Hi all,
I'm working on my M4000 here, and as is almost always the case, schematics, board layouts and the actual equipment have differences. This M4000 is interesting to me that in the the bias circuitry is quite different from the schematic:
.
The SV02 dual diode D401 in the base circuit of Q401 the bias spreader transistor was jumpered out from the factory, and other M4000s I see in YT vids have the SV02 installed and touching the heatsink. I know SV02s are known to be unreliable, so that is potentially a good thing, but only if the bias is thermally stable elsewise.
In this M4000 the heatsink mounted bias spreader Q401 is not heavily bypassed as is the case with just about every other amp I have worked on. The output board layout and silkscreens and schematic as shown above are calling for a paralleled 0.22uF (C401) and 330uF (C402) cap serving the function, but they were never loaded from the factory. The only cap bypassing the bias spreader transistor is a small 0.22 uF (C216/316) on the driver board.
This would cause quite a bit of AC conduction by the bias spreader, especially at lower frequencies, and that 2SC1904 is only rated at 50 mA. The result would be bias modulation with signal amplitude, of course perhaps that bias modulation was found to be desirable in this amp? Alternatively could the factory have accidentally left off the bypass caps from the output boards?
I have looked on the web to see if any other M4000 output boards have C401 & C402 loaded on the output modules, and the couple of M4000 videos I have found look to have a smaller axial cap than the outline on the silkscreen for C402, but I cannot see the value printed on the cap.
Before I go to the trouble of analyzing this with LTSpice I thought I would ask the brain trust if any of you knew the reason for the different versions of M4000 bias circuits. Any ECOs from Luxman on this issue are lost in the mists of time...
Thanks for any help,
Howard Hoyt
I'm working on my M4000 here, and as is almost always the case, schematics, board layouts and the actual equipment have differences. This M4000 is interesting to me that in the the bias circuitry is quite different from the schematic:
The SV02 dual diode D401 in the base circuit of Q401 the bias spreader transistor was jumpered out from the factory, and other M4000s I see in YT vids have the SV02 installed and touching the heatsink. I know SV02s are known to be unreliable, so that is potentially a good thing, but only if the bias is thermally stable elsewise.
In this M4000 the heatsink mounted bias spreader Q401 is not heavily bypassed as is the case with just about every other amp I have worked on. The output board layout and silkscreens and schematic as shown above are calling for a paralleled 0.22uF (C401) and 330uF (C402) cap serving the function, but they were never loaded from the factory. The only cap bypassing the bias spreader transistor is a small 0.22 uF (C216/316) on the driver board.
This would cause quite a bit of AC conduction by the bias spreader, especially at lower frequencies, and that 2SC1904 is only rated at 50 mA. The result would be bias modulation with signal amplitude, of course perhaps that bias modulation was found to be desirable in this amp? Alternatively could the factory have accidentally left off the bypass caps from the output boards?
I have looked on the web to see if any other M4000 output boards have C401 & C402 loaded on the output modules, and the couple of M4000 videos I have found look to have a smaller axial cap than the outline on the silkscreen for C402, but I cannot see the value printed on the cap.
Before I go to the trouble of analyzing this with LTSpice I thought I would ask the brain trust if any of you knew the reason for the different versions of M4000 bias circuits. Any ECOs from Luxman on this issue are lost in the mists of time...
Thanks for any help,
Howard Hoyt