SOAP - Simple OpAmp Pre

zei · 11th December 2007, 08:10 AM

* 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.

Schematic
Copper
Silk

Best regards

/Zei

Nuuk · 11th December 2007, 01:03 PM

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.

BWRX · 11th December 2007, 01:37 PM

Quote:

Originally posted by zei
Made a layout (my first)

Nice job on the layout Zei.

zei · 11th December 2007, 01:48 PM

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 · 11th December 2007, 03:43 PM

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 · 11th December 2007, 03:47 PM

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

/U.

theAnonymous1 · 11th December 2007, 04:49 PM

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 · 11th December 2007, 05:52 PM

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 · 11th December 2007, 05:55 PM

Quote:

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 · 11th December 2007, 06:19 PM

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

Just to show how small it realy is.

Regards
/Zei

11th December 2007, 08:10 AM	#1
zei diyAudio Member Join Date: Feb 2005 Location: Gothenburg, Sweden	SOAP - Simple OpAmp Pre * 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. Schematic Copper Silk Best regards /Zei __________________ Tala är tiga, silver är guld

11th December 2007, 01:03 PM	#2
Nuuk diyAudio Member Join Date: Feb 2003 Location: Somerset, SW England	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. __________________ The truth need not be veiled, for it veils itself from the eyes of the ignorant.

11th December 2007, 01:48 PM	#4
zei diyAudio Member Join Date: Feb 2005 Location: Gothenburg, Sweden	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 __________________ Tala är tiga, silver är guld

11th December 2007, 03:43 PM	#5
Nuuk diyAudio Member Join Date: Feb 2003 Location: Somerset, SW England	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. __________________ The truth need not be veiled, for it veils itself from the eyes of the ignorant.

11th December 2007, 04:49 PM	#7
theAnonymous1 diyAudio Member Join Date: Feb 2004 Location: Anonymityville	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. __________________ "If you don't like funerals don't kick sand in Ninja's face." - Ninja


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11th December 2007, 03:47 PM	#6
Nisbeth diyAudio Member Join Date: Jan 2001 Location: Denmark	Yes, it does. Details here: http://tangentsoft.net/audio/opamp-bias.html /U.

11th December 2007, 05:52 PM	#8
Nuuk diyAudio Member Join Date: Feb 2003 Location: Somerset, SW England	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. __________________ The truth need not be veiled, for it veils itself from the eyes of the ignorant.

11th December 2007, 06:19 PM	#10
zei diyAudio Member Join Date: Feb 2005 Location: Gothenburg, Sweden	Picture of an earlier layout before i settled with the one on the first post. Just to show how small it realy is. Regards /Zei __________________ Tala är tiga, silver är guld

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