Friday, March 4, 2016

Harman Kardon 250 Rectification Close Up

Harman Kardon 250 Rectification Close Up

Heres another focus we did on a customers Harman Kardon 250 Power Amplifier, same unit from our posting here but we decided to update the rectification supplies.

Ideal diode equation

Original and new Dio's

New Dio W/ Bypass Films
The original used an odd ball Silicon device at a typical Vfd .7V similar to most silicon devices with an unknown V(P) rating. A note to Silicon diodes; they're pretty simple, a modern Silicon device has a nominal forward voltage drop of .7V, which for the most part is consistent, hence a "nominal voltage at .7", ( forward voltage drop can slightly skew over a band of current applications but is still relative in parameters.) Modern Ultra Fast 4007 diodes were used in placed and bypassed with a 47PF high grade WIMA polypropylene film capacitors and benched again at a 10Kz 8 ohm load. 

Audio Notizen 10Kz @ 8OHM
4.996  (3.12W)
11.329 (16.04W +12.14dBW) (AVG RMS 11.34V)

11.708 (17.13W +12.33dBW) (AVG RMS 11.2V)

5.017 (3.21W) 
10.927 (14.92W +11.73dBW) (AVG RMS 11.34V)
11.052 (15.26W +11.83dBW)  (AVG RMS 11.2V)

A new 16 AWG UL rated power cord was added and some final buffing on the chassis side plates, turned out great on the sides!

All in all one of my favorite tube amps yet to work on, the integration of high grade polypropylenes, silicon by passed UF diodes and upgraded .22MFD bypass capacitors really took off. 

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