(3) TDA7284 47 dB! ALC / Speech Compressor OP-AMP < > 47 dB Compression

Description

TDA7284 Hi-Performance Op-Amp Speech Compressor / ALC / AGC
1008
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(3)
Speech Compressor Op-Amps
ALC provides
47
dB
of Compression
Very Low 1.2 µV Input Noise
Datasheet for TDA7284:
https://linearparts.com/documents/TDA7284.pdf
Application Schematic:
https://linearparts.com/images/TDA-sch-1024x768.jpg
Price is for
(3)
pieces
Dual Low Noise Op-Amp
Internal Auto-Level Control
Dynamic ALC Range: 47
dB
Single Supply: +3 V
to
+12 V
Requires Few External Parts
External Enable / Disable Control
Datasheet for TDA7284:
https://linearparts.com/documents/TDA7284.pdf
Lab Test Results
We were intrigued by the datasheet claims for Automatic Level Control (ALC a.k.a Automatic Gain Control, AGC) and wanted to see if this simple looking Op-Amp was as good as promised. We bench tested the basic circuit above and were very impressed with the results! We've included all the documentation we have for this part. Please review it and ask questions before you bid. We'll let you take it from here.
COMPRESSION vs CLIPPING
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 average circuit gain over a
very wide
range to maintain a virtually constant amplitude, distortion-free output. When the output level rises, the TDA7284 responds like having a very quick hand on the volume control, and 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 was for use in auto cassette players. It functioned as low distortion pre-amp that could automatically and smoothly compensate for the wide ranging levels between cassette tapes or between songs on a tape.]
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 mV
RMS
. As the input rose 39 dB from 1 mV to 90 mV, the amplifier output level (at approximately 1 V
RMS
) 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 V
RMS
producing about 1.4 V
RMS
out.
Simply put, cranking up the input approximately 500x, the output voltage didn't even double (2x).
APPLICATIONS
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.
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.
Use an electret microphone as input and hook the output to an LED driver for a visual sound display.
Get your QRP transmitter modulation up to 100% without splattering.
Perfect for recording circuits (the original application) where input levels vary tremendously.
Reduce the effects of fading on your shortwave radio using the 7284 as an audio pre-amp.
Use in Intercoms and Speaker Phones.
Boost your baby room monitor performance.
Oh the possibilities! Here's a few ideas, but please review the datasheet for more info.
COMPRESSION
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
Ω resistor with a 5M
Ω pot for a variable compression control.
MAXIMUM GAIN
The maximum gain is fixed by the resistance ratio of the 56K
Ω / 56
Ω voltage divider at pin 5. Typical Open Loop Gain is 78 dB. The values shown produce 60 dB gain. Replace the 56K
Ω resistor with a 50K
Ω pot for adjustable loop gain.
INTERNAL SWITCHES
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.
FREEBIE
(on *request* but only while supplies last)
Those 4.7M
Ω resistors (pin 4) are not common in most experimenter junque boxes so if you need some, while our supply lasts, we can include some samples at no charge. You are not bidding on them. They are NOT part of the auction and are offered only as a courtesy with your payment. In order to receive them you
*must request them with your payment*.
Please include your request in the "Note to seller" field area of your PayPal payment to make sure we get the message.
7284-3
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Unless otherwise noted above:
NOS (New Old Stock) or better, clean, unused
30 day Non-DOA, Good-on-Arrival Guarantee
Untested, but NO JUNK or suspect parts
PCB parts are thru-hole, NO SMD/SMT
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IMPORTANT: The parts we sell are categorized as new because they are unused, but many of our parts are those rare, hard-to-find, discontinued, manufacturing surplus and
/
or old stock commonly known in the parts business as 'NOS' (New Old Stock). Some of our stock pictures show date codes which may vary from the actual product shipped.
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We strive to give full and accurate descriptions to earn your 5-Star Feedback. Any known deficiencies are listed in the detailed description. Datasheets are provided when available. Some datasheets may be close but not a perfect match because of part revisions or because they may be from a different manufacturer. If you have any questions concerning our products please email us before purchasing. Your complete satisfaction is important to us before and after the sale. Thank you.
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