Lehle Buffer/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. Both of those devices are built around audio isolation transformers that allow us to “break” the ground by galvanic isolation of input and output signal, avoiding noise caused by ground loops. So I got an idea – why not combine them into one device and build an ultimate recording utility?

Some of the most popular brands that come up when looking for quality transformers are Lehle, Jensen and Radial. To best of my knowledge, Radial doesn’t sell transformers separately, only built into their products, so it’s down to Lehle and Jensen. Both have impressive specs and cost similarly (65 USD for Lehle and 75 USD for the Jensen). I opted for the Lehle LTHZ (high impedance) isolation transformer which makes is a perfect candidate for DIY projects.

While investigating transformers for this project, I remembered that Xicon small signal transformers are sometimes used in DIY projects because they are readily available and very cheap. I’ve seen some used for signal splitting, some can be used in Tycobrahe Octavia and sometimes even as cheap replacement for a wah inductor. They have a few different models with different winding ratios and impedances. The one that seems the most suitable is Xicon 42TM018-RC 10K:10K transformer which costs a fraction of the price of Lehle, but has less desirable specs on paper, having the bandwidth of only 300 Hz to 3.4 kHz. So I a Xicon 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.

Another feature I wanted to add is a switchable buffer, for two reasons: it can add color to the sound and be an “always on effect” in the signal chain and it can lower the impedance and potentially drive the transformers better.

Features
  • One input, one non-isolated output and one isolated output.
  • 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. The buffer comes before the transformers in the signal chain.
  • 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.
  • 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 (100 x 100 x 40mm) 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.

To finish it off, I laser etched a panels describing what each control does.

Click here to download the PDF version.

Finally, the finished “product” with the front panel installed.

How Does it Sound?

I wanted to find answers for few questions that came up while designing and building this.

To answer these questions, I recorded a few audio clips playing the same riff with my new Bancika JPM guitar. All the clips are recorded using bridge and neck pickups in parallel with split coils, making it essentially two single coil pickups in parallel. The sound is captured straight from the audio interface and no effects or equalization is added on top of it, other than volume normalization.

As the baseline, here’s how the guitar sounds plugged into the interface directly, without the box in the signal chain.

Without the buffer engaged, does the transformer primary coil load the guitar pickups down and affect the sound in any way?

That appears to be true for both transformers, but much less noticeable with the Lehle. Below are the two recordings taken from the main output (not the ISO!) that are supposed to sound identical with the direct recording, but are not, due to the load of the transformer primaries. In both cases I’m hearing a loss of high end sparkle.


Is there a noticeable difference in the isolated output sound quality between the two transformers without the buffer?

There is definitely a huge difference between the the transformers when recording non-buffered signals out of the isolated output. The Xicon sounds thin and dead in comparison, but it’s hard to tell how much of the difference is there because of the primary load on the pickup and how much of the signal is lost in the transformer between the primary and the secondary. It may be cool as a lo-fi effect, but for general use, I would not use Xicon in a passive box.


How does the buffer affect the sound?

The buffer makes all the difference. It is subtle, but adds a bit of the clarity to the sound and lowers the impedance of the signal, so it’s safe to say that it can easily act as an “always on” effect. But more importantly, it helps drive the transformers better and avoid tone loss caused by the primaries. Below are two recordings taken from the main output with the buffer engaged. Compare them directly to observe the effect of the transformer primary load on the buffered signal, or compare them against the direct recording to observe the effect of the buffer on the clean signal.


The next pair of clips is recorded from the isolated output with the buffer engaged. Buffer helps to bridge the gap between the two transformers.


Is the cheap Xicon good enough for guitar use?

Electric guitar is hardly a hi-fi instrument anyways and we often do things with guitar signal that are unimaginable by hi-fi standards. We intentionally add distortion, dramatic equalization and other effects to add character and transform the sound of the guitar. So standards are a bit different compared to the hi-fi world. Without the buffer at the front, I’d say a definite NO, but with the buffer included, Xicon could be a good budget alternative.

Is there a difference in noise levels between the two transformers?

I haven’t noticed any noise at all in any of the recordings.

Performance Analysis

To accompany the sound clips, I wanted to measure the frequency response and plot it on a chart, hoping that it would back up what I’m hearing.

To do that, I came up with few semi-scientific tests. For the first test, I played all notes on the guitar neck once and recorded the output. For the second test, I played a song with a few arpeggio chords across all six strings, but limited to the first five frets. In each test I would record the output taken from the Main (direct) output and Isolated output simultaneously, and would record four different combinations of settings: buffer OFF, Hi-Fi; buffer OFF, Lo-Fi; Buffer ON, Hi-Fi; Buffer ON, Lo-Fi. That should give me enough data points to answer all the questions above.

