开云体育

ctrl + shift + ? for shortcuts
© 2025 开云体育
  1. QRPLabs
  2. Topics

Does the QMX need a mic preamp? #qmx #QMXplus #SSB Firmware beta 1_01_003 release #qmx #QMXplus #ssb

 

Hi All,
?
Just wanted share my experience with you.
?
I have experimented with 3 electret mic capsules so far. 2 that I scrounged off of other boards and one utilising the mic from my PC gaming headset.
?
The mic from the head set resulted in low volume and unsatisfactory modulation. When I turned up the mic gain on the QMX+, the QMX also amplified internal noise creating unacceptable background noise in the modulated output.
?
The other two capsules were a little better but still unsatisfactory.
?
One of the capsules that I had took off was from a MAX9814 Arduino mic amplifier module.?? I thought, "Hey, why not use the amplifier module to use with my headset!"
?
Well, I did just that. I wired up 2 x 1.5v AA's in series to power the board with and wired the mic from the headset as the input to the amp. I then wired the output of the amp to the mic input of the QMX via a 1uF capacitor to block the 3.3v dc from the QMX.
?
Well, that done the trick! I'm getting much better results now. But I did have to reduce the mic amp module gain as it was way too high.
?
As it currently stands, I am unable to achieve a satisfactory calibration with any of the bands on my QMX+.? As a result of this, my QMX+ is in default calibration settings.
?
The mic gain on my QMX+ is now set at 35 and have made a few contacts with good reports with this setup.
?
Now, the question is, how do I wire up the mic amp module to the QMX using the 3.3v supply on the TRS rather that using batteries?
?
I? did try taking the 3.3v via 2k ohm resistor from the QMX TRS terminal to the vdd of the mic amp module, and connect the output of the module back to the same TRS terminal via a 1uf dc blocking capacitor, but sadly that didn't work. I'm clearly missing something...
?
?
Cheers,
?
Mel. M0KMD.
?
?
?
 

I also struggle with background noise that is present in my modulation. I have tried all the options and the only thing that is certain is that the noise is reduced by reducing the gain. The noise gate helps a lot in removing noise in moments of silence, but it contributes to distortion at the beginning and end of words, especially if it is set high. It seems to me that this background noise is currently the weakest point of SSB on the QMX.
Maybe not everyone has this problem to the same extent. I have two QMXs and with the same microphone and the same settings I have different levels of noise.
?
73, Zrinko Ivan, 9A7AM
 

I did a comparison of the microphone input on two of my mid-band QMXs. I used the same microphone. The attached screen shoot shows how significantly higher the noise is on the second QMX.
?
73, Zrinko Ivan, 9A7AM

QMX br. 1 - noise.jpg
QMX no. 1 - dynamics.jpg
QMX no. 2 - noise.jpg
QMX no. 2 - dynamics.jpg

 

Mel,
The MAX9814 tries to draw 3mA so it pulls the output of the QMX to near ground since 3mAx2.2k=6.6V when only 3.3 v is available
Gary
W9TD
 

Hi Gary,
I eventually came to the conclusion that the QMX just didn't have enough 'juice' to drive the MAX9814 from the TRS terminal.
Thanks for confirming.
?
Cheers.
?
Mel. M0KMD
 

That chip requires 3-6ma at 2.7-5.5V.? You are not going to get that with 3.3V behind 2K. ?
No schematic handy to see what that circuit could source if you lowered the resistance.?
Might be best to stick with your batteries or find another place to source the voltage.??
Or use the recommended mic cartridge, which does not require a preamp.
73, Don N2VGU
?
?
?
 

Hi Don,
?
I didn't want to be invasive and have the risk of breaking something in the QMX to source a 3.3v line. I will stick with the 3v battery pack.
?
The idea was to be able to use any generic electret capsule and boost the audio to an acceptable level to drive the QMX. Having to use a specific capsule narrows down options.
?
Cheers.
?
Mel. M0KMD.

1 - 7 of 7