How much voltage gain should I get from this preamp?

Discussion in 'The Projects Forum' started by tracecom, Aug 29, 2012.

  1. tracecom

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    Apr 16, 2010
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    Attached is a schematic of a stereo preamp that I just built. With .4 V p-p in, I get slightly less than .8 V p-p out (per channel), so my voltage gain is less than 2. I expected closer to 3. Is there a problem with the design? Thanks.
     
    Last edited: Aug 29, 2012
  2. Markd77

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  3. tracecom

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  4. #12

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    I figure about 2.6

    Be aware, jfets have variations in idle current. Check the DC levels to make sure your output isn't stuck high or low because of trying to use a "universal" design. You should idle at about 3 volts across the 1500 ohm resistor (2 ma). Adjust the 560 ohm resistor to get the idle current right.
     
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  5. tracecom

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    The supply voltage is 9.10 V. Across R2, there is 2.81 V, and across R6, there is 2.55V.
     
  6. #12

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    That's pretty close. I guess now I have to ask, "what is the load"? The load is in parallel with the 1500 ohm resistor, impedance wise. That can knock the gain down if it's low enough, like 4.4 k would cause this result.
     
  7. tracecom

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    The DC measurements were with no load except the 220k in the preamp. The only additional load on the AC measurements was the scope.

    I put a 4.7 μF cap in parallel with the 560 Ω resistor, which boosted the voltage gain from 1.75 to 5, but I am pretty sure it adds noise.
     
  8. #12

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    Now I'm getting down to, What do you want? Do you want an explanation about why an MPF102 isn't performing as expected or do you want gain?

    Here's a drawing of a jfet pre-amp that has adjustable gain.
     
  9. tracecom

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    I really would like to understand why the voltage gain is low, but maybe I just discovered the reason. I tested 12 MPF-102's before I soldered these two in the circuit, and I thought I picked two of the best. However, as I look back at my records, I actually picked two of the worst. That may be the answer to why the voltage gain is low. Tomorrow, I will desolder them and put in a couple of replacements, and see if that helps.

    I hope to use the preamp in some acoustic stringed instruments, and until I actually try that, I don't know how much gain I really need.

    Now, I'm too tired to do more. Thanks for your help.
     
  10. #12

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    The amplifier I posted was designed for stringed instruments, a Rhodes piano, and it has gobs of gain.

    What I'm thinking now is that the gain of the MPF102 is fairly low. Two of them are doing the same thing so it's our expectations that are wrong. The loss is manifested in the change of gate voltage versus change of source voltage, or more properly, gate to source voltage versus drain current. With a bipolar transistor, that delta Vbe is usually confined to less than a tenth of a volt. It can be a lot worse in a jfet. That's probably where I should have looked, delta Vgs/Id. When you choke down the idle current the slope of the Vgs/Id curve gets pretty flat.

    Your capacitor idea is on the right track but you need to suppress the high frequency noise by putting a capacitor on the drain resistor. You also need to consider the lowest frequency of interest. Study my design. It worked. I sold it. I cashed the check.
     
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