We are on a roll, so lets keep it up.

Up to now we have a functioning receiver. We have audio coming out of
U3, but it needs filtering and lots of amplification. Let's build the
first section.

Gather the following components:
U4 NE5532 and socket
C19 .033 Microfarad
C20 .1 Microfarad
C21 .01 Microfarad
C22 150 pF
C23 150 pF
R2 10k
R3 10k
R4 510k
R7 510k
D3 1n4148
D4 1n4148

Install the components and check your work.

Turn on the rig and look at pin one of U4 with a scope when you key up.
Hey, what's wrong here? I had much more signal at the output of U3 than
I have here. What gives? Well, U4 is an amplifier, but it also has some
limiting diodes in the feedback circuit, D3 and D4. These limit the
output swing of the amplifier as to not saturate Q1, the mute FET (not

Remember that up till now we are cheating with the receiver. We are
using the transmit signal as a signal source to test our receiver. In
the real world we would never have such a large signal on our antenna.

This section provides 30db gain and is differentially driven (like T2 at
the output of U5) to take advantage of the greater signal voltage. It
also acts as a low pass filter, rolling off frequencies that are over
1500 Hz.

R4 DC biases pin 3 of U4. C23 and C22 lower the gain of the stage as the
frequency rises.

Lets get a DC and AC analysis of this stage.

Here are my questions:
1) How can we calculate the DC voltages of this stage. Opamps should be
easy, right ;-)

2) How do we calculate the gain of this stage. The manual says 30db.
What is the voltage gain?

3) Why is C21 smaller than C20?

4) What is the purpose of C19? Is this a terminator of some kind?