Unnecessary DI Box

[zpyder]

Could there be any ill effect to anything in the signal chain or the signal itself if one were to take an XLR microphone and put it through a DI box with XLR out before going to a PA?  I'm asking if placing an unnecessary DI in a chain between mic and PA could cause any problems?  Obviously the mic's pin 3 would have to be dropped to ground before the DI, therefore negating the CMR, but other than that.........

thanks!
zpyder

[roknjohn]

Why are you dropping pin 3 to ground? Does the mic use a 1/4" plug?

A typical DI (not all) is designed to convert a line level signal to a microphone level, which is far lower in level. So, you would be dropping the level of the mic even lower at the output. Nowadays some folks call just about any type of interface a DI, so it's hard to know for sure.

[zpyder]

Why are you dropping pin 3 to ground? Does the mic use a 1/4" plug?

The mic has XLR and I am converting it to 1/4", hence pin 1 goes to shield, pin 2 goes to tip, and pin 3 is dropped to ground (shield) - common passive conversion from balanced to unbalanced from what I read.  pin 3 is is the negative or "cold" signal while 2 is the + or "hot".  we keep the hot and toss the cold.  could do it the other way, but then we're 180 degrees out of phase...

A typical DI (not all) is designed to convert a line level signal to a microphone level, which is far lower in level.

well, this may be true, but also a DI converts from High impedance (instrument) to low impedance (mic).  I am more concerned with impedance than with volume.  So really another way to ask my question would be: If you use a high-to-low impedance converter on a low-impedance microphone, what kind of output impedance will you get?  Low impedance?  Lower impedance than before?

thanks...
zpyder

[Tubebass]

So you're putting a 1/4 inch plug on the mic cable and plugging it into the hi-z side of the DI? The XLR out from the DI will still be low-z, but the level will be very low..... maybe too low to be useful.

[zpyder]

So you're putting a 1/4 inch plug on the mic cable and plugging it into the hi-z side of the DI?

yes...

The XLR out from the DI will still be low-z, but the level will be very low..... maybe too low to be useful.

well damn...

I just want to put a low-z mic through an effect (rebote 2.5 delay) which is intended for hi-z input, and then have a low-z output... I can't seem to find an affordable way to do this and time is ticking.  If I could find the proper transformers to convert from low-z to high-z at input, and another for high-z to low-z at output that should do it, but I don't know where to get them... perhaps I could rewire the input and output of the rebote for low-z......

help anybody?

thanks,
zpyder

[Pushtone]

Nothing described so far could cause any damage. Try it and report back please.

A low-Z mic is designed to plug into a pro PA system WITHOUT a DI.

Care to enlighten us as to what you want the DI to do?
Is it to split the mic signal AND avoid ground loop hum like in a mic splitting transformer?
Are you sending the mic to the PA and to a guitar amp?

Just curious.



Hey spyder,
aren't you the person who wanted to build an effect loop box
for a mic to allow vocals to be routed through guitar pedals?
Still a good idea in my mind and surprising it hasn't been marketed.


Opps, never mind, you replied while I was typing.

[Satch12879]

Yes, why indeed? Most PA mixers have both XLR AND 1/4" balanced jacks so the need to go to 1/4" puzzles me.  Further, DIs convert TO balanced.

Also, you can plug a mic in to guitar effects but the S/N response would be horrid.  I'd use some sort of mic pre and then feed the effects.

[Pushtone]

I just want to put a low-z mic through an effect (rebote 2.5 delay) which is intended for hi-z input, and then have a low-z output... I can't seem to find an affordable way to do this and time is ticking.  If I could find the proper transformers to convert from low-z to high-z at input, and another for high-z to low-z at output that should do it, but I don't know where to get them... perhaps I could rewire the input and output of the rebote for low-z......

help anybody?

thanks,
zpyder

What you are talking about here is really TWO passive DI's.

The mic out connects to the first DI's XLR and it is used backwards. (Passive DI can be used backwards, not active ones).
The Hi-z out of the first DI connects to your effect pedal. The out of the pedal connects to the Hi-z input of the second DI. The XLR out of the second DI connects to the PA.

