VocalMaster A Speech Processor for Low Power Operators
Build this versatile and attractive accessory for the Yaesu FT-817 to give you more talk power.
I have had a lot of fun operating low power with my Yaesu FT-817 portable HF transceiver, but have often wished that it had some audio or RF
processing to add a little SSB punch. There are some excellent audio processing accessories available commercially, but I thought this would make a great homebrew project reminiscent of the early Heathkit days. With this in mind, I began to look for suitable building blocks to bring this idea to life.
I wanted to incorporate several design goals. First, the enclosure should be smaller than my radio, aesthetically pleasing, and made of steel or aluminum to provide RF shielding. Second, all of the components should mount on a printed circuit board with no point-to-point wiring to jacks or controls. Third, the input and output jacks must match the rig's microphone connector. Fourth, I wanted some visual feedback on what was going on inside the box, such as an output level meter. And fifth, a built-in signal generator for antenna tuning would be an added bonus. The end result would combine all of these features into a small package that includes RF filtering and front panel controls, and is powered from the rig's mic voltage.
One of the first challenges was finding a compressor IC that would operate on the 5 V dc available at my radio's mic jack. Several ICs are available that are used in professional audio equipment but they require ±12 V dc. The Analog Devices SSM2165/2166 series of ICs will operate on 5 V dc and I found several articles on their implementation. Unfortunately, the manufacturer has discontinued the dual in-line package (DIP) version of this device. They now only offer the chips as tiny surface mounted devices (SMD) that are much harder to install. After much consternation, I decided to prototype the circuit using a solderless breadboard and then lay out a printed circuit board.
The fact that the SSM2166 combines a mic preamplifier, noise gate, compressor and limiter into one IC explains its popularity. A low-noise voltage controlled amplifier (VCA) provides gain that is adjusted by a control loop to provide compression. The compression ratio can be varied from 1:1 to 15:1 relative to a rotation point. Signals above this rotation point, or limiting threshold, can then be limited to eliminate overload. A noise gate with adjustable time release prevents amplification of noise or hum. The unit performs all of this while boasting low noise and distortion over a 20 kHz bandwidth. The resulting microphone audio signal is thus optimized for communication use. Figure 1 shows the input-output response at different compression settings. Figure 2 provides the schematic and parts list.
All user controls are accessible from the front panel. While they may end up being set-and-forget controls after they have been properly adjusted, I prefer to have them within easy reach for tweaking. The potentiometers were selected to fit in the small enclosure and do not require knobs. BYPASS switch S1 toggles the processing IN and OUT to let you get signal report feedback and for FM use. Switch S2 establishes the level detector averaging time constant. With S2 in the FAST position, the attack time is 22 ms and the release time is 240 dB/s. With S2 the SLOW position the attack time is 220 ms and the release time is 12 dB/second.
The processor, U1, an SSM2166,2 has its input buffer gain initially set to 6 dB and the VCA gain is set to 0 dB. The NOISE GATE control sets the noise gate threshold. Turning clockwise reduces the threshold and sets the level below which input signals are downward expanded at a ratio of 1:3. The COMP RATIO control establishes the compression ratio over the range of 1:1 to 15:1. Turning clockwise increases the ratio. The LIMITING THRESHOLD control determines the level at which limiting begins. Turning clockwise reduces the level at which limiting occurs.
Output Level Meter
The output of U1 (pin 13) is fed to U2, an LM324. Amplifiers U2A, U2B and U2C form a precision full-wave peak detector that drives the input of U4, an LM3915. U4 is set for dot mode and drives low current LEDs to minimize the current demand. Illumination of the green LEDs with an occasional yellow LED indicates output levels in a "safe" area of operation. The red LED indicates a distorted signal and should rarely illuminate if everything is adjusted properly. Trimpot R19 allows the meter to be adjustable.
Mic Input and Output
The RJ-45 microphone input jack at J1 matches the FT-817 microphone. The microphone output at J2 uses a similar connector and is connected to the FT-817 microphone input via a standard CAT-5 computer network patch cord. The microphone and patch cables are not shielded. Instead, they rely on the fact that twisted-pair wires inherently reject noise. All eight microphone wires employ ferrite beads to attenuate any RF. If your VocalMaster is for a radio with different mic connectors, make the appropriate substitution of connector types to fit your radio and microphone.
A novel circuit for providing an 800 Hz tuning signal is provided by U3, a CD4093. U3C forms an astable oscillator with the frequency determined by 2/(R27 x C14). A square wave will be produced at pin 10 whenever pins 2 and 6 are pulled low. R28 and C15 form a simple 160 Hz low-pass filter that modifies the output into a triangular wave. U3D powers down U1 when the signal generator is active and mutes the microphone. To activate the generator, press and hold the microphone push-to-talk (PTT) button and then press the DOWN button. The functionality of the microphone UP, DOWN, and FAST buttons are not affected.
