Feeding mono input with 2 outputs???

[Speedskater]

RaneNote
Why Not Wye?

Why Not Wye?

[Lotus 7]

Originally Posted by Speedskater

RaneNote
Why Not Wye?

Why Not Wye?

Because a "Y" cable/adapter connects two outputs together with no isolation. Bad engineering practice and never a good idea. "Y" cables are intended for connecting (1) output to (2) inputs, not the other way around.

For a simple resistive summer as the O.P. needs, the optimum resistor values depend on the input impedance of the input circuit. For a typical line level input with a 2k to 10k input-Z a couple of 5K to 10K resistors will work.

[Speedskater]

Lotus did you look at the Rane Note?

Isn't it just about what you wrote?

[dualflip]

In MHO you need a summing buss, it can be done with opamps and resistors... such as this http://en.wikipedia.org/wiki/Operati...ming_amplifier however you may want a diff amp to "unbalance" the signals first.

[frecuencializer]

I got this from the link above "RANE".

i guess i need to solder a 475 ohms resistor to both + and - in the XLR inputs, but i don´t understand why i should tie together with a resistor of 20K between + and - in the output XLR... Since it´s balanced operation, + and - shouldn´t be connected together, they cancell each other, isnt it?

[brianroth]

I would bump those 475 Ohm resistors up to 2K, and forget about the 20K.

Bri

[Lotus 7]

Originally Posted by Speedskater

Lotus did you look at the Rane Note?

No, I just shot from the hip when I saw the statement "Why not Wye?" because people on this forum are always suggesting to do it that way (just short the outputs together with a "Y" cable, because you can!). Speedskater: sincere apologies!

It exactly addresses the issue and is a good way to go. The 475 ohm resistors will give reasonable isolation with minimal attenuation. Going to 2K resistors as brian suggests will give more isolation and less loading of each source output, but, of course, will give more attenuation.

There is no real need for the 20K resistor, unless the connected input has a very high impedance (like an instrument input).

The attenuation depends on the input impedance of the load that is being driven.

UAD does not specify the input impedance of the Apollo line TRS inputs (at least I couldn't find it in the Apollo spec sheet).

If you do the math for a worst case of an assumed 1K ohms input-Z, and using 475 ohm resistors in each leg for one source signal the attenuation from one input to the Apollo input is -5.8 dB
1000/((475*2) + 1000) = 0.512 = -5.8 dB

If the input-Z is higher (assume 2K), then the attenuation for each input leg (still using 475 ohm resistors) is -3.47 dB
2000/((475*2) + 2000)= 0.678 = -3.47 dB

If the series resistors are all increased to 2K and the load is assumed to be 1K then the attenuation is -13.9 dB
1000/((2000 *2) +1000)= 0.2 = -13.9 dB

But if the input-Z is assumed to be 2K then the attenuation is: -9.6 dB
2000/((2000*2)+2000) = 0.33 = -9.6 dB

The actual attenuation is slightly higher (about 1.5 dB) because of the loading back into the other signal output feed.

This sort of passive summer will work fine as long as you have enough extra gain that the loss of 4 to 14 dB can be tolerated.

[brianroth]

The Apollo manual states a 10K input impedance. I suggested using 2K resistors to keep the load impedance on each of the (unknown) preamps reasonably high, since a lot of "modern" gear starts choking when driving into loads less than a few K Ohms.

If the load Z was VERY high, there will be a 6 dB signal loss through the resistor networks. But in this situation, each input goes through a voltage divider consisting of 4K series resistance and 4k in parallel with 10K for the shunt leg which works out to 2.86K. Thus, the voltage drop will be approx 7.6 dB.

Each mic preamp output will see a load Z of 4K + 2.86K, or 6.86K which should be OK for any type or pro or semi-pro gear.

(Someone double check my math...I'm really tired this evening!)

Bri

[dualflip]

Originally Posted by brianroth

The Apollo manual states a 10K input impedance. I suggested using 2K resistors to keep the load impedance on each of the (unknown) preamps reasonably high, since a lot of "modern" gear starts choking when driving into loads less than a few K Ohms.

If the load Z was VERY high, there will be a 6 dB signal loss through the resistor networks. But in this situation, each input goes through a voltage divider consisting of 4K series resistance and 4k in parallel with 10K for the shunt leg which works out to 2.86K. Thus, the voltage drop will be approx 7.6 dB.

Each mic preamp output will see a load Z of 4K + 2.86K, or 6.86K which should be OK for any type or pro or semi-pro gear.

(Someone double check my math...I'm really tired this evening!)

Bri

Yeap your math is correct 4k in parallel with 10k is 2.86K

voltage divider with 4k series gives a damping factor of 0.417, which gives -7.6dB attenuation

[Lotus 7]

brian,
Sounds good. With a 10K input-Z, 2k or so, series resistors are an ideal value.

To frecuencializer: Although not terribly critical, to maintain the best common-mode noise rejection, the resistors on each input leg should ideally be the same value. If you can get them, use 1% resistors. Using 5% resistors will work fine, but can slightly unbalance the signals. The match is more important than the absolute value: (2) 2.0K ohm resistors are OK or (2) 2.2K. resistors. Metal film or carbon film resistors are generally more stable than carbon composition.

[frecuencializer]

Thanks a lot everybody, your answers have been a great help.

As I can reed, I should use a 2k resistor in each + and - to match the correct Z:

Input XLR 1

+ (2 Kohms) >>>>
- (2 Kohms) >>>>
S >>>>>>>>>>

Input XLR 2

+ (2 Kohms) >>>>
- (2 Kohms) >>>>
S >>>>>>>>>>

Output TRS

(both + tied together) >>>>> T
(both - tied together) >>>>> R .................>>> Apollo TRS - 10K input impedance
(both S tied together) >>>>> S

[brianroth]

Yes, use the Rane diagram but use larger resistors as I suggested. 2K is just a ballpark number...anything close to 2K is fine.

And I also suggest using 1% metal film resistors to ensure decent balancing.

Bri