DIY Line level box

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PhilipMarlowe

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Nov 28, 2019
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My mixer has two outputs. One master line output and one monitor line output. The problem is that I can have 2 more stereo input channels if I mix through the monitor output. Unfortunately only the master line output is transformer balanced. The monitor mix is not. My idea was to simply build my own transformer balanced Box. I could get a good offer on a pair of used Lundahl 1539 which would fit the description right?
Now if I'm not mistaken all I'd need to do is simply find a donor metal encasing and somehow fasten those two transformers to the encasing, connect the wire leads to XLR inputs and outputs and I should have a working balancing unit- am I missing anything there?

Thanks a lot frens.
 

ruffrecords

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Personally I would not recommend using the Lundahl 1539 for this application. It has a relatively low inductance which is why it demands a driving source impedance of less than 10 ohms for adequate low frequency response. It is not clear if your mixer's monitor output meets this criteria. A more conventional output transformer would be the Carnhill VTB2281.

Cheers

Ian
 

PhilipMarlowe

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ruffrecords said:
Personally I would not recommend using the Lundahl 1539 for this application. It has a relatively low inductance which is why it demands a driving source impedance of less than 10 ohms for adequate low frequency response. It is not clear if your mixer's monitor output meets this criteria. A more conventional output transformer would be the Carnhill VTB2281.

Cheers

Ian

The impedance is 20 ohms on the monitor out- would it be unsuitable?
 

PermO

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PhilipMarlowe said:
Would I need a groundlift? And would it be able to build a switch to pad down the output on the end?

You could always add a groundlift switch, just in case.

You could create a output pad (L Pad) with some resistors and a switch.

Someone is offering Jorgen Schou line transformers in the black market at 70,- a pair, these could also be very nice.
 

PhilipMarlowe

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PermO said:
You could always add a groundlift switch, just in case.

You could create a output pad (L Pad) with some resistors and a switch.

Someone is offering Jorgen Schou line transformers in the black market at 70,- a pair, these could also be very nice.

Already bought the Lundahls now. How does one Add a ground lift switch?
 

PermO

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On the three pin balanced output, pin 2 is hot (+), pin 3 is cold (-) and pin 1 is ground (chassis)

If you connect pin 1 via a switch to the chassis, you'll have a ground lift switch on the output side.

So all it does is connect chassis to signal ground or not, since it's a passive box with no powersupply that is also grounded to chassis it does not make much sense as there won't be a groundloop anyway.
I would just connect pin 1 to chassis and leave it like that, no need for a lift switch in this case.


I would just do this;
0DbtrrW.jpg

 

PhilipMarlowe

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PermO said:
On the three pin balanced output, pin 2 is hot (+), pin 3 is cold (-) and pin 1 is ground (chassis)

If you connect pin 1 via a switch to the chassis, you'll have a ground lift switch on the output side.

So all it does is connect chassis to signal ground or not, since it's a passive box with no powersupply that is also grounded to chassis it does not make much sense as there won't be a groundloop anyway.
I would just connect pin 1 to chassis and leave it like that, no need for a lift switch in this case.


I would just do this;
0DbtrrW.jpg

Thank you so much! This is my first yolo diy project tbh. I'm gonna use a effects pedal metal donor case and will keep this in mind.
 

PhilipMarlowe

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Nov 28, 2019
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ruffrecords said:
Personally I would not recommend using the Lundahl 1539 for this application. It has a relatively low inductance which is why it demands a driving source impedance of less than 10 ohms for adequate low frequency response. It is not clear if your mixer's monitor output meets this criteria. A more conventional output transformer would be the Carnhill VTB2281.

Cheers

Ian

One more question Ian: the unbalanced monitor output is 0db while the transformer balanced one in my mixer has two stereo outputs: one being +6db and +15dbu. According to the manual the signal path of the monitor output is the same as the master output, only it does not pass the output transformers of the master stereo out.

So would it be better off using the Lundahl 1539 as a step up or step down transformer in this scenario? I'm new to circuit design and I'm still a bit confused by what you mean with too high of an impedance for the 1539. I do have the schematics of this mixer- would I need to know how the master output transformers are wired?

Thank you!
 
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