Constructors based in the USA will find this
Audio AGC USA parts list
from John O'Loughlin useful.

 

PIN No Voltage Ohms to
GND
1
0
0
  
2
0.13
1K5
  
3
2.54
40K
  
4
2.53
5M
  
5
2.27
4M9
  
6
2.27
4M4
  
7
1.54
238K
  
8
0.54(varies)
3M3
  
9
4.5
1M5
  
10
2.1
27K8
  
11
4.89
54K
  
12
0
0
  
13
2.33
3.77M
  
14
5.1
4.65K
  
.


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AUDIO LIMITER/COMPRESSOR

 

 

 
 



 

 

 

 


Please click circuit to get latest version PDF .

 

Microphone AVC/Compressor with 60dB range & adjustable noise floor.
Designed to give >1v 1k audio level for Comtech or Digital encoders from standard or electret microphone. Double sided board size: 40mm x 35mm (approx 1.5" x 1.75").
DC input is 8v-12v .

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Contact me at

 

 

 

 

 

'T' Attenuator
values for
audio use
(75 ohms)

 

NOTES: Only C16 & C13 need to be 0.22uF. The others C1,C2,C3,C7 are just decoupling and 0.1uF appear to work ok. Alternatively it is possible to piggy back two 0.1uF to save money. C8 can be any value up to 100uF. C4: 100uF will be OK. D1 can be smd, remove the coating over the tracking to fit. If not using another input on JP1 pin 9 then simply link together JP1 pins 9 and 10 and s/c C16. Presets can be 3mm format if 4mm are not available. Some capacitor physical size descriptions in the original parts list were not correct. C4,C5,C11,C12 are larger than 1206, and can be aluminium SMD if preferred.

 

 

 

 

 

 

 

Not working? Here are the readings you can expect on IC1. Set preset pots to midway. Some readings may depend on the pot settings so use this as a general guide. Most problems encountered have been due to solder bridges particularly on IC pins.

 

 

 

 

 

 

 
no menu? click here
  R1,R3 / R2 Preferred Values
75ohm Actual E12 E24
1dB 1304 / 8.6 1200 / 8.2 1300 / 9.1
2dB 654 / 17.4 680 / 18 680 / 18
3dB 438 / 26.4 470 / 27 430 / 27
4dB 331.4 / 35.7 330 / 33 330 / 36
5dB 267 / 45.5 270 / 47 270 / 47
6dB 225 / 56 220 / 56 220 / 56
8dB 174.2 / 79.2 180 / 82 180 / 82
10dB 144.3 / 106.2 150 / 100 150 / 110
12dB 125.3 / 139.8 120 / 150 130 / 130
18dB 96.6 / 29.3 100 / 270 100 / 300
20dB 91.6 / 371 100 / 390 91 / 360

 


TOP and BOTTOM Views of the PCB.

Overall dimensions are 40mm x 35mm.(= roughly 1.5"x1.75")

Professionally manufactured double-sided printed circuit boards are now available from this website. The pcb is as shown in the photo (bottom left).
Boards are available from me. See the STORE page.

A pdf with build details and description of the circuit is HERE.* Please note the physical sizes of tantalum capacitors in the parts list are not all correct (too small). C6,C8 are correct. C4,C5,C11,C12 should be larger than 1206, and can be aluminium SMD if preferred.

Sorry, I cannot supply individual components or kits.
PayPal me or email for more details


 

Initial Setting Up:
 Set output level R7 to 100% clockwise. Set R9, R11 to  counter-clockwise.
R10 to 75% clockwise. Connect microphone & headphones, speak (or shout!) into mic and adjust R9 to avoid distortion. Whisper near mic to adjust R10. If in a noisy background environment adjust R11 to mute the sound when you are not speaking. It can take a while to get settings that suit the conditions. Feed a line input from your pc audio via a 100 ohm pot to ground to test the low/high range of compression. Some microphones may need around 50pF (Adjust On Test) across the input socket to reduce hiss & treble and coupling C10 can be up to 10uF if mic is low Z type. Check for about 1.5v for mic on JP1 pin 6. Drop R13  down in value,eg 5k ,to give 1.5 to 2v DC for your electret mic.

Other Options:
If you are using it for larger signals than a microphone you may need to pot down the input with about a 10k pot and reduce the value of R2 to about 2k7. If the noise floor is too high it can cause unwanted triggering or clicking on a quiet passage, you can increase the value of R5 to 100k or more and increase the value of C6 to 4u7F or 10uF.