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Very simple Audio compressor for FM transmitter

Discussion in 'Electronic Projects Design/Ideas/Reviews' started by Willen, Dec 6, 2013.

  1. Willen

    Willen Well-Known Member

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    1st time Audioguru said me about a trick to broadcast loud audio from FM by compressing audio using audio compressor. Professional compressor is very complicated and expensive. But here on PiRa.cz I got very basic circuit of a compressor. As before I was ready to make it but again few components stopped me.

    There are two components i didn't find. I think I can use any dual opamp (like noisy LM358 or TL072) instead of TLC272; can't I?

    Main problem is I didn't get BF245C FET. I just have two FETs- 596 and BS170. If I can't use them too, can't I use transistor by simply modifying?

    So excited!!

    ( Alternatively, found more basics other compressor too but I am not sure they will work better or not. electroschematics.com )
     
    Last edited: Dec 7, 2013
  2. audioguru

    audioguru Well-Known Member Most Helpful Member

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    For other people reading this thread:
    Like I said in my reply to your "communication" questions about compressors, the one at PiRa.cz does not spec its distortion which is probably fairly high. Also, its circuit must use those special parts.

    The circuits at electroschematics are very simple and are known to produce distortion.

    That is why I linked to this excellent compressor/limiter project: http://www.sound.westhost.com/project67.htm
     
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  3. Willen

    Willen Well-Known Member

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    The link recommended by you looks more easy to understand too. But--

    - Why they didn't recommend which diode should use? Any silicon around 0.7 forward voltage?
    - They said "This 'audio peak limiter' employs a FET as a variable resistance to attenuate the inputsignal according to a control voltage (CV)." Then can't I use any FETs like 596 or BS170? (I have these, and I think they give audio speed).

    Also, I checked few specs of these FETs and got 2N5459 and BS170 are little same. But in real I learnt more about BJTs and BJT biasing but I am very unknown person about FETs and its bias.
     
    Last edited: Dec 8, 2013
  4. dave

    Dave New Member

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  5. MrAl

    MrAl Well-Known Member Most Helpful Member

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    Hi,

    In your first circuit, the op amps look like they are used in a very general way so you could use other op amps as long as you get the power supply right as per what the op amp needs. Some op amps need a much higher voltage than others. The FET type op amps should have 10v or more.
    There's a questionable node in that circuit though that looks like it may not be biased right, but i'd have to spend a few hours on it to find out if it is right or not.

    For the FET itself, you will have to compare data sheets. See if the threshold voltage and stuff like that is similar.

    I agree that the circuit that audioguru linked to is a much simpler circuit and should be easier to build and there's nothing questionable about it.
    Maybe try that first and see how you like it.
     
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  6. audioguru

    audioguru Well-Known Member Most Helpful Member

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    Use a low current fast signal diode (1N4148 or 1N914) not a slow 1A rectifier.

    A 2N5459 Jfet is very common and inexpensive in The West. A BS170 is not a Jfet, instead it is a Mosfet that is COMPLETELY DIFFERENT.
    I do not know what is a "596" because some letters or numbers are missing.
    Since you cannot obtain common electronic parts then do gardening instead of electronics.
     
  7. Willen

    Willen Well-Known Member

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    hee hee hee I even don't have garden so trying search equivalent components. Wish to be there on USA/Canada, I would make almost anything! :) But now I am in a Electronics desert... :(

    596 is a JFET, mostly electret capsules (condenser mic) use it. Fairchild has 'KSK596' marking. Comparatively it is low powered than 2N5459, But I can't decide this will perform here or not.
     
  8. MrAl

    MrAl Well-Known Member Most Helpful Member

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    Hi,

    Well maybe if you plant the 596 it will grow into a 2N5459 given enough water and proper lighting, har har har.

    I'll see if i can find data sheets for these two guys and see what the difference is. One thing is for sure though, a MOSFET will not be the same.

    Hey, taking another look at the linked to circuit, it appears that there may be a problem with the FET biasing in that circuit. It looks like the source is being biased more positive than the drain. Is the 2N5459 a symmetrical device? But something still doesnt look right.
     
