小磨建伍 KT-5020

goto2003

KT-5020收音头在海外的评价很高，是80榜单中的第7位。最近入手了一台，声音素质比较高，HIFI感强，但和手里的马机比较显得声音平淡。所以就按照下面的帖子小磨一下。

http://www.fmtunerinfo.com/DIY.html#KT-5020

We are aware of three versions of this tuner. My DIY'ed version has switchable voltages and de-emphasis, but some do not. Starting with the de-emphasis: For Europe, England and Australia, C102 and C103 are 1000 pf. For Canada and the U.S. C102 and 103 are 1500 pf. For KT-5020s with a switchable de-emphasis, R119 and 120 (2.2 meg Ohm) are in circuit for Canada and the U.S. 75 µS and switched (shorted) out for 50 µS. For my North American version, I bypassed the de-emphasis switch and 2-4 inches of signal trace by removing R119 and R120, the 2.2 meg Ohm resistors, and replacing with shorting wires. Coming into the MPX chip is a 10 µF 35V cap (C90). I changed it to a 10 µF Black Gate. Leaving the MPX with L and R channels are two 10 µF 35V caps (C101 and C102), which were replaced with 22 µF Black Gates. The originals were 10 µF but I saw no reason not to go to 22 µF starting with C101 and C102. Always observe polarity with these electrolytics.

I cut out the old audio op-amp (IC 12) and cleaned the holes using a soldering iron and toothpicks, to keep them open and clear. Unless you wish to solder a new op-amp in place, add a new socket for the op-amp of your choice and solder that in place. I always finish all other soldering tasks before plugging the new op-amp in. Continuing along the audio circuit, I changed the two 10 µF 35V caps, C107 and C108, to 47 µF Black Gates. Now, the final two coupling caps have virtually no DC offset, so I chose two NP Black Gates, 33 µF NPs to replace the original two 10 µF 35 volt bipolar electrolytics (C111 and C112). Enlarging the capacitance of the caps following the MPX IC was chosen to extend the minus dB rolloff of the bass. I stress again, always observe polarity. On the KT-5020's circuit board, there is a + sign and you will see a small circle intersecting the larger circle where the positive (+) side of the cap inserts. Also, on the NP Black Gates, 33 µF, the longer lead should be considered the positive side even though it's non-polarized.

One final step is to replace C194 10 µF 35V with a 10 µF Black Gate. Do not increase the value of this cap. Also, I cut the rear to fit an IEC connector, and ran two ferrite cores between the AC cable and the power transformer and another ferrite bead between the multiple AC wiring out of the transformer and the board. No power supply changes have been tried at this time. We will update when it's tried and proven. I lifted muting transistors Q41 and Q42 but couldn't live with the harsh noise generated when switching between stations, so they were reattached. I have only tried an OPA2604 so far. Good luck with your projects. jim...

我没有完全照上面的步骤，只是把几个模拟部分的耦合电容换了。C101,C102，C107，C108原来是10UF35V的ELNA，换成10UF50V的nichicon MUSE KZ, C111,C112原来是10UFELNA, 换成22UF6.3V的BG NX电容。运放原来是NJM4560， 换成OPA2134. 声音的提升非常明显，也基本把5020的潜力给发挥出来了。现在的声音是甜美轻松饱满，很有味道。和我的马机比较味道上差不多但细节声场安静胜出。

18876

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