Line-in preamp with opamp

[vasko]

I made this preamp and with higher gains it is oscillating, you can hear 10kHz hiss. When I remove 100 nF pointed capacitor the hiss is gone and sound is crisp. There is 100 uF filtering. I am using guitar as the input. But still I wonder, maybe I should put 1 uF instead 100n and add 500 pF feedback capacitor in parallel with 100 kOhm resistor to remove hiss on higher gain?

 

[DF96]

Try putting a 1k resistor from the junction of the 47k to pin 3.

The problem is probably stray capacitance in your circuit layout.

 

[mchambin]

Put a 100nF as close as possible to the Vcc pin of the Op-Amp.
It does not matter for the 100µF being close.
The big mistake of this schematic is the lack of a 100nF decoupling ceramic capacitor.

 

[29285]

Input impedance is 1kOhm, gain = 100x.
For a standard guitar this circuit is sub optimal.

 

[vasko]

I tried soldering capacitor directly to the IC hiss is the same. Where should that decoupling capacitor be, and what resistor values should I use to match the guitar? It is a little bassy from mismatched impedance.

 

[mchambin]

Decoupling cap directly soldered at the IC pin is best.
Tough luck this did not remove the hiss, there is something else.
In any case, you do need such 0.1µF ceramic cap at the IC to prevent instability, oscillations, impredictable behaviors.

 

[Mark Tillotson]

Adding a small, 10 or 22pF, capacitor in parallel with the 100k feedback resistor is likely to help.

Gain of 100x is a lot for one opamp stage, often causes problems, and reduces bandwidth significantly.