making Headphone Amp -- ChuMoy Amp --


Japanese version here.


I bought a headphone, ATH-PRO5 by audio-technica, believing I could be DJ.
Then I got to want a good headphone amplifier.
I can't help making something.
It's like a hopeless illness, but making audio is interesting.

I want to put a headphone amp into my pocket, so I adopted a circuit by Mr.ChuMoy.
He makes pretty headphone amps.
Mr.ChuMoy's homepage


I imitated his circuit.
ChuMoy Amp is easy to make because it needs only one OpAmp to amplify.
It will be enough for my use.
I determined the gain as 14dB(about five times).

ChuMoyAmp FET Input circuit
(If you click, the figure will expand.)

This circuit is for FET OpAmp.
Do not use for bipolar OpAmp because the resistor of positive input which is 100kohm is too big for Bipolar OpAmp.
The bias current of FET OpAmp is some hundred pA, so we can select over 100kohm.
But the one of Bipolar OpAmp is some hundred uA, so we should choose about 2kohm.

If we adopt FET OpAmp, we can select a capacitor which has a small capacity such as film capacitor because we will use resistor of a high value in that place and we can easily construct input High Pass Filter.

I forgot about this firstly, I used NJM2114D which is bipolar input OpAmp and it sounded as if it was effected something.

OPA2134 and OPA2604 by BurrBrown are famous for FET OpAmps for audio .
They are a bit expensive, but they are nice.


ChuMoyAmp making1

I used many parts which were already in my stock.
MKT1826 (ERO) 0.68uF
KP1830 (ERO) 220uF
Riken 50ohm
NIKKO others

It was a making of clearance of my stock.

ChuMoyAmp making2

I put sockets in order to be able to exchange OpAmp and resistances easily.

It was my mistake that NJM2114D was on board.
I had no idea that I had to exchange OpAmp for OPA2134PA when I took the picture.

ChuMoyAmp making3

The resistor of Riken has golden lead lines. I used them for wiring.
It looks luxuriously.

Finally I put it into a case.

ChuMoyAmp set into a case

This case is SW-75B by TAKACHI.
(50mm * 30mm * 75mm)
It will be the smallest size if you use a battery of 006P.

I inserted a variable resistor between a input terminal and a coupling capacitor in order to control volume.
It is 20kohm 2 accompanied A model made by ALPS.


ChuMoyAmp complete

I made it as small as that I could put into my pocket.

HeadPhone ATH-PRO5 and ChuMoyAmp

This is my potable audio system.
ATH-PRO5, I bought in admiration for DJ.
iPod nano 4GB, my girlfriend gave me as a Christmas gift.
CMoy Amp.

This is my favorite system.

CMoy Amp is nice with its clear sound.
It is mysterious that the sounds which are from CMoy Amp are more clear than that of directly from iPod.

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