FT-818 microphone dynamic compressor

| junio 21, 2018

FT-818 microphone dynamic compressor

Construction Dynamic Compressor for FT-818 based  SSM2167 Analog Devices

When transmitting SSB, a dynamic compressor increases average transmitted power, and thus the audibility of a weak signal at the distant receiver, by at least one S-unit. This accessory is of special interest to amateur radio operators who work with lower transmitting power or wish to be better heard in a pile-up.

The kit for this dynamic compressor was originally introduced in based on an idea of Phil Salas, AD5X. It was developed especially for the FT-817 (now FT-818), but can also be used for other transceivers with the same microphone interface, such as the FT-857, FT-897, FT-900 or FT-991. But never use the external compressor in combination with the internal of a transceiver!

The dynamic compressor is plugged in between the microphone and the transceiver. It does not require a separate power supply, and can be switched on or off as needed. No modifications to the original microphone or to the transceiver are required.

The integral AF tone generator, which facilitates antenna tuning in SSB mode, can be keyed via two existing buttons on the microphone. As a result, the general practice of whistling for tuning is no longer required.

With the exception of one tantalum capacitor, the SMD components are preassembled. Only a few wired components remain; these can be easily installed even by less experienced builders.


Figure 2 shows the AF dynamic compressor’s schematic. Its central feature is the special SSM2167 Analog Devices microphone preamplifier. This IC has a fixed gain of 18 dB, which is compensated for by the R14/R18 voltage divider.

The adjustment resistor P1 in the negative feedback branch of the low-noise preamplifier IC1 serves to raise the level of the microphone signal until it exceeds the noise gate of the SSM2167. Quiet (disturbing) noises can be effectively suppressed in this way.

The Switch S1is used to turn the compressor function on and off, and along with potentiometer P2 sets the desired compression ratio. Three of the four NAND gates on CMOS chip IC2 form the AF generator, which can be activated by simultaneously pressing the PTT and Down microphone buttons.

The compressor is powered from the internal 5V DC rail of the connected transceiver. Current consumption is approximately 10 mA.

Speech Compressor for FT-818 schematic


After checking all parts against the parts list, you may begin assembly of the circuit board. You will need a temperature-controlled low-voltage soldering iron, 1 mm dia. rosin-cored solder, a pair of tweezers and a small pair of wire-cutters.

Warning! When working on the board, be sure that it is not exposed to undue mechanical stress. Strong pressure, which causes transverse deflection on the board, should be avoided. Lengthwise rotation (torsion) should be avoided as well.

Both RJ45 sockets are already inserted on the board, so only the eight pins of each socket need to be soldered to the board

Next, tantalum capacitor C17 is installed. Its positive pole is marked with a dash, and its correct positioning can be seen in Figures 3 and 5 of the assembly guide.

When soldering C17, proceed as follows: First, tin the solder pad of the terminal to the board. Then, center the capacitor and its solder connections over the board’s two solder pads. These have been intentionally designed to be slightly larger than neces-

sary in order to facilitate soldering the capacitor. Next, heat the previously tinned solder pad with a soldering iron and, in this way, “tack» it to the capacitor. If the capacitor is sitting incorrectly, it can still be straightened at this stage. If everything fits, solder the negative pole and subsequently, the positive pole.

This is followed by the two potentiometers P1 (2,5 k) and P2 (250 k) and the miniature slide switch S1. Along with the box, the latter should sit flat on the circuit board so that the horizontal lever later fits through the opening in the box.

Now, the protruding leads of P1, P2 and S1 located on the solder side in the immediate vicinity of the board edge, must be carefully cut flush with the board using wire cutters so that the board fits nicely into the bottom half of the box when assembled.

FT-818 microphone dynamic compressor

Figure 3: Assembly diagram (PCB, top side)


FT-818 microphone dynamic compressor

Figure 4: Assembly diagram of the preassembled SMD parts (PCB, bottom side)

FT-818 microphone dynamic compressor


Before the compressor is connected to the transceiver for the first time, current consumption should be checked. To do this, you will need a 5 V laboratory power supply and a multimeter with a current measuring range.

