How to Wire a 4-pin XLR for HP output?

[jiggad369]

Hi All,

I'm in the process of building my crack and I was wondering how one goes about wiring the HP output on the BHC for a 4-pin XLR output.

I know the amp itself cannot do balanced output but my main question is regarding how to wire the 2.49k resistors on the pins.

[Paul Birkeland]

Pins 2 and 4 need to be connected to terminal 3.  The 2.49K resistors will go between pins 1 and 2, as well as pins 3 and 4.

The amp will work a lot better with the 1/4" TRS jack installed, as it shorts the outputs during warmup and keeps the voltage spike at the headphone jack from appearing at all.  By installing the 4 pin XLR jack, that function is defeated.

[jiggad369]

Pins 2 and 4 need to be connected to terminal 3.  The 2.49K resistors will go between pins 1 and 2, as well as pins 3 and 4.

The amp will work a lot better with the 1/4" TRS jack installed, as it shorts the outputs during warmup and keeps the voltage spike at the headphone jack from appearing at all.  By installing the 4 pin XLR jack, that function is defeated.

Thanks Paul, so you would def. recommend building a 4pin XLR to 1/4" adapter instead?

[Paul Birkeland]

I would build or buy one instead.  They are useful to have around anyway!

[Mucker]

FWIW, I run 4 pin XLR jacks on my two Cracks and S3X amps and haven't experienced any negative effects. In fact, it's totally uneventful, the plugs go in and out with not even the slightest hint of noise or pop.

I've measured the startup voltage at each of the signal pins of the XLR jack. (black probe on T12) The voltage will rise to 0.3 volts max and then immediately go to zero, all within a split second of switching on with volume at zero.

My usual practice is to start the amp at zero volume and let it warm up for at least 5 minutes, plug the phones in and listen. When done, set the volume to zero, unplug, and turn off amp. Hundreds of insertions and hopefully many more ....

 

[Paul Birkeland]

The voltage spike with the Speedball on a Crack with no shorting jack is a lot higher than 0.3V.  I seem to remember it being more like 9V.

[Mucker]

Thanks for prodding me to recheck. I think I discovered my measuring flaw. I wasn't waiting long enough for the Speedball boards LED's to light up. There is indeed a voltage spike happening, but so fast I can't accurately see exactly how high, but yes, well above 0.3V. The good news is that it immediately (in the blink of an eye) goes to zero and remains there. As I mentioned, I have no issues at all and always wait for a warmup before plugging the phones in and always disconnect before shutting off. I remember on my first Crack, the warning in the manual about the 9V TRS occurence, and also experienced pop when plugging into the TRS jack (before the mod was suggested). The nice thing with this XLR jack is no pop or noise at all.