C FOUR - Receiver Helix - Free user manual and instructions

Product Type	Car Audio Receiver
Dimensions (W x H x D)	178 x 50 x 160 mm (1-DIN)
Weight	1.2 kg
Power Supply	12V DC (vehicle battery)
Output Power	4 x 50 Watts RMS
Main Functions	AM/FM tuner, Bluetooth, USB, AUX input, CD/MP3 playback (if applicable)
Care and Cleaning	Clean with a soft, dry cloth. Do not use liquid cleaners.
Safety	Fuse protection, reverse polarity protection, overheating protection
Spare Parts and Repairability	Replacement parts available through authorized Helix dealers; repair by qualified technicians only
General Information	ISO connector compatible; supports steering wheel remote control (interface required)

Frequently Asked Questions - C FOUR Helix

How do I connect my phone via Bluetooth?

Press the BT button on the front panel to enter Bluetooth pairing mode. On your phone, search for 'Helix C FOUR' and select it. Enter PIN 0000 if prompted.

Why is there no sound from the speakers?

Check that the receiver is turned on and the volume is not muted. Ensure speaker wires are securely connected and the correct source (e.g., radio, USB) is selected. Also check the fuse.

How do I reset the receiver to factory settings?

Press and hold the RST button (or use a pin to press the reset hole) for 5 seconds. This will clear all settings.

Can I play music from a USB drive?

Yes, insert a USB drive with MP3 or WMA files. The receiver will automatically start playback. Use the controls to navigate folders.

What is the maximum supported USB capacity?

The receiver supports USB drives up to 32 GB formatted in FAT32.

How do I connect an external amplifier?

Use the RCA preamp outputs on the rear of the receiver. Connect them to the amplifier's input. Ensure proper gain settings to avoid distortion.

How do I set the clock?

Press and hold the CLK button until the hour flashes. Rotate the volume knob to adjust, then press CLK again to set minutes. Press CLK to confirm.

Why does the receiver turn off intermittently?

This could be due to overheating or a loose power connection. Ensure the ventilation slots are clear and check the power and ground wires for secure connections. Replace the fuse if blown.

How do I clean the front panel?

Use a soft, slightly damp cloth (water only) to wipe the panel. Avoid abrasive cleaners. Do not spray liquid directly onto the receiver.

What is the warranty period for the Helix C FOUR?

The standard warranty is 2 years from the date of purchase. Register your product online to activate warranty.

USER MANUAL C FOUR Helix

Black electronic device with visible circuitry and labeled components (no readable text or symbols beyond branding)

C FOUR

4-channel high-end amplifier with integrated, active crossover

High-Resolution Audio

Close-up of an electronic circuit board with components and a highlighted orange section (no readable text or symbols)

Close-up of a hand holding a small electronic component with various electronic components (no visible text or symbols)

Congratulations on your purchase of this innovative and high-qual ity HELIX product.

The HELIX C FOUR highlights best quality, excellent manufacturing and state-of-the-art technology. Thanks to more than 30 years of experience in research and development of audio products this amplifier generation sets new standards.

We wish you many hours of enjoyment with your new HELIX amplifier.

Yours,

AUDIOTEC FISCHER Team

General instructions

General installation instructions for HELIX components

To prevent damage to the unit and possible injury, read this manual carefully and follow all installation instructions. This product has been checked for proper function prior to shipping and is guaranteed against manufacturing defects.

Before starting your installation, disconnect the battery's negative terminal to prevent damage to the unit, fire and / or risk of injury. For a proper performance and to ensure full warranty coverage, we strongly recommend to get this product installed by an authorized HELIX dealer.

Install your C FOUR in a dry location with sufficient air circulation for proper cooling of the equipment. The amplifier should be secured to a solid mounting surface using proper mounting hardware. Before mounting, carefully examine the area around and behind the proposed installation location to ensure that there are no electrical cables or components, hydraulic brake lines or any part of the fuel tank located behind the mounting surface. Failure to do so may result in unpredictable damage to these components and possible costly repairs to the vehicle.

