Pelham Hall 003 - BACK TO THE BENCH

Last week was a lost weekend since I was in training all week and was catching up on work in the evenings. Come the weekend, I did manage to get more done.

After stopping at Radio Shack and picking up a small proto board, I got busy with planning and building stage 1 (Left Channel??). Fortunately, I had held on to my construction tools over the years:

• Soldering Station
• OK Brand Wire Wrap (WW) tool
• OK WW unwrap tool
• Various WW wire spools
• BOURNS brand trimmer adjustment tool
• WW IC sockets and component headers

These became invaluable as I planned the build (see the picture later…). I figured I would get back to this type of work at some point, even if it was for hobby.

First, I needed a 3.5mm jack to accept the Table headphone output. This was something I decided to buy from a hobby shop (Jameco). Along with those, I ordered some additional LM741 IC’s, an assortment of resistors and capacitors to ensure I had enough parts to tweak my design.

I forgot that planning is everything

There were some challenges getting the components located in a reasonable layout to accommodate everything on this small board. After locating the headphone jack, I salvaged a power connector from another device (old DSL modem) and mounted it next to the jack. The mounting was bugging so I tried to super glue it to the board (bad idea!). Anyway, in the end, I pulled it off and went with a 2 pin connector. Once this was in place, I realized this arrangement would be perfect for the Left/Right output.

To the bench!!

 

Thinking about where to get the 2 pin connectors for this, I realized that I have a few old PC chassis around that used these two pin connectors to connect to the front panel LED’s and switches, so off came the case of one (I should have thrown this out years ago!!) and the 2 pin cables were snapped and interfaced (two for Left/Right to RCA plugs, one for the now removed power connector).

The first stage was built and tested, with some component values being adjusted to provide good dynamic range and variable gain. Actually, in the end, the gain adjust is anywhere from 1 to 80 via a 100K ohm, 10-turn trimmer. This allows for the tablet volume to be set at about 50% and 4 turns of the trimmer to get a decent range.

The first stage was completed. Notice that the power connector is on the board at this stage.

Once this was mostly tested, the second stage was built and tested. Below is a picture of the entire setup with the Right/Left RCA Jacks near the bottom right. Note that the power connector is no longer on board...more packaging work.

And here is a close up of the [mostly] completed preamp.Right/Left Line out cables are on top, with the power connector bottom right, and Headphone Jack in on the left (white cable).

Packaging

 

I have been thinking about packaging for the better part of two weeks and after some false starts with “round-the-house” boxes, I am still searching for the best fit. More when I figure that out.