passive summing circuits

[pucho812]

So I got asked if there was a way to passively sum without need of makeup gain for the following a RND 5059. They literally want to take stereo 1 and stereo 2 outputs and sum them down to a single output. No volume control, just a bare bones summing of 2 stereo signals into 1. The purpose is to be able to use both busses at mix down to a single output in the analog domain. Then they can compressor or eq that before going back into the DAW.

Unfortunately on the RND website they don't list the output Ω

So I know for my inputs 2 X 5K resistors per channel. Which if my math is right will give me a buss Ω of 10K. If I divide that by the number of channels 2 I get 5K

Using math to get me a shut resistor Rsh = (Rbus * Z)/(Rbus - Z) and an expected output of 600Ω, I get RSH = (3000000 * 5000) / (5000 - 600) which is roughly 680Ω. I am thinking for the shut resistor, I would just stick a 1 K trim pot in there and go to town.

 

[Brian Roth]

Not quite following this. Summing 2 sources passively will be at least a 6 dB loss, and hence need a makeup amp.

Bri

 

[abbey road d enfer]

Unfortunately on the RND website they don't list the output Ω

You can bet on typically 50-100 ohms.

So I know for my inputs 2 X 5K resistors per channel. Which if my math is right will give me a buss Ω of 10K. If I divide that by the number of channels 2 I get 5K

All is correct up to that point.

Using math to get me a shut resistor Rsh = (Rbus * Z)/(Rbus - Z) and an expected output of 600Ω, I get RSH = (3000000 * 5000) / (5000 - 600) which is roughly 680Ω. I am thinking for the shut resistor, I would just stick a 1 K trim pot in there and go to town.

That's where you're misleaded. Why would you need to knock down the impedance (and signal level)?
It's very likely that the signal is going to a 10-20k impedance.
Unless the input has a very temperamental transformer, give your signal a chance to reach it with minimum attenuation.
Of course, each signal will be attenuated by 6dB. If the signals are correlated, they will um at unity, if they are not, they will sum at -3dB. The typical input gain range of any outboard can easily handle that.

 

[pucho812]

You can bet on typically 50-100 ohms.

All is correct up to that point.

That's where you're misleaded. Why would you need to knock down the impedance (and signal level)?
It's very likely that the signal is going to a 10-20k impedance.
Unless the input has a very temperamental transformer, give your signal a chance to reach it with minimum attenuation.
Of course, each signal will be attenuated by 6dB. If the signals are correlated, they will um at unity, if they are not, they will sum at -3dB. The typical input gain range of any outboard can easily handle that.

Thanks for the clarification.