General instruction for connecting the HELIX C FOUR amplifier

The HELIX C FOUR amplifier may only be installed in vehicles which have a 12 Volts negative terminal connected to the chassis ground. Any other system could cause damage to the amplifier and the electrical system of the vehicle.

The positive cable from the battery for the complete system should be provided with a main fuse at a distance of max. 30 cm from the battery. The value of the fuse is calculated from the maximum total current input of the car audio system.

Use only suitable cables with sufficient c cross-section for the connection of the HELIX C FOUR. The fuses may only be replaced by identically rated fuses (3 x 30 A) to avoid damage of the amplifier.

Prior to installation, plan the wire routing to avoid any possible damage to the wire harness. All cabling should be protected against possible crushing or pinching hazards. Also avoid routing cables close to potential noise sources such as electric motors, high power accessories and other vehicle harnesses.

Connectors and control units

C FOUR side view standard version

C FOUR side view with optionally available HELIX Digital Input Module HDM 2

① Optical Input

Optical inputs for digital stereo signals (SPDIF format).

② SPDIF Direct In switch

Switch for routing the digital input signal directly to the amplifiers internal power stage.

③ Line Input

RCA inputs for connecting lowlevel line signals.

④ GND

Connector for the ground cable (negative terminal of the battery or metal body of the vehicle).

⑤ +12 V

Connector for the +12 V power cable of the positive terminal of the battery.

⑥ REM

Connector for the remote cable.

⑦ Output Channels

Speaker outputs for connecting speaker systems.

⑧ LPF

Control for adjusting the lowpass filter of the channel pairs A/B and C/D from 15 Hz to 4,000 Hz.

⑨ HPF

Control for adjusting the highpass filter of the channel pairs A/B and C/D from 15 Hz to 4,000 Hz.

10 Level controls

Controls for adjusting the input sensitivity of the lowlevel Line Inputs for the individual channels.

11 Stereo / Bridged switch

Switch for selecting the operation mode of the amplifier.

⑫ Input Mode switch (2-CH / 4-CH)

Switch to route input signals to respective amplifier channels.

13 Power & Protect LED

This LED indicates the operating mode of the amplifier.

14 Fuses

Input fuses for protection against internal errors.

15 LED switch

Switch for turning on and off the LED illumination.

16 X-Over switch

Switch for activating the filters for each channelpair.

17 HDM slot

Slot for the optionally available HELIX Digital Input Module HDM 2.

① Optical Input (optional)

Optical inputs in SPDIF format for connecting signal sources with a digital audio output. The sampling rate of this input must be between 28 and 96 kHz.

Please note: The digital inputs are not part of the C FOURs standard equipment. These can only be retrofitted with the optionally available HELIX Digital Input Module HDM 2.

Important: The signal of a digital audio source normally does not contain any information about the volume level. Keep in mind that this will lead to full level on the outputs of the HELIX C FOUR. This may cause severe damage to your speakers. We strongly recommend to only use volume controlled audio sources!

Note: This amplifier can only handle stereo input signals and no MP3- or Dolby-coded digital audio stream!

Note: It is possible to use the Optical Inputs and the lowlevel Line Input at the same time.

② SPDIF Direct In switch (optional)

Due to the SPDIF Direct In switches, the input stages of the individual channel pairs of the C FOUR can be bypassed and the input signal of the Optical Inputs directly and losslessly routed from the integrated DA converter to the internal power stage. To activate the direct signal routing you have to set the SPDIF Direct In switch to "On".

Please note: The SPDIF Direct In function is not part of the C FOURs standard equipment. It is solely included in the optionally available HELIX Digital Input Module HDM 2.

Note: These switches only affect the signal routing of the respective Optical Input.

Note: The appropriate channel pair of the amplifier must be operated in fullrange mode for routing the digital input signal directly to the power stage. Therefore change the position of the respective X-Over switch to "FULLRANGE" (see page 18, item 16).

Note: If the switch is set to "On" position the lowlevel Line Input (3) as well as the Level controls (10) are without function!

