The low monitor audio level is a common complaint on the Jupiter. Apparently some radio amateurs have just about enough audio feedback and are quite satisfied, while others claim it does not work (at all).
I took the time to make an investigation of the microphone audio chain in the rig and it appears to me that the differing views on the monitor audio level is down to not fully understanding the underlying mechanism. The manual is of not much help here either as there are several settings that interact.
This following discussion is only relevant for the microphone input from the front panel and not the LINE IN on the rear jack.
From reading the schematics and after a series of experiments this is how I think the microphone signal is manipulated:
My guess of the microphone signal path in the Jupiter.
Desscription: The microphone signal “Mic in” is input from the 4-pin front panel jack to a “Buffer Amp” into a “Mic Gain” potentiometer on the bottom printed circuit board internal to the Jupiter. The output from the potentiometer is A/D-converted to digital samples and the rest of the signal processing is done in software.
The signal path is split after the front panel control’s MIC setting (0–100%): One path is to the speaker/headphone via a “MON”, monitor level control, accessible from the from panel, another path is to the transmitter section of the transceiver. The “MON” level depends thus directly on what level actually leaves the “MIC GAIN” on the PCB.
The transmitter section begins with a software controlled “MIC BOOST” control where an added 0, 6, 12 or 18 dB ensures sufficient signalling levels for the digital signal processing steps in forming the audio before it is D/A-converted and fed into the hardware mixing stages and final HF amp. After this initial boost a series of audio adjustments are optionally applied (all of them through MENU settings) “TX WIDTH”, “ROLL OFF” etc.
Note that the boost is performed in software, meaning that a very low input signal that is A/D-converted into a lesser number of steps still has the same resolution even though it may be of a higher amplitude after the boost. Best audio quality is achieved if no boost is necessary as the signal’s full dynamic will be used in the following digital processing stages.
In short, to maintain fidelity, it is better to start off with a large signal and attenuate that, rather than starting with a low signal and amplifying it. Without access to the software it is hard to see how this is implemented in detail but it appears that a large input amplitude is necessary and the software controlled “MIC” and “MON” have very limited range — perhaps they only attenuate what enters the DSP with different amounts?
This is also where the low monitor level enters the scene. The monitor feedback appears to be a version of the direct microphone input. For example, the “MIC BOOST” control has no effect on the perceived monitor audio. No matter how I turned the other controls (TX WIDTH, ROLL OFF,…) did the audio quality of the monitor change even a bit. A high internal MIC GAIN will also give a higher MON level. A high input level also ensures good “MON” audio!
My specific case
In my case with an electret microphone I turned the MIC GAIN on the PCB to its maximum. This way I have the maximum usable signal into the rig and the pre-DSP boost can now be a couple of levels lower than before (used to need +18 dB). With MIC at around 50% and MON at 100% I get a usable monitor level in my headphones.
Opening the Jupiter 538 the PCB is located on the under side. The aluminium bay for the ATU is at the very top right in this picture but luckily the blue MIC GAIN potentiometer is accessible without having to remove anything. Note that maximum gain is achieved with the potentiometer turned most CCW, i.e. rotated to the left, contrary to what one may expect!
With MIC GAIN level set, the rest is manipulating the sound for the transmitter. Usually I settle for a transmitted bandwidth of 2.7 kHz (menu option “TX WIDTH” 2700) and chose a “MIC BOOST” of just enough to get the ALC lamp light up at sound peaks. For best transmitted sound (though I have not been able to verify this) one should back off “MIC BOOST” yet another notch. This way the power amplifier is run without risk of splatter due to over modulation.
The other transmitter settings are adjusted to taste, microphone used and your voice. You will need a remote receiver to adjust these settings as none of them have any effect on the monitor audio. Nowadays there are a number of SDR receivers on the internet one can use to check sound quality.