The required operating voltage connection can be made with two thin wires provisionally soldered across C17. The multimeter, connected in series, should show a current of about 10 mA. A significantly different value points to an assembly error. Before moving on, it is necessary to find and fix this error. Particular attention should be paid to proper mounting and clean solder joints.

Warning! If you connect the dynamic compressor to the transceiver despite excessive current consumption, the transceiver may be damaged.

Potentiometers P1 and P2 are set at midrange from the factory. P1 and P2 already produce a clearly perceptible effect in this position. For this reason, the first tests should be conducted at this setting. If the microphone level is too high or low at this

stage, it can be corrected with P1. Refer to the guide in the section on operating instructions when adjustingthe compression ratio with P2.

Box installation

To install the unit inside the box, the assembled board is placed inside the lower half of the shell (Figure 7). The upper half of the shell is placed on the lower half. Both halves of the box have corresponding sockets and pins to ensure firm cohesion. It is not necessary to affix the board to the box with additional screws. After pressing the two halves together and applying the label, the dynamic compressor is ready for use.

FT-818 microphone dynamic compressor

Operating Instructions

The AF dynamic compressor DYC-817-3 is looped into the microphone lead of the original Yaesu MH-31A8J microphone. It is not suited for use with the optional DTMF microphone.

To connect to the transceiver, one of the two plugs on the supplied short cable must be inserted into the dynamic compressor’s TRX socket. The other plug must be inserted into the transceiver’s microphone jack. The plug on the microphone cable is then inserted into the MIC port on the compressor. When the transceiver is turned on, the DYC-817-3 is ready for use. The position of slide switch Sldetermines whether the microphone signal is compressed (ON) or not (OFF).

The compression ratio can be adjusted to individual requirements by varying the resulting resistance between Pin 8 and Pin 10 of IC1 and levels in the range of 15 k^ to 178 k^. The result is that the compression ratio’s adjustment range runs from 2:1 to 10:1, which is sufficient for all cases.

Especially at low SSB signal received strength, dynamic compression has a positive effect on speech intelligibility. Distant stations will notice a signal strength increase of about 1 S-unit (6 dB), and the increase in loudness may be even higher.

The dynamic compressor should not be turned on for FM, as there are no resulting benefits. On the contrary, the signal quality will be degraded on the receiving end. The same applies to SSB paths with good audibility. The compressor comes into its own with signals at the limit of audibility. It should not be turned on in any other case.

When using 100W transceivers, it is particularly important to note that any shortcomings in the station setup cannot be compensated for with the use of the dynamic compressor. This mainly concerns poorly engineered antennas without adequate RF ground and EMC issues such as RF feedback via the microphone cable, which can cause problems especially when used with a linear amplifier.

Additionally, the DYC-817-3 contains a simple AF generator. This can be used to generate a stable test tone for tuning antennas, ATU’s or linear amplifiers, for example. The AF generator is activated by pressing the Down button on the microphone while the PTT button is already depressed. This can be done with one hand. Please be sure to keep such tuning signals as brief as possible and at the lowest possible RF power level, so as not to interfere with other radio communications.

The standard functions of the microphone buttons (Up, Down and Start Search) remain intact.

At high AF gain or when using the maximum compression level, be sure to listen to your own transmitted signal and ask the distant operator for an audio report. High AF gain can cause background noise to overwhelm the compressor circuit’s filter threshold and be transmitted, especially when a high compression ratio has been set. This should be strictly avoided.

Good luck and have fun putting the kit together and using your dynamic compressor on the air!

Parts list

Name Model/Value Number Comment
C17 220 ^F/10V 1 Tantalum
P1 Potentiometer 2,5 kQ 1
P2 Potentiometer 250 kQ 1
S1 Slide switch, 1-pin 1
Bu1, Bu2 RJ45 jack 2 on the board
Circuit board Circuit board 1 SMDs mounted
TRX connector cable 1 with 2 x RJ45
Box 1 processed
Label 1
Construction manual 1


Box 73 Amateurfunkservice GmbH

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