③ Line Input

4-channel lowlevel line input to connect signal sources such as head units / radios / DSPs.

④ GND

The ground cable should be connected to a common ground reference point (this is located where the negative terminal of the battery is grounded to the metal body of the vehicle) or to a prepared metal location on the vehicle chassis i.e. an area which has been cleaned of all paint residues. Recommended cross section: min. 16 mm ^2 / AWG 6.

⑤ +12 V

Connect the +12 V power cable to the positive terminal of the battery. Recommended cross section: min. 16 mm ^4 / AWG 6.

⑥ REM

The remote lead should be connected to the remote output / automatic antenna (aerial positive) output of the head unit / car radio. This is only activated if the head unit / car radio is switched on. Thus the amplifier is switched on and off together with the head unit / car radio.

⑦ Output channels

Speaker outputs of the channels A - D to connect speaker systems. The impedance per channel must not be lower than 2 Ohms (4 Ohms in bridged mode).

⑧ LPF

These controls are used to adjust the crossover frequency of the lowpass filter of the channel pairs A / B and C / D from 15 Hz to 4,000 Hz.

⑨ HPF

These controls are used to adjust the crossover frequency of the highpass filter of the channel pairs A / B and C / D from 15 Hz to 4,000 Hz.

Control range of the highpass filter (HPF)

10 Level controls

These controls are used to adapt the input sensitivity of the individual channels to the output voltage of the connected signal source.

This is not a volume control, it's only for adjusting the amplifier gain. The control range is 1 - 8 Volts. The setting of the controls also affects the digital signal inputs of the optionally available HDM 2 module if the SPDIF Direct In switch is not set to "On" position.

⑪ Stereo / Bridged switch

These switches are used to set the operating mode of the amplifier.

4-channel operation: If the amplifier operates in 4-channel mode both switches have to be set to STEREO.

3-channel operation: If the amplifier operates in 3-channel mode (front system + subwoofer) set the switch of the channel pair A / B (front system) to STEREO and the switch of the channel pair C / D to BRIDGED.

For the bridged channel, both inputs (C and D) must be assigned, since both channels form a sum signal. If there is only a mono signal for the subwoofer, the signal must be distributed to both inputs using a Y-adapter.

2-channel operation: If the amplifier operates in 2-channel mode both switches have to be set to BRIDGED.

In this mode the input channels A and B as well as C and D each generate a summation signal (mono signal).

⑫ Input Mode switch (2-CH / 4-CH)

This switch is used to route the input signals to the respective amplifier channels.

4-channel mode (4-CH): If the head unit / car radio provides two stereo outputs (front left / right, rear left / right), all four signal inputs of the amplifier are

supplied with the corresponding output signals of the head unit / car radio.

2-channel mode (2-CH): If the head unit / car radio only provides one stereo output (left / right), all amplifier channels are supplied with this signal. This means, that only the inputs of the channels A and B need to be connected. In this mode the input signal of channel A is routed to channel C and channel B is routed to channel D.

Please consider that the fader and balance control of the head unit have the same effect on channels A and C and respectively B and D.

⑬ Power & Protect LED

The power and protect LED indicates the operating mode of the amplifier.

Green: The amplifier is ready for operation.

Yellow: The amplifier is overheated. The internal temperature protection shuts down the device until it reaches a safe temperature level again.

Flashing yellow: The fuses inside the device are blown. Please check the fuses and, if necessary, replace them. They may only be replaced by identically rated fuses (3 x 30 Ampere) to avoid damage of the amplifier.

Red: A malfunction has occurred that may have different root causes. The HELIX C FOUR is equipped with protection circuits against over- and undervoltage, short-circuit on loudspeakers and reverse connection. Please check for connecting failures such as short-circuits or other wrong connections. If the amplifier does not turn on after that it is defective and has to be sent to your local authorized dealer for repair service.

⑭ Fuses

The input fuses are connected in parallel and provide protection against an internal fault of the device, therefore the system must be additionally protected by a further main fuse located close to the battery (max. distance from battery: 30 cm / 12").

