ALC and the IC-7300 – About talk-power and modifications

“My IC-7300 has lower power output compared to my previous transceiver.”

Yes, with compressor off that’s true. Is the IC-7300 broken? No, it’s not. So what is wrong here? Often, the resulting discussion is about peak meters, PEP, settings, the weather and cosmic radiation.

By chance I noticed, when changing tune power levels in WSJT-X, (the slider on the right) that the IC-7300 ALC has very fast attack time. An input level increase above max output level is corrected very quickly (to avoid overshoot). But then the reverse action, the same input level decrease, is followed very slowly by the ALC. It takes up to 3 seconds to recover and settle at the starting point. So power output follows the input level decrease, thus dropping below max output, and then gradually increases back to the max output level.

So in SSB, the input speech envelope is not really followed, it’s more an automatic level set function, where max output power is matched to peak input speech level. The result is a very lineair transfer from input to output in SSB. This makes sense, after all, the SDR architecture makes clean transmitters possible, so better not let the ALC spoil it.

Back to talk-power. For the average human voice, the ratio voice-peak to average-speech-level is about 10-16 dB. 10 to 16 dB below 100W peak power is 2.5W to 10W average. So a peak-power meter will show 100W out, but your Birds needle does not really swing on a few watts average. The Icom page on the site of AB4OJ/VA7OJ has some excellent articles on speech and compression.

Commercial broadcast uses all kinds of compression, limiting etc. to make the DJ’s voice sound louder (Unfortunately, currently the same is applied to music, resulting in the so called loudness war). It’s not a bad thing to enable the compressor in the IC-7300. The manual advices 10-20 dB on the comp meter at voice peaks. The compressor in the IC-7300 is completely implemented in the digital domain and should not increase spurious emissions.

The famous ALC mod disables the Icom style ALC function, something you can verify with the schematics, it’s one of the few analog design parts of the IC-7300. Look closely at the mod-videos and you will see the external tuner peak power meter hit 100 W before and 150 W after the mod. I have also seen the IC-7300 current meter peak at 23A, a value normally never reached.

With the mod, speech peaks are filtered out by the added capacitor, thus not triggering the ALC. On continuous input such as WSJT-X, the ALC will still react and output power settles at 100W. With the mod, speech peaks are limited by other protection mechanisms that are part of the same circuit, such as over-current protection or simply by drive and/or final transistor power limits. Of course will the mod result in increased talk-power. But this comes at a cost !

The ALC circuit around POCV and FORLP in the IC-7610 and the IC-9700 has an extra op-amp. The IC-9700 service manual even talks about a fast and slow circuit. So I did the same power level change test with WSJT-X on the IC-9700. Here the recovery is much faster, the 3 s. recovery time has decreased to about 1 s. So slightly more talk power with compressor off ? Why Icom changed the circuit, only Icom can tell…