On 10/15/2018 7:03 AM, Scott Dorsey wrote:
My sig gen signal only appeared before R20, and on the output pin 1.
I recall learning about virtual ground in this topology.
** Yes, that is what you had.
Think about it this way. R20 and R24 are identical, and both are coming
from sources that have effectively infinite current available.
Pin 1 is reverse polarity from the input, so pin 2 is part of a summing
junction with equal parts of the input and inverted input summed together.
So in normal operation there's nothing there.
Correct. There is a "virtual ground" on pin 2.
On the other hand, pin 3 has the inverted input coming in from the source,
so you should see signal there. R21 and R25 just form a divider to halve
the voltage.
Pin 3 had no signal either, but that could have been because
I was only driving it unbalanced, from pin 2.
It's not really a very good differential input topology, because you wind
up having to use fairly high value resistors in order to get the same load
impedance on both sides (which you want for noise rejection). But it works
well enough for the job and it only takes one op-amp. Line level is high
enough that noise isn't a concern.
--scott
Here's a good video on "virtual ground":
https://www.youtube.com/watch?v=0Maaa6gcXpw
The subsonic filter looks to be a unity-gain, non-inverting
topology. The series cap C6 is obviously a high-pass filter, but
how are the values for R10 and R11 chosen? Where is the "RC time
constant" that determines when the low frequencies start rolling off?
I assume it's usually -3dB at around maybe 20Hz? Or something
like that?