An older microphone preamplifier; 2 channels.

Versions #

Seems like earlier units like mine used a THAT1512 preamp IC; later I believe an INA163. I believe there are also at least two implementations of the input filter section.

Schematics #

I found partial schems of the input section just around the preamp IC, but I was interested also in the phantom power generation and ended up tracing essentially the whole thing:

Concerns #

Preamp input protection #

No protection on the delicate inputs of the preamp IC ! There is litterature about this :

• THAT1512 datasheet
• https://www.thatcorp.com/datashts/AES5335_48V_Phantom_Menace.pdf
• https://thatcorp.com/datashts/AES7909_48V_Phantom_Menace_Returns.pdf

Regulator protection #

Seems like the 7815 can go into latchup, presumably if the negative rail settles slightly faster. Datasheets often recommend a diode between OUT and GND, which this version lacks. I always thought that was to protect the regulator from destruction in case of shorts on the input or other anomalies.

60hz #

I found some 60Hz was getting into channel B (none measurable on ch.A), due to its proximity to the AC input section. My experimentation showed that the power switch area was the main source of this, and I improved things by building a small shield around the switch, with leftover RF shield (tinned steel) from probably an old gutted TV tuner…

Phantom multiplier #

The DMP3 uses a ne555 to generate 100-110kHz 15V signal that is multiplied up to 48V. I found that those 100kHz edges are getting into the +15 rail and the ground plane, not to mention the +48v as well. Adding a 100nF MLCC right on the ne555 power pins was not as effective as I hoped.

I also tried slowing down the edges with a resistor in series with ne555.3; barely measurable difference. I may try adding an extra RC filtering stage after the multiplier. To be continued, maybe.