Unusual Microphone Amp needed.

Discussion in 'General Electronics Chat' started by HalloweenBob, Apr 21, 2014.

  1. HalloweenBob

    Thread Starter New Member

    Nov 27, 2013
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    I will try to describe what I need and also what I have tried that I expected to work, but didn't. Maybe someone can lead me in the right direction.

    I need a small amplifier (Mono or Stereo) that accepts a mic input, perhaps for a mini-electret mic. The amp can run off a small battery. The most important part is that the very lowest end of the output power needs to be able to be adjusted. I will use this amp to drive a set of headphones. I need to be able to turn it on and make a VERY small adjustment to the volume control so that the output can bearely be heard. The problem I have been having is that as soon as the amp is turned on, it is already too loud. Most people aren't concerned with the low output that is difficult to even hear, but I need that part of the range for what I'm doing.

    I was using a cheap amplifier kit (http://www.ebay.com/itm/FA672-SMALL-POWER-AMPLIFIER-2W-MICROPHONE-WITH-SPEAKER-KIT-/320993538753). This is a good example of the size and quality that I need, but the adjustment POT on the board was so sensitive that the slightest movement raising the volume skipped right past the range I wanted and was too loud. Keep in mind, that when I say too loud, I am still talking about sound levels that you would have to strain to actually hear, so too loud is relative here.

    It was a 10K POT, so I thought if I replaced it with a special 5 turn 10K POT, I would have more control and would be able to make more exact, small incremental adjustments, but it still didn't work any better. It seemed as though the chip on the board had a threshold point at which it turns on and you can't get any softer than that.

    Is anyone familiar with an amp or pre-amp/amp combo that I can control the output level in a way that I can control the very lowest end of the volume spectrum in a linear fashion from no sound at all up to a point where you can first hear a sound. The range I need is just below the point where you consciously hear it, but there is volume there.

    I know no one else cares about this part of the output range, but I do need it for a project.

    Please help me if you can.

    thank you.
    Bob
     
  2. Alec_t

    AAC Fanatic!

    Sep 17, 2013
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    Assuming you still have the original 10k pot you could use that to attenuate the output from the 5-turn pot.
     
  3. THE_RB

    AAC Fanatic!

    Feb 11, 2008
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    Adding another pot at the input may do what you need but will worsent he signal to noise ratio, and you may still get hiss and hum even where there is no sound.

    A better method would be to put a pot on the output, in series with the speaker. Since you only need barely audible sound levels, try a 3.3k (3k3) resistor in series with the speaker, that will make it very quiet. That is the "loud" end of your range.

    Then add a 10k pot in series with the speaker and 3k3 resistor (all three in series like a chain).

    That will give you an adjustability range from inaudible to just audible.

    You may need to change the 3k3 resistor value depending on your amp's volume and gain. But that should be a good starting point.
     
  4. GopherT

    AAC Fanatic!

    Nov 23, 2012
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    Could your application benefit from a compressor circuit. These circuits amplify small signals (exponentially more as signals are weaker and weaker). They do not do anything to signals in the mid-volume range and they compress (attenuate) more and more as signal gets stronger. The output is a bit distorted but still comprehendable.

    The chip you need, a compander, compresses on half of the chip and expands on the other half (to return the sound to the normal (un-distorted) waveform).

    As I write this, I realize that you may be using linear pots, and hence your difficulty. Make sure you have "A", "log" or "audio" pots to adjust volume.
     
  5. HalloweenBob

    Thread Starter New Member

    Nov 27, 2013
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    0
    I did indeed use a linear POT. the 5 turn POT I replaced the one on the kit with was a Linear POT. I don't know what the original was.

    I will look into all the suggestions in this thread

    Thank you
     
  6. AnalogKid

    Distinguished Member

    Aug 1, 2013
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    This is a very common circuit, a one-transistor mic preamp and an LM386 power amp, although the 386 can not make 2W into 8 ohms so it either is a different chip with a very similar pinout or the product details are wrong.

    If the original pot has a B on the back, like B10K, that is an audio-taper pot. Putting a resistor in series with the speaker probably is the easiest way to shift the audible output power range.

    If you don't like that approach, another one is to renstall the original pot with only two legs going through the pc board. Viewed from the front of the pot, the left leg is GND, the center leg is the output. Those go back into the pc board. The right leg is the input. Bend it up so it is free of the board, and solder a resistor between it and the hole it supposed to go into. If you use a 39K resistor, the full range of the pot now is only 20% of the gain it used to have.

    If you can trace out a schematic, it would help. If the chip is a 386, it has gain adjust pins. Getting less gain may be as simple as removing a capacitor. Page 5 has the applicable circuits.

    ak
     
  7. GopherT

    AAC Fanatic!

    Nov 23, 2012
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    B10k is a linear pot in most parts of the world. A10k would be the audio (log) taper.
     
  8. AnalogKid

    Distinguished Member

    Aug 1, 2013
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    oops. been too long...
     
  9. ian field

    Distinguished Member

    Oct 27, 2012
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    For the ultimate in simplicity you could use the TL431 programmable zener/adjustable shunt regulator. Its basically a comparator with its own 2.5V reference built into a TO92 package, if yoy roll off the comparator with enough nfb, it works in linear mode.

    Its not a new idea - there's various examples on the web, but usually depicting a simple resistor from cathode to the control input stabilising the DC operating point.

    The feedback resistor provides DC nfb to stabilise the operating point - but it also provides AC nfb, so the gain is not much more than you'd get wiring up a small MOSFET the same way.

    The trick is to split the feedback resistor with the top bit a few times the cathode resistor and the bulk of the resistance in the section to the control input. Then AC short the tap in the resistor to GND with a capacitor. You can of course put any simple network of components in series with the AC nfb shunt cap to tailor the gain.

    The 2.5V you have to set at the control input suits an electret mic very well, I suggest a cathode resistor around 2k2 to Vcc as a starting point, connect the electret from control input to GND and a 47k pot from cathode to control input, adjust the pot to set 1/2 Vcc on the cathode.

    You'll probably need about 5k6 for the top tap resistor, its best to have this in situ while setting up the DC point to select a standard value of resistor.

    With just a capacitor shunting the AC nfb, the gain will be huge! You can simply moderate the amount of capacitor with some resistance in series with the cap, or you can use those molded RF chokes to tail off rising frequencies (local radio breakthrough can sometimes be a problem). You can also add a CR network direct from cathode to control as long as it doesn't add a DC path.
     
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