You could also buy a good quality Hi-Z Shure mic like the PG58 that already has a Hi-Z 1/4" output. That would eliminate one of the DI's

I seem to remember writing this a couple of months back. Did the two DI thing put you off this?

I would'nt build one. I would just use two DIs like the DOD 260 passive DI.
Or you could buy a couple of cheap Yorkville DIs and take the transformers out.

If you can find an affordable transformer you want one that is 10k ohms on one side and 150-600 ohms on the other. Remember they can be pass signal in either direction so you only need the one.

[zpyder]

What you are talking about here is really TWO passive DI's.

yea, that's what I'm seeing here, problem is buying two DI's pretty much triples the cost of the project... It seems I should be able ot buy the same or similar transformers used in commercial DI's before markup, but where!?

I seem to remember writing this a couple of months back. Did the two DI thing put you off this?

Yea, I brought this up before, and after doing quite a bit of research I spent a few days toying with several active designs... I couldn't get any of them to work... and I never really wanted to to it actively to begin with, so I'm back to square one it seems.  And the price of 2 DI's did put me off, but if I could get just the XFMR's at a reasonable cost that'd be great... doesn't seem like they're really out there though...

thanks,
zpyder

[roknjohn]

"(Passive DI can be used backwards, not active ones)." I was about to mention this, but Pushtone beat me to it.



" If I could find the proper transformers to convert from low-z to high-z at input, and another for high-z to low-z at output that should do it, but I don't know where to get them... perhaps I could rewire the input and output of the rebote for low-z......"


At one time RATSHACK carried these, but I'm not sure anymore as the last time I took a look in my local store, the wasn't much for components and the like, and was pretty much consumer electronic products. Your local music store should (Should being the key word!) carry standard barrel type tranformers in a variety of connector and gender types that match hi to low impedance.

[roknjohn]

Heres one for $12.


http://www.musiciansfriend.com/product/AudioTechnica-CP8201-InLine-Transformer?sku=270570

[Tubebass]

You could try this: Use a passive DI backwards to get to hi-z (Either put a female XLR on your mic cable, or use a gender bender),go into your effect from the 1/4" jack on the DI, then plug the effect output into the line input on your mixer.

[Barcode80]

I'm shocked no one has suggested this, but does your PA have an effect send on the board? if so, you plug a cable from the send to the pedal input, one from the pedal out to the effects return, then turn up the effect on the track. it works, i've done it numerous times.

[ashcat_lt]

you may have volume trouble.  you won't have impedance problems.  I don't think you mentioned what delay pedal you're using, but it's probably got a Hi-Z buffer at its front end anyway.  Probably the same 1M that the DI would present. 

[Pushtone]

Don't be shocked.
this is kinda of a rehash of a thread in September that covered all the basses on this.

Still, I think it's a good idea.

The Aux send and return is the easiest way to go fer sure,
but I can see a need for such a mic-loop stompbox thing.
When it came up in September I was totally for it.
So build it already will ya! 

It basically lets a vocalist use guitar effects like guitarists do.
Stomping on it when they want to turn it on/off.

And stompboxes belong on the stage, not at FOH.
Just try to get a house tech to plug in your stompbox at FOH
when they most likely have an SPX990 in the rack. I wouldn't.
On the stage, the band rules and anything goes for instruments.

I can totally see myself singing the cadence of a nice slow cover song,
then stomping on a DS-1 to distort my voice as I count off 1, 2, 3, 4 for the next Skinny Puppy cover song.
Better have it (the stompbox) adjusted just right so not to freak out the FOH mixer.

And why shouldn't vocalists get in on the stompbox action.
Hey there might even be a few more vocalists (sans instrument)
hanging out here building effects just for their voice.

I can imagine reading...
" Hi, I'm a tennor. What value output cap should I use in the XYZ phaser to give my voice that fat Pavarotti tone?"




OK having fun on zpyders thread-time.




Back to the Mic-Loop Box...

