How to Build a Boutique Passive DI June 29, 2012 19:12 31 Comments

If you can solder one component to a couple of jacks, you can make the best passive DI money can buy. That's no exaggeration--at it's core, every passive DI is just a transformer, and some of the best winders in the world, Cinemag, Sowter, Lundahl and Jensen make transformers designed specifically for direct box applications.

Passive DI Basics

Let's start from the top: a DI, or "direct inject" or "direct box," is a device that allows one to plug an instrument output directly into a microphone preamp. A DI achieves this by doing two conversions: 1) from high impedance (Hi-Z) to low and 2) from unbalanced to balanced. Electronically, there are two ways (at least) to perform these conversions: 1) actively, with buffering/balancing circuitry or 2) passively, with an audio transformer. Active DIs are fairly complex, require power (usually in the form of +48Vdc "phantom power"), and are ideally suited for hi-Z signals such as those from passive guitar and bass pickups. Passive DIs are extremely simple, require no power, and are best suited for lo-Z inputs such as synthesizers and active pickups (however, they have been known to sound great on passive sources such as the "Billy Jean" bass line).

Why Go Passive?

From a technical perspective, active DIs outperform passive DIs in almost every respect, and at a fraction of the price. The performance of passive DIs is restricted by an inherent property of transformers: the closer you bring the input and output impedances to their ideal values, the more signal you loose. An active buffer, on the other hand, can present an exceptionally high input and low output impedance while maintaining unity gain. Yet here I am telling you how to build a passive DI. Pourquoi? They sound good. A good passive DI will thicken and smooth the sound in a way that a transformerless, active DI will not.

The DI Transformer

In terms of sound quality, the only component that matters in your passive DI is the transformer. And, unlike a reamping box where a budget transformer performs admirably, a direct box is a sensitive application that demands a high-quality transformer. There are a couple of essential features that take quality DI transformers out of the "budget" range: 1) mu-metal case for magnetic shielding and 2) Faraday shields to isolate each winding.

Building Your Direct Box

Gathering Components

Nothing exotic here. Transformers must be custom ordered from the manufacturers and the remaining components are standard at Mouser, Digikey, Farnell, etc. Depending on the transformer you choose, the entire BOM will cost you between $75 and $115 plus shipping from at least two locations.
Qty  ↓ Value  ↓ Note  ↓ Approx. Price  ↓
2 1/4″ TRS jack Solder terminals $1
1 SPST (or SPDT) Toggle Switch Solder terminals $1.25
1 DI Transformer Cinemag CM-DBX, Lundahl 1935, Jensen JT-DB, Sowter 4243 $55 - $85
1 Male XLR jack Solder cup terminals $3
1 Aluminum chassis Hammond 1590N1 $11.50
1 Ring Terminal $.25

Assembly

Wiring

The schematic and wiring instructions below are based on the color coding of the Cinemag CM-DBX. However, the circuit itself applies to any DI transformer with a mu-metal case and shield wires for each winding. This includes offerings from Jensen, Sowter, and Lundahl.

DIY passive DI

  1. Begin by twisting the brown and red transformer leads together. (Making differential wires a twisted pair can reduce electro-magnetic interference.)
  2. Mount the transformer and jacks on the case. Stretch each transformer lead to where it will be soldered and trim the leads, leaving an extra 1/2" for wiggle room. Strip and tin the leads.
  3. Trim, strip, and tin two lengths of heavy-gauge wire for the ground connection. We'll call this wire "Green".
  4. Solder the leads to the jacks in the following configuration:
    • Yellow -> Input Tip
    • Orange, White, Black -> Input Sleeve
    • Gray, Green -> XLR 1
    • Red -> XLR 2
    • Brown -> XLR 3
  5. Solder the green wire from XLR 1 to the center pin of the ground switch. Solder one end of the remaining length of green wire to one of the other pins of the ground switch, and the other end to the ring terminal.
  6. Fasten the ring terminal to the case to secure the chassis ground connection.

How Does it Sound?

I built my passive DI with a transformer from Cinemag--I'm calling it "Ferrite" after the magnetic core of the transformer. To compare the sound of the Ferrite to something familiar, I reached for my personal favorite DI, a Hamptone HJFP2 with a built-in active DI. This has been my go-to for warming up direct synths since the day I fired it up on the bench. I compared the two DIs by setting both preamp channels to the same gain setting, and using the stock DI on one channel and the Ferrite on the other. Initial impressions: The Hamptone with the stock DI is already a very rounded, "solid" sounding signal path to my ears. However, compared to the Ferrite it has some rough edges, especially in the upper mids where the Ferrite really "irons" things out. Both had excellent, punchy bass response. But enough mumbo-jumbo from me, check out the 16-bit/44.1kHz .wav files below:
Hamptone DI: Download the WAV
Ferrite DI: Download the WAV

 

Questions? Impressions on the sound? Please, let's discuss...