mixing down two preamp outputs

Hi,

Due to limited inputs in my sound card, I would like to mix down two preamp outputs to send them into one input of my sound card.

Would it be as simple as soldering each outputs together into one plug, or is it not that simple ?

Thanks for your advice.
Alex

Almost but not quite that simple. Did you check the mixer META? You should find everything there.

Samuel

oops! sorry I did look enough, thanks.
I did find some good info, including this mathematical functions to calculate the values of the resistors that I need to use.

R= (N-1)/(N+1)*Z
Where N is the number of inputs to sum up.
Along this, there is:
db=LOG(1/N)*20.

I want to sum up two balanced channels, so I get to use 100 ohms resistors. That is fine, what I found a bit strange is that doing this would make me lose 6 db which is a lot, is not it ?

Anyway thanks.
Alex

No, losing 6dB when mixing two inputs is normal. The loss is 1/N.

1/2 = 0.5

20 log 0.5 = -6

Remember that "Z" is the impedance you want to present to your sources. 100-ohm mixing resistors sound a bit low for most sources.

If the network is balanced, the resistor value is R/2

> doing this would make me lose 6 db which is a lot, is not it?

6dB for ONE input. For two identical inputs, no loss. For two similar but not identical inputs, 3dB on average, 0dB on very rare coincidental peaks.

It works. What you get out is as unpredictable as your music. Most inputs will have enough reserve gain to make-up the losses.

The mixer impedance can be higher than the minimum source loading but smaller than the input impedance. If sources can drive 600 ohms, and the load is a typical 10K-20K input, a couple of 2K2 (or four 1K for balanced) is good for mixing two sources in many situations.

Yeah, the loss formula assumes uncorrelated inputs.

Once again, NewYorkDave and PRR, thank you.
I am taking on a online electronic class so please excuse me for my lack of knowledge at the moment.

I am getting out of those modified Ampex 351, -remember ?. I went into using PRP's solution for my output attenuator and use a 1k single pot in place of the 600/600 o/t attenuator.

Now I have two ampexes and I want to mix them down to one track.
Therefore my source impedance is 600 ohms. As far as the impedance destination, I am not sure, I have a MOTU 828mkII and I don't know what impedance the inputs are expecting.

Still searching.
Thanks
Alex[/i]

I wouldn't bother too much about calculating the loss as in your case (with only two sources) noise is uncritical. Use four 3.9k resistors (or something close you have at hand) for the summing network and go to the two mic inputs of the MOTU to restore levels. If the line inputs have adjustable gain as well (don't recall at the moment) you can try these too.

Samuel

Well actually, there is no adjustable gain in the MOTU. Appart for the two inbox preamps.

ok, after some research,

I have some TLaudio Quad mic preamp that have output impedance of 47ohms per leg, and the inputs of my card require 10k ohms impedance, so how do I calculate my Z now ?
Thanks
Alex

[quote author="alexidoia"]ok, after some research,

I have some TLaudio Quad mic preamp that have output impedance of 47ohms per leg, and the inputs of my card require 10k ohms impedance, so how do I calculate my Z now ?[/quote]

Fortunately, the inputs of your cards don't really require 10k impedance; this isn't an impedance-matching situation (you don't find a lot of those anymore). What the spec says is that the card has an input impedance of 10k. That's your load. If you use a pair of 2k resistors:

In1-----2k-----|
In2-----2k-----|--------Out into 10k load

then your loss will be:

Rshunt / (Rshunt + 2k)

And Rshunt, in this case, is the parallel combination of 2k and the 10k load, or 1.666...k. So loss = 1.666....k / 3.666...k = .454545...

The equivalent of that in decibels is about 6.85dB, not a big deal. As someone pointed out, you'll get back about 3dB adding the two channels.

Total load for each channel will be 3.666...k. This'll work fine with your TL Audio circuit. If you use it with the Ampices, though, you'll find they really do want to see a 600 ohm load (they're from back in the old impedance-matching days), so hang a 715 ohm resistor from each input to ground.

Peace,
Paul

I don't know if this helps in your particular case Alex, but if you have two balanced outputs (ideally from the same device) and a balanced input, there is a simple way of making use of these to form a simple 2:1 mixer.

Use the balanced signals of the two outputs to make two unbalanced outputs; the first one by using the hot pin (non-inverted) and connecting the cold pin (inverted) to ground (pin 1). This is the conventional way of creating an unbalanced output from a balanced source. The second output needs to be out of phase, so therefore we need to connect the hot pin (non-inverted) to ground (pin 1), and the cold pin becomes our inverted output. The second output needs to be a 'properly' balanced and floating output; whether electronically or with a transformer; ground compensated types will not work as they have no actual inverted output. Also, some early electronically balanced outputs didn't like their hot pin shorted to ground...if this is the case for your device, leave the hot pin unconnected.

At the balanced line receiver input, connect the first output signal to the non-inverting input, and the second output to the inverting input. This effectively means that you have two ground compensated outputs being summed at the balanced input (ground compensated as long as the outputs are both from the same device).

If you only have unbalanced outputs, then the inversion on the second output will have to happen earlier; with mic pre's typically at the phase switch. However, this method does require a balanced input. I kept writing non-inverting and inverting in this post because I'm not sure how solid the XLR standard is; as far as I know it is now fairly universal that pin 2 is hot/non-inverting and pin 3 is cold/inverting.

This method does work on the bench, but I have not tried it in real world conditions; if anyone has further opinions, feel free to comment.

Bjorn

experiment with a simple Y cord first. you may not need any mixer crap at all.

Bad idea, CJ. It works reasonably well with consumer stuff that has large (~1K) series resistors built in, but it could cause damage to pro or semi-pro gear with low output impedance. Even if it didn't cause damage, it would be likely to cause distortion and poor frequency response.

Remember, the impedance looking back into the outputs of Alex's mic preamp is 47 ohms!

Yeah, right.

A trip to crap shack for a Y chord and 20 minutes later, I am listening to a stereo to mono mix while reading the funnies.

$2.99

no soldering!

Every thing is beautiful.....