Those Audio Technica ones are perfect.
Tear them apart while keeping track of what wire went to what connector pin.
Then get cleaver on mounting them.

If your looking for  a PCB mount package...
When I think transformers, I think www.alliedelec.com.
They seem to have the best selection of little tiny transformers as well as tube amp
power transformers with heaters, a bunch of telecoms isolation
transformers and many general purpose audio transformers for PC audio.

Finding the right one is daunting.
So lets look at a DI schem to see what the transformer spec is.

When I wanna see a schem for a DI, I got to...
http://www.jensentransformers.com/as/as066.pdf
http://www.jensentransformers.com/apps_sc.html

Looking at the specs for the JT-DB-E transformer used in that.

SPEC:

Model #    Price    Impedance Ratio    Turns Ratio       Max 20Hz Input    THD 20Hz    
JT-DB-E    $ 80     200k:1.5k            12:1              +22dBu 0.036%    <0.001%    


Wow!
A 200k:1.5k impedance ratio.
Why is the output impedance so high at 1.5k?
I thought it would be around 600-150 ohms.

Anyway, the closest match to that $80 Jensen transformer I can find in 5 min of searching is...

http://www.alliedelec.com/Images/Products/Datasheets/BM/TRIAD_MAGNETIC/SP-4.pdf

from the alliedelec.com site. I didn't search there for long. Used the search words "audio transformer". 
There could be a better match. This one has a freq response of 300Hz to 1000kHz so
it WILL roll of some bass from male vocals. (that might not be a bad thing for this).

I wonder what Whirlwind would say to you if you asked to buy a few transformers from them?
And what about Rapco.com. They use transformers in their DIs. Maybe they would sell?

[Gilles C]

I would just use XLR<->1/4" adapters from Radio Shack or the equivalents.

http://www.radioshack.com/sm-a3f-xlr-jack-to-1-4-plug-adapter-transformer--pi-2062443.html

They have a transformer inside, and they are cheap enough that you can kust take the transformer out and use the things separately.

I have one with a switch "lo-hi Z" that just take the transformer out of the circuit to use only the XLR-1/4" as needed.

Gilles

[ulysses]

The mic has XLR and I am converting it to 1/4", hence pin 1 goes to shield, pin 2 goes to tip, and pin 3 is dropped to ground (shield) - common passive conversion from balanced to unbalanced from what I read.  pin 3 is is the negative or "cold" signal while 2 is the + or "hot".  we keep the hot and toss the cold.  could do it the other way, but then we're 180 degrees out of phase...

give it a go. you will see that it really does sound terrible when you do this.

i tested it with a xlr-> 1/4inch mic cable recently. even when plugged into my preamp low-z socket, my shures sounded like turd.

it should be illegal to sell xlr -> 1/4 inch cables. becasue people presume it will sound just as good as xlr->xlr when really it sounds no where near what it should.

cheers
ulysses

[moosapotamus]

I hashed out one way of doing it in this thread...
Microphone Stompbox Blender/Looper

Haven't had time to build one yet, but our singer is still bugging me for it.
Compared to using the aux send/return on your mixer, something like this would make it easy to have everything on the floor right in front of you on stage... Master Of Your Own Domain.

~ Charlie

[Barcode80]

i disagree about the difficulty of a stompbox on the aux. if you are playing a small venue where you bring your own, the sound is usually by the stage, and if not and you are using a snake (as i would hope is the case, otherwise you are doing some lousy cable runs ) you can run a 1/4 channel of the snake to and from. have the sound guy so a quick check for unity gain and VOILA! stage customizable on-the-fly effects.

[ashcat_lt]

I disagree with the FOH effect loop all together.  This would work if you wanted the effects in parallel with the vocal sound.  Actually would probably work fine for the OP's application.  It will not work so well with effects you want in series, like the distortion that dude was talking about above.

The ideal way to do it would be to have an actual mic pre before the pedal chain.  ART makes some good cheap tube driven ones that are not much bigger than a stomp box.  One of their models even includes a sort of "one size fits all" compressor.