral and versatile. It does not impose any strong character on the music material, vastly increasing the number of studio and music production applications. If required, the vintage mode will introduce classic/vintage harmonics, converting the CP554
to a vintage/classic Diode Bridge Compressor. Therefore, CP554 is 2‐in‐1.

Stereo Buss Operation

*The Stereo Link cable is provided together with a pair of the CP554.

CP554 | Diode Bridge Compressor  vs. DAW Plugins

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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.

CP554 | Diode Bridge Compressor vs. Dione VCA Compressor

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CP554 Specification

Input Type / Input Impedance Balanced Electronic 20k
Output Type / Output Impedance Balanced HRK Active Transformer Less than 10R
Max Output Level +25dBu@0.1%THD with 1k Load * Balanced Signal
Recommended Load Balanced 10k Minimum Supported Load 600R
Initial Gain 0dB Gain Make-up set to 0dB
Phase Reverse No CP554 is not introducing phase reverse in any mode of operation.
Frequency Response 5Hz-80kHz@-1dBr Compressor disengaged.
Static Noise Level -95dBu@10Hz-22kHz Compressor disengaged.
Power Consumption Typically 45mA per unit 60mA max. per unit
Format API 500 Format Compatible with most of the 500 Racks
Type of Compressor Engine Diode-Bridge Symmetric Gain Cell
Compression Curve Adaptive Soft-Knee Depends on the Threshold and Ratio knob settings
Threshold Range -25dB / + 20dB Smooth Continues Potentiometer
Ratio Range 1.5:1 - 8:1 Smooth Continues Potentiometer
Attack Time 250uS - 100mS Smooth Continues Potentiometer
Recovery Time 50mS - 2.5S Smooth Continues Potentiometer
Side Chain HPF 150Hz@6dB/oct Use the HPF Switch
Frequency Response 5Hz-30kHz@-1dBr Compressor engaged.
Recommended Use
CP554 is a line-level compressor. CP554 is not equipped with a microphone preamplifier.

The input signal should be connected to the rear XLR input.

It is recommended to use -12dBu input signal level initially and increase the level signal if required.

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 / RegionShipping CostCustoms FeeVAT
All 27 European Union Countries Delivery Time: 2-7 Days– 14.99USD for orders below 299USD- FREE shipping for orders over 299USDNO 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 299USD- FREE shipping for orders over 299USDNO Customs FeesNo VAT charge. HMRC collects the VAT.
USA, Canada Delivery Time: 5-10 Days– 14.99USD for orders below 299USD- FREE shipping for orders over 299USDNO Customs Fees for orders up to 800USD – charged by the USA Customs OfficeNo VAT charge.
Australia Delivery Time: 5-14 Days– 19.99USD for orders below 399USD- FREE shipping for orders over 399USDNo info available – refer to local regulations.No VAT charge.
Switzerland Delivery Time: 3-7 Days– 14.99USD for orders below 299USD- FREE shipping for orders over 299USDNo info available – refer to local regulations.No VAT charge.
Norway Delivery Time: 3-7 Days– 14.99USD for orders below 299USD- FREE shipping for orders over 299USDNo info available – refer to local regulations.No VAT charge.
Worldwide destinations Delivery Time: 3-14 Days– 29.99USDRefer 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.