[proid]

I want to try the buffer design in SOHA II with opamp gain stage to make a simple build. With the original buffer, do i need to change anything to make it work with opamp? I'm intend to run it with dual rail +-12V power supply/

 

[nikongod]

I am not sure if anything *in* the buffer its self needs to be changed as I can not be bothered. Hopefully someone else can help here. 
 
But, I will say this - be very careful with global feedback loop(s). The cap between the gain op amp and the buffer will fuxor the DC stability of the op amp if you do not respect it. 
This may be a good place to use a Jung multiloop feedback arrangement, or just let the buffer run open loop. 

 

[Avro_Arrow]

 
Eliminate input cap, eliminate servo, insert gain stage.

 

[nikongod]

   
Eliminate input cap, eliminate servo, insert gain stage.
 

 
I feel like we have done this before. 

 

[Avro_Arrow]

   
I feel like we have done this before. 

 

[proid]

But as i have read, the servo is there to somewhat bias the base of input bjt?

 

[Avro_Arrow]

 
It might do that as a side benefit, but it's main purpose is to

control the output offset.

 

[proid]

   
It might do that as a side benefit, but it's main purpose is to
 
control the output offset.

 
So if i remove the dc servo, do i need to add a resistor to the input of the buffer?

 

[Avro_Arrow]

 
I don't really think one is needed, but a value up to around 1k would not hurt.

 

[Avro_Arrow]

 
The purpose of an input resistor for the buffer would be to isolate

the op amp from the capacitance of the input transistor. The BC550
shown has very low capacitance and should be well within the
tolerance range of most op amps that would be used with this.
You can add one in for safety if you like. You can always jumper it
if it is not needed.

 

[proid]

Today i built this as an buffer only, no gain stage as the schematic below and i got 2VDC at output, any suggestions? I use BC550B because i don't have BC550C, R1 changed from 820R to 1K, voltage between R2 is 120mV.
 

 

[nikongod]

:facepalm:
 
Build EXACTLY what you see in the cavalli schematic - most importantly with the DC servo. The advice you were given about removing the DC servo op amp from the Cavalli design only applies when the buffer is nested in the feedback loop of a "gain" (even if configured for unity gain) op amp. 
 
:facepalm:

 

[Avro_Arrow]

  I want to try the buffer design in SOHA II with opamp gain stage

 
 
Insert gain stage.

 

[Avro_Arrow]

 

 
Your R2 is in the wrong place.

 

[proid]

Thanks guy, i added a gain stage then put the buffer in a global loop, it's working now and sound very good