DIY-Central.com - Go Create something! - PCB Design, Electronics

This is a DIY Signal Isolation/Unbalanced to Balanced Converter Box. I'm using high-quality MADE IN USA Audio Output transformers to create this SI/UnBalanced-to-Balanced (or Balanced-to-Unbalanced) Converter Box. In addition, a Ground Lift switch is provided.

On this picture, you can see the 4 TRS jacks (2 per channel x 2 = 4), the 2 large audio output transformers, and the ground lift switch in the middle.

And this is the picture of the finished project. The switch on top is the GND LIFT switch.

The jacks from left to right, are Unbalanced in/out, Balanced in/out (for Channel 1), Unbalanced in/out, Balanced in/out (for Channel 2).

The unit can perform as a Unbalanced to Balanced converter, or as a Balanced to Unbalanced converter.

On a previous post, I posted a schematic of a passive direct box using Jensen Transformers.

I have a ROLLS DB25 passive direct box in my studio so I decided to open it up to see it's "guts".

The ROLLS DB25 is an inexpensive direct box using all passive components. That's right, no need for a 9V battery or wall wart to use this thing. Plus, it's got a transformer inside that takes care of converting from an unbalanced Hi-Z connection (from a guitar) to a balanced Lo-Z connection for connection to your mixer or preamp via XLR jacks.

In addition, it has a -20dB and -40dB pad, and a ground lift switch for "stubborn hum" reduction.

This is a schematic plan for building a passive, transformer-based DI Box (Direct Box) from Jensen-Transformers.com. View the original PDF here.

You plug your hi-Z (or high impedance) guitar to jack J1. From here, it goes to a pad on/off switch. You also have 2 pad choices... -10dB and -20dB depending on the position of switch S2.

S3 is a hi-cut filter switch. If S1a (i.e. the pad switch) is switched ON, and the hi-cut filter is ON, the high frequencies are shunted to ground via C3.

The signal going back to your guitar amp via jack J2 is unaffected by the pad and hi-cut switches.

Transformer T1 is a JT-DB-E transformer. This takes care of matching impedance between your guitar and the mic preamp of your mixer. It also converts the guitar signal from unbalanced to balanced connection.  The red-brwn wires of the transformer goes to pin 2 and 3 of the XLR which forms the HOT (+) and COLD (-) connections.

Ground is connected to Pin 1 of the XLR. A ground lift switch is provided, S5, which isolates the pin 1 ground of the mixer from the rest of the circuit.



If you don't want to build one, here are some passive DI boxes.

ART ZDirect Passive DI View more products from ART The Zdirect is a high-quality, totally passive interface that lets you connect instrument-, line-, or speaker-level signals to a mixer or other balanced input through a high-performance audio isolation transformer. The high impedance single-ended 1/4 in. input is converted by the transformer into an isolated balanced low impedance signal source. Read Reviews... List Price: $30 Click for Sale price   Shipping cost: $4.99

Radial JDI Duplex Stereo Passive DI Box View more products from Radial A stereo version of the Radial JDI. Ideal for recording, broadcast, and live sound where extreme dynamics such as those produced by digital sampling devices and keyboards are encountered. Completely passive, the JD4 employs two Jensen JT-DBE transformers for 100% isolation and low phase distortion. Read Reviews... List Price: $350 Click for Sale price   FREE SHIPPING!

Radial JDI MK3 Passive Direct Box View more products from Radial The Radial JDI is considered by many to be the world’s finest direct box. It is a passive DI that employs a Jensen isolation transformer for optimum audio performance offering outstanding linearity at all frequencies, combined with extraordinary level handling without introducing distortion. Read Reviews... List Price: $200 Click for Sale price   FREE SHIPPING!

Radial Pro DI Passive Direct Box View more products from Radial The Radial ProDIs are high-quality, full-range passive direct boxes equipped with custom-made audio transformers for exceptional signal handling without saturation and with extremely low phase distortion in the critical bass and mid regions. The result is exceptional clarity and definition at an attractive price point. Read Reviews... List Price: $100 Click for Sale price   Shipping cost: $4.99