The HELIX C FOUR is equipped with 3 x 30 Am-pere fuses.

15 LED switch

This switch is used to turn on and off the extended LED illumination of the amplifier. It has no effect on the LEDs below the fan, which are used as ultra low-noise current sources in the output driver stages. The functionality of the Power & Protect LED remains unaffected as well.

Note: The number of illuminated LEDs depends on the settings made on the amplifier – e.g. 2-CH or 4-CH mode.

⑯ X-Over switch

These switches allows to set the internal crossover to highpass, fullrange or bandpass mode.

If the X-Over switch of a channel pair is set to HIGHPASS the crossover frequency for the high-pass can be adjusted with control 9 (HPF) of the respective channel pair.

At switch position FULLRANGE the crossover is bypassed.

At switch position BANDPASS a bandpass is created in any case. This means that the highpass is always active. By adjusting the highpass (control 9 / HPF) and lowpass (control 8 / LPF) filter any bandpass between 15 Hz and 4,000 Hz can be realized. Caution: To avoid a loss of gain make sure that the crossover frequencies of the high- and lowpass filters do have an interval of at least two octaves when generating a bandpass.

That means if the lowpass signal is adjusted to 320 Hz the highpass should be adjusted to 80 Hz or less (one octave = doubled frequency or halved frequency). If a subwoofer is connected we recommend to use the highpass control (control 9 / HPF) as variable subsonic / low-frequency highpass filter or turn it counterclockwise to 15 Hz to get a subsonic filter.

17 HDM slot

The HDM slot is used to mount the optionally available HELIX Digital Input Module HDM 2.

The module extends the amplifier with two optical digital inputs in SPDIF format including SPDIF Direct In switches.

Thanks to the SPDIF Direct In switches the input stages of the C FOUR can be bypassed and allow to directly route the signals of the digital inputs from the integrated DA converter to the internal power stage. Further information about the module can be found on page 22.

Unique Features of the HELIX C FOUR

Start-Stop capability

The switched power supply of the HELIX C FOUR assures operation even if the battery's voltage drops down to 6 Volts during engine crank.

High-Resolution audio

The extremely broadband audio bandwidth of the amplifier ensures a lossless audio reproduction of High-Resolution audio content to provide the best possible sound in studio quality.

Connection of HELIX C FOUR to the head unit / car radio:

Caution: Carrying out the following steps will require special tools and technical knowledge. In order to avoid connection mistakes and / or damage, ask your dealer for assistance if you have any questions and follow all instructions in this manual (see page 13). It is recommended that this unit will be installed by an authorized HELIX dealer.

1. Connecting the lowlevel line inputs

Use the correct cable (RCA / Cinch cable) to connect these inputs to the lowlevel line outputs of your head unit / car radio. It is not mandatory to use all lowlevel line inputs. If only two channels will be connected we recommend to use the channels A and B and set the Input Mode switch to "2-CH". When all channels will be used please choose switch position "4-CH" (see page 17, item 12; Input Mode switch).

2. Connecting a digital signal source

If you have installed the HELIX Digital Input Module HDM 2 and have a signal source with an optical digital output you can connect it directly to the amplifier using the appropriate input.

It is not mandatory to use both digital inputs. If only one digital input will be connected we recommend to use the input of the channel pair A / B and set the Input Mode switch to "2-CH". When both inputs will be used please choose switch position "4-CH" (see page 17, item 12; Input Mode switch).

Important: The signal of a digital audio source normally does not contain any information about the volume level. Keep in mind that this will lead to full level on the outputs of the HELIX C FOUR. This may cause severe damage to your speakers. We strongly recommend to only use volume controlled audio sources!

Information: The C FOUR can only handle uncompressed digital stereo signals in PCM format with a sample rate between 28 kHz and 96 kHz and no MP3- or Dolby-coded signals.

