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Old 31st August 2014, 07:35 PM   #1
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Default Best Buffer to Sum L&R Without Adding to Crosstalk

Hi Everyone, I would appreciate some advice.

I am Y-jacking from an audio source, left and right.

I am going to sum one of the left/rights to mono to feed a subwoofer eq. The other will stay stereo to feed an AVR. In doing this, I don't want to add any crosstalk that will make the signal more mono out of my AVR.

Which of these active buffers would be the best choice?
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Old 31st August 2014, 07:46 PM   #2
MOER is offline MOER  United States
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Default SUB

You cannot join the outputs of two opamps

simply use an inverting stage as a summer and then reinvert with another unity gain inverter
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Old 31st August 2014, 07:51 PM   #3
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Combine figure 3 with figure 2 and reduce the 25kOhm resistors to 2kOhm for better noise performance.
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Old 31st August 2014, 08:07 PM   #4
Mooly is offline Mooly  United Kingdom
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The one with two opamps is perfect.

(not quite sure what Joachims seeing in those pictures I suspect combining two unity gain followers into an inverting stage is what is meant No noise penalty with the 25k because its shunted by the source impedance of the driving stage)

Edit... or do you mean the 10k's in the inverting stage ?
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Old 31st August 2014, 08:13 PM   #5
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OK, i thought the 25kOhm resistors increase the noise.
Buffering and lowering the 25kOhm would have solved that probelem.
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Old 31st August 2014, 08:15 PM   #6
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The second stage reads 10kOhm. Maybe lowering this is an advantage.
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Old 31st August 2014, 08:20 PM   #7
Mooly is offline Mooly  United Kingdom
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no problem, I was trying to figure the "combine fig 3 with fig 2".

The 25k's on the non inverting buffers do increase noise until you connect them to the source component and then the impedance of that shunts the 25k. Only with the input open is there a noise penalty.

(A phono stage is "noisy" until you connect the cartridge, then its quiet as its shunted by the low impedance)

The two 10k's in the inverting stage are low enough in practice but you could theoretically gain a better noise figure by going lower and using suitable opamps.
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Old 31st August 2014, 08:22 PM   #8
Mooly is offline Mooly  United Kingdom
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Quote:
Originally Posted by Joachim Gerhard View Post
The second stage reads 10kOhm. Maybe lowering this is an advantage.
A little advantage, yes, but the opamps have to be suitable. Something like the NE5532 or LM4562. That's not just to take advantage of the lower noise offered but also needed for drive ability because the second stage then has a 2k input impedance and a 2k feedback network to drive plus the load.
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Old 31st August 2014, 08:27 PM   #9
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Sure.
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Old 1st September 2014, 12:09 AM   #10
sreten is offline sreten  United Kingdom
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Hi,

The first option is the simplest by far, but you need to add resistors,
say 4.7K in series with L and R before combining them. Do that at
the driving end to allow a relatively long single sub cable not to
load down, due to capacitance,* the shorter direct drive feeds.

Anything more complicated is simply not needed.
It will not compromise channel seperation.

rgds, sreten.

* The long sub cable will lose a lot of treble
due to its capacitance and the 4.7K/2 driving
impedance, but hey, its a sub input, who cares ?
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Last edited by sreten; 1st September 2014 at 12:29 AM.
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