SOAP - Simple OpAmp Pre

* Flamesuit on. :rolleyes:

Thought i should give something back to the forum.

I wanted a small (tiny) preamp and found loads of schematics on the web.

Made a layout (my first) based on one of them and this is the result. It worked perfectly and sounds powerful and clean with my LM4780 chipamp.

On my prototype build i did,nt use the input/output caps. Neither the resistors on the output to gnd. Workes without them.

Be my guest to use it as you wish. :)

[IMGDEAD]http://bildarkiv.hififorum.nu/Zei/SOAP3/SoapOverview.JPG[/IMGDEAD]

Schematic
Copper
Silk


Best regards

/Zei
 
Nuuk, a few questions for you.....

I read on the tangent site that the lower the load R the higher the bias needs to be. What load range is the 1K5 good for with the OPA?

What is the purpose of the 33pf cap across RF?

I see your schematic doesn't use input or output resistors. What is the purpose of having a ~1k resistor on the input and under what circumstances should it be used?

I know what an output resistor is for, but why have you excluded one?

Thanks.
 

BWRX

diyAudio Moderator Emeritus
2005-01-17 5:29 am
Pennsylvania
Note: These responses are for the circuit Nuuk posted.

theAnonymous1 said:
What is the purpose of the 33pf cap across RF?

That capacitor across the feedback resistor lowers the gain at higher frequencies. The impedance of a capacitor is 1/(2*pi*freq*C), so as frequency increases the impedance of the cap decreases. This in turn decreases the feedback impedance because the cap is in parallel with the resistor. 33pF will start to gradually reduce the gain around 50kHz and will reduce the gain to about half around 2MHz.

theAnonymous1 said:
What is the purpose of having a ~1k resistor on the input and under what circumstances should it be used?

Resistors in series with inputs of op amps are generally used in conjunction with caps for RF filtering but are also a form of protection since they limit the current that can be injected into the inputs.

theAnonymous1 said:
I know what an output resistor is for, but why have you excluded one?

The option to use an output resistor would be very handy but may not be needed depending on the load. If the load has very little capacitance then a series resistor is usually not necessary. The datasheets for some op amps will tell you the maximum capacitance the op amp can handle on its output.
 
BWRX said:
Note: These responses are for the circuit Nuuk posted.



That capacitor across the feedback resistor lowers the gain at higher frequencies. The impedance of a capacitor is 1/(2*pi*freq*C), so as frequency increases the impedance of the cap decreases. This in turn decreases the feedback impedance because the cap is in parallel with the resistor. 33pF will start to gradually reduce the gain around 50kHz and will reduce the gain to about half around 2MHz.



Resistors in series with inputs of op amps are generally used in conjunction with caps for RF filtering but are also a form of protection since they limit the current that can be injected into the inputs.



The option to use an output resistor would be very handy but may not be needed depending on the load. If the load has very little capacitance then a series resistor is usually not necessary. The datasheets for some op amps will tell you the maximum capacitance the op amp can handle on its output.

Thank you sir. :D