# M-Audio Mobilepre Mk II connections



## htsnube (Nov 28, 2013)

Hello,
I tried the search feature and was unable to find the answers I am seeking. If there is a thread or resource that I should be directed to, I would like to know about. I apologize in advance if these questions are redundant on this forum. 

I have the M-Audio Mobilepre MkII, which I plan to use with REW to produce a correction filter for the room response.
I want to ensure I acquire the proper cables so that the interconnections will be made properly. 

Firstly, I realized there are multiple ways to make the connections to the stereo receiver as well as the loopback for the calibration. Upon further digging, I found that the Mobilepre MkII has balanced line outputs that could be damaged if improperly connected to unbalanced equipment. Most of the information I found regarding the connections were for the older Mobilepre (unbalanced line outputs), not the Mobilepre MkII (balanced line outputs).

I used the info at the following link to help formulate my questions regarding how to make the connections:
http://www.presonus.com/news/articles/balanced-unbalanced


1.) How should the connections be made between the (balanced) 1/4" TRS line output of the Mobilepre MkII into the (unbalanced) RCA line input of the stereo receiver? Depending on the circuit topology of the outputs, it may be required to have the ring wired to ground or leave the ring open.


A.) What is the circuit topology of the Mobilepre MkII balanced outputs?

i.) Transformer balanced?
ii.) Electronically-balanced / “active-balanced”?

a.) unity-gain inverting operational amplifier?
b.) two op-amps—one configured as a non-inverting amplifier, the other wired in the inverting configuration?
c.) “cross-coupled” / “servo” balanced?
iii.) “impedance balanced” / “single-ended balanced"?


Connecting (600ohm) headphones to one of the 1/4" TRS line outputs on the Mobilepre MkII reveals that both headphone channels have the "same" signal. Using a balance adjustment to fade to one side will leave the signal unaffected in both headphone channels and completely fade out both headphone channels if fading to the other side.


2.) How should the connections be made between the (balanced) 1/4" TRS line output of the Mobilepre MkII into the (unbalanced) 1/4" TRS line input of the Mobilepre MkII? This connection would be used for calibration.


A.) Does the Mobilepre MkII (unbalanced) 1/4" TRS line input jack wire the ring contact to ground?

I believe if the 1/4" TRS line input jack internally wires the ring to ground, then I should be able to use a standard 1/4" TRS to 1/4" TRS interconnect cable to connect the loopback for calibration.


3.) Is the reduction of line output voltage when going to an unbalanced input an issue? It seems probable I will see a reduction in voltage by one-half, but I would think there is plenty of headroom.


Any information is appreciated, thanks!


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## Wayne A. Pflughaupt (Apr 13, 2006)

htsnube said:


> Upon further digging, I found that the Mobilepre MkII has balanced line outputs that could be damaged if improperly connected to unbalanced equipment.


First time I’ve ever heard that one. If that’s really the case with this piece, that would be an automatic deal-breaker for me.

The loopback is easy: Just use a cable with TRS plugs on both ends.

The playback connection to an AVR appears to be a bit trickier. Typically any component with TRS jacks can also utilize TS plugs, but the Mobile Pre manual doesn’t specifically mention TS cables at all, if they can be or should not be used for the output (Item 11, Pg. 5). In light of that you’d better get or make a RCA-TRS cable with the sleeve connection lifted.

Regards, 
Wayne


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## htsnube (Nov 28, 2013)

Wayne A. Pflughaupt said:


> First time I’ve ever heard that one. If that’s really the case with this piece, that would be an automatic deal-breaker for me.
> 
> The loopback is easy: Just use a cable with TRS plugs on both ends.
> 
> ...


Thanks for the response. I really didn't even notice the Mobilepre MkII had balanced outputs until after I ordered the unit!

Regarding the loopback. I would expect the Mobilepre MkII's TRS input jack would wire the ring conductor to ground. Otherwise, I would expect the the input jack to use a TS connector intead.
It seems logical to me that the manufacturer would design the balanced outputs to be able to connect to the balanced inputs of the same unit. In that case, a TRS to TRS cable would be perfectly fine for the loopback connection.

From my limited understanding, it seems likely that the balanced outputs are not transformer-based as that would normally be advertised and is more expensive. It also seems unlikely that the outputs are "single-ended" because I can hear the music signal in both headphone channels.
It seems probable that the balanced outputs on the unit are one of the "active-balanced"/"electronically balanced" configurations:
a.) If unity-gain (single op-amp), then I expect that ring should be open.
b.) If inverting/non-inverting (usually two op-amps), then I expect that the ring should be open.
c.) If cross-coupled (usually three op-amps), then I expect the ring should be shorted.

I decided I will do a test to see if the outputs are cross-coupled or not. I will cut into a TRS interconnect so I can connect the volt-meter. I will then short the ring (cold) to sleeve (ground) and see if the voltage on the tip (hot) changes at all.

This is all pretty interesting, and I'm slowly getting closer to the "end" goal of correcting the room response.


