Differential preamp wiring - diyAudio
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Old 3rd May 2012, 12:57 AM   #1
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Join Date: May 2004
Location: NJ
Default Differential preamp wiring

Just wondering if anybody could give me a quick lesson on how to wire the input for a differential preamp.
Then how would you connect a switch that would allow you to switch polarity easily between the positive and negative connections.
I have seen the vacuum state electronics FTP 3d schematic and was wondering how would you easily connect the input.
My oppo has a balanced output so I could use this for the input, I suppose, with a balanced input. The phono easily has a balanced connection.
Just wondering about the actual wiring.

Gary
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Old 4th May 2012, 09:58 PM   #2
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What I meant to ask was about the phase switch- not polarity.
Gary
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Old 9th May 2012, 03:25 PM   #3
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It is polarity, not phase
How about a single-line diagram to show us what you want to accomplish? E
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Old 10th May 2012, 02:13 PM   #4
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Mickey,

Thanks, please refer to this link at the vacuum state electronics website.

Vacuum State - High End Hifi Equipment

The issue I mean is exactly where the wires from the input connection are wired.

I also understand from Allen's Preamp Cookbook that the advantage of these differential designs, besides the noise canceling ability, is also that you can connect them for absolute phase correction with a switch. I was wondering about how to wire that switch as well. My assumption is that it goes after the volume control, but what kind of switch and how is it wired.

Thanks for any help.

Gary
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Old 10th May 2012, 02:58 PM   #5
DF96 is offline DF96  England
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To wire a balanced source to a balanced input, just connect the two signal wires to the two input points. However, this is so trivial that it can't be what you are asking.

To swap polarity (and phase - in this context they are the same thing) use a DPDT switch. Wire one side to the centre connections (poles). Wire the other side to one set of end connections (ways), then extend to the other end connections but swap the wires over. Again almost trivial so possibly not what you are asking. It doesn't matter too much which way round you wire the switch (input or output to the poles).

Differential inputs reduce noise picked up externally, although this is rarely a problem domestically, but may increase internally generated noise by up to 3dB. They also double the cost of circuitry.
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Old 10th May 2012, 05:01 PM   #6
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Df96,
I am just asking about the DpDt switch. I have never actually seen a circuit that uses a polarity switch and wanted to hear your confirmation.
This particular design has intrigued me for the past few months, as I have not been happy with many of the preamps that I have heard. I have been listening to audible illusions preamps recently and they are fine, but don't have the soundstaging presence that my old Theta slider gave me. The review of this real time preamp from vacuum state have been superlative in terms of those real terms for listening to records that I enjoy so much.
I'd like to try build that preamp and am looking for basic help in wiring a differential amp since I have never seen the design elsewhere.
Thanks for your help.
Gary
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Old 10th May 2012, 05:30 PM   #7
AndrewT is offline AndrewT  Scotland
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Download some articles on balanced connections. Jensen and Jung and Self are good starters.
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regards Andrew T.
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Old 10th May 2012, 07:06 PM   #8
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Wow! That Vacuum State is quite a preamp!
Polarity reversal switches are often found in "pro"-sound gear, as many years ago there were different standards to connecting XLRs. For program material it is not really important (dont chide me for this!) but speech has a distinct pattern that, if inverted, may require correction
When chosing your switch make sure it is not as toggle switch (preasure contacts) but a self-cleaning slide switch or a relay with bifurcated contacts. E
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Old 11th May 2012, 06:58 PM   #9
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This preamp appears to be something and maybe a bit more than I should probably bite off, but I have completed tube amps that sound wonderful and appear to be working fine, at least both sides appear to be very similar in stats, from a voltage standpoint.
I just want to make sure that I think through how to lay it out and how to put it together . I suspect the layout is 90 % of the battle.
There is a lot of discussion about the thinking that went into this preamp in Alan's preamp cookbook, so there is some minor help on getting the power supply together and functioning.
I just wasn't absolutely sure about the wiring on these parts.
Gary
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