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Wiring an autoformer for RCA+XLR in/out?
Wiring an autoformer for RCA+XLR in/out?
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Old 14th February 2020, 02:46 PM   #1
andyjevans is offline andyjevans  United Kingdom
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Default Wiring an autoformer for RCA+XLR in/out?

I have a pair of autoformers for a tube stage or possibly stand-alone passive pre which I haven't wired up yet,
but I'd like to do so. Wiring for RCA inputs and outputs looks straightforward. But how do you wire in the added option of XLR in and out?

I always assumed that you put the variable resistor or I guess autoformer between pins 2 and 3. So I'm at a bit of a loss to see how you combine
phono and xlr inputs and outputs.

Can anybody clarify with a wiring diagram?
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Old 14th February 2020, 03:11 PM   #2
JMFahey is offline JMFahey  Argentina
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XLR in principlen implies floating / not directly referenced to ground.

Not sure how an *auto* transformer might achieve that.
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Old 14th February 2020, 04:38 PM   #3
audiowize is offline audiowize  United States
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If you want to do this, set the autoformers aside and switch to TVCs.
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Old 14th February 2020, 08:31 PM   #4
andyjevans is offline andyjevans  United Kingdom
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Quote:
Originally Posted by audiowize View Post
If you want to do this, set the autoformers aside and switch to TVCs.
OK, how would you wire up a TVC for both RCA and XLR?
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Old 14th February 2020, 09:13 PM   #5
audiowize is offline audiowize  United States
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Quote:
Originally Posted by andyjevans View Post
OK, how would you wire up a TVC for both RCA and XLR?
A TVC input is balanced unless you provide an unbalanced earth reference on your own. For XLR input, pins 2 and 3 go to the input side of the TVC. The RCA jacks connect similarly, and you have the option of providing a DPDT switch to enable or disable the galvanic isolation of the TVC. To maintain the balanced input, you'll want to use a 4 pole switch to let you switch hot/ground on the RCA jacks and +ve and -ve on the XLRs as a function of the input switching.

The output side is a little simpler, as you can have a toggle switch that either earths one of the ends of the secondary winding to allow for unbalanced outputs or floats the winding for balanced output.
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Old 14th February 2020, 10:43 PM   #6
andyjevans is offline andyjevans  United Kingdom
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Thanks, that's helpful.
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Old 15th February 2020, 10:17 AM   #7
andyjevans is offline andyjevans  United Kingdom
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how to build a pre using S&B TX-102 transformers

Wiring diagrams post 57. Excellent thread about building a TVC. The S&B TX102 was popular at the time of the thread and Thorsten Loach has good input as usual.
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