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## jcmusic (Oct 26, 2012)

I use the Mobile Pre II with XLR connections to RCA's and have had no issuse's!!! For the connection from the system to the Mobile Pre I use a cable store bought that is a TRS split to two RCA's works perfect....


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## EarlK (Jan 1, 2010)

jcmusic said:


> I use the Mobile Pre II with XLR connections to RCA's and have had no issuse's!!! For the connection from the system to the Mobile Pre I use a cable store bought that is a TRS split to two RCA's works perfect....


I would recommend that you use a multi-meter to the determine if your TRS ( to twin RCA outs ) is indeed a splitter cable ( as you describe it ) .

It's as common ( for cables presenting this visual arrangement ) to be "Insert Cables" which one wouldn't want to use when driving 2-chnls simultaneously, ( since one channel would be polarity "flipped" when compared to the other ) .
- A person should test one channel at a time ( which will avoid this type of cabling error ) .

You don't want to use a cable wired like the following to run REW tests ( if testing two channels at a time ) .











BTW ; 

"Best Practice" is for language to descriptively mimic the functional reality of a situation .

In this case, follow the flow of the audio signal when describing ones cabling efforts ( Output to Input ) .

ie; A person cables ( makes a connection ) from the output of the soundcard to the AVR's input ( not the other way around ). 


:sn:


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## jcmusic (Oct 26, 2012)

OK Earl thanks for the tip I will check them tonight. BTW how should they be wire for using with 2channels?


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## Wayne A. Pflughaupt (Apr 13, 2006)

htsnube said:


> I would expect the Mobilepre MkII's TRS input jack would wire the ring conductor to ground.


A balanced TRS connector is wired tip (+), ring (-), and sleeve ground. If the ring is shorted to ground then it is not a balanced connection.

Regards,
Wayne


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## Wayne A. Pflughaupt (Apr 13, 2006)

jcmusic said:


> For the connection from the system to the Mobile Pre I use a cable store bought that is a TRS split to two RCA's works perfect....


Are you using the headphone jack for the output to your system?

Regards,
Wayne


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## htsnube (Nov 28, 2013)

I just tested the output of the balanced 1/4" TRS connections.
The RMS voltage of a sine wave on both the tip (hot) and the ring (cold) with respect to the sleeve (ground) are both identical in voltage.
When I short the ring to ground, the voltage between the tip and the sleeve remains exactly the same.

Because shorting the ring has no effect on the voltage of the tip, it appears that the output topology would either be "unity-gain" (single op-amp) or "inverting/non-inverting" (usually two op-amps).
If this were indeed the case, I suspect that the proper method of interconnecting the balanced 1/4" TRS outputs of the Mobilepre MkII to unbalanced equipment would be to use a cable that allows leaving the ring (cold) as open.
This would prevent shorts from damaging the op-amps of the balanced output stage. It does seem plausible that the op-amps could be resistant to damage from shorts, and may be able to be connected with the ring shorted.

Regarding the loopback connection. If the unbalanced 1/4" TRS input jack of the Mobilepre MkII internally shorts the ring to ground, then I believe it is a good idea to connect the balanced 1/4" TRS output jack with a cable that would leave the ring open. If the unbalanced 1/4" TRS input jack internally leaves the ring open, then a 1/4" TRS to 1/4" TRS cable could be used without causing shorting of the ring.

In summary, it appears that the Mobilepre MkII uses either a single or double op-amp configuration for the balanced outputs (which in some equipment may be susceptible to damage). If true, then I suspect that the ring should be wired open (for both going into the loopback connection as well as into other unbalanced equipment) unless the Mobilepre MkII uses op-amps that are resistant to damage from shorts.

I'm going to go ahead and order these for the connections:
http://www.amazon.com/gp/product/B000068O4W/ref=ox_sc_act_title_3?ie=UTF8&psc=1&smid=ATVPDKIKX0DER
Then I can just use RCA cables to complete the connections.


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## jcmusic (Oct 26, 2012)

Yes Wayne it is a headphone jack or 1/4 standard plug which ever you like to call it. Can I put a meter on the RCA's to determine that the polarity is the same in both?


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## Wayne A. Pflughaupt (Apr 13, 2006)

jcmusic said:


> Yes Wayne it is a headphone jack or 1/4 standard plug which ever you like to call it. Can I put a meter on the RCA's to determine that the polarity is the same in both?


If you’re using the headphone output, then you have nothing to be concerned about. I believe Earl thought you were using that cable with one of the balanced TRS outputs.

Regards, 
Wayne


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## jcmusic (Oct 26, 2012)

Wayne in the front of the unit there are 2 XLR inputs that also will accept a standard size phone jack. In the rear you have 2 outputs that only accept the the standard size phone jack or 1/4 size plug as some call it. In Earl's post with the pic of the one he plans on buying, mine look just like that except the wire is longer.


