Lehle Splitter/Re-amp

Introduction

As part of an effort to improve the quality of my home recordings, I realized that I’ll be needing a splitter box that can allow recording “wet” signal that comes from my amps/cabs as well as dry signal that I can manipulate further after it’s recorded. Another utility that could be useful is a re-amp box that would allow passing the recorded dry signal back through the pedals and amps and record wet again, potentially with different amp settings and/or effects. Two most popular brands that come up when looking for quality solutions are Lehle (from Germany) and Radial (from Canada). Each serious spliiter or re-amp box has a high impedance isolation transformer in its heart that allows us to galvanically isolate two signals and avoid ground loop noise. It’s possible to get the original Lehle LTHZ isolation transformer which makes is a perfect candidate for DIY projects, so I got one and started building the circuit around it. Another popular choice in the DIY community is Xicon 42TM018-RC which costs a fraction of the price of Lehle, but has less desirable specs on paper, having bandwidth of only 300 Hz to 3.4 kHz. So I threw Xicon in the mix as well, partially to be able to go between “lo-fi” and “hi-fi” sounds and partially to demonstrate the differences between the two transformers and justify or debunk paying extra for a top shelf transformer from Lehle.

Features
  • One input, one non-isolated output and one isolated output.
  • Switchable record/re-amp modes. Re-amp mode features dedicated Level and Impedance knobs, similar to Jensen’s schematic for “Balanced line level to unbalanced guitar guitar converter”. In record mode, those potentiometers are out of the circuit and do not load down the signal.
  • Switchable Klon-style op-amp buffer that affects both outputs. With the buffer disengaged, the rest of the circuit is passive and needs no power. Even when not splitting the signal into two paths, it’s nice to have a buffer at the front of the signal chain.
  • Phase and Ground-lift switches for the isolated output.
  • A switch to go between lo-fi (Xicon) and hi-fi (Lehle) modes.
Layout

I went for a compact (100x100x40mm) enclosure similarly sized to Lehle P-Split and tried to pack everything inside. All controls are mounted at the front panel and all connectors are on the back panel.

Build

In order to make it less painful to cram all the circuitry into small space, I got a cool aluminum enclosure that has removable front and back panels, as well as removable top and bottom covers. That allowed me to work on front and back panels separately before connecting them with the rest of the circuit.

There are two internal rails that keep all the panels together and also have mounting holes for circuit boards and other internal parts. And after connecting the panels with the main circuit board, it’s still much easier to work on the circuit without top and bottom covers attached.

And here’s now it looks with top and bottom covers installed.

Note that the “Isolated Output” (leftmost) jack has plastic thread to keep it isolated from the rest of the jacks. This is crucial.

Performance Analysis

Other Transformer Options

I haven’t had a chance to try any other transformers, but I’ll mention few more that are often used in DIY projects. Jensen is said to be the first maker that offered suitable transformers for re-amping and splitting and their Jensen JT-11P-1 Line Input Transformer which promises flat frequency response across the spectrum, but is even more expensive than the Lehle. More reasonably priced options is the Edcor PC10K/10K transformer priced halfway between the Lehle and Xicon. Finally, Hammond 140UEX is the most expensive of the bunch. For the price difference I’d rather buy the enclosure and all the other parts than pay extra for the transformer alone.

Possible Modifications

Instead of the hi-fi/lo-fi switch, it’s possible to run both transformers at the same time and output two isolated signals through separate output jacks simultaneously. The upside would be the ability to record both the hi-fi and lo-fi outputs at the same time, but the downside is that this arrangement would require a buffer in front of each transformer to make sure that transformer primaries don’t load each other down. In that case we would lose the ability to have a simple 2-way passive splitter because buffers would need to run all the time. Furthermore, we could use the same approach to add more (hi-fi or lo-fi) isolated outputs if needed by adding more buffers and transformers in parallel.

Another idea that comes to mind is the ability to go between op-amp and discrete buffer. If the space is not a constraint, another switch to select the buffer would be a nice addition.

Finally, some folks would benefit from battery operation. Simple buffer doesn’t draw a lot of current, so it should last for years, but it would need to be switched off when not used, either with another switch or using a stereo jack for input that would break the battery ground connection when unplugged. I didn’t want to bother with that and was already lacking space, so I opted for external power supply only.

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    The idea behind this site is to share my experience with Do It Yourself approach to guitars, amplifiers and pedals. Whether you want to save a couple of bucks by performing a mod or upgrade yourself instead of paying a tech, or want to build your own piece of gear from scratch, I'm sure you will find something interesting here. Also, this is the home of DIY Layout Creator, a free piece of software for drawing circuit layouts and schematics, written with DIY enthusiasts in mind.