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Glen Trew

Mystery Distortion Solved: SD-442 to Zaxcom TRX-900

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A while back an issue was brought up on this group and on “ramps” by someone having peak distortion problems (severe crackling during dialog peaks) occurring somewhere between their Sound Devices SD-442 mixer’s balanced TA-3 outputs and their Zaxcom TRX-900 transmitter’s stereo input adapter. The same issue was reported (and confirmed) when using the unbalanced TA-3 outputs of the Cooper CS-104.

The symptoms were:

There was no problem when using a transformer-balanced or active-balanced line level output when using a normal, shielded pair, straight through cable. Testing with the SD-442 transformer balanced outputs and the SD-302 active balanced outputs confirmed this (no problem). In these cases, aligning a 0VU reference line level tone to -20dBfs on the Zaxcom transmitter’s meter, the output at the receiver would be clean even when levels went up to and even past full scale.

The problem occurs when using a normal straight-through cable with an impedance balanced output (like on the SD-442 TA-3 balanced line ouputs) or unbalanced output (like the Cooper CS-104 TA-3 unbalanced outputs) feeding the Zaxcom transmitter’s balanced stereo input. In this case, the tone aligns normally and sounds clean, but dialog peaks that are well below the 0dBfs (full scale) create severe crackling distortion that is heard on the output of the Zaxcom receiver.

This was not a typical “hot level” over-voltage clipping problem because, when reducing the output level of the source (in this case the SD-442 mixer) and making it up with the Zaxcom’s input trims, the problem remained.

The sound and behavior of the distortion reminded me of the input distortion the old analog Nagra IV-S would have when being fed from a normal low impedance source. In that case, the fix was to have what Nagra referred to as “current limiting” resistors in line with the audio conductor (not really a pad). Nagra’s spec called for 47K Ohms. So…

After a lot of experimentation with the Remote Audio bench guys, we determined that installing in-line resistors of 27K inside the cable solved the problem, allowed headroom well beyond full scale, and still left enough range in the Zaxcom’s trims to align a 0VU tone at -20dBfs.

From this point forward, when ordering a Remote Audio cable with two TA-3F inputs and a TA-5F output for the Zaxcom TRX-900 stereo adapter, it will have these resistors included. We have verified that this cable works properly with both the SD-442 impedance balanced outputs and the Cooper CS-104 unbalanced outputs.

Anyone who has previously purchased these cables for this use is encouraged to notify Remote Audio for an RA#. Upon receipt, Remote Audio will modify and return them at no charge.

Glen Trew

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hmm. in anticipation of a cl8 for my 788t. i just spent 3 hours making up a cable to feed my zaxcom trx900/sta100 from the outputs from the 788t. (the remote audio cables are worth every penny, i have since decided).

and guess what. whilst the cable with the 27k resistor works with the 442. its un-necessary for the 788t! how i wish id tried to see for myself if the distortion would occur on the 788 before making up the cable.

it also seems that the ta3 outs on the 788t are about 4db hotter compared to the 442, though that can be changed through the menu on the 788.

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hmm. in anticipation of a cl8 for my 788t. i just spent 3 hours making up a cable to feed my zaxcom trx900/sta100 from the outputs from the 788t. (the remote audio cables are worth every penny, i have since decided).

and guess what. whilst the cable with the 27k resistor works with the 442. its un-necessary for the 788t! how i wish id tried to see for myself if the distortion would occur on the 788 before making up the cable.

it also seems that the ta3 outs on the 788t are about 4db hotter compared to the 442, though that can be changed through the menu on the 788.

The 27K resistor cable is only necessary when the Zaxcom stereo adaptor is being fed by the impedance-balanced outputs such as those found on some of the Sound Devices TA-3 outputs and the tape output of the

Cooper 104. At this point, I am assuming that the 788 output you are using is an active-balanced type.

Glen Trew

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you mean i should have read the specs in the instructions more carefully before making up the cables with resistors in for the 788?

'I am assuming that the 788 output you are using is an active-balanced type.'

that being the specific bit i should have paid attention to and noticed how active balanced is spelled differently from impedance balanced.

oh well. i'll blame every production company out there for not making me work which meant i decided i needed a project for today.

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you mean i should have read the specs in the instructions more carefully before making up the cables with resistors in for the 788?

'I am assuming that the 788 output you are using is an active-balanced type.'

that being the specific bit i should have paid attention to and noticed how active balanced is spelled differently from impedance balanced.

oh well. i'll blame every production company out there for not making me work which meant i decided i needed a project for today.

Well, the bright side is that the cable with the resistors should be OK for either output type. You may have a few dB drop, but nothing to worry about.

Glen Trew

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Just ran into this yesterday for the first time. Not with a Zax xmtr but with an FR2. I was using the TA3s to XLR inputs. I tried line level, -10, and mic level (Which worked the best) but all of them peaked out the FR2 preamps (Not the meters) even at low levels.

Any idea if the 27k resistor could be wired into the 442s TA3 connectors, and avoid having all these cables made?

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Curly,

Re: the FR2

The combo input XLR's are at mic level and the TRS are line level. (can't remember if -10 or +4)

Much like a lot of semi-pro recorders. 

Grant.

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Just ran into this yesterday for the first time. Not with a Zax xmtr but with an FR2. I was using the TA3s to XLR inputs. I tried line level, -10, and mic level (Which worked the best) but all of them peaked out the FR2 preamps (Not the meters) even at low levels.

Any idea if the 27k resistor could be wired into the 442s TA3 connectors, and avoid having all these cables made?

Use the 1/4" line level input on the FR2 works just fine on mine.

Eric

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Curly,

Re: the FR2

The combo input XLR's are at mic level and the TRS are line level. (can't remember if -10 or +4)

Much like a lot of semi-pro recorders.  

Grant.

Hi Grant,

If the FR2 has the same issue in handling impedance-balanced outputs, then the isolation resistors should solve the problem. 27K may or may not be the optimum value (depending partially on the input impedance of the FR2), but my guess it will be fine. You will probably have a just a few dB of insertion loss to make up with the input pots.

Glen Trew

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