LINE2AMP Design Journey February 15, 2012 20:14 1 Comment

This all started when my grandfather decided to dissolve his woodworking shop and give me his gorgeous old Sears drill press. I couldn't let a tool like that sit idle in my basement, so I thought, hey why not make a DIY kit? That was last August (2011), and while the The LINE2AMP reamping box is about as simple a piece of gear can get, it's been a rather long and circuitous journey to bring it to fruition. Doubtless, someone more experienced could have whipped the LINE2AMP up in a weekend, but I've learned a ton about electronics, product design, and DIY in the past few months. I'll be using this page to share what I've learned with you, as well as to let you in on the design process of the LINE2AMP as it progress.

 

August 2011: Researching Reamping

My original goals for the LINE2AMP were that it should:
  1. Perform as well as any commercially available unit
  2. Cost less than $50 to build
  3. Be simple enough for newbies to build successfully.

I starting looking around at other reamping designs available on the internet and found three popular schematics: Jensen, Recording Magazine (Scott Dorsey), and NYDave. Looking at schematics the schematics, I realized that all of them shared the same essential topology:

reamp schematic

I ordered a couple of transformers from Edcor USA to start experimenting with circuits.

September 2011: Prototyping

Four weeks later transformers arrived and I started soldering. 30 minutes later I had a working reamping box. Huzzah! Goal #3 was in the bag. My first prototype hewed almost exactly to NYDave's design: I used the Edcor PC10k/150 transformer with a volume pot and 200 Ohm terminating resistor. The only variation was that I used a 1M Ohm variable resistor to test different output impedances. Testing the prototype with a variety of amps, pedals, and DIs revealed a few things:
  1. The unit was dead quiet and imparted practically nothing on the tone. At first, I had been skeptical about how the unshielded Edcor would perform, as both the Jensen schematic and the Reamp use shielded transformers. While a shielded trafo would certainly be better on paper, in practice the Edcor added no noticeable noise, even when run via DI into a clean preamp.
  2. The impedance resistor had no noticeable effect on the sound until it was cranked beyond about 200k Ohms, after which the signal started to lose some high frequencies and become distorted in the bass.
  3. In almost all cases, there was less noise when the connection from ground pin of the input jack to the case was disconnected. Only with some pedals did it sound better connected.

Overall, I was really happy with how the circuit worked. I didn't have a commercial reamping box on hand to shoot it out against, but it did everything a reamping box should and without any noise or sonic artifacts of its own. In fact, I was pretty blown away by the transparency of the Edcor transformer. In a blind test between A) a signal sent first to the reamping box, then the DI input of my Hamptone preamps and B) a signal sent right from my interface to the line input of the Hamptones, I couldn't make any conclusive distinction between the reamped and non-reamped signal.

 

Paring Down to Essentials

After playing around with my prototype, I realized that the LINE2AMP could be made even simpler than any of the other reamping designs and do the job just as well. The first thing to go was the impedance knob. From one perspective, it makes sense to have a range of output impedance available to emulate the source impedances of different guitars (typically between 10k-50k Ohms) and pedals (as little as 200 Ohms with a buffered output). However, in testing there was no audible change in tone until about 200k Ohms. From a technical perspective this makes sense, as any source impedance below 100k or so would be easily bridged by most guitar amp and pedal inputs. Goodbye to that pot! Next to go was the volume knob. In theory it's nice to have a tactile way to control the level, but in practice I found myself almost always doing it from the DAW. I also rarely found myself wanting less signal, and without adding active circuitry the volume control can only attenuate, not boost. What's left is the bare essentials of a reamping device: a transformer for impedance/voltage conversion and ground isolation, and a resistor to set the input/output impedances. That's it!