Load resistor for JFET preamp?

Discussion in 'The Projects Forum' started by tracecom, Aug 15, 2012.

  1. tracecom

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    Is a load resistor necessary to measure the output from this JFET preamp? If so, what value should it be?

    Thanks.
     
    Last edited: Aug 15, 2012
  2. #12

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    Do you mean R4?
    R4 is a load resistor. You can measure the output without R4 being connected.
    As for what value it should be, it depends. What standard that requires a certain load are you trying to comply with?
     
  3. tracecom

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    Yeah, I see that now that you point it out...stupid question. From what I have read, preamps have a high impedance output because they are not intended to drive speakers. I am planning to use a stereo version of this preamp between an MP3 player output and a circuit something like this. The objective is to reduce drain on the MP3 player battery by lowering output volume. However, I am not sure exactly how much gain to expect from the preamp.
     
    Last edited: Aug 15, 2012
  4. bertus

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  5. #12

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    The TDA1517 requires 15.4 microwatts of power at its input. (.96 volts)squared/60,000 ohms.

    Are you really trying to save .0000154 watts by building a preamp for a power amplifier that is already designed to work with an MP3 player?

    hang on. That was peak voltage. You're trying to save .0000077 watts.
     
  6. tracecom

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    I haven't done any measurements, but my experience is that I have to run the MP3 player at maximum volume in order to get the volume I want out of the TDA1517. I thought about boosting the power on the TDA1517 amp from 12 to 15 volts. Would that be a better idea?
     
  7. tracecom

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  8. Audioguru

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    The datasheet for the TDA1517 says that its voltage gain is 20dB which is 10 times.
    Its maximum output power is fairly low but much more than an MP3 player that drives 32 ohm headphones with a low level so it doesn't make you deaf.

    The output voltage of an MP3 player is probably about 1.8V RMS max. The output of the TDA1517 with a 14.4V supply is about 4.5V RMS (5W into 4 ohms).

    Then the TDA1517 has more than enough voltage gain without using a preamp.

    It is difficult to predict the voltage gain of a Jfet preamp because of its very wide range of voltage gain (gm). You don't need its very high input impedance used for an electric geetar.
    Use a normal transistor or opamp preamp instead.

    Maybe you want 100W from the 5W amplifier IC? Impossible.
     
  9. Audioguru

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    I hope you are not powering your amplifier from a transformer. The diode is not a full wave bridge rectifier. The diode probably prevents damage if a battery is connected backwards.
     
  10. tracecom

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    Thanks. The maximum output from my MP3 player is (as you say) about 1.6 V, however, I would like to operate it with the volume set at midrange instead of maximum. That setting reduces the output to about 100 mV. Would a preamp not solve that issue? That way, I could increase the play time on the MP3 player by minimizing its battery drain since the preamp and the TDA1517 amp would be powered from a separate power supply.
     
  11. tracecom

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    No, I am using a separate regulated supply and included the diode in the amplifier for just the reason you suggested.
     
  12. Audioguru

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    The output amplifier in an MP3 player is class-AB for very low supply current instead of a class-A heater that uses a huge supply current and gets very hot.

    In your case the class-AB amplifier is not driving a 32 ohm headphone which takes a small but substantial amount of power, it drives the input of your amplifier which is high impedance so the supply current of the MP3 player is very low whether its volume is high or low.
     
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  13. #12

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    The MP3 player is driving a load of 60,000 ohms when it is driving the TDA chip.
    The MP3 player is providing about 43 microwatts of power to drive the TDA chip.
    A single "AAA" alkaline battery can provide that much power for 28,000 hours.
    Obviously, something besides the volume control is using up your batteries.
    Turning the volume down to a tenth of the maximum will not have any significant effect.

    (Same as what AG just said.)
     
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  14. Audioguru

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    Maybe the MP3 player has an LED that wastes about (6V x 10mA)= 60mW.
     
  15. #12

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    Almost off-topic, but...

    Once upon a time, I had a customer that insisted I install an LED "on" indicator on a preamp very much like this one. 1 ma for the jfet, 10 ma for the LED, and power it with a 9V battery. The battery life dropped from 440 hours to 40 hours, and he thought he was going to save on batteries because the LED would remind him to turn the preamp off. I couldn't even talk him in to a switch that shows red paint when it's on.

    Anyway, what applies to this case is that an LED uses a lot more power than a volume control.
     
  16. tracecom

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    There is no LED on the MP3 player, only a small LED display which times out quickly and goes dark. As I previously posted, it was only my speculation that running the volume at max would cause the battery to be more quickly depleted. After reading the explanations by AG and #12, I now understand that is not the case, due to the high input impedance of the TDA1517 amplifier. I am going to put together a 15 V power supply for the TDA1517 amp and see if that increases its output.

    Thanks.
     
  17. Audioguru

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    The datasheet shows a single-ended stereo amplifier has an output of only 3.3W with 1% clipping distortion into a 4 ohm speaker with a 12V supply. Its output power is almost halved into an 8 ohm speaker. Your diode in series with the power supply reduces the voltage and reduces the power.

    The mono bridged output is 5W with low distortion into an 8 ohm speaker with a 12V supply.

    They do not spec a stereo amplifier with a 15V supply. It might melt.
    Instead they show a mono bridged amplifier producing 7W with high clipping distortion into a 16 ohm speaker.
     
  18. tracecom

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    The NXP datasheet for the TDA1517 indicates a maximum operating voltage of 18 V, and a typical of 14.4 V. So, 15 V regulated supply should put 14.4 V on the IC, as opposed to the 11.4 V it's now getting. Should be okay, don't you think?
     
  19. Audioguru

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    We don't know your speaker impedance.
    We also don't know if you are making a single-ended stereo amplifier or a mono bridged amplifier.

    A rectifier diode has a voltage drop of 1V or 1.2V when it has the high current of an amplifier in it. Its voltage is only 0.6V when its current is very low.
     
  20. tracecom

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    The amp circuit is in post #3, and the speakers are 4 Ω each.
     
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