The current requirements (measured values) for this circuit are 13.9 mA during normal operation and 4.4 mA when the signal generator is on. The FT-817 mic power lead was found to deliver up to approximately 15 mA before the voltage dropped below 4.85 V dc, indicating that it should power the VocalMaster without any problems. For other radios, the mic supply voltage may or may not be up to the task. If not, a different source of 5 V must be found from within or outside the radio.
Printed Circuit Board
I was fortunate to have an SSM2166S on hand, courtesy of Analog Devices, and this prompted me to start the prototype. I made an SMD to DIP adapter to allow me to use a solderless breadboard designed for DIP devices. After I had completed the PCB layout I discovered a source for the DIP package, the SSM2166P,6 for which I completed a second version. Both versions of the PCB employ the manufacturer's advice on star grounding and short lead lengths to minimize instability. Builders should get the desired chip before ordering one of the two versions of the PCB.
I ordered prototype printed circuit boards from ExpressPCB.7 I chose the most economical approach by ordering boards that are double sided with plated through holes but do not have a solder mask or component silk screening. Regardless of the manufacturer, groups of builders or clubs should get together and order the boards in quantities to reduce the cost. You can see the various cost breakdowns by installing the free PCB software and loading one of the layouts, VocalMaster-DIP.pcb or VocalMaster-SMD. pcb. If the PCB does not have plated-through holes, be sure to solder the components on both sides of the board. Additionally, if the pad is a feedthrough, insert a short length of wire and solder on both sides. The inside top view of the completed VocalMaster is shown in Figure 3. Check with the author to determine the availability of fabricated boards.
Printed Circuit Assembly
Install the components by referring to the printed circuit board (PCB) assembly drawing for part numbers and locations, and the materials list for corresponding values. For the DIP version, use a small soldering iron of 25 W or less with a standard small tip. For the SMD version, a very tiny tipped iron (1/16 inch) and a magnifier are mandatory for soldering the SSM2166P.
A beginners guide to surface mount technology is a good starting point to get you acquainted with the tools and methods involved.8 Work with plenty of light and use small gauge solder sparingly, as the PCB pads are small. Too much solder will increase the chance of a solder bridge, especially on boards without a solder mask. Cleaning the tip on a damp sponge before every solder joint will decrease the chances of a faulty connection. Although not mandatory, I encourage the use of IC sockets to aid in testing and troubleshooting. Observe polarity on all ICs, diodes and electrolytic capacitors. All components should be mounted close to the board to minimize lead lengths.
Trimpot R19 will be adjusted following assembly to illuminate the red LED on audio peaks that produce distortion in the transmitted signal.
Save the clipped ends of component wires for installing ferrite beads FB1-FB8. Install all components except U1, U2, U3 and U4. Take your time and check your work. If your eyes are as bad as mine, a magnifying glass, headband or worklight may come in handy. When you are satisfied, connect MIC OUT to the radio microphone input with a short jumper cable. Attach the negative lead of a dc voltmeter to the PCB common connection. Turn on the radio, and, using the voltmeter positive lead, check for 5 ±0.2 V dc at the following locations: U1, pin 14; U2, pin 4; U3, pin 14, and U4, pin 3. If all voltages are okay, remove power and connect the mic to the MIC IN jack, J1. Place S1 in the OUT position and ensure that the radio is functioning normally, including the microphone audio, UP, DOWN and FAST functions. If you experience problems go back and inspect the PCB for incorrectly installed components, reversed polarities, solder bridges, and faulty solder joints. When the test is successful, remove power and install the ICs in their sockets. Pin 1 is identified with a square pad on the PCB. Make sure all pins are aligned before pressing them into the sockets or you may bend them.
Setup and Operation
The following instructions are FT-817 specific. For other radios, check the manual for the appropriate control function. FT-817 menu 46 (SSB MIC) should initially be set to the default value of 50. Plug one end of a standard 1 to 3 foot CAT-5 patch cable into MIC OUT jack J2 and the other end into the FT-817 microphone jack. Plug the MH-31 microphone into MIC IN jack J1. (Please note that the VocalMaster is designed to work only with the standard MH-31 microphone. The optional DTMF microphone is not compatible.) Connect a dummy load to the radio, and monitor the transmitted signal on a second rig using headphones. It is very important to monitor your transmitted audio until you have adjusted R19 and are familiar with the operation of the VocalMaster, as improper settings may produce a noisy or distorted signal.
While monitoring your transmitted signal, adjust the controls as follows: TIME CONSTANT to SLOW, NOISE GATE to 10, COMP RATIO to 10, and LIMIT THRESHOLD to 0. At these settings you should hear distortion in the transmitted signal. Adjust R19 to illuminate the red LED on these audio peaks...