Hi-Fi TX audio using the ACC2 port on the Kenwood TS-950SDX

Written by:

Bill Paul N1GPT wpaul@windriver.com

There are mods available for both the Kenwood TS-850SAT/DSP-100 and the Kenwood TS-950SDX to achieve high fidelity transmit audio. Both these mods are similar in that they involve injecting audio directly into the DSP unit to bypass the mic pre-amp circuits. Unfortunately, they're also both similar in that they involve drilling holes in your precious Kenwood equipment.

For example, the VooDoo Labs audio site has a TX mod that's documented here:

As you can see, it calls for drilling a 3/8" hole in the back of the rig to mount a 1/4" jack.

For the TS-950SDX at least, I think I've found a reasonable alternative that doesn't require such drastic measures. To understand how it works, a brief description of the audio input paths is required.

The front panel mic line passes through two transistor amplification stages, as shown in the TX Mods section on the main VooDoo Labs site. The first is formed by Q251 on the MIC AMP (X59-3710-01), and the second is a buffer stage formed by Q17 on the SW UNIT(A)(X41-3240-00)(H/10)).

Meanwhile, the ACC2 audio input passes through a separate pre-amp stage formed by Q42 on the IF unit, which then also connects to the Q17 buffer stage on the SW UNIT. Thus, both the ACC2 audio and the mic audio pass through two buffer stages.

However, the ACC2 audio circuit differs from the mic pre-amp in two significant ways: it has several small value series coupling capacitors, and it has a smaller emitter shunt capacitor. Both of these cause the low end frequency response to roll off drastically at a few hundred Hz. This is as designed, since the ACC2 audio input is intended for packet data, but it sounds awful for speech. The mic pre-amp response is much better by comparisson.

The ACC2 circuit has one major advantage over the mic-preamp though: it's much easier to modify it. The mic pre-amp is on a separate mini daugherboard that's attached to the control unit, which is jammed behind the face of the radio. Accessing it is a major headache. By contrast, the ACC2 circuit is on the IF unit, which is just under the bottom cover of the radio.

The VooDoo Labs discussion forum has a modification for modifying the ACC2 circuit to improve low-end response, which can be seen here:

The mod that was originally suggested was ok, however I followed up with a suggestion to improve it. See the next to the last entry for the full details. The short story is that I made the following changes to my radio:

On the IF unit (X48-3100-00):

1) Change C154 from .1uF to 4.7uF

2) Change C158 from .1uF to 4.7uF

3) Change C161 from 1uF to 4.7uF

4) Change C159 from 10uF to 470uF

On the signal unit (X57-4130-00):

5) Change C152 from 1uF to 4.7uF

Before you close up the radio, you should also adjust the 12Kc LC filter: adjust L28 and/or L29 on the IF unit for the loudest noise in the receiver. This mod is documented at:

Once you've completed the above hardware mod, you need to perform a bit of special menu setup to get the radio to let you transmit in HiFi mode. Without these changes, the low-end response will be improved but the high-end (treble) will still be limited by the DSP to about 3KHz. With the changes, the bandwidth increases to about 6.7KHz.

Allow setting of analog filters in TX and well as RX:

1) Start with the radio off

2) Press and hold the SCAN key and the right M.CH key while turning the radio on

3) Use the small encoder knob to select menu option 1

4) Press the UP arrow key to change the setting from "off" to "on"

5) Press the CLR key to exit the menu

Enable "off" setting of LPF menu option (21):

1) Start with the radio off

2) Press and hold the FINE key (to the left of the VFO knob) while turning the radio on

Once these procedures are complete, you can configure the radio for Hi-Fi audio as follows:

1) Press the MENU key

2) Set menu options 20 (HPF) and 21 (LPF) to "off"

3) Key the transmitter in the desired mode (LSB, USB, AM)

4) Keep pressing the 8.83Mhz IF filter select key until the "bypass" setting is reached (all filter lights off)

You will now have 6.7Kc of TX audio frequency response.

Note that out of the box, this will work for SSB mode only. To operate in HiFi mode in AM, you need a modified firmware chip. You can find details about that here:

I feed the radio via the ACC2 port from my audio rack using a Radial Engineering J-ISO isolation transformer. This unit has the same Jensen transformers in it that Mike WZ5Q recommends for his voodoo mod. I got mine from here:

but you can find them at a number of distributors.

Feeding the radio this way requires that you make up a patch cable. The ACC2 port uses a 13-pin DIN connector. You can order these from a number of suppliers, but I got mine from DigiKey: