Passive Attenuator/Downmix Question

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synthetic

Well-known member
Joined
Oct 23, 2004
Messages
218
Location
Glendale, CA
I want to build a passive downmixer and attenuator for a Super Audio CD player. I want the Center channel reduced by -3dB and sent to the left and right speakers. (No center in this listening system.) I also want a stereo attenuator for the surround channels.

passive.gif


All of the signals are unbalanced -10 level. Can someone help me with the values for R1 and R2? My electronic textbooks are buried somewhere. Thanks for your help.
 
Maybe it's blocked by my employer's sysadmins. They've been going nuts with blocking sites lately, some of them for no apparent reason.

Could you email it to electronic_dave

at

ya

hoo

dotkom

or post it on twin-X, perhaps?
 
Thanks for emailing the image.

There's a problem with the center channel mixing circuit as you've drawn it. The amount of center channel attenuation is overly dependent on the source impedance of L and R. And, with the resistors chosen to give 3dB attenuation of the center channel working against the source impedances of L and R, the loading on L and R will be excessive.

The circuit below will mix in the center channel with the center down by about 3dB relative to left or right, while maintaining source and load impedances that should work reasonably well with typical "consumer" equipment.

centerchannelmixer.png


Problem is, it reduces your overall signal level by one half. But that's the name of the game with passive mixing! Most CD players have fairly hot outputs, though.

the amount of crosstalk between L and R will depend on the source impedances of L, C and R. If it's excessive, then the 3.3K and 2.2K resistors can be increased in value--as long as you maintain the same ratio between them. The price will be increased insertion loss, and also a higher source impedance driving the cable to your amplifier. In any case, the mixing network should be located close to your amplifier, connected with a reasonably short cable if possible.
 

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