Recording Unbalanced signal through Balanced Mic pre??

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travis

Well-known member
Joined
Jul 8, 2011
Messages
64
Location
Portland, OR
I just recently built a 1290EZ mic pre (Martins project) and I am trying to find a solution for direct recording from unbalanced outputs (synth, sampler).  Some folks have already given me very helpful advice on the 1290 build thread, but I decided to start a new thread because I think that the discusion might not belong there and I still am a little unsure of how to procede.

From my understanding my options are to either add DI boxes to the chain or to use TS> XLR cables.  I would prefer to not have to buy/build anything that is not absolutely necessary at the moment so my inclination is to just use the un-balanced cables, but forum member tmuikku brought up the concern that I have a chance of shorting out Phantom power (if switched on):

Both xlr pin 2 and 3 would have the +48V so basically your phantom supply could still burn since it would be dumping current to the synth output ground potential?

If anyone could else could confirm this or advise me on a possible solution I would greatly apreciate it :)

Page 8 of martins guide shows how the phantom is connected to input XLR pin 2 and 3 if this helps:
http://s3.amazonaws.com/EZ1290/assembly4.pdf

Thank you, I would love to figure this out ;D 
 
easiest solution is to build in a DI and put a 1/4 jack in the front. Or buy a DI and use it stand along along side the mic pre. Can easily get  a passive decent sounding DI for on average 30 dollars.
 
travis said:
I just recently built a 1290EZ mic pre (Martins project) and I am trying to find a solution for direct recording from unbalanced outputs (synth, sampler).  Some folks have already given me very helpful advice on the 1290 build thread, but I decided to start a new thread because I think that the discusion might not belong there and I still am a little unsure of how to procede.

From my understanding my options are to either add DI boxes to the chain or to use TS> XLR cables.  I would prefer to not have to buy/build anything that is not absolutely necessary at the moment so my inclination is to just use the un-balanced cables, but forum member tmuikku brought up the concern that I have a chance of shorting out Phantom power (if switched on):

Both xlr pin 2 and 3 would have the +48V so basically your phantom supply could still burn since it would be dumping current to the synth output ground potential?

If anyone could else could confirm this or advise me on a possible solution I would greatly apreciate it :)

Page 8 of martins guide shows how the phantom is connected to input XLR pin 2 and 3 if this helps:
http://s3.amazonaws.com/EZ1290/assembly4.pdf

Thank you, I would love to figure this out ;D
There is no risk of burning the phantom power supply because current is limited to about 14mA by the phantom injection resistors.
But there's a risk of injecting DC in the synth output. The output capacitors of the synth may not withstand long term application of DC.
But the major issue is the risk of impedance mismatch. The synth outputs are designed to be connected to medium-to-high impedance inputs, not to low-impedance inputs like mic pres. It may introduce level loss, which may or may not be without consequences, and narrowing of the frequency response. It is always advisable to have dedicated instrument inputs.
In the case of synths, it is preferable to have medium-impedance (about 10k) balanced inputs than high-impedance unbalenced inputs (typical DI), which are more suited for guitar and bass (Fender Rhodes too).
 
There is no risk of burning the phantom power supply because current is limited to about 14mA by the phantom injection resistors.

Good! ;D

It is always advisable to have dedicated instrument inputs.
In the case of synths, it is preferable to have medium-impedance (about 10k) balanced inputs

Does this basically mean that the 1290 is not ideal for this aplication or is there something that I could build/buy to add to the chain that would achieve a medium impedance?

I should point out that I live in an apartment and my vocal booth is the walk in closet 8), so while I do want to do everything that I can to get good recordings, Im not really in a position to get it perfect!  My biggest concern is that I do not want to cause any damage, which it seems there is still a potential for.  Would a DI box at least eliminate that risk if there is not an easy alternative?

Thank you for the responses! 
 
A DI box is a solution, but you should avoid the passive type.
Another, much simpler possibility is to put a resistive pad between the synth and mic input. One needs to know the basic level and impedance characteristics of the synth to calculate the values.
 
Another, much simpler possibility is to put a resistive pad between the synth and mic input

I like this idea!  Will the L pad described here work for me along with the calculator?    http://diy-fever.com/misc/l-pad/

I think that I have or can find the impedance and level characteristics I need.

