Tuesday, February 11, 2014
REAMP with schematic
Here's a "reamp" I built for a friend. I'll be doing more, including a stereo "Amp /OR/ Mic Preamp (balanced)" version for myself. If you can follow this badly drawn schematic on note paper, that's what I used. I took most of my tips from Peterson Goodwyn's LINE 2 AMP and LINE 2 PRE devices on diyrecordingequipment.com, he's got some great articles posted. The balanced input jack must not be metal, grounded to the chassis, that is, because you want to be able to lift the ground with a switch if necessary for noise/ground loop isolation. I used the Jensen reamp schematic which you can find on their website, for the RC to ground in lift position, which helps with radio frequency noise rejection. The transformer is an unshielded 10K:600R Edcor that was an !excellent! value and really does sound fantastic. You might need to keep it away from large guitar amp transformers, etc, in use to minimise noise. A fancier Jensen, Cinemag etc. that has mu-metal shielding, in a can, would obviously be ideal but I wanted to save money. I think this entire project was about $20, give or take. You could use an XLR input, but I wanted a 1/4" since my friend will probably be using an unbalanced 1/4" line output from his mixer into this Reamp. To save him from having to use a special cable, any 1/4" instrument cable will work with the TRS input, it will just short the "cold" signal, or that end of the transformer primary, to ground, which is fine. Obviously you can adjust the output impedance by tweaking the 15K series output resistor, or use a pot there to get real tweaky (as in the Jensen). You want to be in the neighborhood of a guitar or bass pickup. You could also wire up an output attenuation pot like Jensen did but I wanted to keep it simple. New York Dave also posted an article with a similar reamp to look at. Another idea is to use a lower ratio transformer if you want a hotter output. You just want to get the output impedance to simulate a typical instrument, and the input to properly load a line output, along with knocking down the level a bit. Peterson really explains this well on his website and on Youtube.