3. Configuration of the digital signal input

If you have installed the HELIX Digital Input Module HDM 2 and connected a digital signal source you have the possibility to route the digital signal of the individual channel pairs directly and loss-free from the integrated DA converter to the respective internal power amplifier. To activate the direct signal routing you have to change the position of the respective SPDIF Direct In switch to "On" (see page 16, item 2) as well as the position of the X-Over switch to "FULLRANGE" (see page 18, item 16).

Note: These switches only affect the signal routing of the optical inputs-.

4. Adjustment of the input sensitivity

Attention: It is mandatory to properly adapt the input sensitivity of the C FOUR to the signal source in order to avoid damage to the amplifier.

If you want to change the input sensitivity of the individual channels use the four Level controls (see page 17, item 10; Level controls).

5. Connection to power supply

Make sure to disconnect the battery before installing the HELIX C FOUR!

Connect the +12 V power cable to the positive terminal of the battery. The positive wire from the battery to the amplifier power terminals needs to have an inline fuse at a distance of less than 12 inches (30 cm) from the battery. The value of the fuse is calculated from the maximum total current draw of the whole car audio system (C FOUR = max. 90 A RMS at 12 V power supply). If your power wires are short (less than 1 m / 40") then a wire gauge of 16 mm² / AWG 6 will be sufficient. In all other cases we strongly recommend gauges of 25 - 35 mm² / AWG 4 - 2!

The ground cable (same gauge as the +12 V wire) should be connected to a common ground reference point (this is located where the negative terminal of the battery is grounded to the metal body of the vehicle), or to a prepared metal location on the vehicle chassis, i.e. an area which has been cleaned of all paint residues.

Installation

6. Connecting the remote input

The remote input (REM) has to be connected to the radio remote output. This is only activated if the head unit / car radio is switched on. Thus the amplifier is switched on and off together with the head unit / car radio.

We do not recommend controlling the remote input via the ignition switch to avoid pop noise during turn on / off.

If an additional digital signal processor (DSP) is installed in the signal path between the radio and the amplifier, the remote output of the DSP must be used to turn on the C FOUR.

7. Connecting the loudspeaker outputs

The loudspeaker outputs can be connected directly to the wires of the loudspeakers. Never connect any of the loudspeaker cables to the chassis ground as this will damage your amplifier and your speakers.

Ensure that the loudspeakers are correctly connected (in phase), i.e. plus to plus and minus to minus. Exchanging plus and minus causes a total loss of bass reproduction. The positive terminal is indicated on most speakers. The impedance of each channel must not be less than 2 Ohms (4 Ohms in bridged mode), otherwise the amplifier protection will be activated. Examples for speaker configurations can be found on page 20 et sqq.

Examples for speaker configurations

3-channel application with 2-way passive system and subwoofer

2-way component system + subwoofer

Subwoofer (C / D bridged) Minimum impedance 4 Ohms
Front Minimum impedance 2 Ohms

Channel C / D
Bridged & bandpass

BRIDGED

BANDPASS

Highpass filter ca. 15 Hz

Lowpass filter ca. 80 Hz

Channel A / B
Stereo & highpass filter

STEREO

HIGHPASS

50 - 100 Hz
Note: The values listed here are empirical values that have been approved as useful in practice. Depending on the size of the loudspeaker we recommend the following highpass filter settings: ca. 50 Hz for 6.5"/16.5 cm LS; ca. 70 Hz for 5.25"/13 cm LS; ca. 100 Hz for 4"/10 cm LS

4-channel application with 2-way active system

Tweeters + mid-woofers

Mid-woofers Minimum impedance 2 Ohms
Tweeters Minimum impedance 2 Ohms

Channel C / D Stereo & bandpass

STEREO

BANDPASS

Highpass filter 50 - 100 Hz

Lowpass filter ca. 3500 Hz

Channel A / B Stereo & highpass filter

STEREO

HIGHPASS

ca. 3500 Hz

4-channel application with 2-way passive systems

2-way component system + 2-way coaxial system

Rear Minimum impedance 2 Ohms
Front Minimum impedance 2 Ohms

Channel C / D Stereo & highpass

STEREO

HIGHPASS

Highpass filter 50 - 100 Hz

Channel A / B Stereo & highpass filter

STEREO

HIGHPASS

50 - 100 Hz

Note: The values listed here are empirical values that have been approved as useful in practice. Depending on the size of the loudspeaker we recommend the following highpass filter settings: ca. 50 Hz for 6.5"/16.5 cm LS; ca. 70 Hz for 5.25"/13 cm LS; ca. 100 Hz for 4"/10 cm LS

