ต้ดบางส่่วนของรีวิว Ayre K5xeMP Preamp มาให้อ่าน Ayre เป็นผู้ผลิตเครื่องเสียงที่ใช้ JFET เป็นเกนดีไวซ์เป็นหลัก Charles Hansen เป็นนักออกแบบวงจรออดิโอที่ได้รับการยอมรับในความสามารถ
ยาวนิดนึงครับ แต่มีข้อเท็จจริงที่น่าสนใจเกี่ยวกับ ความแตกต่างของเสียงจากการใช้ BJT ไปเป็น JFET และคุณภาพของ Toshiba JFET
The primary difference between the MP and its predecessor concerns the amplifying devices. I'll let Charlie Hansen explain, as the story is a fascinating glimpse into how a high-end audio engineer develops a product:
"The K-5xe was the only non-power amp in our entire product range that did not use JFETs for the output (buffer) stage. The reason was that when we designed the K-5, we had just switched from FET outputs to bipolar transistors (BJTs) in our power amps and were extremely happy with the results. We figured that, since they worked so well in the power amps, we would also use them in the preamp.
When we were designing the analog circuitry for the QB-9, I was determined to make a circuit with bipolar transistors that sounded as good as our JFET-based circuits, the reason being that Toshiba had discontinued the low-noise P-channel JFETs that had been a staple of our designs. We made a lifetime buy of 500,000 pieces, which should last us somewhere between 50 and 100 years, but I still only want to use these parts when absolutely necessary.
When it came time to design the output (buffer) stage for the QB-9, we decided that the easiest thing for experimentation would be to start with the circuit used in the K-5xe rather than build something from scratch. We soon found (for reasons I still don't understand) that the simple complementary BJT emitter follower used in the K-5xe was holding back the performance of that whole unit. The easiest solution was to replace it with a pair of complementary JFET source followers, as found in, for example, the C-5xeMP, the K-1xe, or the KX-R.
Rather than use up the Toshiba JFETs, we found some US-made parts that sounded nearly identical. That is when I originally notified JA of the upgraded unit. We were just getting ready to ship it out to him when we ran it through the battery of measurements to make sure that all was well with the unit. That is when we found that the US-made JFETs caused the THD of the unit to double compared to either the Toshiba JFETs or the previous bipolar output stage. Their transconductance is only about one-third that of the Japanese parts, and this is apparently the source of the problem.
I wasn't happy about a decrease in the measured performance of an 'upgraded' product, no matter how much better it sounded. So we went through a long period of developing a circuit using bipolar transistors that would sound as good as the JFETs. We were finally able to do this, but at the cost of an extremely complex circuit. The final result was a great-sounding design [it's used in the QB-9—Ed.], but it was simply too complex to retrofit to the K-5xe in a manageable fashion.
So we are plundering our precious stash of Toshiba JFETs to make the MP upgrade to the K-5xe. It measures great, sounds great, and is easy to retrofit to existing units."—Charles Hansen