    Last edited: Dec 8, 2013
  9. Willen

    Willen Well-Known Member

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    Can't we use 596 jfet by modifying few resistors?
     
  10. ronsimpson

    ronsimpson Well-Known Member Most Helpful Member

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    I have done compressors, limiters with JFETs and MOSFETs. It has been 30 years. The bias is different.
    I switched over to ICs most of which are not in production now.
    Here is another choice.
    View attachment 82554
     

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  11. ronsimpson

    ronsimpson Well-Known Member Most Helpful Member

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    The first schematic: http://pira.cz/hyperlme.htm is for FM broadcast. The other ones are for audio like recording and not for FM.
    If you are to use this for broadcast you must have Preemphasis!
     
  12. audioguru

    audioguru Well-Known Member Most Helpful Member

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    The spec's for a KSK596 Jfet are COMPLETELY DIFFERENT to a 2N5459. For example a 2N5459 with zero G-S voltage conducts 4mA to 16mA. But a KSK596A conducts only 100uA to 170uA. A KSK596E conducts 440uA to 800uA. The cutoff voltages are also very different.
     
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  13. ronsimpson

    ronsimpson Well-Known Member Most Helpful Member

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    MOSFET are used differently as a VCA.
    See US patent 5808516 A
    I have used this method to get better distortion with a MOSFET used as a voltage controlled amplifier. Resistor 41 & 45 along with C43 solves some problems. If you don't want to use the JFET then use this method to linearize the signal.
    upload_2013-12-8_18-46-53.png
     
  14. audioguru

    audioguru Well-Known Member Most Helpful Member

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    Feeding half of the signal from the drain to the gate is also used for slow Jfet VCAs like in sinewave oscillators. But as explained in the Fast Compressor project I posted in post #2 its attack time is too slow for an audio compressor. The first part of a loud sound BLASTS until the capacitor charges.
     
  15. ronsimpson

    ronsimpson Well-Known Member Most Helpful Member

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    If I had to make a JFET limiter for broadcast; I would use the pre-emphasis from post #1 with the Fast Compressor in post #2.
     
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  16. MrAl

    MrAl Well-Known Member Most Helpful Member

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    Hi,

    I like the idea of the resistance controlled opto coupler, which would give very true resistance limiting. Alternately a cadmium sulfide cell coupled with an LED enclosed in a home made enclosure that is light proof.
     
  17. ronsimpson

    ronsimpson Well-Known Member Most Helpful Member

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    35 to 40 years ago I made a cadmium sulfide cell limiter. The cell responded too slow.
    I tried several way of speeding up the cell. Just too slow.

    attached is a datasheet showing 5 to 10mS.
     

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    Last edited: Dec 8, 2013
  18. MrAl

    MrAl Well-Known Member Most Helpful Member

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    Hello there,

    Are they any faster today, and how fast do you think we need here for an audio limiter?
     
  19. audioguru

    audioguru Well-Known Member Most Helpful Member

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    The fast compressor has an attack time of 5ms, the slow one is 100ms.

    The audio compressor at my local TV station sounds like its attack time is real slow at half a second. When the announcer takes a breath before speaking then you are deafened by the hurricane. The first syllable of every word sounds like a shot gun.
     
  20. ronsimpson

    ronsimpson Well-Known Member Most Helpful Member

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    Audio compressor limiter attack and release time:
    Here is a page in a manual of a product that I have many of.
    In manual mode the attack time and release time can be set over a range of times shown. In auto mode the time is dependent on the audio. The response is complex and has both fast and slow times, at the same time.
    For broadcast set the attack time short. 1 to 5mS.
    upload_2013-12-8_20-33-31.png
    The peak limiter is another function at the end. Peak limiting is important in broadcast.
    upload_2013-12-8_20-40-2.png
     
  21. ronsimpson

    ronsimpson Well-Known Member Most Helpful Member

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    More on Audio compressor limiter attack and release time:
    upload_2013-12-8_20-48-52.png
     

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