Hello to everyone.
I was repairing an Aiwa XR-AVH1000 mini component system with a 5.1 amplifier. In the schematic i came across something interesting in the front amplifier section. There is a circuit called "VOLTAGE SELECT SWITCH". I was wondering how this works.
The front L+R amp is fed with a constant ±21V, however it has to power lines called ±VL and ±VH, probably for low and high. The ±VL is fed directly to the two power darlingtons, ±VH is �±42V which goes in two MOSFETs, 2SK2937 for the negative and 2SK2957 fot the positive line.
What I think is that at high volume the 2 MOSFETS turn on the ±VH, but I don`t understand how this works and why was it implemented. I`ve attached the amp section schematic, i`ve circled the positive voltage switch.
I was repairing an Aiwa XR-AVH1000 mini component system with a 5.1 amplifier. In the schematic i came across something interesting in the front amplifier section. There is a circuit called "VOLTAGE SELECT SWITCH". I was wondering how this works.
The front L+R amp is fed with a constant ±21V, however it has to power lines called ±VL and ±VH, probably for low and high. The ±VL is fed directly to the two power darlingtons, ±VH is �±42V which goes in two MOSFETs, 2SK2937 for the negative and 2SK2957 fot the positive line.
What I think is that at high volume the 2 MOSFETS turn on the ±VH, but I don`t understand how this works and why was it implemented. I`ve attached the amp section schematic, i`ve circled the positive voltage switch.
Attachments
Looks like some simple form of Class G amplifier where the higher rails are switched in to power the amp as the speaker output voltage increase. It can work well and enables the amp to run cool for most of the time on its lower rails. Nad used this technique a lot and I guess many others did too, Pioneer for one.
So it`s something like Technics or Panasonic used to use but instead of a second pair of output transistors, Aiwa implemented an "active" switch for the power rails?
I`ve measured the collector voltage of the main amp at high volume with a digital multimeter but the max voltage I measured was ±24V. Could the changes in voltage be so quick that the multimiter can`t pick it up?
You'll probably find it only switches when the output is approaching clipping on the lower rails and it won't remain on once the peak has passed.
The way to test it is with no speakers attached and playing a 1kHz tone. Turn the volume up and at the point the rails switch measure the AC voltage across the speaker terminals. You can work out then the point at which it changes over.
The way to test it is with no speakers attached and playing a 1kHz tone. Turn the volume up and at the point the rails switch measure the AC voltage across the speaker terminals. You can work out then the point at which it changes over.
My Harman Kardon HKTS7 100w sub amp had a bipolar triple in each supply line to switch from 24v to 47v at high signal levels. It wasn't even shown on the schematic, I traced it when looking for a standby fault that turned out to be elsewhere in the end.
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