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Seeking schematic for Rotel RB-890 power amplifier
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<blockquote data-quote="Shonver" data-source="post: 92874" data-attributes="member: 34"><p>Finally got this one resolved.</p><p></p><p>I had replaced the existing long-tailed pair input transistors with new ones, as I had identified this as the most likely cause of the output DC offset. But it was months before I got to install the module and test it. I did that last night. But I still had an offset that started at 120mV and settled at 80mV. Not a happy situation. Did a lot of head-scratching and resistor-measuring. Finally, I considered what off-board influences could cause the fault... and there it was!</p><p></p><p>The amp has a bridge mode. This is achieved by feeding the output of left amplifier channel into the inverting input of right channel amplifier's long-tailed pair via a 12K resistor, and grounding the normal (non-inverting) input of that channel. This arrangement means that the right channel amplifier is now effectively DC coupled. The left channel has a -10mV offset (very good) and this, when bridge mode is activated, is enough to upset the quiescent DC conditions of the right channel. <em>I had, evidently, been testing the amp with the switch in bridge mode position</em> <img src="data:image/gif;base64,R0lGODlhAQABAIAAAAAAAP///yH5BAEAAAAALAAAAAABAAEAAAIBRAA7" class="smilie smilie--sprite smilie--sprite7" alt=":p" title="Stick out tongue :p" loading="lazy" data-shortname=":p" /> . When I set it to stereo mode, the offset went to 38mV. Not great, but good enough.</p><p></p><p>Schematic here http://www.elipse.co.za/Docs/RB-890%20sch.pdf if you are interested.</p></blockquote><p></p>
[QUOTE="Shonver, post: 92874, member: 34"] Finally got this one resolved. I had replaced the existing long-tailed pair input transistors with new ones, as I had identified this as the most likely cause of the output DC offset. But it was months before I got to install the module and test it. I did that last night. But I still had an offset that started at 120mV and settled at 80mV. Not a happy situation. Did a lot of head-scratching and resistor-measuring. Finally, I considered what off-board influences could cause the fault... and there it was! The amp has a bridge mode. This is achieved by feeding the output of left amplifier channel into the inverting input of right channel amplifier's long-tailed pair via a 12K resistor, and grounding the normal (non-inverting) input of that channel. This arrangement means that the right channel amplifier is now effectively DC coupled. The left channel has a -10mV offset (very good) and this, when bridge mode is activated, is enough to upset the quiescent DC conditions of the right channel. [i]I had, evidently, been testing the amp with the switch in bridge mode position[/i] :P . When I set it to stereo mode, the offset went to 38mV. Not great, but good enough. Schematic here http://www.elipse.co.za/Docs/RB-890%20sch.pdf if you are interested. [/QUOTE]
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Seeking schematic for Rotel RB-890 power amplifier
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