LF roll-off on mic amp

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Walrus

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Jul 18, 2006
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Could someone explain why this mic amp has a 2dB roll of at 20 Hz when the gain is at maximum, but is flat when gain is less than max??
There is a 10 Ohm resistor in series with the gain pot that I forgot to add to the drawing....
 

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The 2dB roll-off when the gain is maximum is due to a capacitor inside the loop.  Reducing gain by increasing negative feedback will flatten the response of the amplifier.  I would suspect something the size of the 22uF capacitor, which looks like about 500 ohms at 15Hz, and greater below that frequency; but I wouldnt worry about -2dB at 20Hz at max. gain unless you are recording VERY LARGE pipe organs or earthquakes (which will probably be loud enough to need the gain turned right down)!

Yes, it is usual to have a capacitor in series with the level control pot in a circuit like this.

David
 
Walrus said:
Could someone explain why this mic amp has a 2dB roll of at 20 Hz when the gain is at maximum, but is flat when gain is less than max??
There is a 10 Ohm resistor in series with the gain pot that I forgot to add to the drawing....

Is there also a cap in series with the 10 ohm and 5k pot? That is the common reason for similar preamp designs losing LF response at max gain. 

I don't see a dedicated LF pole in the high gain part of the schematic you drew, and the input pole should be reasonably independent of gain, while the effective input impedance of the transistors will drop some with max gain. so there could be some interaction in the margin. 

  JR
 
Oops.  :-[  Yes, there is indeed a capacitor in series with the gain pot. It's value is 2200uF. I thought that this could be the culprit, so I shorted it out, but it had no effect still had 2 dB roll off.

By the way, it's an MCI mic amp.
 
If you're absolutely sure that the cap in series with the gain pot is not the culptry (also make sure that shorting it does not cause too much DC offset!), then I'd first review the measurement technique. Is there any chance that the measurement setup could cause the frequency variation? Is there perhaps the mic preamp clipping somewhere at the highest gain setting? Verify with another amplifier. If this fails, short the 15 uF input caps. As JR noted, the input resistance of the preamp will drop somewhat at high gains. I'd be surprised if that'd make the difference, but then I have not investigated this in detail and thus my guess at the order of magnitude of the effect is a shot in the dark.

Samuel
 
After reading your post Samuel, I decided to try shorting out the capacitor again. Guess what, the croc-clip cable I used was broken  :-[  :-[ (it has mini clips covered in a plastic boot and I couldn't see that the wire was broken but still being held by the crimp on the insulation!!)
Taking the capacitor out and wiring direct to the pot brought the 20Hz up to -0.25 dB. There is only about 5mV DC offset across the pot, so I may well leave the cap out altogether, if it doesn't cause noise when changing gain.

Thanks for all the suggestions and circuit description. I'm still learning the hard way after all these years.... ;D
 
Samuel Groner said:
Broken cable sounds familiar!

There is only about 5 mV DC offset across the pot.

With 60 dB gain, this makes 5 V at the output...

Samuel

But surely in this topology, the 5mV will always be there from LM394 emitter to emitter, whether there is a cap in series with the pot or not.
The cap just serves to stop any dc scratchiness in the pot. Or am i missing something again? ;D
 
The cap reduces the DC gain to unity, so you would only get 5 mV of DC at the output instead of the 5V with the cap shorted and 1000x gain. The AC gain for signal is still 1000x but this cap needs to be a pretty large value to deliver a LF pole in combination with low R gain limit resistance.

JR
 
Yep, I was missing something again!  :)

On the basis of a bit of experimentation with a few caps, I'm going to use a 15,000uF 6.3V cap.
I think that should give me only a minor roll-off at the bottom.
 
Walrus said:
On the basis of a bit of experimentation with a few caps, I'm going to use a 15,000uF 6.3V cap.
  • Make sure it is connected the right way round to deal with your 50mV
  • Don't use a Tantalum
Having it the right way round is good for long life in Aluminium electrolytics.  Tants die INSTANTLY if there is ever a reversed voltage; eg if you ever have a very LF signal or maybe even bump a mike with good LF.
 
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