SOAP - Simple OpAmp Pre

[zei]

* Flamesuit on.

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]

Zei, as opamps go, I like the OPA134/2134! This is a circuit that I built a while back. Note the 1K5 resistor going from output to the negative rail.

[IMGDEAD]http://myweb.tiscali.co.uk/nuukspot/decdun/propods/propod%20pre.gif[/IMGDEAD]

 

[BWRX]

Made a layout (my first)

Nice job on the layout Zei.

 

[zei]

Thanx Brian.!

Nuuk, Please explain Why did you put the resistor between out and - supply?

Did i miss something important?

The cirquit is tested and it works very well But i can always make changes to the layout of course.

Regards

/Zei

 

[Nuuk]

I was told that the 1K5 resistor would bias the opamp nearer to class-A operation, It did sound very slightly better after I did so.

 

[Nisbeth]

Yes, it does. Details here: http://tangentsoft.net/audio/opamp-bias.html


/U.

 

[theAnonymous1]

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.

 

[Nuuk]

TAO - there are a few more details here but basically I just took advice from Carlos Machado and don't fully understand how it all works. I just try suggestions, and if I can hear an improvement I include them in the final version.

 

[zei]

I was told that the 1K5 resistor would bias the opamp nearer to class-A operation, It did sound very slightly better after I did so.

Aah.. yes i read that but never tried it my self. It should be fairly easy to solder directly to the OPA pins i guess.

Thanx for the input Nuuk!

/Zei

 

[zei]

Picture of an earlier layout before i settled with the one on the first post.

Just to show how small it realy is.

[IMGDEAD]http://bildarkiv.hififorum.nu/Zei/SOAP3/SoapMK3.JPG[/IMGDEAD]



Regards
/Zei

 

[Burnedfingers]

zei

Very nice job!

If you would care to sell any boards please let me know.

 

[zei]

burnedfingers; I was not planning to have these pro made but i guess if a bunch would be interested we could arrange something.

I,ll keep u in mind if something comes up.


Regards
/Zei

 

[nickthevoice]

PCB

YesSSss me too !

if you sale PCB let me know !

thanks

[email protected]

 

[BWRX]

Note: These responses are for the circuit Nuuk posted.

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.

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.

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.

 

[theAnonymous1]

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.