Icom IC-705 BHI DSP

The ICOM IC-705 modification described in this article is a neat, clean, internal solution needing no external wiring or power supply.

Upgrading the ICOM IC-705 with BHI Noise Reduction Module

Icom IC-705 BHI DSP. I admit it! I’m in love with BHI Ltd.’s DSP noise reduction accessories. I’ve owned most of their popular models like the DSP Desktop Speaker, and have installed BHI low-level audio modules in six different receivers and transceivers.

How is it that an audio-based DSP noise reduction accessory can be so effective? Only BHI knows, but they clearly have top-notch algorithms that rival the best of noise reduction circuits in contemporary Yaesu transceivers.

(Personal bias alert: I find Yaesu’s approach to noise reduction (“DNR” in Yaesu-speak) to be quite superior to ICOM’s, and this is what got me thinking about improving the transceiver with an internal BHI NEDSP1901-KBD module in the first place.)

The noise reduction feature in the IC-705 and its IC-7300 base station counterpart is merely “OK” in my opinion, but the addition of BHI’s NR makes a significant difference in S/N and intelligibility of signals. It’s simple enough to use an external BHI product and connect it to your rig’s speaker or headphone’s audio path, but it adds wiring and complexity. The ICOM IC-705 modification described in this article is a neat, clean, internal solution needing no external wiring or power supply.

As a medium wave DXer of overseas, 9 kHz-spaced broadcasters from my West Coast USA location, I chase DX targets which fight to get past powerhouse domestic station splatter on adjacent 10 kHz channels.

There’s often the local QRM and RFI from neighborhood sources to contend with too. In fact, the struggle is not unlike a ham radio contester seeking to hear and work a DX station on a crowded, interference-plagued band. What DXer–ham or broadcast–wouldn’t love to improve the clarity and audibility of their transceiver or receiver? That’s what this mod is about.

As with any construction project, this IC-705 modification is done totally at your own risk. Warranties will be voided, and your ICOM might become an expensive paperweight. SMD rework and careful soldering skills are essential to operate on the tightly packed, complex IC-705.

With that in mind, let’s begin!

This mod uses BHI’s newest low-level audio module, the BHI NEDSP1901-KBD. It replaces the earlier NEDSP1061-KBD and offers even greater noise and tone reduction (up to 40 dB, in eight selectable levels). At today’s exchange rate the ex-VAT 99 GBP price of the module is $139 USD (plus shipping) direct from BHI Ltd. in the UK.

The basic idea is to interrupt the IC-705’s audio path just upstream of the audio power amplifier (IC273, Main Board) and insert the BHI module, and power it from 5-15 volts inside the IC-705.

The BHI manual above has clear block diagrams outlining the addition of the module into the low-level audio path.

Icom IC-705 BHI DSP

For optimum performance provide the module with a constant amplitude signal, for example before the volume control.

Icom IC-705 BHI DSP

Note: On some modern receivers/transceivers the volume control is digital and doesn’t operate in the audio path. Contact the manufacturer for advise for the most suitable position for the DSP module.

Fitting it all in. The space between the IC-705’s Display Board and the rest of the transceiver’s stacked PCBs is the only logical spot to mount the 28mm x 37mm x 9mm noise reduction module. BHI provides a piece of double-sided foam tape for mounting.

The position is slightly to the right of bottom-center (when looking at the Display Board from the back). It’s important to avoid any interference with the flexible ribbon cables between the Main and Display Boards, or other wiring, when the modified radio is closed back up.


With this mod you can enjoy the BHI noise reduction through the internal speaker as well as with headphones. However, for headphones you need to use a mono adapter for stereo headsets, or connect an old-school monaural headphone. You’ll need to set menu item CONNECTORS > SP JACK FUNCTION to SPEAKER and adjust the “Phones Level” setting if needed (I use the default “0” setting).

The reason SPEAKER is chosen is because the BHI NR modification affects the audio path for the speaker. The default audio path for headphones is different out of the CODEC chip and additional connections to the Main Board would be needed to use this (see pins 22 & 23 of the AK4951AEN CODEC chip).

Setting the SP JACK FUNCTION to SPEAKER lets you use still use headphones if desired, as long as they are mono, or stereo with a monaural adapter. This is a work-around approach which functions well.

The BHI NR is not available through the IC-705’s Bluetooth audio. The audio is digital throughout the transceiver when BT is active and is transmitted digitally from the radio to the wireless device. The signal is not routed through the analog I/O leads of the BHI module.

The BHI manual describes use of the noise reduction module in detail, but basically it’s dead simple to use. A quick push of the button engages or disengages the noise reduction, and overall volume is controlled with the radio’s AF gain knob as usual.

Longer presses on the module’s small button cycle through all eight of the levels (each level getting more “aggressive”). Higher levels of NR can remove more noise, but at the expense of some audio quality and potential “watery” DSP sounds. However, I find the noise reduction artifacts of BHI to be less annoying than the stock IC-705 noise reduction.

The net result–to my ears–is more effective, more legible audio compared to ICOM’s NR. Stronger AM and SSB signals become virtually “full quieting” with levels 1 – 3. In practice I rarely use a setting higher than 4, but all eight levels are available if needed. I have not tried the BHI noise reduction with CW signals but I understand it’s considered very effective.

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