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Another LM1875 Chipamp Execution
Another LM1875 Chipamp Execution
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Old 12th July 2009, 03:19 AM   #1
swin1 is offline swin1  Australia
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Default Another LM1875 Chipamp Execution

Another LM1875 Chipamp Execution…

I bought a 1970’s Jorgen AMI A-333 Integrated Amplifier on ebay about 6 months ago. The amp was a neat, compact solid state unit with very nice components, all alps faders, switches etc. It sounded clean at low volume levels, but had a very weak EI core transformer, probably around 30VA. I had considered upgrading the supply only, but as the whole system was a single rail power design, I decided to gut the whole amp, removing the power amp, preamp and phono preamp boards, and inserting a whole new chipamp setup.

The only additions I made to the chipamp design were increasing capacitance to 3,000uF per rail. This was after some testing and I found a nice balance of bass output in my room with more filtering. The transformer used is a Nuvotem 120VA 18v toroid, which is plenty for this application. I moved the power switch from front to back, to keep AC in the back corner, so I need to do something about that hole in the front. The amplifier boards are mounted one on top of the other, and the heatsink is isolated from the chassis, by using thermal padding underneath the heatsink, and bolting down the heatsink with bolts wrapped in heatshrink and separated from metal at both ends with silicone washers. This is a good way of achieving good heat transfer without having to use mica washers I think.

Only the input selector and volume pots are used in the signal line, all other switches and pots have been removed from the line. I may add headphone capability, but am unsure whether to run it off the LM1875 or implement a separate headphone amplifier.

The decorative blocks of wood replaced some thinner, less attractive ones. As you can see the saw burnt one end of one of the blocks, I’ll address this later.

Absolutely thrilled with the sound, no noise or hum at any volume level, so grounding must have worked out (no star ground on this one.) Currently they are powering my 7” 2-ways with a similar wood finish -

Comments or suggestions appreciated!

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Old 12th July 2009, 07:23 AM   #2
akis is offline akis  United Kingdom
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How do you switch it on and off ? :-)
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