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電學原理一開始就告訴我們V=I x R、P=I x V=I2 x R，喇叭上是個可變的電容和電感組合體。但是在推動它時因為線圈、分音器以及振膜的阻力等因素，它也是有阻抗的東西，我們稱之為特性阻抗。一般喇叭的設計者都會取某個頻率下的值為代表阻抗（這個之後會再談），目前主流大多是8歐姆的喇叭，意思就是當我們通入1KHz交流訊號時，會看到8歐姆的阻抗。
這個範圍有什麼差異呢？我們看紅色直線對映到的縱軸I，就是輸出電流量，若是我們用Vg=-10～-45，電流會從240mA左右變化到10mA左右，也就是輸入端電壓變化35V會產生230mA的電流變化量。而變壓器在一次側感應到的電壓變化則等於1500 x 0.23=354V。當輸入訊號最低的時候，真空管會放過10mA的電流。但以上選取範圍只能當參考，不是實作使用，因功率過大且Va變化會超過範圍。
若是我們想使用品質好一些的範圍，那麼就使用Vg=-15～-35的範圍，此時輸入端變化只能有20V，輸出端電流變化則會等於130mA，因此輸出變壓器感應到的電壓變化會是1500 x 0.13=195V，且功率等於25W，約是這支管子的極限了。但重點在於，在這個條件下，訊號在最低電準位時，真空管會放過50mA左右的電流量，而訊號電準位為零時，Vg=-25V，系統電流量約在110mA。也就是真空管的電流變化量和系統靜態電流量不會差太多。
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前幾個月我們談過元件與音色的關係，恰好水電工自己正在設計的產品要用到2組濾波器，想要使用比較高檔的元件，這一報價可讓水電工的客戶嚇了一跳。不是水電工要老王賣瓜，而是很多非量產型的高級零件原本就是這麼貴，1顆電容幾千元台幣都算是合理價。
