Member #1192200

Member Since: October 27, 2017

Country: United States

  • Possibly a better op amp to try is the OPA2132. Single supply operation down to 5V and 40mA output current ability. I do think you are going to want a higher supply voltage to get the headroom you need though. Note from the OPA2132 datasheet that for low RL, the output swing becomes asymmetrical with respect to the voltage rails, so the ideal input bias point may be a little higher than Vcc/2. Easy to calculate based on the gain you end up with. I would recommend as a minimum ditching the regulator and just add some capacitors if needed to filter the 9V supply. Check out the CMoy pocket amplifier: for other ideas and op amp options..

  • In your original circuit with the LM358, I assume this was operated on a single supply? Did you directly connect an AC coupling capacitor to the opamp to supply the headphones? If so, you probably could have fixed your original circuit by connecting a 10K resistor directly from the opamp output to ground, then have the series cap to the headphones. The output needs a DC resistive path to ground (or its negative supply) for the output section to operate properly without crossover distortion. This is buried in TI application note AN-116. Bottom line - there’s really no reason to need the push-pull transistor output stage.

    Also the OP275 is specified for a minimum supply voltage of +/- 4.5V (9V single-supply), so if you have a fairly clean 9V supply, you should eliminate the regulator all together to ensure you are running the OP275 within its minimum supply spec, otherwise, I’m not sure if you are getting optimum performance out of that device.

    For other improvements you should look at building an input filter to replicate the relatively narrow bandwidth of a typical guitar amp speaker which rolls off below 100Hz and then pretty sharply above about 3-5 KHz. You can use a properly sized input ac coupling capacitor to simulate the roll-off below 100Hz, then a 2nd order active low-pass filter to simulate the high end roll-off. You will want a filter Q around 1- 2 to simulate the peaking that most guitar speakers have just before they roll-off.

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