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Unbalanced to Balanced Converter

Unbalanced to balanced converter, my Transformer coupled interface
After having made possibly hundreds of these over the years, my Transformer Coupled interface Type 1 is now available to everyone.

Many musicians, engineers and producers appreciate why signals should be kept as 'hot' as possible. High signal levels can reduce your noise floor and of course a lot of professional, studio equipment is designed to run at +4dBu (about 1.25 volts). What many might not appreciate is the virtue of also running balanced signal lines. My unbalanced to balanced converter Transformer Coupled Interface (or TCI) does just that. It converts a +4dBu unbalanced signal to a +4dBu balanced signal (no gain).

Running an analogue audio signal over a balanced line severely increases immunity to noise. I won't go into the physics of just why that is yet, although I'll no doubt end up adding to this post at some time in the future.

Unfortunately there's a lot of older gear out there including stuff we all love dearly, which has unbalanced +4dBu outputs. The Marshall JMP-1 that was featured in a recent post, is a prime example. The Roland RSP-550, an excellent nineties multi-effects processor, is another example.

Rear of Roland RSP-550 showing level selector switches
Unbalanced but switchable -20dBm/+4dBm inputs and outputs on an old Roland RSP-550

So, the traditional way to convert an unbalanced signal to a balanced signal is to run it into a D.I. box. We're all familiar with those little boxes that cover many a studio and stage floor, right? They work just great but while they balance the signal, they also knock down the level of the signal to a few millivolts which means that you have no option other than to run the other end into a mic. pre-amp, either as a separate unit or built into a desk. That's not always convenient and it seems stupid attenuating a signal, only to amplify it again, anyway!

Jack inputs on the TCI Type 1.
The inputs on the TCI Type 1 are conventional 2-pole 6.35mm jack sockets.

Have you noticed another downside to trying to use a D.I. box to solve this problem, yet? Earlier I gave the example of the Roland RSP-550. It's an effects unit. It has +4dBu inputs! So you ain't easily gonna be able to send signal from your desk to it via a D.I. box! The inputs on the RSP-550 are looking for either -20dBV or +4dBu. The microphone level output from a D.I. box just won't be enough to drive the unit. On top of that, the D.I. box output is balanced. The inputs on the RSP-550 are unbalanced. That's actually two downsides. 🙁

To convert an unbalanced signal to a balanced signal and maintain the level of the original signal was a problem I decided to solve once and for all, many years ago. I developed the transformer coupled interface (TCI). It's not rocket science (as they say) but boy, do these things come in handy.

Here's what's inside my Transformer Coupled Interface unbalanced to balanced audio converter
Here's a basic representation of what's inside my TCI unbalanced to balanced converters (single channel shown).

The TCI is a non-powered device (like a passive D.I. box) which basically comprises a single transformer on each channel. For an unbalanced to balanced converter, that's all you need, really.

Having said that, you of course, want the highest quality transformer you can get. You need a good frequency response but also, good phase coherency. Phase distortion, especially down the bottom end, is characteristic of cheaper transformers and if not addressed, makes bass guitar for example, sound woolly.

Similar to a passive D.I. box, one hidden benefit to using the TCI, is that it offers something called galvanic isolation which means that there is no physical connection like a piece of wire for example, between between your source and destination equipment. With the ability to easily lift the earth of the source equipment, the TCI Type 1 is just great for helping to get rid of ground loop problems!

TCI Earth Lift Switches
The TCI is equipped with separate earth-lift switches on each channel, to provide full galvanic isolation.


All equipment has what’s called an input impedance and an output impedance. Impedance, like resistance is measured in Ohms but differs in that it's frequency dependant. Impedance varies with frequency and so the impedance at say 500Hz will be different to the impedance at 5kHz. Having said that, due to the nature of things, a common resistor is often used on for example, the output of an electronic circuit to provide the output impedance of a system.

Output impedance schematic

Most guitarists will be quite aware that everything to do with guitar electronics is high impedance and that the first thing you plug your guitar into should have a very high input impedance. This reduces the ‘loading’ on the guitar pick-ups and allows for a good frequency response further on down the chain.

Unbalanced outputs like those found on a lot of consumer electronics such as hi-fi gear, have a high impedance, perhaps in the order of thousands of Ohms. Balanced outputs in contrast, are always low impedance; just a couple of hundred Ohms. Again, I won't go into the physics suffice to say that this is why amongst other things, low impedance outputs allow for long cable runs.

There are exceptions however and while unbalanced, a lot of studio gear has low impedance outputs. This makes life a lot easier!

The TCI is classed as a low input impedance device and while it will technically work on something with a high output impedance, you might notice some high-end roll-off. Basically, you’ll need to check the output impedance of your gear before considering a solution like the transformer coupled interface. That’s why D.I. boxes are so popular. D.I. boxes have a high input impedance which means you can plug in just about anything. Like my transformer coupled interface, passive D.I. boxes also have a transformer to convert the input to a balanced line. The turns ratio of the transformer however, is such that while presenting a high impedance at the input and low impedance at the output, the signal voltage is greatly attenuated. Hence, the output of D.I boxes is mic. level.

Rear sockets on the Marshall JMP-1
The outputs on my favourite MIDI valve guitar pre-amp, the Marshall JMP-1 can be switched to +4dBu but... they're unbalanced. Grrr...

Housed in an aluminium enclosure, TCI's transformers are individually contained in MU-metal cans, all of which contributes to very high noise immunity.

Inside the TCI Type 1
TCI Type 1 is a double screened device considerably improving noise rejection.

So, if you've got problems interfacing some of your gear with your mixing desk or DAW and you reckon that an unbalanced to balanced converter might help, please don't hesitate to contact me.

UPDATE - 22nd June 2021

Weeks after launching this post, I received so many inquiries about my TCI, that I decided to make a production version. I'm pleased to announce that this amazing little problem solver is now available to buy.

Non-slip feet on the TCI Type 1.
If you don't like feet, please turn away now!

Plasma Music Limited -

Like all my stuff, TCI is designed and made by me, here in Hemel Hempstead, UK. I do however, use sub-contractors for making enclosures, PCBs, etc. Unlike many, I only use local companies and here's why...

I'm deeply concerned about the environment and the exploitation of labour and so  I always use local manufacturers in preference to the Far East, with the following in mind:

  1. I can be confident that workers are treated fairly and earn a proper wage.
  2. I can be confident of the standard of quality of each item that is delivered to me.
  3. Communication is important and using local manufacturers, all correspondence is quick and understandable.
  4. I believe in supporting the local economy.
  5. I can be confident that the disposal of manufacturing waste is managed properly and in accordance with national and EU law.

Plasma Music uses local manufacturers

Using local manufacturers isn’t the cheapest option but the above points are important to me. I hope that they’re important to you too.