Please note that I do not have access to a lab or high-tech professional gear, so all tests are performed using my guitar plugged into the splitter box that goes into an audio interface hooked up with a laptop. Variations in pick attack between the takes is possible, but I think that results are decent enough to draw educated conclusions.

Starting with easier questions, by comparing frequency responses of buffered and non-buffered signal coming through the Lehle transformer, it’s easy to see how the buffer affects the sound. There’s a slight dip in the higher mids, above 1KHz and then a slight boost of highs above 2K, improving the clarity of the sound without making it sound piercing high. The other test matched the results closely.

Now, comparing the two transformers, it’s clear that there is some bass roll-off happening close to where the Xicon datasheet suggested it would – between 200Hz and 300Hz. Open strings on guitar ring at frequencies between 82Hz and 300Hz, so this range is crucial for fundamental tones of the guitar. The data matches with my empirical findings – that Xicon sounds less full. It is not terrible sounding, but the bass is noticeably tighter. In some situations, this could even be a good thing!

Moving on to the fun part, by engaging the buffer before the transformers, the difference between the transformers becomes less apparent. The roll-off point moves to 70-80Hz, right around the fundamental frequency of the open low E string and around the roll-off point of most guitar speakers. It is still possible to hear that Lehle sounds fuller, but the difference is smaller.

As far as the noise level is concerned, there was no detectable noise added by either of the transformers, with or without the buffer engaged.

Another aspect worth mentioning is the distortion, although I do not have a way to measure it. To me it sounds like the little Xicon is adding a bit of dirt to the sound without the buffer at the front, but I may be mistaken due to the narrower bandwidth.

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. However, in that case we would lose the ability to have a simple 2-way passive splitter because buffers would need to run the buffer 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 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.

Comments
3 Responses to “Lehle Buffer/Splitter/Re-amp”
  1. Patrick says:

    How much is one of these. I plan on running a mixture of old tech and hi and eventually future tech if I get good enough but right now I’m running a headphone jack amp with 3.5mm AUX CORD output into a 1/4″ 6.5mm guitar plug into my pedal board. Problem is my effects cant ALL Handle the jack amps overdriven output very well. I dont know specific numbers but my EHX 2013 hot tubes NANO plays well with it.

    I was gonna put everything in a 12-13 stomp isolated true bypass also with switchable buffered capabilities. Then I also was thinking about having a signal splitter, a few of them actually thru out the board that I can combine with stuff to get dif tones and whatnot. I have a ZOOM MRS-1608CD RECORDER. IT HAS RCA Ins and outs a digital out and 1/4″ headphone outs inuse with 10-15w junk practice amps at the moment. I also have a PEAVEY VIP3 amp. So I need signal going to a few dif places a few dif ways EVENTUALLY!! BROKE Right now but I did just get $80 so I’m trying to build and buy some components and strip tag board with it too, but this piece of machinery I one day need in my life in a pedal board friendly re design!! Please let me know if you would help me a lil bit at least draw a simple turret tag board schematic of an ABY true bypass or whatever and also buffered IC as well as switchable. I need my setup back online cuz I’m tryin to change my life. Sick and tired of being sick and tired AND STUCK in the lower end of society you know!! And my ex destroyed a bunch of my already crappy ass gear in January right before we split cuz she knew my guitar keeps my alive EVERYDAY. She went for the jugular and the heart with one fell swing of my effects chain spread out on the floor, all connected 6 pedals cords power supply, grabbed the cords closer to pedals and started swinging like kurt cobain!! So now I’m broke and trying to diy with a bunch of components I’ve saved from OLD electronics I’ve stripped down carefully over last 20 years. In storage I got some good stuff 😉 so any help would be greatly appreciated. Thank you so much for your time,

    Sincerely,

    Patrick Butcherine
    1870 Branchton Rd
    Hilliards, Pa 16040

  2. Antonio says:

    Are there big enough differences in sound between both transformers to justify the price of the Lehle LTHZ?

    • bancika says:

      It’s good that you reminded me that I need to finish up the article and add some performance analysis charts. For guitar use I found the cheap Xicon to be surprisingly good. Had definitely more low end than the 300Hz that the data sheet suggested. Guitar is not a hi-fi instrument, so imperfections can add character to the sound. We distort the heck out of guitar signal anyways. So it depends on what you want to achieve. Lehle is superb and neutral. Xicon adds more character, but the output is not the same as input.
      Cheers

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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.