I just purchased a used Sony ECM-55 wired lav and made a quick test with my Studio1 adapter, normal speaking voice with the mic clipped at lapel level. The Canon GL-1 camera does not have manual gain control and only the 1/8 miniplug input for external mics. The signal coming to the camera, as audited on headphones plugged into the camera, is very high with the Studio1 attenuator set at full, and I hear some distortion and LOTS of ambient, as well as some sort of regular rumble-hum, not affected by the ground/lift switch. Turning the camera's 2-position attenuator on is too drastic; it must be for rock concerts. Turning down the Studio1 attenuator seems to do the trick. Should I just trust my ears, or do I need to run some tape, upload it into my NLE program and check the wave form? What might be causing this hot signal, since the camera has AGC?

Run some tape in the different situations you plan to shoot and check the waveform. Anything else is trusting the accuracy of your headphones and ear, which for most folks are not well calibrated.

The Gl1 has audio AGC that is better than what is found on most low-end camcorder, but it is still AGC and will, within the limits of available gain, try to make all audio about the same average level. Thus ambient background sound level will increase during lulls in program material. There are ways to reduce the apparent pumping of background noise, but that goes beyond the scope of this post.

Using headphones to judge the sound quality can be misleading because the close coupling of the headphones will tend to increase the noticeability of any sound defects such as noise, hiss, etc. It helps your assessment to give the audio a listen in the target environment as well.

The external mic input sensitivity of the GL1 is about -55 dBV, and -35 dBV with MIC ATT enabled. This should allow a reasonable match to the ECM-55 but I do not have the specs of the ECM-55 handy. I believe that it is omnidirectional, which will tend to increase ambient sound pickup.