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eBay mono LM1875 kit
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Old Today, 12:23 PM   #391
rabbitz is online now rabbitz  Australia
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Location: Newcastle, Australia
Default LM3875 minimum

Bear with me here as this is background to my next post about the latest LM1875 build. I want to try a minimum component LM1875 but first I tried a AudioSector LM3875 to see if it was worthwhile.

I was surprised that the amp board was a Chipamp.com PCB and not that usually supplied by PD but shouldn't matter as they are both similar. The power supply PCB was PDs.

Build 1 was a bit "meh" and not what I remember from Peter's kits of years past. I've used a Chipamp.com PCB over a decade ago but can't remember what it was like.

Build 2 tossed out the power supply and made a new one with a bridge rectifier and 2 x 10000uF caps. The balance was all wrong like a BSC circuit with the response running down hill (strong bass and recessed mids and top).

Build 3 kept the new power supply but tossed out the Panasonic FC 1500uF caps near the chip and replaced with 100uF Nichicon KZ and X7R by-pass caps. The sound now came together and is a keeper. I now remember that all the AudioSector kits I built had 100uF caps on the amp PCB and larger caps on the power supply PCB.

The crunch with the minimal approach is horrible DC offset values... 60mV, -90mV with open inputs and 5mV, -11mV with shorted inputs. Depending on the source, the offset can vary such as an active pre gave high offset values and an Ipod gave low offset values.

This put me off doing a minimal LM1875 as it really needs the feedback and input caps so did a full Brian Bell LM1875. I've attached pics of the LM3875 just for reference only.
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File Type: jpg RZ38752019b.jpg (154.1 KB, 0 views)
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