Ok folks, let's make this thing hear!
We are going to complete the last section of the receiver that consists
of the mute circuit and the final audio amp/bandpass filter.
During key down we want to reduce the signal to the final audio amp.
Remember that a portion of our transmit signal is fed into the receiver
to give us a side tone. We want to reduce the level to a comfortable
listning level. On key down Q1 acts like an open circuit, so the only
audio getting through is that through R9. This is why if you reduce the
value of R9 that the side tone gets louder.
The second part of the circuit that consists of the other half of U4 is
an amplifier / bandpass filter. It has approximatly 30db of gain and a
center frequency near 800Hz. It also provides the power to directly
drive headphones without needing and additional audio power amp. Those
LM386s are noisy ;-)
The parts gathering should be getting easier as the contents of the
parts bags dwindle ;-)
R6 10 ohms
R8 1 meg
R9 4.7 meg
R10 22 k
R11 510 k
R12 1 meg
R13 1 meg
R14 10 ohms
C24 .1 micro farad
C25 820 pf
C26 .0022 micro farad
C27 47 micro farad
C106 47 micro farad
C107 .1 micro farad
Install the above components. Be aware of the polarity of C27, C106 and
Connect a set of low impedance headphones or a small speaker (good
headphones are MUCH better) to J3 pins 1 and 2. Connect a temporary
antenna to the side of C40 that does not connect to the 4 diodes D7-D10.
The antenna should be long enough to pick up a strong signal, maybe 10
or more feet. Remember that we don't have the RF gain connected yet, so
watch out for strong signals. This receiver is VERY sensitive ;-) When
signals are too high remember that they are clipped in the first audio
stage by D3 and D4, so it shouldn't blow your ears out.
Try this in the evening when 40 meters is usually very active. Power up
the rig. You should hear some hiss from the headphones. Tune around and
listen for some CW signals.
Cool, It hears!
Glen Leinweber has done an outstanding circuit description of the audio
sections. It is a detailed analysis of both the first and second stage
audio circuits. Find it at:
For details on the transmit / receive switching, which includes the mute
Q1, look here:
Glen has done some excellent work here. Please read through it and be
prepared, bring your thinking cap ;-)
Also, Glen has done an analysis of the crystal filter portion that has
up till this point totally confused me. Please read:
A plethora of information. I'm going to go make some coffee and read the
rest of the cohn filter analysis.
We are really close to being done, folks. All we have left is the power
amp and output filter. Glen has already finished the analysis on these
portions so I'll give you folks a few days to do section 12 then we will
finish this baby up and get it on the air!! I can't wait!
In case anyone lost it (like I did) the url for Glens site is: