AC Coupling Audio Buffer

Discussion in 'General Electronics Chat' started by Nescafe, Oct 30, 2014.

  1. Nescafe

    Thread Starter New Member

    Mar 21, 2014
    Hi All,

    I have some questions regarding op amp for audio buffer from the given schematic:

    1. If the C1 around 220-1000uF & R1 100K, which is the better op amp for U1, BJT or FET input?
    2. For C1 and C2, with value around 220uF-1000uF what is the right/correct electrolytic capacitor rated voltage needed (I have try with 6.3V, 16V & 35V and all are seems work?)?

    Any reply, help, and explanation are really appreciated.

    Thank You.
  2. MrChips


    Oct 2, 2009
    Use your low frequency limit to determine the value of C1 and C2. 1000uF is overkill. 10uF/16V should be enough.

    BJT or FET doesn't matter. Depends on the application. Try TL071.
    Nescafe likes this.
  3. Nescafe

    Thread Starter New Member

    Mar 21, 2014

    Thank You for the reply, I'm aiming the phase shift at 10Hz as low as possible that's why I use large value cap but I'm not sure what voltage is the correct one, any direction how to calculate it?

    As with the op amp, with the Z behaviour of cap along with the frequency change, would You like to inform me what is the ideal one BJT input (low noise voltage) or FET input (low noise current)?

    Thank You.
  4. ericgibbs

    AAC Fanatic!

    Jan 29, 2010
    hi N,
    I would recommend that you lower the +/-18V supply values, these are the absolute ratings for the TL701 series.
    So, +/-12V or +/-15V would be OK, always check the datasheet for the OPA, as the supplies are different on many types.
    Nescafe likes this.
  5. Nescafe

    Thread Starter New Member

    Mar 21, 2014
    Hi ericgibbs,

    Thank You for the suggestion, right now I'm not decide which op amp to use since I'm still lookin for the ideal one like I ask in previous post, and also the rest of the circuit is already works and made including the supply (+/- 18V) fron the quick search maybe I will use OPA1642 (FET) or OPA1611 (BJT). The buffer is the additional I should make to isolate impedance.

    Thank You.
    Last edited: Oct 30, 2014
  6. josip


    Mar 6, 2014
    There was page with comparison of all available chip opamps but it is not active anymore ( Most popular in entry level are BB / TI OPA2132 or OPA2134.

    If you need buffer, than there is no need for opamp. Ask google for SK170 based buffer. Or for example OPA861 (used in Pedja Rogic AYA II ouput stage).
  7. Veracohr

    Well-Known Member

    Jan 3, 2011
    You'll have a high-pass pole determined by C1 and R1, so figure out both those values to determine the phase shift at 10Hz.
  8. AnalogKid

    AAC Fanatic!

    Aug 1, 2013
    For best long-term reliability and lowest change in capacitance value over time, the rated voltage for the capacitor should be at least twice the value of the peak voltage across it. So if you think there might be 5VDC on the incoming audio signal, use a 10V cap. If you think the opamp might fail and present 12 or 15VDC on the + input pin, use a 35V cap.