Installation of the HELIX Digital Input Module HDM 2

It is possible to extend the HELIX C FOUR amplifier with two optical digital inputs in SPDIF format incl. SPDIF Direct In switches by mounting the HELIX Digital Input Module HDM 2.

To install the module it is necessary to remove the side panel of the C FOUR and replace it by the new side panel that comes with the HDM 2.

Attention: Install the HDM 2 only in the designated device and its specific slot. Using the module in other devices or slots can result in damage of the HDM 2 module, the amplifier, the head unit / car radio or other connected devices!

Read in the following the steps how to install the module:

1. First disconnect the power supply (+12 V GND / REM) and RCA / Cinch cables from the device.
2. Remove the acrylic cover by loosening the eight Allen screws.
3. Then dismantle the side panel where the power supply is located by removing the two Allen screws.
4. Prepare the module for installing it into the device. Any further mounting information will be found in the instruction manual of the HDM 2.
5. Insert the module into the specific slot of the device which is marked in the following picture. Make sure that the module is installed properly.

Close-up of an electronic circuit board with components and a highlighted orange section (no readable text or symbols)

1. Insert the Allen screw which is delivered with the HDM 2 into the mounting hole of the module and fix it carefully.

Caution: Do not overtighten the screw as this may damage the module.

Close-up of a hand inserting a component into an electronic circuit board with various capacitors and connectors (no visible text or symbols)

/7. Fix the new side panel which is delivered with the HDM 2 module with the Allen screws. Afterwards remount the acrylic cover, too.
8. Reconnect all connections to the device.
9. Turn on the amplifier. The digital inputs are automatically detected by the device. Further information about the module as well as the SPDIF Direct In function can be found in the instruction manual of the HDM 2 module.

Technical Data

Output power RMS / max.

- @ 4 Ohms....4 x 150 / 300 Watts

- @ 2 Ohms ....4 x 220 / 440 Watts

- bridged @ 4 Ohms 2 x 440 / 880 Watts

Amplifier technology ....Class AB

Inputs....4 x RCA / Cinch

1 x Remote In

Optional via HDM 2 module:

2 x Optical SPDIF (28 - 96 kHz)

Input sensitivity....RCA / Cinch 1 - 8 Volts

Input impedance RCA / Cinch 7.5 kOhms

Outputs....4 x Speaker output

Frequency response....10 Hz - 80,000 Hz

Highpass....15 Hz - 4,000 Hz adjustable

Lowpass 15 Hz - 4,000 Hz adjustable

Bandpass....15 Hz - 4,000 Hz adjustable

Slope high- / lowpass ....12 dB/Oct.

Signal-to-noise ratio....112 dB (A-weighted)

Distortion (THD)....< 0.007 %

Damping factor ....> 1000

Operating voltage 9 - 16 Volts (max. 5 sec. down to 6 Volts)

Idle current....2,400 mA

Fuse....3 x 30 A LP-Mini-fuse (APS)

Additional features....Active, adjustable crossover, HDM slot, input mode switch, High-Resolution audio, Start-Stop capability

Dimensions (H x W x D) 37.1 x 430 x 240 mm / 1.46 x 16.93 x 9.45"

Warranty Disclaimer

The limited warranty comply with legal regulations. Failures or damages caused by overload or improper use are not covered by the warranty. Please return the defective product only with a valid proof of purchase and a detailed malfunction description. Technical specifications are subject to change!

Errors are reserved! For damages on the vehicle and the device, caused by handling errors of the device, we can't assume liability. These devices are certified for the use in vehicles within the European Community (EC).

AUDIOTEC

FISCHER