Eric:
Assuming that everything is working correctly in the first place, it is more
likely that the receiver does not have enough overall gain. For headphone
operation, about 80 db of overall receiver gain will produce significant
audio levels. For speaker operation, another 20 db or so is required.
If the receiver does not have enough gain, it is strongly desirable to place
that gain behind the receiver filtering to prevent overload from strong
adjacent signals, especially on 40m where high ambient band noise makes a
high sensitivity receiver unnecessary. From this viewpoint, audio
amplification will be much more useful than an RF amplifier. If this were a
10m receiver, that would be a different story.
A low noise, high bandwidth op-amp such as a NE5532 or a LM833/LM837 can be
used for this additional audio amplification. If these are not readily
available, op-amps such as a TL072 can be used, but these have significantly
more noise, and have less usable gain since they have a significantly lower
gain bandwidth (4 MHz vs. 15 MHz). Op-amps such as a 741 should be avoided
for audio applications (high noise/low gain bandwidth).
Most people do not understand why a 15 MHz op-amp is good in an audio
circuit. Excess gain is used in an op-amp circuit as extra feedback, which
reduces distortion in the amplified waveform. At 3 KHz, a 1 MHz op-amp can
have at most a gain of 1000000/3000 = 333x or 50 db of gain. In a circuit
requiring 30 - 40 db of gain, there is only 10 - 20 db left over for
feedback. As a result, audio distortion will be higher. Using a 15 MHz
part, a gain of 15000000/3000 = 5000 or 74 db is available, giving an
additional 24 db of feedback margin to reduce distortion. Since the gain
decreases with frequency, you can readily imagine why most audio vendors
specify distortion at 1 KHz rather than 20 KHz where the available feedback
gain is reduced by 26 db.
I suggest that you start with an circuit with 10x gain, and increase that up
to 50x or 100x only if needed. Excess gain will contribute to excess
receiver background hiss when the volume control is reduced to zero, so
don't put in more than you need. For receiver overload protection, it would
be preferable to place this additional gain on the back side of the volume
control.
- Dan Tayloe, N7VE; Phoenix, Az; Az ScQRPions
>I recently completed building a 40 meter receiver listed in the 2000 ARRL
>Handbook from an article titled "The Simple Superhet."
>As the name implies, the receiver is not complex, however it does work. I
>have noticed that the receive sensitivity seems to be low.
>Any thoughts or comments?
>Eric NM5M
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