Dual PCM1794A Mono Switch? - diyAudio
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Old 17th August 2014, 07:13 AM   #1
druu is offline druu  Australia
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Default Dual PCM1794A Mono Switch?

Hi,

I have a dual 1794A DAC/preamp that i'd like to install a mono/stereo switch on. Can someone advise on the best way to do this?

Thanks.
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Old 17th August 2014, 07:15 AM   #2
druu is offline druu  Australia
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Also how difficult would it be to add a dim function? Say, -20dB?
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Old 17th August 2014, 04:34 PM   #3
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I'll assume that toggling the stereo/mono configuration setting via PCM1794A register programming in software is not an option. An schematic of your DAC would enable the most correct answer for an hardware solution. However, you likely can simply solder an SPST (open/closed) switch directly across the PCM1794A current outputs of the two channels. The reason the PCM1794A's current output pins can be directly switched together this way without damage is because they feature an high output impedance. Do not solder wires directly to the DAC chip's pins, as these are easily damaged, or inadvertently shorted to adjacent pins via solder bridge. Solder wire leads to the PCB pads or traces connected to the chip's output pins instead. The leads should be as short as is possible in reaching the switch mounting position. Solder the wire leads to the mounted switch.

In the switch open position, the DAC will function normally in L and R stereo. In the switch closed position, the DAC chip's L and R current outputs will be paralleled to form a single summing node, blending the L and R outputs as mono. Because of the typically very low input impedance of the I/V stage which follows, the cross coupling impedance of the switch, even though very low, still may have the effect of preventing an perfect mono coupling between the channels. Perhaps, delivering an stereo/mono blend.
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Last edited by Ken Newton; 17th August 2014 at 04:48 PM.
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Old 17th August 2014, 11:02 PM   #4
druu is offline druu  Australia
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Hi Ken,

This sounds like the solution I was after, thanks. I wanted to make sure no damage would come from simply joining the channels. So to confirm, simply shorting the outputs without resistors is fine?

Unfortunately I don't have a schem as the item is (somewhat) commercial. Any suggestion on the dimming function?
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Old 18th August 2014, 12:42 AM   #5
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Quote:
Originally Posted by druu View Post
Hi Ken,

This sounds like the solution I was after, thanks. I wanted to make sure no damage would come from simply joining the channels. So to confirm, simply shorting the outputs without resistors is fine?
Correct, you want to connect the channels through a zero ohm path for mono operation. Such shorting will not harm the PCM1794A current outputs.

One other point. Because the PCM1794A features differential outputs you will need to be certain to wire the positive phased L and R outputs together, and likewise, the negatively phased L and R outputs together. A single DPST switch can be employed to conveniently switch the two paths at once. If you reverse those connections they will cancel the in-phase inter-channel signal content, and produce the out-of-phase inter-channel signal difference content.

Quote:
Unfortunately I don't have a schem as the item is (somewhat) commercial. Any suggestion on the dimming function?
Not sure what dimming function you are referring to. If you mean display illumination dimming, then connecting the display power supply feed through a resistor may do the job, but you would likely need the schematic to do this; which you've said you don't have access to. You might try applying neatly cut translucent tape or colored plastic film over the display window. Not very elegant, but should be effective so long as the tape or film is not too dark.
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Last edited by Ken Newton; 18th August 2014 at 01:05 AM.
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Old 18th August 2014, 12:57 AM   #6
druu is offline druu  Australia
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Thanks Ken. If I'm using balanced then I'm assuming I need to switch both + and - through a DPDT switch?

With the dimming function I was referring to something like a pad, to reduce the level around 20dB or so if possible. How would I go about this?

Thanks again!
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Old 18th August 2014, 12:51 PM   #7
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Quote:
Originally Posted by druu View Post
Thanks Ken. If I'm using balanced then I'm assuming I need to switch both + and - through a DPDT switch?
Yes.

Quote:
With the dimming function I was referring to something like a pad, to reduce the level around 20dB or so if possible. How would I go about this?

Thanks again!
I'm still uncertain as to what dimming you're refereing to. Confirm if you mean the display illumination, because your specific mention of a 20dB reduction makes me wonder whether you may be referring to something signal related.
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Old 18th August 2014, 12:58 PM   #8
druu is offline druu  Australia
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I'm referring to volume dimming, allowing instant reduction of the monitoring level.
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Old 19th August 2014, 12:40 AM   #9
druu is offline druu  Australia
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Perhaps a switched L-pad?
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Old 29th August 2014, 08:22 AM   #10
druu is offline druu  Australia
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I tried the mono solution but for some reason it would only output to one speaker. Would tying the XLR pins 2 and 3 together with a DPST switch on the channels be ok?
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