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> (6) alc / speech compressor op-amp [47 db compression]
(6) alc / speech compressor op-amp [47 db compression]
ALC provides 47 dB of Compression
Very Low 1.2 V Input Noise Optional ALC Disable Pin
Datasheet: TDA7284 (163K PDF) Application: Schematic
External enable / disable control
You may have heard of 'Speech Clippers'. Clippers limit audio peaks to a fixed level. But clippers don't regulate the gain; they just abruptly chop off the audio peaks. Clipping is a poor choice in controlling output level by sacrificing quality. Clipping produces severe distortion commonly heard as the popular 'fuzz' sound on guitar amps. Obviously, fuzzy speech is not desirable in voice communications. The TDA7284 does NOT employ clipping.
In strong contrast, the TDA7284, though simple to use, is much more sophisticated. It actually monitors the constantly changing output peaks and then varies the circuit gain over a very wide range to maintain a virtually constant amplitude, distortion-free output. When the output level rises, then like having a very quick hand on the volume control, it reduces the gain to keep the output constant but without distortion. Similarly, when the output falls, it turns the gain up to keep the output level constant. [The original application of this IC in auto cassette players kept the music distortion free while maintaining a constant volume from various recording levels between songs.]
As expected, we found exceptional sensitivity and a wide dynamic input range. Our lab tests of the circuit shown above were impressive. The output quickly leveled off with a very low input signal and didn't change much after that. At very low levels, before compression kicked in, we measured 59 dB gain. ALC action began with an input below 1 mVrms. As the input rose 39 dB from 1 mV to 90 mV, the amplifier output level ( approx 1 Vrms ) rose less than 1 dB. So we drove it harder to it's limit. Output distortion became noticeable on our scope when driven with a whopping 0.5 Vrms producing about 1.4 Vrms out. Simply put, cranking up the input approximately 500x, the output voltage didn't even double (2x).
1. Surveillance transmitter "bugs" benefit greatly with ALC to pick up weak distant voices at maximum sensitivity while compressing (Not clipping!) strong nearby voices to prevent distortion.
2. Make an audio signal tracer that can follow the signal from the front-end to the back-end of any amp without having to fuss with volume controls.
3. Use an electret microphone as input and hook the output to an LED driver for a visual sound display.
4. Get your QRP transmitter modulation up to 100% without splattering.
5. Perfect for recording circuits (the original application) where input levels vary tremendously.
6. Reduce the effects of fading on your shortwave radio using the 7284 as an audio pre-amp.
7. Use in Intercoms and Speaker Phones.
8. Boost your baby room monitor performance.
Oh the possibilities! Here's a few ideas, but please review the datasheet for more info.
With our test configuration, we achieved compression of about 40 dB. Experiment with the R/C values on pin 4 if you need more or less. These parts also affect the attack and decay times. Grounding pin 4 disables the compression and the amp will run at the gain determined by the voltage divider at pin 5. Replace the 4.7M resistor with a 5M pot for a variable compression control.
The maximum gain is fixed by the resistance ratio of the 56K / 56 voltage divider at pin 5. Typical Open Loop Gain is 78 dB. The values shown produce 60 dB gain. Replace the 56K resistor with a 50K pot for adjustable loop gain.
Optional DC controlled switches can be wired to remotely disable the ALC, mute the output, or change circuit gain without ground loops or added noise.
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