DI box with stereo to mono summing

[00940]

I need a pair of DI boxes to connect smartphones or similar portable music players to mic inputs. Sound quality is second to price, size and isolation.

Would this work ? The transformer will be either a neutrik nte-1 or a monacor ltr-110.

 

[AndrewT]

Use a summing circuit.
i.e. an inverting stage with two input resistors instead of only one.

mic input/s are probably too sensitive for the line out of portable players/.

 

[00940]

Use a summing circuit.
i.e. an inverting stage with two input resistors instead of only one.

no active circuit allowed

mic input/s are probably too sensitive for the line out of portable players/.

the 1k/68r divider takes care of that.

 

[rayma]

no active circuit allowed. the 1k/68r divider takes care of that.

Should work ok, if the source can drive 1k.

 

[Nigel Goodwin]

It's fine - you don't even really need summing resistors, the outputs are quite happy just shorted together - it's designed that way.

But including the two summing resistors is 'nicer' than not having them.

As an experiment you might try shorting across the LH side of the 1K resistors, see if you can hear any difference.

 

[00940]

Aren't the summing resistors necessary to avoid the outputs trying to drive one another when the signal is very different in between L and R, effectively shorting them ?

Why Not Wye?

 

[rayma]

Aren't the summing resistors necessary to avoid the outputs trying to drive one another when the signal
is very different in between L and R, effectively shorting them ?

There may be some internal impedances in the source outputs that would allow shorting the two together,
but why take the chance? The 1k resistors certainly won't harm anything, and they may prevent harm.

 

[Nigel Goodwin]

Aren't the summing resistors necessary to avoid the outputs trying to drive one another when the signal is very different in between L and R, effectively shorting them ?

It would be on the output of two power amplifiers, with low impedance outputs - but this is a line/headphone output with relatively high impedance outputs, and very possibly even resistors actually feeding the socket.

So it normally makes no difference, but as I said before it's certainly 'nicer' to use the two resistors, even if it's not strictly required.

As I also said - try it with the two resistors, then short the inputs to the resistors together, and see what difference it makes.

 

[00940]

Well, these boxes will be used with all kinds of laptops, smartphones and the like, so I must take extra care. Safety is more important than best sound in this case

 

[Nigel Goodwin]

Well, these boxes will be used with all kinds of laptops, smartphones and the like, so I must take extra care. Safety is more important than best sound in this case

All such devices are (or should) be designed for just such a use, however adding the summing resistors does no harm whatsoever, and won't reduce sound quality - and for the third time - it's a 'nicer' solution

 

[sreten]

Hi,

The summing resistors are needed for the attenuation at least.
There is no need for the transformer for a floating device input.

rgds, sreten.

 

[00940]

There is no need for the transformer for a floating device input.

True.

For longer performances, it's quite likely that the laptops will end up to be running on mains though. Hence the transformers.

 

[wg_ski]

There is no need for the transformer for a floating device input.

YES THERE IS. Laptops and such are notorious for introducing the nastiest of ground loops. I had a horrible digital "Squeeeeeeeeeee" trying to run a laptop and lower-end DJ controller (with unbalanced 1/4" outputs). There were only 2 solutions - either to run a clean box or pull the power supply and run off battery only. And the battery won't last for the full gig, of course. You either need a clean box or a transformer to run form all possible sources. If you don't need/want the amplification the transformer is a better solution.