Camcorder XLR Adapter/Preamp: Input Balance Technique

Transformer taken from Beachtek, along side a Lundahl microphone transformer.

Transformer taken from a Beachtek device.

This section describes the approach and justification for the input balance technique implemented in the mixer.

Professional grade microphone transformers do a great job as the input balance circuit. They have good characteristics for frequency response, linearity, and common mode rejection. Since they are windings of wire and basically can act as antennas for picking up interference, they also take special care for shielding. They use MuMetal shields that are effective for not only electric fields, but magnetic fields as well. They also implement a Faraday shield with shielding between windings. Unfortunately, they are also very large, heavy, and expensive.

Many manufacturers of camcorder adapters use much smaller transformers. This saves cost, size, and weight. But, it compromises frequency response, linearity, and common mode rejection. Also, these transformers are generally not shielded.

Take a look at the first photo. On the left is a transformer that was taken out of a BeachTek camcorder adapter. On the right is a professional microphone transformer from Lundahl. Notice the difference in size. Both are single transformers.

Take a look at the second photo. This shows what BeachTek has attempted to do in order to shield the transformer. They stuff 2 transformers under a single shield, and glue the shield on top of them, and solder a wire on one side of the shield to ground

At juicedLink, we did not want to take the route which compromises performance for size, then kludge together some kind of shield. However, there is no practical way to fit 4 Lundahl transformers into such a small form factor, and still have it considered an “ultra portable mixer”. It would just be too big and heavy for camcorder applications.

We choose to take advantage of advances in integrated circuit technologies, where our preamp is precision laser trimmed to achieve balance. It’s small, lightweight, very low noise, has great linearity, and excellent frequency response. Additionally, having low noise preamp gain provides for some other advantages in our architecture, yielding better overall system signal-to-noise performance relative to passive adapters for low level signals (such as when dynamic microphones are used).