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QMX+ with HardRock 50+ amplifier

 

I am about to order a QMX+ which I plan to use with my HardRock 50+ amplifier/ATU.? The HR-50+ will
operate in "QRP mode", in which the ampifier is switched off and the ATU is still operating; this will allow
running the QMX+ at 5 watts while utilizing the ATU in the HR-50+.
?
With the HR-50+ amplifier enabled, the amp will crank out 100 watts.?? My experience using it (with
my Elecraft K2) is that it can take as little as 2 watts input to achieve 100 watts output.? If the HR-50+
thinks it's being over-driven, it will take itself off-line.
?
So, to get to my question: I presume that I can decrease the QMX+ output power by decreasing the
power supply voltage.? In that scenario, does it make a difference whether I build the QMX+ for 9V
or 12V?? A few years ago, I built the QDX and opted for the 9V supply (I think my reasoning was that
cautions about blowing out the BS-170s were associated with the 12V supply) and I would probably
make that choice for QMX+ as well ... if that is consistent with lowering the supply voltage when used
to drive the HR-50+.
?
Is there a point at which the QMX+ will cease to function if the supply voltage is dropped too low?
?
Would it be better to construct some sort of variable pad to decrease the QMX+ output power?
?
Thanks all!!
?
- Bruce K1FFX
?
 

The QMX/QMX+ work fine down to a supply voltage of about 7.5V.? Somewhere around 7V it will cease to function.?
 

You can decrease the output of either a 9V or 12V QMX by decreasing the input voltage. For any given input, a 9V build will produce more output power than a 12V build.

I have neither tested it nor seen reports, but the minimum is likely to be around 6V. Below that, the buck regulator for the 5V rail in the QMX will drop out of regulation. 5V is used in the receiver circuit (for better dynamic range), in the ADC and PWM, in the USB interface, and to drive the LCD. Most of the digital circuitry uses the 3V3 rail.

On Sat, Nov 30, 2024 at 12:05?PM Bruce K1FFX via <rosen.bruce=gmail.com@groups.io> wrote:
I am about to order a QMX+ which I plan to use with my HardRock 50+ amplifier/ATU.? The HR-50+ will
operate in "QRP mode", in which the ampifier is switched off and the ATU is still operating; this will allow
running the QMX+ at 5 watts while utilizing the ATU in the HR-50+.
?
With the HR-50+ amplifier enabled, the amp will crank out 100 watts.?? My experience using it (with
my Elecraft K2) is that it can take as little as 2 watts input to achieve 100 watts output.? If the HR-50+
thinks it's being over-driven, it will take itself off-line.
?
So, to get to my question: I presume that I can decrease the QMX+ output power by decreasing the
power supply voltage.? In that scenario, does it make a difference whether I build the QMX+ for 9V
or 12V?? A few years ago, I built the QDX and opted for the 9V supply (I think my reasoning was that
cautions about blowing out the BS-170s were associated with the 12V supply) and I would probably
make that choice for QMX+ as well ... if that is consistent with lowering the supply voltage when used
to drive the HR-50+.
?
Is there a point at which the QMX+ will cease to function if the supply voltage is dropped too low?
?
Would it be better to construct some sort of variable pad to decrease the QMX+ output power?
?
Thanks all!!
?
- Bruce K1FFX
?
 

Thanks, Stan and Shirley ... that's very helpful.
?
And it got me looking at the "40m power output vs Voltage, transformer windings" chart in the
QMX+ assembly manual.? Assuming that the rig still functions at 7VDC, using the 3:2 winding
gives me about 1 watt output at 7VDC with a nice smooth curve to around 2 watts at 9VDC,
which will mostly drive the HR-50+ to 100 watts output. For QRP usage, the QMX+ will do
around 4 watts at 12VDC, which will do nicely.
?
Thanks again!
?
- Bruce K1FFX
?
 

Bruce, I was going to recommend a 12V build.?
Another option is the Smart Power Attenuator from WA2T. This allows adjustable attenuation from 1-15db, and it saves settings for each band, and changes can be made via wifi. It goes between the rig and the amp, which would allow you to run a constant supply voltage but not overdrive the amp. Considering how much the power changes by band on the QMX+, I'd assume you'd want to change the supply voltage otherwise on 80m vs 10, etc.
?
-Nate?
N8BTR
 

Hi Group,
?
Here is a thread with info on the Smart Power Attenuator (SPA). ?They are in stock and available for immediate shipment.
?
Now that I have finished and released firmware version 2.0, I will turn my attention to creating and publishing online info on the SPA.
?
?
73 and HNY,
?
Robert, WA2T

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