QRPPなCWトランシーバのオーディオアンプはやはりQRPPなケースが多いが、その出力にロスの多いパッシブのLPFなどを突っ込むから一層QRPP化が進むことになる。昨今のスマホ用イヤホンは音質が良い上に音圧効率も高いからこの様なケースにはジャストミートするが、常にイヤホンだと少々煩わしくもなる。自然、小出力でも良く鳴ってくれるスピーカーが欲しくなってくる。このSPユニットは、バスレフの様なダクト構造を設ければ、その共鳴周波数では音圧効率もグッとアップするのではないかとインスピレーションが湧き、作ってみたものである。参考となる設計事例が有った訳ではないから自信は無かったが、実際に作ってみたら結果は予想以上にＦＢ。共鳴効果により、緩やかなピークを示しながら朗々と鳴るではないか。正にイメージ通りのものができたワイッ！と内心小躍りした。SPユニットはΦ60mm、SPボックスには手持ちの小型の桐箱（内寸100mm×100mm×52mm、板厚7mm）を使った。出来上がり品の共鳴周波数がヘルムホルツの式で計算した値とピッタリ合わなくても、容易に現物調整できるから心配は要らない。自分の好みの音調に合わせ込んで行くのも、楽しみの一つとなろう。追試を乞うため、この製作過程をJARL
QRPクラブ会報Vol.60-3に投稿した。製作は2017年。
An audio amplifier of a QRPP CW transceiver is often QRPP to save battery.
And because passive LPF with losses is put in the output, audio QRPP-ization
will be developed further. Earphones of a recent mobile phone are with
good sound quality and also have high efficiency. So I think it’s very
nice for a QRPP rig, but if they're always earphones, it's becoming a little
complicated. In other words, I was coming to want the loudspeaker through
which even small output resounds efficiently. One day, when I was seeing
a site, I received a hint by a concept of a bass reflex loudspeaker and
designed this unit. The inspiration sprang that the acoustic efficiency
may be improved by the effect of duct resonation. Therefore I made it right
away. Because there wasn't a suitable design example which becomes reference,
so there were no confidences. But the result of my test was good beyond
expectation. I found out that this newly designed loudspeaker rang very
well while indicating a gentle peak. As I understood that the one which
agreed with my aim was really made, I was glad to dance. The diameter of
the speaker unit is 60mm, and a small wooden case (100 mm x 100 mm x 52
mm of inside dimension, thickness is 7mm) was used for a speaker box. Even
if the resonant frequency isn't fitly to the result of Helmholtz calculation,
you don't need to worry because it can be adjusted easily. It’s fun to
adjust the frequency response to your favorite tone by yourself. To request
a makeup examination, this making process was contributed to JARL QRP club
bulletin Vol.60-3. Making was in 2017.
