KA 71 – Channel mixing headphone amplifier

Purpose

KA71 is a stereo headphone amplifier with additional function of stereo channels crossmixing. As such it is designed for Amiga computers. Amiga sound chip Paula sends two of its four audio channels to stereo left channel, two to the right channel. They are fully separated, which is annoying especially when listening with headphones. KA71 can mix stereo channels at a level adjustable with a knob, from full separation to mono output. It can be also used with computers having monophonic audio, or to mix two computers simultaneously playing parts of the same song. Of course one can also use KA71 as a regular headphone amplifier with smartphones, music players, computers and other sources.

WARNING: KA71 amplifier, depending on input signal, connected headphones and volume set, can produce sound pressure levels higher than 85 dB. High sound pressure level is harmful and may cause irrecoverable damages to your hearing sense. 85 dB is considered safe for continuous listening. Be careful turning the volume up and switch off the amplifier when changing signal sources!

Features

Photos

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User's Manual

Front Panel

KA71 front panel power switch power LED headphones jack input jack USB power socket volume knob stereo mix knob on off

Power Supply

The amplifier may be powered from USB or from batteries. It has a standard micro USB type B socket, the same as used in most of smartphones and tablets. Power requirements of KA71 are low, so it may be powered from any desktop computer, laptop, USB hub, phone charger or powerbank, using standard USB type A to micro USB type B cable. The amplifier only uses USB power, data lines are not connected inside.

In case when batteries are inserted and USB power is connected at the same time, batteries are automatically disconnected, so the device works on USB power. Then there is no need to remove batteries each time USB power is used.

Batteries

KA71 uses three AAA size batteries. Accepted types are single use zinc or alkaline batteries, as well as rechargeable NiMH ones. To insert or replace batteries:

Remember:

Specification

Physical

Input

Output

Power Supply

Schematic Diagrams

Power Section

L1 22µ L2 22µ D3MBR0520 D1MBR0520 SW1 R147k C2330p R2 100k CN1 U1LMR62014 D21N4148 R313k B13×AAA 5 4 V+ C12µ2 C34µ7 R43k3 D4L934LID-TNR C41000µ16V 2 3 1 1 5

Input Stage

CN2 V+ V+ C9 33p C10 33p R10 150k R9 150k C1110µ50V C1210µ50V R7100k C5330n R8100k C6330n A B V+ C72µ2 C82µ2 R6100k R5100k R17A10k lin.* R17B * In some units R17 is 2×22k linear, R15 and R16 are 22k then. U2AMC33178 U2B GND 8 1 4 7 R14100k R12100k R1610k* R1510k* R11100k R13100k R R-SW L-SW L 3 2 6 5

Output Stage

V+ V+ C17 33p C18 33p R22 150k R21 150k CN3 C191000µ25V C201000µ25V R19100k R20100k C13330n C14330n U3AMC33178 U3B A B 1 7 C152µ2 C162µ2 V+ 8 4 R L GND R25100k R26100k R24100k R23100k R18B R18A22k log. 3 2 6 5