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Old 28th May 2015, 07:02 AM   #1
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Default Aiwa amplifier with power supply electronic voltage switch?

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.
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Old 28th May 2015, 07:08 AM   #2
Mooly is offline Mooly  United Kingdom
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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.
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Old 28th May 2015, 07:15 AM   #3
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Originally Posted by Mooly View Post
Looks like some simple form of Class G amplifier

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?
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Old 28th May 2015, 07:57 AM   #4
Mooly is offline Mooly  United Kingdom
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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.
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Old 29th May 2015, 01:55 PM   #5
sbrads is offline sbrads  United Kingdom
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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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Old 29th May 2015, 02:15 PM   #6
Mooly is offline Mooly  United Kingdom
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Sounds like a little change in the production run
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