Arrangements for increasing the modulation strenght
Circuit of a preamplifier for amateur radio microphones
If the output voltage of the microphone is not sufficient to entirely drive the modulator of a transmitter, a preamplifier is usually needed. The circuit shown here is suitable for two-port electret capsules. It is important to pay attention to correct polarity. For three-terminal capsules, the 10kΩ resistor can be inapplicable. The third connection should then be connected to the positive pole of the 100μF electrolytic capacitor. Without the resistor, the circuit is also suitable for dynamic microphone capsules. The polarity does not need to be respected in such microphones.
Despite the low circuit complexity, the circuit causes a quite significant increase in the microphone signal. The gain is dependent on the transistors respectively in particular their current amplification factors and is approximately between 30 and 40 decibels. It can be expected with an output level of 0.1 to 0.5 volts, depending on the used microphone, the speech volume and the distance. It can be reduced with the output potentiometer and thus adjusted to the value suitable for the modulator.
A dynamic compressor for amateur radio microphones
An independent of speech distance and speech volume consistently strong and therefore good understandable modulation is achieved with a compressor microphone. Simple amplification microphones merely cause an overall higher level at the microphone jack of the transmitter. If the speech is too loud or the speech distance is too tight, it speedily overdrives and the modulation sounds distorted. In the case of the compressor microphone, a special circuit ensures that quieter passages are raised in level or, in relation to this, louder sounds are lowered, the audio signal so is compressed. For simple amplification microphones, the modulation sometimes may be distorted, other times still too low, depending on the distance and volume of the speech. With a compressor microphone, however, it is possible to always achieve the optimum degree of modulation.
An appropriate signal compression can be achieved by the microphone amplifier with a AGC (Automatic Gain Control) is equipped, which works much like an automatic recording level control in old tape recorders. There, a part of the output signal is diverted from the amplifier and fed to a rectifier. The resulting DC voltage is proportional in value to the magnitude of the output signal from the amplifier. It is now used to attenuate the audio signal with increasing strength. Unlike the level control in music recorders, the control of the compressor microphone works with much less time delay. With music that would lead to clearly audible distortions. Speech, on the other hand, is only sounding compressed in this way, but much stronger. Intelligibility increases considerably, especially in the case of interference or higher noise.
The circuit shown, I have developed specifically as a dynamic compressor for radio operation. It can be inserted between a standard PTT microphone and the modulation input of a transmitter. Another option is to build a complete compressor microphone yourself. I built such a circuit in a metal console case, where I mounted a disused gooseneck microphone from a taxi radio control center. In this way arised a universally applicable compressor microphone for radio use, which can be optimally adapted to almost any device.
For the loudest possible modulation with low distortion at the same time, the level with the output controller is matched to the sensitivity of the microphone input in the transmitter used. The 10-kiloohm trimmer is adjusted once with the sensitivity pot turned up so that there is only a marginally greater volume at close-range than when talking from a distance. Once set, only the input-side potentiometer will be used. It is to be adjusted to the respective requirements. Here an excessive regulator position should be avoided in the interest of good intelligibility: Due to the compression characteristics otherwise disturbing noise, such as caused by a power amp fan, a multimedia device of the neighbors, by birdsong or by passing cars outside, quickly be as loud as the speech to be transmitted. Also it can come in the transmission to an undesirably strong reverberation. Should this be the case even when the sensitivity is set at a fairly low level, this can be counteracted by attaching sound-absorbing materials (for example felt or foam) to the walls that are close to the microphone. A remaining, slight space reverberation can be accepted with pleasure: The abundance of the modulation thus created can even be helpful for the intelligibility.
Speech-clipper using single components
A so-called speech clipper can be used to make the modulation appear louder not only in quieter passages, but also overall. Here we amplify the modulation signal so that the signal peaks are cut off. The resulting distortions in the form of harmonics are reduced by a following low-pass filter. The signal density, which increases in this way by given a enough suitably high input level, leads to a less beautiful sounding, but penetrating and, with lower reception field strengths, more understandable modulation. Less strongly driven, such a limiter can be used in FM transmitters in order to prevent the maximum intended frequency deviation from being exceeded when signal peaks occur.
In the circuit shown, the input signal is amplified with a transistor pre-stage. At the collector of this stage there is a trimmer with which the sensitivity can be adjusted to the microphone used. It should be set so that only the peaks of the microphone signal are limited in the following amplifier. For FM transmitters, the frequency deviation is set on the output side to the value that matches the channel grid used. Another trimmer is used to adjust the symmetry of the limitation. So that the transmission signal does not become an inadmissibly large bandwidth, the harmonics resulting from the limitation here are suppressed at the output by an LC low-pass filter.
Modulation limiter with the IC TA7061
The integrated circuit TA7061 can be used to build a modulation limiter that works very well with little effort. This is actually designed to use as FM IF amplifiers in FM radio receivers and in analog TV receivers. Its outstanding limiting properties can also be used in the low frequency area. Therefore, it has also been found in many 2m FM two-way radios as modulation limiters, such as in the Trio TR-2200G, Trio TR-7200G, Icom IC-22A, and several Grundig CB radios, such as the model CBM-200. This moved me to use such a circuit in a self-built transmitter. Out of it came the circuit shown below, which brings quite excellent results. The IC limiter is followed by a low-pass for the voice frequency range, minimizing the harmonics that occur during clipping. In my transmitter the AF output is directly connected to the FM modulator, which works with a capacitance diode.