4-8 Ohm Switch

[Chris H]

I am considering fitting a switch to change between 4 and 8 ohm impedance depending on usage.
I've come up with the attached but am fairly new to anything other than following instructions so thought it prudent to check it will work (dashed line is striped wire, 2 and 5 are common for switch).
Diagram is for one channel using a DPDT switch so if correct a single 4PDT switch for both channels is all that is needed?
Also, any placement issues I need to consider?
Any advice much appreciated.

[Paul Joppa]

There is a way to do this with one DPDT switch for both channels. I'll need to dig out my notes to be sure, but will post back when I'm sure I've got it right.

[Paul Birkeland]

That should work fine.  You can use very small light duty switches for this application as well since the voltages present are small. 

[Chris H]

Thank you both.
I couldn't do it with a single switch without causing a connection between transformer lugs that wouldn't otherwise be connected.
It may be that there's a connection that is ok but without understanding the internal wiring of the transformer I couldn't be sure.

I intended to use an on-off-on switch to avoid both being momentarily on.
For that matter, is there any risk to the amp if switched while powered up?

[Paul Birkeland]

Use an on-none-on.   This is not something that requires a whole lot in the way of precautions.  You could get a single 4PDT switch to do the job for both channels and put that up by your headphone jack.  This is similar to what we do in the Mainline.

[Chris H]

Thank you Paul, good to know regarding precautions.
And thank you for the correct terminology.  I did mean on-none-on and see that on-off-on would get me soimething else.

4PDT was the plan and placement makes sense as can follow original intended path and minimise variation from original layout and design.

Thanks you again for the advice.

 

[Paul Joppa]

OK, looking at my notes, you can connect 2 to 7 for common, 3 to 6 for 4 ohms, and 1 to 8 for 8 ohms. A single 2-throw switch can select the 4-ohm or the 8-ohm tap, thus a 2P-2T switch can handle two channels.

This is only for the OT-2 at 8K primary impedance, as used in the S.E.X. amp. (At 4K ohms as in Stereomour, it would be 2 or 4 ohms output, which is not all that useful.)

[Thermioniclife]

I used a 4PDT switch on my S3X and placed it by one of the
oT-2's https://forum.bottlehead.com/index.php?topic=12217.0

[Chris H]

Thanks Paul.
I had been assuming an extra pole (on each channel) was required to open 1 to 8 or 3 to 6.
But, If I understand correctly, both 1 to 8 and 3 to 6 can be wired permanently and just the single pole required to select either 6 or 8 out (so a DPDT switch for 2 channels).

Lee, I have read some of your posts with interest but was looking at an earlier thread where the switch was not so clear.
Thank you for the link - the fog is starting to clear.

[Paul Birkeland]

But, If I understand correctly, both 1 to 8 and 3 to 6 can be wired permanently and just the single pole required to select either 6 or 8 out (so a DPDT switch for 2 channels).

I hadn't considered that, thankfully PJ is around!

Yes, the OT has two secondary coils, and at the start you connect the bottom of each coil together (2 & 7), then for either 4 or 8 ohms you parallel together the majority of the winding (for 4 ohms) or the whole winding (for 8 ohms).  If you use both parallel connections, that works too and the transformer won't mind.

[Adrian]

Thought I might add to this thread with what I did to wire my S.E.X. 3.0 for 4 and 8 ohm output.
I wanted the 4 ohm for HPs and the 8 ohm for speakers.
I used a 4PDT on-none-on switch.
Here are some pics including the wiring diagram I developed for the project.
(edited for spelling!)

[Chris H]

Thanks Adrian, reason for doing this is the same (HP and speakers).
Your diagram corresponds to mine, although mine is for 1 channel only (1-1, 3-3, 6-6, 8-4).
I'm assuming all worked ok but any issues to flag?
I am thinking I should build stock first, then mod, which seems the prudent course but wonder how tight it is to work if doing this mod after completion.
2 builds done with no issue (a few clarificaiton questions) so I am getting more confident but don't want to be over confident.

Best
Chris

[Adrian]

Your welcome, Chris.
No problems with this.
Yes - stock first and give it good run-through and enjoy - then do any mods you like.
The reason I put the switch in the back is because with the C4S up front there was not enough room (I put in a Goldpoint attenuator that is a little larger than the stock one).
Good Luck.
Adrian