Kanzen amp III


Current positive feedback is employed because I study bass reflex system
these days. As you look at this circuit this is a kind of voltage feedback
detecting output current. Amount of feedback is variable but I don't understand
this circuit well which is similar to inverted amplifier. It seems to be difficult
to innovate this amp for I don't know about phase compensation.

This circuit is named "Bridged feedback" which I've never seen before.

Making of Kanzen amp III

This is the looks of Kanzen amp III. It looks rather dull but practical. Large
sized circuit board is prepared because it makes easier to add the circuit
of positive feed back.

At final stage Toshiba C3281,A1302 is employed, because they are easy
to buy and its cost is reasonable. Mark Levinson No.383L employs these
devices too. At first stage Toshiba K170 is employed. This has higher gm
than K117, therefore open loop gain becomes larger. Capacitors are Okaya
Vx this time.

Hfe of PNP transistor is necessary to be measured. By this measurement
the value of first stage current is decided. Measurement of NPN transistor
is not necessary.

  Idss of K170

Idss of K170 was measured. 25 pieces were measured and 8 pieces were

L ch 1 4.06mA for first stage
  2 4.07 for first stage
  3 4.08 for fixed current
  4 about4 for bias of NPN
R ch 1 5.06 for first stage
  2 5.07 for first stage
  3 4.76 for fixed current
  4 about4 for bias of NPN

By this measurement idle current of the final stage will be

  Lch     (4.08/2)*172=350 mA

  Rch    (4.76/2)*151=359 mA

Lch board was assembled successfully. There is no oscillation. Music is played
clearly. Idle current is 350mA. Current positive feedback is not applied yet.
For a while I have to confirm the sound of Toshiba.

Both channels were completed. The sound is warmer tha NEC. When I listened
to Steve Reich'Us CD, I found this sound was that of familiar Kanzen amp. The
anxiety was relieved. idle current was settled to the planned value.

Pwer transistors whch are now available will e Tosiba's orSanken's. Sanken's
transtor have highher ft. They may have diferent sound charactor.


Ouput impednce of Kanzen amp is approximatey 8 ohm, so iis a mild
crrent output amplifer. It never breaks speakers by overdrive at fo.
It has milder magnetic damping than voltage output amplifier, so they
have different transient characteristics.

Final circuit

At VR=50K, Z=7.0 ohm and at VR=0 ohm, no sound is heard. This time VR is
tuned for Z to be 5.7 ohm. Without current positive feedback Z=8.28 ohm.
Minus impedance is impossible at Kanzen amp, so this project is finished.

July 30 , 2000