01-10-2016, 01:17 PM
Well, obviously the input impedance can be anything the designer chooses - whether using valves or transistors or whatever.
I can see that a high input impedance is required with the various "lo-fi" crystal microphones that were about at the time (and similar arguments apply to cartridges, of course), but for dynamic microphones, high impedance working is not required. So it strikes me that a hi-Z microphone of the sort we're discussing is simply about getting some voltage gain "free of charge" from the internal transformer, as I said earlier. This obviously saves cost in the amplifier design, but gives problems with long cable runs. Everything is a compromise.
I've seen this approach done in professional mic amps. One elderly example used a 1:3 stepup transformer to provide 10dB of practically noiseless gain (in addition to the usual balanced-to-unbalanced conversion, and allowing phantom power injection). This fed into a 741, which was able to make a better job within its modest abilities, having been relieved of that 10dB by the transformer.
I'm also wondering if this hi-Z option was to provide a "crossover" product that allowed a user to upgrade from crystal to dynamic? Without knowing the typical output levels of crystal mics back then, that's just speculation, but you can't blame me for fishing around for logical reasons to do something that is a bit silly on the face of it. After all, this is audio, so no need to worry about transmission line effects (never send mic-level any further than you have to), and no need to match impedances (that 6dB signal loss matters when struggling with the tiny signal from a microphone). The loading impedance can alter the frequency response of the capsule, of course, which is why mic preamps have settled on being in the 1.2-2k region (roughly 10 times the source impedance) so designers have something to reference their designs to.
I can see that a high input impedance is required with the various "lo-fi" crystal microphones that were about at the time (and similar arguments apply to cartridges, of course), but for dynamic microphones, high impedance working is not required. So it strikes me that a hi-Z microphone of the sort we're discussing is simply about getting some voltage gain "free of charge" from the internal transformer, as I said earlier. This obviously saves cost in the amplifier design, but gives problems with long cable runs. Everything is a compromise.
I've seen this approach done in professional mic amps. One elderly example used a 1:3 stepup transformer to provide 10dB of practically noiseless gain (in addition to the usual balanced-to-unbalanced conversion, and allowing phantom power injection). This fed into a 741, which was able to make a better job within its modest abilities, having been relieved of that 10dB by the transformer.
I'm also wondering if this hi-Z option was to provide a "crossover" product that allowed a user to upgrade from crystal to dynamic? Without knowing the typical output levels of crystal mics back then, that's just speculation, but you can't blame me for fishing around for logical reasons to do something that is a bit silly on the face of it. After all, this is audio, so no need to worry about transmission line effects (never send mic-level any further than you have to), and no need to match impedances (that 6dB signal loss matters when struggling with the tiny signal from a microphone). The loading impedance can alter the frequency response of the capsule, of course, which is why mic preamps have settled on being in the 1.2-2k region (roughly 10 times the source impedance) so designers have something to reference their designs to.
