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Jan 01, 2017, 03:21 PM
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Preamp rebuild

After 6 years on the wagon train taking heroic efforts to defeat noise, the preamp was redesigned again. The discrete transistor amplifiers are now an LF353. The discrete transistor was nifty, but impractical for fully balancing 2 channels. The LF353 subtracts the differential inputs & amplifies the result with minimal components. This eliminated the noise completely & finally allows the Zoom to record 4 channels. It also seems capable of sharing the power supply with the Zoom without any noise, but it's not soldered for it.

The mane problem is the pots being too close together. The mane cost in going to 2 balanced channels is 2 stereo pots & 8 wires to connect them. Previous work with digital pots ran into a lot of noise from the SPI & a display to show the current level. The balanced inputs may defeat the noise, but the analog controls have proven simpler.

The LF353 circuit had to deviate from the goog slightly. The 1k's had to be fixed. Only the 100k's could vary. It needed 1k's on the outputs to isolate it from Zoom noise. Spudger diodes prevent the 48V pop from blowing it up. The virtual ground is 5V. It needs 12V to be happy.

All support for 2V microphones & dynamic microphones is gone. It's now just a phantom powered 48V preamp.

The moral of the story is any microphone is going to need a balanced amplifier in its 1st stage, right down to your cell phone mems microphone. The balanced signal needs to be retained all the way until the line level or power supply noise will always be a problem.
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Jan 01, 2017, 08:08 PM
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Pretty cool. Your knowledge just blows me away. I had an MXL mike with a phantom power pre-amp but just bought a 5v USB Blue Snowball mike that blows the old one away and less noise to boot. Won't go back to phantom power again.

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