Impedance board for sex 3.0?

sharktopus · 1212

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Offline sharktopus

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on: August 07, 2019, 05:35:10 PM
I see a number of places here that there is (was?) and impedance switching kit for the sex.  Is this still available?  If not, I've seen pictures of the mainline amp where it has a physical toggle switch on the front that does the same function.  Would you be able to wire such a switch into the sex?

Drew Sanford


Offline Paul Birkeland

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Reply #1 on: August 07, 2019, 05:57:37 PM
The Impedance Switch Kit is no longer available.  While it was a great idea, we have many different versions of the SEX amp and two different versions of the Stereomour amp, and the complications of making a manual to work with four or five different kits got to be too much (especially considering how many of these we were selling).  The small PCB toggle switches also frequently went out of stock.  There isn't a whole lot of use for the 16 and 32 ohm settings, and we found ourselves explaining over and over again that 32 ohm Grado headphones absolutely should not be used with the 32 ohm tap.  This is why you see the recommendation to use the amp on the 4 ohm setting if you aren't sure, or the 8 ohm setting for speakers.

That is the short version of why you don't see this kit available anymore.  The Mainline circuit is a bit different and uses a different transformer. You could very likely wire up a rotary switch on the SEX amp to go from 4 to 8 ohms, but that's not the kind of project where we can give you three simple steps to get the job done!

Paul "PB" Birkeland

Bottlehead Grunt & The Repro Man


Deke609

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Reply #2 on: August 08, 2019, 03:32:58 AM
If the S.E.X. 3.0 uses the same OT-2 output transformers used in the Stereomour II, as I believe it does, you can build your own impedance switches using PJ's schematic for a 2-switch solution (or 3 switches, if you want to be able to switch from balanced to unbalanced). [/size]You'll find it in the following thread: http://forum.bottlehead.com/index.php?topic=11130.0

I built and installed the impedance switches on my SII-2A3 amp and they work great.[/font]

If you decide to build them, make sure you get low profile switches. I failed to check dimensions on the datasheet before ordering and wound up with very tall switches.[/font]

cheers, Derek[/font]



Offline sharktopus

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Reply #3 on: September 29, 2019, 05:48:06 PM
So from what I can tell, if I am only interested in switching between 4 or 8 Ohms then I can get away with only one 4PDT On/On switch.  This would choose between the two stock wiring setups.  Only terminals 1, 3, 6, and 8 of the output transformer would be switched between.  Terminals 2 and 7 would be hardwired together and out for either impedance setting (unchanged from the stock instructions).  The wiring schematic is attached.

And I would wire it like so:
Switch     FunctionImpedance
6Left OT terminal 64Ω
3Left OT terminal 34Ω
12Right OT terminal 64Ω
9Right OT terminal 34Ω
4Left OT terminal 88Ω
1Left OT terminal 18Ω
10Right OT terminal 88Ω
7Right OT terminal 18Ω
2Left Out (Green Striped)Common
5Bridge to switch 2Common
8Bridge to switch 11Common
11Right Out (Brown Striped)Common


For each channel the two outgoing terminals on the switch would be wired to each other (eg 2 to 5 and 8 to 11)


Do any of you see any issues with wiring it like that?

Drew Sanford