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ヘッドホンアンプを作った

https://blog.with2.net/link/?1965529

ヘッドホンアンプを作った

https://blog.with2.net/link/?1965529

たまにはArduinoを離れての電子工作をやってみたくなってきました。

ということで今更ながらヘッドホンアンプを作ってみようということです。以前にもChuMoyアンプなるものを作ってみたことはありましたがオペアンプ一個ではそれなりのものにしかならず興味をなくしていました。

以前に作ったCHUMOYアンプ

もうちょっと音質の良い方式はないかと探してみるとA47型と言われるアンプがあるようです。

ChuMoyアンプではオペアンプの出力電流が大きく取れないため2回路の出力を合成させて出力電流を2倍にしようとしたのがA47型です。
2つのアンプの出力電流を均等に1/2づつ受け持たせるためにオペアンプの出力同士を直接繋ぐのではなく同じ値の抵抗を介して合成しています。
最初に公開されたのがこの合成用の抵抗に47Ωを採用した事からA47型と命名されたようです。

製作にあたって参考にさせていただいたサイトは以下です。

http://kirylabo.blog.fc2.com/blog-entry-134.html

・回路

回路図を描いてみました。

電源は12VのACアダプタを使います。電池交換が面倒なこととACアダプタを電源とすることでサージノイズなどに対しては多少は効果があるかもしれません。一般的に交流信号を増幅するアンプ回路では正負電源が必要になります。電源電圧を半分に切り分けて使用するオペアンプをつかったレールスプリッター回路により±6Vをとりだします。レールスプリッターのオペアンプにはNJM4556ADを使ってR1,2で生成した中間電圧をオペアンプで増幅(1倍バッファ)して分圧します。オペアンプによるフィードバックがかかるため、仮想GNDが安定しやすいという利点があります。またC6,7コンデンサの容量が大きいほど安定性が増すということなので1000μFを使っています。

音声回路部分のオペアンプは一段目はMUSES8920、二段目がMUSES8820としました型番が異なるのはそれしか部品箱になかったからで意味はありません。

基本増幅倍率はR3,4の値で決まり約5.2倍とやや大き目かもしれません。

基本増幅倍率=(2.4k+10k)÷  2.4k=5.2

入力部分は入力カップリングコンデンサC1(1uF)とR1(20kΩ)で、出力部分は出力カップリングコンデンサC4(470uF)とR9(10KΩ)でハイパスフィルタを構成しています。入力側は約8Hz、出力側はインピーダンス32Ωのヘッドホンを接続した場合に約10Hzがカットオフ周波数になります。

・製作

回路は決まったので製作にとりかかりますが基板サイズは72×95㎜には収まりそうなのでこのサイズでケースを選択してみます。aitendoさんにて以下のアルミケースを購入しました。

http://www.aitendo.com/product/10714

外形寸法:75.5x22x122mm

裏ブタにはACアダプタ用の孔が表ブタにはボリューム、音声入出力コネクタ用の孔が加工されています。追加工で裏ブタにRCA入力端子孔を加工すればよいだけです。

購入した基板の全長がケースに対して短いので表ブタに取り付ける入力部、裏ブタに取り付ける電源とRCA端子それにメイン基板の三分割とします。(二分割でもよかった)

表面

裏側

音声回路部

電源回路部(ヒートシンクは飾りの意味しかない)

左からイヤホンジャック、入力、スイッチ付きボリューム

RCA入力、AC電源端子

試聴した感じではやや低音域が不足気味かなという感じです。各常数の値をいじるか低域バッファ回路を追加すればとも思うが、やり始めると深みにはまりそうな気もするので今回はこれにて収めておきます。

追記:いつものロボット製作ではあまり必要を感じないオシロスコープですが、オーディオ関連となるとオシロなしでは暗闇を手探りで歩くようなもので辛い。

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