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Old 18th May 2011, 10:52 AM   #1
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Default Hafler DH220 quiescent current

Ive just bought a hafler DH220 aged over 20 years. It has been used in Europe on 220 V mains and the transfo cabling is on a 220 V configuration Putting on some cheap speakers it seemed to perform well, apart from a 100hz mechanical noise from the transformer. I put the transfo on rubber studs , which resolved the noise problem. However I had some suspicions and did check the quiescent currents on both channels at nominal 65 V powersupply voltage. I found 375 mA against 275 mA indicated in the manual. If there would have been misuse of the equipement in the past it seems very unlikely that this would have occurred on both channels in the same way ,except for excessive input power disturbing the input transistors on both channels. I mesured on one channel the Zero offset: it was 0.3 v but could be adjusted to zero via a P1 1/3 of a turn correction, the quiescent current still being 375 mA. I tried to identify if Hafler had changed the output transitors. On the Q12 top it says 551049 8205 and 571134 8205. On Q12 was measured Vc +65 V, Ve 0.9, Vb 1.05
Could someone help me out? To me its enigmatic !
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Old 19th May 2011, 07:28 PM   #2
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Additional info and problemsolution : 1)my previous statement that Q12 is the outputtransistor is wrong :Q401/402 Q403/404 are the output powers. 2) All voltages have been measured by now and are ok except quiescent current. I decided to readjust P2 on both channels and quiescent currents came down to the nominal 275 mA.
Why deviated both channels in the same mannor? The powertransformer was wired for 220 V but since 1986 mains in Europe are 230 V , which accelerated aging and so the need for readjusting.The mechanical 100 hz noise generated by the transformer has also disappeared. Amp is stable and silent problem solved
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Old 19th May 2011, 07:47 PM   #3
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The zero temp. coefficient point of Vgs for the 2SK134 / 2SJ49 Lateral MOSFET devices used in the DH-220 is approximately 100mA Id. According to the service manual, the DH-220ís bias current is set for a total positive rail quiescent current of 275mA per channel. The DH-220 clearly does run itís output stages relatively hot (about 30W per channel) at a quiescent Id of approximately 100mA per device.
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Old 19th May 2011, 08:22 PM   #4
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I believe when this amp was rather new some people subjested turning the bias up
higher than the 100ma per device called for in the service manual. It was suposed to
sound better that way at the expence of stressing the outputs a little more.
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