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Audio Compressor AGC Schematic

Audio Compression Amplifier /AGC

This is an audio compressor/AGC (automatic gain control) with an astonishing 75db input voltage range. A P-Channel JFET is used as a variable resistance element that is applied as a variable attenuator. Distortion is minimized by keeping the signal voltage across the JFET extremely low (1.3mV). This is an improvement over similar circuits that place the variable gain element in the op amp feedback circuit where it supports high signal voltages and subsequently introduces significant distortion.
AGC attack time & recovery time may be adjusted by altering resistor values. Signal input levels cover the range of 1.5mV to 8.3V. The output signal level is close to the consumer audio standard of -10dbV.

Schematic of Audio Compression Amplifier Circuit

Audio Compressor AGC Schematic

Circuit function

The circuit consists of four basic sections: Variable attenuator (R2 & Q1), fixed gain amplifier (U1) and amplitude detection (Q2) & Q1 bias control.

R1 provides a resistive load for the dynamic microphone – it may vary from about 150 to 600Ω for low impedance microphones to about 50K for high impedance microphones. If a transformer is used to convert the balanced line to unbalanced, R1 = square of the turns ratio times the microphone impedance.

The attenuation caused by R2 and the dynamic resistance of Q1 is a function of the JFET DC gate voltage: +6V provides no attenuation and 0V provides maximum attenuation.

The amplifier is a textbook non-inverting amplifier using a TL081 op amp. Note that there are many suitable operational amplifiers that may be used and some are better for audio than others. R4 & R5 set the single-supply amplifier bias point midway between 0 and 12V. C2 bypasses this point to ground potential and R3 provides a DC path for the extremely low op amp bias current (perhaps a few picoamperes).

The amplitude detector is very crude as it detects only the positive going signal excursion. When this signal voltage exceeds 0.65V, Q1 turns on and starts to discharge C3 through R10 – the lower the value of R10, the faster the attack time. The attack time must not be so fast that it responds to every peak, nor so slow that it takes too long to bring the signal level down to normal.

When no peaks are detected, C3 continues to charge via R9 thus reducing the attenuation of the variable attenuator. The resistance of R9 controls the recovery time. The attack time is generally one or two orders of magnitude faster than the recovery time. I experimented with a voltage clamp circuit that prevented the gate voltage from exceeding 6V, but found that it was unnecessary.

The JFET (Q1) has a sloppy range for the Vgs off parameter (3 to 6V). This tends to vary the bias clamp voltage, attack time and recovery time. For repeatable results, it is recommended that devices be selected for a tight range of Vgs off.

db website http://www.sengpielaudio.com/calculator-db-volt.htm

This is a great reference website that covers db signal level units (dbV in this case) and db signal ratios (voltage gain and voltage attenuation).

For a primer, check out this table of common ratios:

Voltage gain Voltage attenuation
1 db – 1.1 -1db – 0.9
3 db – 1.4 -3db – 0.7
6 db – 2 -6db – 0.5
10db – 3 -10db – 0.3
20db – 10 -20db – 0.1
30db – 30 -30db – 0.03
40db – 100 -40db – 0.01
50db – 300 -50db – 0.003
60db – 1000 -60db – 0.001

Oscillographs

79 Oscillographs

Observe the nice sine waves – minimal distortion for both low and high input voltage signals.

Photos

Conclusions

This was a fun experiment – the results far exceeded my expectations and I believe that it will likely become a favorite among audiophiles. Going with a split power supply (±15V) would enable it to output studio level signals with satisfactory headroom.

For the future

Intelligent gain control – a means of reducing short term cyclic gain variations (“breathing”).

Undocumented words and phrases – for our ESL friends

headroom (or headspace) –noun –literally, the space about one’s head such as in an automobile –in electronics it refers to excess signal level range that may be required for faithful reproduction of audio sound transients.

62 Comments

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  • cdevilscute624gmail-com

    hey can you please help me out with the part of the circuit where the signal gets compressed and how to control the compression ratio and the attack time of the signal.
    Moreover could you suggest me some good articles for understanding the basics of compression
    Please reply as soon as possible

  • prakashilumi-co

    Hey Thanks.
    1. I have selected MMBFJ177 (Vgs (off) 0.8 to 1.5V) mosfet, can you please verify whether this will work or not for 3.3V operation

    2. The Max attenuation formula I think is, Maximum attenuation = min RDSon of Q1 / (R2 + min RDSon of Q1), can you please verify

  • prakashilumi-co

    Hey,
    I would like to apply 3.3V to the circuit, can anyone know what all are the changes and calculations needs to be done.

    • Jim Keith

      It seems to me that C7 may be unnecessarily large for high frequency response–I may not have actually measured frequency response. Reduce the size or eliminate C7 to see what happens.

    • singh-gurbaj5124871gmail-com

      Hey jim, I have a doubt, while using sine wave as input, it has zero dynamics, there is no need for compression only amplifier is enough, so when i use .wav as input it is not working as ac transfer characteristics of amp gives gain only till 1k freq but i need at least 10k. Please help….

    • Jim Keith

      woops, I meant P-Channel. The J176 has a gate-source cut off voltage range of 1 to 4V. A selected device is required.

    • Jim Keith

      This can be made to work if we can find /select an N-Channel device that pinches off @ 3.3V. The output level will remain the same, but the available headroom will suffer. By substituting an LM339 or equiv comparator for Q2 the output level may be reduced (or even made settable via a pot)–in that case, the headroom will be preserved. Note that a low voltage op amp must be selected–I have little experience with low voltage op amps.

  • serge

    Hi! What changes if any to use 9V battery instead of 12V?
    Thanks!

  • Larry Henson

    How about a Corrected Schematic (FET P vs N and of course, a CURRENT Op amp IE LM324 quad, single voltage). LEHenson (at) msn com AE6JI need for an Electronics teaching program. 73’s

    • Jim Keith

      Check out the updated schematic with the correct P Channel JFET symbol. On the op amp, the TLO81 is a more recent device than the old LM324 and is superior for audio applications –reduced crossover distortion and better frequency response. Of course, there are many other suitable and perhaps newer devices that will work well.

  • Jianguo

    Good job. How to adjust it for single 5V power supply system?

  • Sergey Moryakov

    Clear and helpful, thanks!

  • Guru

    how to adjust the peak detector voltage (ex: 1.5v). When it crosses only gain should be adjusted?

  • Paul S.

    I the first paragraph of the circuit you refer to Q1 as a P-Channel JFET but the schematic shows it as an N-Channel. Which is correct? Just and old audio engineer here with an expiring mind…

    • Jim Keith

      Yes, it is a P-Channel device and yes, I always seem to get my FET symbols backwards–my apology

  • Devang Khamar

    hi! I am not able to find IC TL081 could you suggest any alternate op-amps that could work here? I used IC OP07CP the simulation results show that the output gets clipped once the i/p voltage crosses 900mV.

    • Jim Keith

      The OP07 should work OK. Troubleshoot your simulation by replacing Q1 with a 125Ω resistor.

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