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[scottfran]

Hello.

I am trying to wire the unbalanced -10db inputs of my old tape deck   into the patch bay of my otherwise balanced system. There are 16 channels and most line matching amps are too expensive.  Can anyone tell me how to construct a simple pad that would knock off the 14 extra db coming from my mic preamps and board?  In the simple H pad plans I've found, I'm not sure what values to use for the in and out impedences.  thanks for any help.

scott

[Brian Roth]

You really need an L-pad when going into unbalanced gear (one resistor in series with the signal, the other shunting to ground at the "output side" of the series R).

My long-time friend Rick Chinn has a very useful paper on this topic:

http://www.uneeda-audio.com/pads/

Bri

[scottfran]

that's great.   thanks alot.

scott

[Wire]

Or a balancing inline transformer will do the trick too.  They run aorund $40 or so I would think.

[Brian Roth]

Inline transformers probably won't correct the mismatch in levels.

Bri

[Wire]

Like a  http://shure.com/accessories/a15bt.asp would do it, as well as any DI box.

[John Ivan]

Very helpful links indeed! At the new room, we have a bunch old Altec gear that is 600ohm's. I need to get them on the patch bay this week. The link to that paper will help a lot. What a cool place this is.

[John Klett]

Dropping level down from +4dBu balanced to -10dBV unbalanced can be done passively with a transformer.  Bringing level back up and being able to "drive a wire" or drive signal into the next device (possibly a "+4" 600 ohm device) will require a line amplifier.  

Level sifter boxes made to interface unbalance consumer cassette decks etc. into a +4dBu balanced recording studio environment will also work for this, BUT, the vast majority of these are transformerless.  Transformerless level shifters work fine for the transition from +4 balanced to -10 unbalanced.  Actively balanced (or differential) input amplifiers can work with balanced and unbalanced sources fairly seamlessly.  The problem comes in with the transition from -10 unbalanced to balanced +4.  Active balanced output amplifiers, no matter how they are designed to correct for level when driving unbalanced loads, will drive some signal into your ground when they are used to drive an unbalanced load... i.e. the insert returns on the majority of consoles made.

If every input to every device in your studio is balanced then active balanced outputs on level shifters and other gear are fine...  this is rarely the case.  

The solution I came up with, since I have these more or less laying around anyway, are things like 10k:600 input transformers to balance and level shift down to -10 devices and line amplifiers like api 325 cards in a card frame or some Neve BA183 / LO1166 line amplifiers mounted in a box (Purple made a batch of 16 channel motherboards to hold BA183/283 boards for making DA's and adding direct outs to things like Kelso's)...  to bring the level back up and balance it.  These items go on to patch to be inserted as needed.  Adding these to every device that could use them can get expensive and they are not always needed anyway.  Other items that are useful are 1:1 600:600 transformers set up as phase reverses, passive phase reverses, passive mults, summing networks, distribution amplifiers, some stepped attenuators and other useful things...  sort of a tool belt on patch.

For your application you could look for a level shifter box that has transformer balanced outs.  ATI (Audio Techologies, Inc. NOT Audio Toys, Inc.)
http://www.atiguys.com/
used to make level shifter boxes that had transformer balanced outs...

[scottfran]

One other solution I came up with is to use old blackface adats.  In input mode they will pass the signal from the unbalanced ins to the balanced outs.  8 channels for about 100 bucks, but 3 rack spaces.

scott

[guitarbth]

Would this work?

http://www.musiciansfriend.com/srs7/g=home/search/detail/bas e_pid/189100/

What are your thoughts?

Brandon

[John Klett]

let's read this...

"The Line Level Shifter uses the physics of inductance and impedance matching to increase or decrease signal voltages without adding noise or employing active electronics. Allows a step up in volume approximately 11 decibels without adding system noise. It also converts back and forth between balanced and unbalanced signals automatically. And as a bonus, it contains Ebtech's Hum Eliminator

[guitarbth]

John.... what do you think of the L-pads vs. a box with transformers for leveling down from +4dBu to -10dBV? Your input is greatly appreciated. Thanks.

Brandon

[Brian Roth]

Obviously, I'm not John <g>, but pads have worked well for me in many circumstances for the past 30+ years.  I "grew up with" some old hands that had worked in Ye Olde 600 Ohm power-matched universe, and pads were part of their universe that also translated into the modern "bridging" systems.  10 cents in parts vs $100 is also a good reason to look at pads, especially in systems with unbalanced I and/or O.

Transformers do have the advantage of isolating equipment with nasty grounding problems.  I use both approaches, but always try KISS first.  It's very inexpensive to try a pad, so if it doesn't work out for whatever reason, you can walk away with minimal out-of-pocket expenses.

Bri

[John Klett]

pads are fine for attenuating "+4" to "-10" however in some situations, mainly where an active balanced or differential output is the source, there might be some issues.  For a $1 in parts it is certainly worth trying a pad.

[guitarbth]

Here's what I went with:

http://www.markertek.com/SearchProduct.asp?item=MATCHBOX& ;off=1

Henry Engineering Matchbox HD

I decided for the money I may as well get something that was truly bidirectional - plus it will fit great in a rack with my soon to come RNP/RNC combo (being a 1/3 rack space unit)

I used their "Lowest Price Policy" and got an awesome price. -$169

Thanks so much for your help.

Brandon