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Frequency response of transformers Transformer Sowter 4603 Wiring Diagram
Frequency response of transformers
Transformer
Sowter 4603 Wiring Diagram

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Build A DI Cable
A DIY cable that's a DI!
By Jules Ryckebusch

Did you build the Project r DIY Cable in the November 2000 issue? Would you like to eke a little more bass out of it?

That project featured a transformer available from Digi-Key that we quoted as having a frequency response of 0.3 Hz to 100 kHz ±2 dB. Actually, that’s a faux pas: the specs are .3 kiloHertz to 100 kHz. That means it rolls off below 300 Hz —not as tremendous a bottom end as we’d hoped for in our exuberant search for an inexpensive transformer.

Yet the transformer works well and sounds good on instruments—so what’s the deal here? After this national scandal was uncovered, I immediately set up some incredibly complicated scientific experiments (in other words, I ran a CD player through the two DI Cables I use). Sure enough, close scrutiny revealed a minor dip in the low end. It’s not horrible, yet it’s real; you don’t particularly notice it on guitar, but you probably would on bass.

Then I descended into my laboratory, which is where I’d designed and built the prototype devices that are shown in the article. Mounds of sophisticated test equipment yielded inconclusive results. After consulting with Scott Dorsey and a couple of other gurus, I added a 600-Ohm load to the transformer (which my test equipment had not been doing) and the specs improved, showing that the transformer was only 3 dB down at 60 Hz. Not great, but not horrible either.

For stage use low-end roll off is probably a good thing! You don’t necessarily want all that rumble going out to the audience. My studio has monitor speakers with 6.5" woofers that start dropping off around 60–70 Hz all by themselves, which is typical for nearfield monitors in a project studio. So losing 3 dB at 60 Hz isn’t the end of the world.

But it’s not the beginning of the world either, so what can we do? Time for more research into transformers. Jensen transformers have great specs, but they’re in the $100 range—which is fine, but that takes some of the DIY cost-saving thrill out of the project.

Then I found Sowter (www.sowter.co.uk), a British company that offers a small 600-Ohm shielded line-level transformer, model 4603, at $22 US. I ordered two (under $50 with shipping), and they took only twelve days to show up.

Both spec sheet and listening comparisons to the original Digi-Key in the article proved that yes, they do indeed have better low-end response. And because they’re shielded, they should pick up less hum and noise—not that I’ve had any problems with this in the original version.

The wiring diagram is included with this update. Note that the Sowter transformer does not have a split secondary but does have a shield, which gets connected to ground on the balanced side. Unlike the original Digi-Key part, this transformer won’t mind phantom power.

If you’ve already built the cable using the Digi-Key transformer, solder a 600-Ohm resistor across the output windings (Pin 2 and 3 on the XLR). This will improve the frequency response considerably, although it will drop the output level somewhat.

My apologies for the confusion, and special thanks to Scott Dorsey for his help (and to eagle-eyed reader Ken Sakara for catching the misquoted specs in the first place).

Jules Ryckebusch (talkback@recordingmag.com) welcomes all suggestions and comments on his DIY designs—including the ones that lead to refinements.

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