When your Polarity switch becomes a Boost switch....

[CurtZHP]

Finishing up the test assembly of my "Heavy" tube preamp.  For the most part, everything works great, but you know the drill...

There's always that one thing.  In my case, that one thing is the polarity switch.

You'll see it on the attached schematic.  It lies between the output transformer and the output jack.  When running a sine wave into the input and looking at the output on my sillyscope, the sine wave does not reverse polarity (flip upside down) when I flip the switch.  Instead, the signal apparently maintains it previous polarity and actually increases noticeably in amplitude.  This is whether I'm looking across XLR Pin 2 and Pin 1 or Pin 2 and Pin 3.

I pulled out the 10K resistor, thinking that was causing the trouble, but that made no difference.

And flipping the switch upside down and calling it a Pad isn't really an option.  I already have a sort of pad earlier in the circuit, so that would be redundant. 

The design for this particular polarity switch was shamelessly plagiarized from Jakob's G9 preamp.  (I renounce myself, and am whipping my bare bottom with a patch cord as I type this...)  Obviously, I've made a mistake in translation, or I've wired it incorrectly.  So my first question is, did I get the design right?  If that's good, then I can try to determine where I goofed on the wiring.

 

[scott2000]

I don't know anything about this so hopefully someone can help you.......

Just a quick glance at the output trafo, looks different from the original drawing.....The secondaries are wired differently and there's a 10k resistor missing ?????

But IDK... I guess you are using a slightly different approach??? The circuit may be different enough to cause the issue....??
..... Good luck!!

 

[ruffrecords]

It is probably working OK. The problem is you should be looking across pins 2 and 3. Pin 1 is not a signal pin.

Cheers

Ian

 

[CurtZHP]

It is probably working OK. The problem is you should be looking across pins 2 and 3. Pin 1 is not a signal pin.

Cheers

Ian

I did that.  Looked across 2 and 3.  Similar result.  No boost in signal, but it doesn't change polarity, at least according to the scope.

Actually, on my scope, there's a Coupling switch that has four positions:  AC, LF rej, HF rej, and DC.  I usually have that in the AC position.  If I switch it to LF rej, I get the result I'm looking for.

 

[ruffrecords]

I did that.  Looked across 2 and 3.  Similar result.  No boost in signal, but it doesn't change polarity, at least according to the scope.

Actually, on my scope, there's a Coupling switch that has four positions:  AC, LF rej, HF rej, and DC.  I usually have that in the AC position.  If I switch it to LF rej, I get the result I'm looking for.

If you just look at the output on the scope you will not see any polarity change.  If the scope triggers on an edge so the picture will look the same no matter what the polarity. You need to compare it with the polarity of the input if you can using a dual trace scope. Then you will see the polarity flip.

Cheers

Ian

 

[CurtZHP]

If you just look at the output on the scope you will not see any polarity change.  If the scope triggers on an edge so the picture will look the same no matter what the polarity. You need to compare it with the polarity of the input if you can using a dual trace scope. Then you will see the polarity flip.

Cheers

Ian

I've got a dual trace scope, but I only have one probe.  I'll have to rig something up.  But, like you said, it's likely working just fine.
Heck, I'm amazed this thing is passing a signal at all!
;D

 

[scott2000]

I've got a dual trace scope, but I only have one probe.  I'll have to rig something up.  But, like you said, it's likely working just fine.

;D

Can you just record  something in a daw both ways, line them up  and look at it???

 

[CurtZHP]

Can you just record  something in a daw both ways, line them up  and look at it???

This thing is currently a pile of guts on the workbench.  Not quite ready for that yet.

 

[Gold]

The easiest way to check polarity is to use a diode clipped test signal. That way you just look for the flat top.

 

[abbey road d enfer]

If you just look at the output on the scope you will not see any polarity change.  If the scope triggers on an edge so the picture will look the same no matter what the polarity. You need to compare it with the polarity of the input if you can using a dual trace scope. Then you will see the polarity flip.

Cheers

Ian

+1 on that. Checking polarity on a 'scope needs to have a reference on the 2nd channel.