Amplifier Project – Part 4

This is the fourth part in the Amplifier Project series.  This covers the entire process of building an audio amplifier based on the information located at Red Free Circuit Designs.  The first three parts of this project are located here:

Amplifier Project
Amplifier Project – Part 2
Amplifier Project – Part 3

Some progress and a rather large setback.  First the setback:  the harvested transformer cannot be used, the voltages were incorrectly measured.  There are three center taps on this transformer, the output voltage is too low with regards to the 18V secondary, or too high and unequal with regards to the two 43/33V secondaries.

Better to cut the losses and search for a replacement.  Some more research is required, but this Edcor transformer or this item from Parts Express look promising.  The Edcor has a lower current rating, but is nicely put together.  The Parts Express item is higher current and is a basic transformer.  Both are the same price.

Work had already begun on the case modification before I realized my goof with the transformer.  The aluminum case was purchased at Fry’s (p/n 1656619), and at $20 it’s a great price for a box of this size.

I used the template for the power connector (made in part 3), traced it out and used a Dremel to cut out the hole.  Some filing was required to clean it up.

AmplifierProject_Case_03 AmplifierProject_Case_04
Next up: power switch installation.  Same process as above, a cardboard template was created and tested with the switch.  That hold was transftered to the case (which has blue painter’s tape laid out) and since I don’t have a large enough drill bit, or a punch, several small holes were made and connected with file.  Tedious but effective.

AmplifierProject_Case_07 AmplifierProject_Case_08 AmplifierProject_Case_09 AmplifierProject_Case_10
The transformer was installed, connected and turned on… and that was when I discovered my error.  Well, at least the connector and switch were not a wasted effort!

The process of translating the schematic to a perfboard layout was started.  There’s a great piece of software called DIY Layout Creator that is written in Java, therefore platform independent.  So far I have found this software to be stable and very helpful getting this project laid out.  There are a nice features, one of them is a bill of material is created as you put components on the drawing.  I use that to double-check against the project BOM to make sure  that a part has not been forgotten.  The only issue is printing is almost useless.  If you want a large printout, screen capture appears to be the best option.



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