Building Passive DI Box

[Harrison]

Currently, my high school just uses adapaters whenever they need to connect 1/8" or RCA to one of the XLR based systems in the building. I know that that's a bad solution, so I'm looking into building a passive DI box, as I want to keep costs down. I've found a cool guide here:

http://www.nfaudio.com/wp-content/uploads/DIY-DI.pdf

But I'm having trouble finding a transformer to use that's within price range. I see DI's with prices between $20-$50, what kind of transformer are they using? What kind should I look into?

 

[JD]

While the 600 ohm output in theory matches mixer inputs, it really should read "600 ohms or less." Depending on if you can live with some attenuation, I've had great luck using old 70 volt CV transformers. The lowest wattage tap (.5 or .25 watts) is usually around 10K and works great for the input. The speaker winding, which is 8 ohms, then drives the balanced output (pins 2 & 3 of the XLR.) The mismatch usually knocks about 10 Db off signal strength, but if you are inputting a line level signal, that works out pretty good. Usually can find these transformers in junk piles, but the new cost is about $5.

Note: You can not read "impedance" on a multimeter as the meter uses DC and will give you very low readings. (Ex- Output will look more like a short.)

 

[Harrison]

While the 600 ohm output in theory matches mixer inputs, it really should read "600 ohms or less." Depending on if you can live with some attenuation, I've had great luck using old 70 volt CV transformers. The lowest wattage tap (.5 or .25 watts) is usually around 10K and works great for the input. The speaker winding, which is 8 ohms, then drives the balanced output (pins 2 & 3 of the XLR.) The mismatch usually knocks about 10 Db off signal strength, but if you are inputting a line level signal, that works out pretty good. Usually can find these transformers in junk piles, but the new cost is about $5.

Note: You can not read "impedance" on a multimeter as the meter uses DC and will give you very low readings. (Ex- Output will look more like a short.)

Okay so I tried to find some stuff, but I must be doing something wrong because this was basically all I could find, and the price is ridiculous. Can you point me in the right direction?

http://www.lowellmfg.com/tinymce/jscripts/tiny_mce/plugins/filemanager/files/1716-TLM600.pdf

 

[Harrison]

While the 600 ohm output in theory matches mixer inputs, it really should read "600 ohms or less." Depending on if you can live with some attenuation, I've had great luck using old 70 volt CV transformers. The lowest wattage tap (.5 or .25 watts) is usually around 10K and works great for the input. The speaker winding, which is 8 ohms, then drives the balanced output (pins 2 & 3 of the XLR.) The mismatch usually knocks about 10 Db off signal strength, but if you are inputting a line level signal, that works out pretty good. Usually can find these transformers in junk piles, but the new cost is about $5.

Note: You can not read "impedance" on a multimeter as the meter uses DC and will give you very low readings. (Ex- Output will look more like a short.)

Okay, I spent another hour or so looking around, and I found a few more products. Would either of these work? The prices are much more reasonable, and it appears they both have a .5w or .25w tap.

http://www.fullcompass.com/prod/136739-Quam-TBLU

http://www.mouser.com/ds/2/177/5c0056-48788.pdf

I'd hope the fullcompass one would work, considering how much cheaper it is.

 

[themuzicman]

By the time you've bought all the jacks, an enclosure, and the transformer you're already approaching the price of a cheap DI. Why not just get something that you know will work? 95% of the time folks are making their own passive DI's to make sure it's of a super high quality using parts they want. You seem to be just going for functional.

The ART ZDirect, Behringer UltraDI, and Pyle Pro PDC are all products that have costs $30 or under, some have stereo options, and more importantly work.

 

[JD]

more like this guy http://www.mcmelectronics.com/product/555-7120
Lowest tap on this one is .63 so I like the one above that goes down to .25 watts.
Not used to paying for these things as usually they are all over the place in any shop that has upgraded installations. Usually a nice pile just waiting to go in for copper recycling.

@themuzicman makes a good point. Don't spend to much making one as they are pretty cheap. In the case I was working on, I put together a 5 line direct for a keyboard player who wanted 5 feeds into the sound system in one box. In that application, I think I spent the most on the XLR jacks! The rest of it was junk lying around.

 

[n1ist]

I have used the Pyle Pro one - $8.99 on Amazon. You can't get the parts for that...

 

[Harrison]

So at this point, I'm trying to calculate costs and weigh the pros and cons of the different ways I could go.

If I buy the materials for a DI box, it will probably cost around $25-30, considering we already have some jacks lying around. Additionally, it becomes a learning experience for students to assemble them, and gives us an excuse to buy a soldering iron (I'm ticked that we don't have one yet - I would like to start repairing some cables). Finally, I would like to do some customization (I really want a DI with 3.5mm and RCA).

The ART ZDirect, Behringer UltraDI, and Pyle Pro PDC are all products that have costs $30 or under, some have stereo options, and more importantly work.

I have used the Pyle Pro one - $8.99 on Amazon. You can't get the parts for that...

I looked a little at some of those on Amazon, and to be honest, I'm not impressed by the reviews. I know I risk something by making one, but at least I know exacrtly how it works, and hopefully even how to fix it (the schematic is ridiculously simple obviously).

Lowest tap on this one is .63 so I like the one above that goes down to .25 watts.

When you said the one above, were you referring to the fullcompass link I had?


I think I will try to come up with a solid estimate for the cost of making one, and weigh the options. If anyone has input on the cheaper Passive DI boxes, I'd love to hear it. Thank you for your help so far!!!

 

[JD]

Yea, the fullcompass one has the 1/4 watt (.25) tap.
So, I should mention that direct boxes vary quite a bit, even the passive ones. The expensive ones use a magnetically shielded transformer. Cheap ones don't and are subject to picking up hum from other magnetic sources that will not go away with the ground lift switch. One thing to keep in mind if you need a few is to buy or make one and try it out to make sure it fits your needs. (As compared to buying or making a batch of them.)

 

[FMEng]

I would avoid an unshielded transformer. A 70 constant volt speaker transformer will work, but it could also be pretty effective at picking up hum from magnetic fields. Here is a transformer more suited to the purpose.

https://www.bhphotovideo.com/c/prod...FRMR_IL_19_XFRMR_Stand_Alone_Transformer.html
That's about as cheap as you'll find a new, good sounding audio transformer.