Project_HeadsetAmp-Universal Notes

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Pins:

BD139, BD140:
knotches in hole facing up, pins pointing right.
---1: base
---2: collector
---3: emitter

NE5532ap/OPA2134 dual op-amp:
      Pin
1out |1 8| V+
1in- |2 7| 2out
1in+ |3 6| 2in-
  V- |4 5| 2in+

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Notes:

Power Select Switch: This should be DPDT so that the battery and wall
grounds don't get connected. This may be paranoia, but if the bridge or
battery gets connected backwards, bad things could happen. This may also
be preferrable if a separate battery charging circuit is added.

Electret: S_epole should have an off position in the middle as Re+Cefilter
will form a high pass filter. Keep Cefilter large to keep the frequency
low.

Electret: After the resistors, the voltage drop should be to around 2v.

I/O: Capacitors on the inputs and outputs are desirable protection for
both DC offset and if phantom power is being used.

Mic Input: Cout directly after the input op-amp was removed because it was
forming a high pass filter. In theory this capacitor was redundant
anyways.

Spk Output: Cout was moved after RVsvol because it was warping low
frequency sine waves in an unusual manner.

Spk Output transistor booster stage could be replaced by a small power
opamp.

Cn_tmp electrolytic capacitors were added in parallel to the inverting
inputs of the op-amps. Combined with the ground resistor, these formed a
high pass filter that was much too high without Cn_tmp. Electrolytics are
undesirable for audio signal frequencies, but there isn't much choice in
the matter as an equivalent film capacitor would be huge and expensive. A
bipolar electrolytic used in speaker crossovers may be more desirable.

Input and output capacitors should be rated at least 63 volts to protect
against studio grade phantom power.

Mic Frequency Response: good to about 300kHz.
Spk Frequency Response: good to about 80kHz.

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Add Ons / Options / Changes / Maybe's:

Built in battery charger. (some way to detect and not charge alkalines?)

Some way to auto-switch between wall power and battery.

Transistor voltage preregulator for input of over 15v.

Belt, phone, and headset clips/carriers.

Diode on electret output to protect it from negative signal swings?

Add snubber on Spk Output?

Use single op-amps instead of dual for better cross talk performance.

Use instrumentation op-amps for higher quality differential input.

4kHz phone cut filter for telephones.

Active noise cancelling headphones.
active noise cancelling mic (switch to flip to normal polarity?)
antinoise mic may need frequency filtering.

VCA type compressor. Make it on/off switchable.

Tone controls (+-6db with sliding freq control).

DPDT, DPTT and such switches to make an input switcher matrix.
   3x DPTT + 1x DPTT for 9 inputs in 3 banks.

Figure out method for regular telephone hand/headset.

Quick cable for playback of recorded conversation/music?

Hack the mic and spk I/O from bluetooth headset for wireless?

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Problems:

Log pots push up the volume too much at the end of the turn.

Cn+Rn high pass filter having to use electrolytics to fix.

High cross talk noted in NE5532a's.

Need shielding from cell phone transmitter noise.