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## Wayne A. Pflughaupt (Apr 13, 2006)

htsnube said:


> I'm going to go ahead and order these for the connections:
> http://www.amazon.com/gp/product/B000068O4W/ref=ox_sc_act_title_3?ie=UTF8&psc=1&smid=ATVPDKIKX0DER
> Then I can just use RCA cables to complete the connections.


Where do you intend to use that cable? Please note the first bullet point on the Amazon page you linked: _“This cable is designed to split a stereo signal.”_ In other words, it cannot be used with a TRS balanced output. See Earl’s diagram above - you will not get a signal at the RCA because _the signal (-) will not be present._ The balanced signal’s ground (sleeve) connection (black wire in the diagram) does not carry any portion of the audio signal.




htsnube said:


> Because shorting the ring has no effect on the voltage of the tip, it appears that the output topology would either be "unity-gain" (single op-amp) or "inverting/non-inverting" (usually two op-amps).
> If this were indeed the case, I suspect that the proper method of interconnecting the balanced 1/4" TRS outputs of the Mobilepre MkII to unbalanced equipment would be to use a cable that allows leaving the ring (cold) as open.


If this were indeed the case, don’t you think M-Audio would make mention of it in the manual? Regardless, I don’t see how it would work, because if the ring connection is open, there will be no signal (-) going to the unbalanced component receiving the signal.




htsnube said:


> Regarding the loopback connection. *If the unbalanced 1/4" TRS input jack of the Mobilepre MkII internally shorts the ring to ground,* then I believe it is a good idea to connect the balanced 1/4" TRS output jack with a cable that would leave the ring open.


As I noted above, if the ring and sleeve connections are shorted, it is an unbalanced, not a balanced connection.




> *If the unbalanced 1/4" TRS input jack* internally leaves the ring open, then a 1/4" TRS to 1/4" TRS cable could be used without causing shorting of the ring.


There is no such thing as an unbalanced TRS jack. If the connection was unbalanced, they would simply use a TS jack.

Really, I think you’re over-analyzing all this. I’ve never seen any component with balanced connections that could not be connected to unbalanced components.

Regards, 
Wayne


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## jcmusic (Oct 26, 2012)

Wayne here are the specs of said device:

24-bit 
Sample Rates 44.1 and 48kHz 
Analog Input 2 x TRS/XLR Combi (front panel)
2 x 1/4" TRS phone Line/Instrument, balanced or unbalance (rear panel) 
Analog Output 2 x 1/4" TRS phone Monitor
1 x 1/4" TRS phone Headphone 
Digital Input No 
Digital Output No 
MIDI In/Out No 
Sync In/Out Not applicable 
Headphone Output Yes 
THD + N Not specified by manufacturer 
Dynamic Range Not specified by manufacturer 
Frequency Response Not specified by manufacturer 
Power Requirements USB bus powered 
System Requirements


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## jtalden (Mar 12, 2009)

I agree with all Earl and Wayne's comments. 

There are several solutions that work. *TS to RCA adaptor* is an easy one. These adaptors are available everywhere including Radio Shack. I just use one of them on the balanced line output of my Focusrite 2i2 audio interface: 

I have used these on 3 different audio interfaces (2 Focusrite and 1 Presonus models). Each were equipped with with balanced line outputs. I experienced no problems over several years of use. 

For a loopback timing connection a TRS/TRS or TS/TS cable works fine for me. I have no experience with M-Audio. We have several users of M-Audio here and users of many other types of USB and Firewire interfaces and I have not seen any issues reported with this type of adapter or similarly wired cables.


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## EarlK (Jan 1, 2010)

htsnube said:


> <<<<SNIP>>>>Because shorting the ring has no effect on the voltage of the tip, it appears that the output topology would either be "unity-gain" (single op-amp) or "inverting/non-inverting" (usually two op-amps).
> If this were indeed the case, I suspect that the proper method of interconnecting the balanced 1/4" TRS outputs of the Mobilepre MkII to unbalanced equipment *would be to use a cable that allows leaving the ring (cold) as open*.
> This *would prevent shorts from damaging the op-amps of the balanced output stage*. It does seem plausible that the op-amps could be resistant to damage from shorts, and may be able to be connected with the ring shorted.


An interesting thread !

I agree ( fwiw ) with your conclusions here .

ie; Since a cross-coupled ( servo ) topology ( which you tested for, by shorting pin 3 to grnd ) would have doubled it's voltage across pin 2 to ground ( and because that didn't happen ) what you have, is as you surmised above .

For the sake of long term safety of the output circuitry, I too would leave the ring open ( when unbalancing this type of output ) . That is easily accomplished by using just the left out of the stereo adapter that you linked to ( assuming "tip" is connected to the left RCA ) .

I must say, I'm like just everyone else who has commented within this thread that "shorting" pin 3 (to ground ) is the de facto, modus operandi ( for un-balancing these outputs ). I have a few audio pieces with this sort of output ( that I have been shorting for decades ), it would be interesting to see if 1/2 of their opamps have given-up-the-ghost after many years of abuse . 

:sn:


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