Impedence Matching Transformer

  • Thread starter Thread starter Aren
  • Start date Start date
A

Aren

New member
Hi,

Can a Radio-Shack Impedence Matching Transformer (Hi Z - Lo Z - Female 1/4" to Male XLR) function as a simple direct box? I want to make the connection between my keyboard and my mixer balanced, and these transformers seem cheaper than "proper" DI boxes, although they seem to be doing the same thing...
Any thoughts?


Oren
 
If you need to convert balanced to unbalanced or vice versa,these trasformers are cool.As far as bringing a mic level to a line level signal it won't help.
 
You may find the results somewhat lacking. Those transformers are a long way from being the best thing in the world for quality, but they are at least somewhat adequate for mic levels (-50dBu). When you start running line levels through them (-10dBu or thereabouts), you'll find that they saturate badly, and their frequency response will get even worse. It's worse yet if the output buffer of your synth has a little DC offset, and runs DC through the primary: that'll partially saturate the core with a DC_fixed_ magnetic field, and lead to asymmetrical distortion (munging positive-going peaks worse than negative-going, for example).

Now, on the other hand, you may _like_ that distortion! I once built an iron-munger box for a keyboard player- it had a pretty decent Ampex input transformer in it, and a nice 6v supply, and a pot that let him just _cream_ the primary with DC: twist the knob increasing the DC bias, and the output level would drop, the signals would get hideously munged, and generally all hell would break loose.... Sounded like dogmeat, but he liked it to add "character" to his Prophet-5. There's no accounting for taste...

Mic-level impedance matching transformers, and line-level impedance-matching transformers, are different kettles of fish. The winding and core designs do differe depending upon the designed operating levels. By all means try it, but be prepared to be disappointed...

Unless you have a long signal run, there's probably not much reason to concern yourself with going balanced from the keyboard output. Are you dealing with noise issues here, such as ground loops (which using a DI would certainly help), or is this just something you want to do because it is often recommended as "better"? For a short run from a bank of keys to the local keyboard mixer, it may not be worth the expense. What problems are you trying to solve?
 
Skippy,

What problems am I trying to solve? Where do I start? :D
Seriously - The line between my keys (A Yamaha PSR-GX76 - great keyboard apart for the fact that the only output it has is really a headphone output and not a real line out) is approximately 3 meters (10 feet) long, but runs across an AC line, and has been known to be occasionally noisy (occasionally - only when it's important ;) ). I hoped that a balanced line would get rid of the ground loop, and maybe, if the impedence will be lower, maybe the mixer would deal better with the "not proper line level" output.

Oren
 
Then doing up a real DI box will probably help a lot more than kluging at it. Last time I looked at one of those RS inline transfromers, it did not isolate the sleeve on the TS side from Pin 1 on the XLR side...

Intermittent flaky noises are no fun at all. Another problem that you (very!) occasionally run into with using headphone outs into a bridging load (like a very high impedance mixer input) is that some headphone buffer amp designs really need a load to be properly stable- you get into wierd behaviors sometimes, for some settings of the level control. In that case, you can always just terminate the headphone driver out into something that looks like a headphone- say 250 ohms, and it'll clean it up a lot. Even 600 ohms is better than running into a 10Kohm input, sometimes... I don't know your mixer, or your keyboard, but this is one of those ~1% possibilities.

If the noises you're getting are primarily 60Hz hums and its relatives, or RF injection/detection, going with a DI so that you can isolate the grounds will almost certainly be a win. if it's a hiss that comes and goes, or sudden odd HF behaviors in the output signal, you might need to load the headphone output properly. If it's a DC offset or intermittent wierdness near DC (sudden rumbles, low whooshes, crud like that), then it could very well be an intermittent solder joint or other mechanical issue, and neither loading nor the DI will fix it...

Flakeys are not our friends. The other thing to remember is that cheapo stereo headphone output jacks are _notorious_ for going flakey. It could be as simple as the mating fit between that jack and the plug you choose, and a good cleaning (or worst-case, replacing the jack) might fix it outright. Happy hunting, and good luck!
 
:)

Skippy - Thanks for the quick and detailed reply!

I didn't really understand what you meant about loading the headphone output properly. My mixer has standard low impedance inputs (Behringer Eurorack). If I understood you correctly, that should be alright. If not, what exactly did you mean?

Oren
 
I was talking about the headphone output on the keyboard- the one that you are using to create a "sort-of" line level output. A low impedance mixer input is still probaby 5-10k ohms, bridging.

A good example of a marginal output is my Casio CZ101's headphone output. Now, the CZ has a line out, so I didn't need to do this- but at one point I decided to use the headphone out to drive another line in (don't ask why). Plugged it all up, and the equipment being driven by the headphone output showed nearly fullscale input, but there wasn't much audible signal- just a lot of hiss. Pulled out the oscilloscope, and discovered that the headphone amp was oscillating, rail to rail, at about 125kHz... The capacitance of the output cable, taken with the very high impedance of the input that I was driving with it, destabilized the output driver. Loaded the output with 200ohms, and the oscillation stopped and the thing behaved just fine.

This is really degenerate, and is probably not what you are seeing. More commonly, you'll see increased distortion and noise for questionably-designed headphone drivers, and sticking a couple hundred ohms across the output right at the keyboard to provide the load that they were designed for can make a noticeable difference.

Real line outs are designed to handle a very wide range of loads. Some keyboard designers sleaze out on that with the headphone jack: if there are no headphones plugged in, who cares what it's doing? (;-)
 
Thanks again Skippy!

So, the final conclusion from this discussion, is to place a resistor right at the keyboard, then plug a patch cable nto a direct box, to prevent ground loops, etc.

I can see a lot of soldering in my future... :)

Oren
 
You must log in or register to reply here.
Back
Top