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/
Soha II buffer with opamp gain stage?
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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.
Edited by nikongod - 10/28/13 at 8:50am
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.
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.