Transmogri-Notes Edition 2:

On-Stage Instrument/Monitor Mixer

In this edition a circuit is presented for use as a small mixer
with 2 instrument inputs, a stereo auxiliary input (for example, mp3 player),
and a transformer isolated input able to handle high power speaker connections.

There is included a power amp section able to deliver 1 Watt "per ear" to headphones.
This is a total of 2 Watts average power, while the "peak" power is approximately 6 Watts,
so there is a great deal of headroom.

The headphone output can be connected to an 8 ohm speaker and is able to deliver 3 or 4 watts
depending on the output transistors used. The suggested "optional" overload limiting circuit
is set to limit for headphone use as the output transistor imagined for the basic headphone amp
are SMT SOT-223 parts, and they should not dissipate more than 1 Watt. Selecting an equivalent
D2PAK part allows for better heat sinking and possibility to deliver approximately 6 Watts to an
8-ohm speaker.

Furthermore a "thru" jack output is supplied for the Inst 1 input. A tap could be taken from "MixOut"
to optionally send this to the output jack. It may be more convenient to make this optional with a
toggle switch.

The main purpose of this device is to allow a musician to control his own monitor mix on stage.

Below is the block diagram of the high-level function.

Next is the schematic by function.

Simulation Results

This simulates as a high-fidelity device having what will appear to be <0.5% THD+N if the real thing
comes out close to simulation.

Frequency response is plotted below to show its wide-band response. In short:
40 Hz to 15.5 kHz, < +/-0.5 dB
-3 dB Bandwidth : 13.5 Hz to 22.5 kHz

R40 is instrumental in extending bandwith beyond 10 kHz on the power amp section.

And finally, a link to the LTSpice Simulation file