EMRFD Message Archive 2434
Message Date From Subject 2434 2008-11-21 21:28:58 cwfingertalker "Buzzy" FET mute switch Greetings to the Group.
I have been repacking my KK7B microR2 receiver. I have decided to add
an FET audio mute switch between the microR2 and a single 2N2222 audio
amplifier for headphone use.
The problem is when I add the FET audio mute switch in the audio line
the quality of the audio becomes degraded quite a bit. It sounds
buzzy. Especially on loud signals.
I am using 0.1 uf capacitors in the source and drain leads. The gate
is biased at the junction of two 1 meg ohm resistors. When the lower
resistor is grounded (by the key) the gate goes from 12 volts to 6
volts. It mutes just fine. Very well. But the original audio
quality sure is "buzzy" or "muddy".
Thanks in advance for any commets.
2435 2008-11-22 17:32:50 bobtbobbo Re: "Buzzy" FET mute switch 2436 2008-11-22 19:44:19 Rick Re: "Buzzy" FET mute switch I used a series mute switch in the signal path of my earlier receiver designs, but left it out
of the R2pro and the microR2 designs because it limited the audio dynamic range. Series
FET switches are not perfectly linear, so it is best to use them early in the signal path. I
was able to pick up at least another 10 dB of audio dynamic range--well beyond 80 dB, by
removing the FET mute switch between the last two audio stages.
FETs are well behaved when biased well beyond pinch-off, so another option is to put a
FET switch across the feedback resistor of an op amp. Then the FET switch is off for
normal operation and on for mute.
It may be possible to obtain satisfactory results with a FET switch between the output of
the microR2 and an external amplifier, but I'd experiment with larger FETs. Try a J310 or
even an IRF510 and see how that sounds. Experimenting is key--make sure your
capacitors are appropriate for the audio impedance levels.
For the R2pro, I didn't want any compromises at all in the signal path, so I used a reed
relay across an op-amp feedback resistor to implement the mute function. It is
completely out of the circuit when signals are passing through the receiver.
2437 2008-11-23 09:29:28 cwfingertalker Re: "Buzzy" FET mute switch Bob,
Thanks for the comments. Just as an update. I took the switch out of
the circuit and put it on the test bench. Ran some audio through it
and watched it on the scope. At some level half the waveform was
clipped or deformed. I was using a J309. Don't know if the FET is bad
or just an unknown parameter I am dealing with. I switched it out
for an MPF102 and that seems to have cured the buzz. I will
experiment with the larger caps and bypass cap too. The switch works
really well. No pops or snaps on make and break.
Next phase is to get the RF amp in the little receiver to switch
electronically and throw the relay in the junk box.
2438 2008-11-23 21:53:31 cwfingertalker Re: "Buzzy" FET mute switch Hello Rick.
Thanks for your comments. I have substituted the original FET a J309
with a MPF102. The buzz is gone. Also put the switch on the test
bench with an audio generator and scope. Verified the clipping is
gone. I will experiment with different capacitors as suggested by Bob
and yourself to possibly improve the audio. Right now it sounds much
better than before.
My circuit differs from the one in EMRFD. Mine uses two 1 meg
resistors in series with the junction of the two at the gate. The top
resistor of the series is at B+. The bottom is attached to the key.
When the key is closed the gate voltage goes from 12 vdc to 6 vdc.
The source/drain channel has no dc voltage attached to it. I wonder
what the difference is between the EMRFD circuit in figure 2.26 is to
mine. Although there are no values for the resistors in the EMRFD
circuit I would assume they would be similar to mine.
I did run my uR2 with relays and was not satisfied with that
arrangement. I am repacking the receiver and modifying the switching
to electronic vs relay. My next project is to use the circuit on page
9.35 figure 9.64 to switch the RF amplifier on and off with the key.
2439 2008-11-24 10:22:59 cwfingertalker Re: "Buzzy" FET mute switch Hello Bob,
I did a little more experimenting. The cap from gate to ground only
added a bit of a time delay on make and break. So I took it out. I
am looking for a very fast QSK switch.
I did some voltage measurements while keying on and off. I noticed
the source drain channel voltage dropped almost as much as the gate.
(You need a high megohm VOM or VTVM to measure this.) I think this
might be part of my problem. So I added a 1 meg ohm resistor from B+
to the drain. This keeps the channel pretty much at B+. Now the
switch cuts on and off very fast. No audio distortion either.
As a side note I did build up the EMRFD circuit on page 2.9 figure
2.26. It works fine but requires a positive voltage to "unmute". I
used 1 meg ohm resistors. I opted to stay with the original design
instead of adding a PNP switch for the needed positive voltage.
I'll play with the receiver for a few days before doing the next mod.
So far I think I chased away the "buzzy" audio.
Tnx and 73