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UNBALANCED TO BALANCED CONVERTER

Transformer coupled interface at Plasma Music

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.

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!

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.

Transformer coupled interface schematic (basic)The TCI is a non-powered device (like a passive D.I. box) which basically comprises a single transformer on each channel. I designed the transformer myself as not only did I want the best linearity I could get over the entire audio spectrum but I also wanted minimum phase distortion, especially down the bottom end, something that cheaper transformers tend to generate. Anyway, I managed to get a bunch of these things made here in the UK and the TCI was born.

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 the signal path between your source and destination equipment is 'broken'. With the ability to easily lift the earth on the output (balanced side) of the TCI, these things are just great for getting rid of ground loop problems!

IMPEDANCE

All equipment has what’s called an input impedance and an output impedance. Impedance, like resistance and is measured in Ohms but differs in that it's frequency dependant. The impedance varies depending on the frequencies in the signal. Taking a guitar signal as an example, the input impedance of an amp let's say, will be different at 500Hz than it will 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 device.

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 exception; unbalanced outputs on a lot of ‘studio’ gear can be low impedance. This makes life a lot easier!

The TCI has a low input impedance 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.