Can this all happen in the cable with some heat shrink or is a box necessary?
 
abbey road d enfer said:
A DI box is a solution, but you should avoid the passive type.
Another, much simpler possibility is to put a resistive pad between the synth and mic input. One needs to know the basic level and impedance characteristics of the synth to calculate the values.

and what would be wrong with a passive DI ???
 
Two 10K, one 220-ohm.

Does this pad work in most unbalanced>balanced scenarios?

should 220 ohm cross xlr pin 2 and 3 and the 10k on pins 1 and 3?

there's a risk of injecting DC in the synth output. The output capacitors of the synth may not withstand long term application of DC.

would a resistive pad correct this?

Andy Peters said:
I'll throw in with the "just use a DI" crowd. Simple, direct. (Hah hah.)

Sounds like a lot of folks go this route and I might too!  My concern is putting a cheap DI the chain and having it do more harm than good.  Also if I can spend $10 insead of $60-$100 (on two boxes)  I will be happy! 
 
pucho812 said:
and what would be wrong with a passive DI ???
A cheap passive DI is crap.
An expensive passive DI maybe good on synths, adequate on bass and active undesaddle, poor on electric guitar, useless on passive undersaddle.
A moderately priced active (dbx, behringer, samson, art...) will be good not stunning) on most sources.
I think the OP is not in a position of shopping an expensive active BSS or LA Audio or Radial.
 
should 220 ohm cross xlr pin 2 and 3 and the 10k on pins 1 and 3?

Pin 1 is the shield connection.  No audio.

The pad PRR described is a U-pad.  The formal term doesn't get used enough and google likes to give you a page on Urban pads, whatever they are.


go here:

http://www.uneeda-audio.com/pads/
 
lassoharp said:
should 220 ohm cross xlr pin 2 and 3 and the 10k on pins 1 and 3?



The pad PRR described is a U-pad.  The formal term doesn't get used enough and google likes to give you a page on Urban pads, whatever they are.

Urban pads: Sanitary protection for a women made out of kevlar for living in the hood?
 
The pad PRR described is a U-pad.

more like this?

http://s556.photobucket.com/albums/ss1/travisttaylor/?action=view&current=hpad.jpg#!oZZ1QQcurrentZZhttp%3A%2F%2Fs556.photobucket.com%2Falbums%2Fss1%2Ftravisttaylor%2F%3Faction%3Dview%26current%3Dhpad.jpg

Why not build a couple Bo Hansen DI boxes?
I like this Idea too!  A great deal Im sure, but I would still opt for the cheaper option if possible.  My budget really comes down to how much I can get for my thrift store finds on Craigslist ;D
 
http://classicapi.com/catalog/index.php?cPath=107

You can try a few low cost DI options from Jeff and the 51X crew. There is a missing adapter for general use that is missing in his offereings for those who need them in other projects.

 
Thanks kazper, that looks like a good option too,
I think I got the pad figured out for now though.  I used the resisters PRR recommended but I would still like to figure out an exact impedance match and just generally wrap my head around the formula!  I think I just need to do a little more reading up on it.  Thanks for all of the input.
 
abbey road d enfer said:
pucho812 said:
and what would be wrong with a passive DI ???

A cheap passive DI is crap.

Agreed.

An expensive passive DI maybe good on synths, adequate on bass and active undesaddle, poor on electric guitar, useless on passive undersaddle.

Depends what style the guitar is playing. 

Few years ago I picked up some NOS Triad transformers with 700K input and 2 x 7K output. Used one as DI and it works great. I also played it on an acoustic with passive undersaddle. Still great. However, I admit it is quite difficult to get a transformer like that.
 
sahib said:
Few years ago I picked up some NOS Triad transformers with 700K input and 2 x 7K output.
What did you connect it to? With a standard 2k mic input, the reflected impedance at the primary would be 200k, which is a poor match for a passive undersaddle. Anyway the sound of these undersaddles is not great, usable only in live applications IMO, so rolling off bass is  rather welcome.
In fact, I don't like the sound of DI'd guitar, electroacoustic or electric, so I use DI's only for bass and keys for recordins.
Live, the performance of a good active DI is so much better and predictable than a passive that I never even think of using one.
 

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