Help with switch wiring for transformer i/o boxes

[whynot]

Hi folks,

I come seeking help.
Please let me know if I'm asking too much, being fairly new around these parts!

so I’m in the midst of building a few 2 channel transformer i/o boxes per this Barry Rudolph article at sonarworks!


I’m using pairs of the following transformers:

Carnhill VTB 2281: Primary & Secondary windings @ 600 & 150 ohms
UTC A-20: Primary & Secondary windings @ 500, 300, 200, 125 & 50 ohms
(I've also got UTC H-25s [Primary @500, Secondary @500 & 125 ohms] currently hardwired to pigtails..)

So far I've got everything but the switching working using test leads

Balanced XLR input connectors are wired to the Carnhill and A-20 primaries. Secondaries are connected to 10k linear pots (trim) which are wired to the output XLR connectors. Everything works as expected... so far so good!

Now I would like to add the switches to access the different windings.

I am able to access each of the transformer windings individually, but the switch wiring has mostly eluded me. I did get the Carnhill 600ohm secondary passing signal correctly through the switch, but could not figure out how to add the @150 winding so it also passed signal when switched in. I realize that I may well be misunderstanding the suitability of a 3PDT switch for this purpose (I’ll presumably need a 3P5T switch for the A-20 if I’m going to access all of it’s windings.. but I’m still figuring out the easy stuff)

The Barry Rudolph article includes hand drawn schematics for the Carnhill VTB 2281 build, which I’m trying to follow. Trying being the operative word.

For example: Carnhill VTB 2281 secondaries:

Series wired secondaries (@600): 5 Hot, 8 cold, 6&7 tied
Parallel wired secondaries (@150): 5&7 Hot, 6&8 Cold


As near as I can tell..
Series (@600) wiring to the switch should be as follows:

• Carnhill terminal 5 (hot) → switch throw 1 hot (→ xlr pin 2)
• Carnhill terminal 8 (cold) → switch throw 1 cold (→ xlr pin 3)
• Carnhill terminal 6 → switch throw 1 pin3 ( → terminal 7) ← this does not work
• Switch OUT pin3 → terminal 7

This works! (with nothing connected to switch T2)

Parallel (@150) wiring as follows:

• Terminal 5 & 7 (hot) → switch T2 hot (→ xlr pin 2)
• Terminal 6 & 8 (cold) → switch T2 cold (→ xlr pin 3)
• Ground (?) → switch T2 pin3

This does not seem to work?


..
Thinking about how a switch takes signal in from one of two inputs, and sends it to the output
(Or conversely, takes one input and sends it to one of two outputs) .. it seems like the wiring should be:

Wiring:
Primary:
Throw 1 (out to carnhill) → @600
Middle (IN from xlr) ← IN
Throw 2 (out to carnhill) → @150

Secondary:
Throw 1 (in from carnhill) ← @600
Middle (out to trim pot) → OUT
Throw 2 (out to carnhill) ← @150


here are the drawings:

I know it's a lot, but can anyone point me towards the light?
I clearly have little idea what I'm doing
..but I'm invested now


thank you!

 

[radardoug]

Its exactly the same wiring as for a two winding power transformer that you can parallel for 115 volts or series for 230 volts.
You need a two pole two position switch.
The wiring has been discussed on this forum.

 

[whynot]

Its exactly the same wiring as for a two winding power transformer that you can parallel for 115 volts or series for 230 volts.
You need a two pole two position switch.
The wiring has been discussed on this forum.

Thank you for the reply. I've tried the search function but must not be very good at it. If you've got a link or any guidance on where to look, I'd very much appreciate it.

 

[rackmonkey]

If you can’t find it using the site’s search function, try using Google and specifying the groupdiy.com domain:

<search terms> site:groupdiy.com

 

[whynot]

Well I appreciate the clues folks, but I must not be very good at using the tools because whether I search using the big G or the site search function, I've combed through dozen and dozens of threads and found nothing that looks like the answer I seek...

I assume the problem is me .. but thanks for trying!

 

[radardoug]

Here you go.

 

[whynot]

Here you go.View attachment 99281

Thank you so much.
Let's see if I can make sense of this.


I'm also gonna reverse engineer your search now that I know what the desired result looks like.. so I can do better next time.

thanks again!!
I very much appreciate it!

 

[whynot]

OK..
Well, I'm starting to think I've gotten in over my head, as I'm not actually very capable at reading schematics. After studying the provided diagram I am somehow no closer to unraveling the mystery of how to wire these switches. If anyone is willing to provide some guidance I'm here for it, but if I'm come to the wrong place, I get it. I suppose I can live with a dedicated, non-switchable wiring scheme, at least until I get better at reading the docs or find someone who can walk me through this stuff.

Anyways, thanks again. Sorry for the trouble.

 

[kbx]

DIY Transformer Box

 

[whynot]

DIY Transformer Box

Thank you very kindly!
I will go try to wrap my head around this

 

[tony hunt]

I advise simplifying your schematic drawing.
Take one side of the transformer at a time.
Draw your switch closer to the transformer schematic.
Like the suggestion in #6 from Radardoug.

Ask youself how to go from a series to a paralell implementation of the windings, on one side of the transformer first of all. Take it one step at a time.
Do one side, test it, then take it from there.

Attatched is a simplification of Post #6 schematic. Focus only on that for now. Understand that one principle, then you can run from there.