Hi guys,
I've been enjoying my Crack+Speedball for a few weeks and just wanted to share a little mod that I've done. (do at your own risk!)
I took the liberty of measuring the headphone jack voltage and saw a -2V spike on startup, followed 15s later by a slow ramp up to +20V. There is a similar ramp to -20V on shutdown. I searched the forums and I see that this is expected behavior. And with a 300Ω load like HD650's in parallel with the 2.49k drain resistors this voltage should drop by factor of 10.
That said, I'm still uncomfortable subjecting my headphones to these voltages so I've been making a habit of removing the headphones before startup and shutdown every time. I won't be the only one using this amp so I decided to make it a little more foolproof with a timer relay to mute the headphone jack.
I've seen the 555 timer chip, but I don't have the means to draw up a PCB and lay out all of the supporting circuits. I found a guy online who sells PCBs for $20 but you still need a laundry list of parts from digikey.
Instead, I went to amazon to buy a "delay on make" timer (search for "ICM Controls ICM102 DOM Timer", can't post link here)
and this DPDT relay with 120VAC coil: ("SCHNEIDER ELCTRIC/MAGNECRAFT 92S11A22D-120 POWER RELAY, DPDT, 120VAC, 30A, DIN RAIL")
This does add 3 screws to the chassis, but I don't think it detracts from the overall appearance. I made sure to thoroughly wipe/blow out any metal shavings after drilling and deburring the holes.
I set the timer to 30-35 seconds (the red timer knob is not very accurate) and wired it in series between the AC switch and the DPDT relay coil.
The red and white wires from terminals 6 and 10 go to the "common" contacts on the DPDT relay.
I wired the "Normally Open" contacts to a set of 2.2k resistors to headphone jack ground (couldn't find exactly 2.5k resistors at Fry's). I'm no EE, so this may or may not be necessary. I wasn't sure if it was safe to leave an open circuit or short the voltages to ground so I figured I can't go wrong mimicking the drain resistors on the headphone jack.
Lastly, the "closed" contacts go back to the headphone jack.
Now the headphone jack is disconnected for about 30 seconds after startup, and immediately disconnected after shutdown. Other than that, everything else sounds the same. I do hear a faint buzzing sound which I assume is due to the 120V coil. But it's not very noticeable, and still quieter than the crack w/o speedball.
In retrospect I could have just added a parallel path to ground instead of routing the headphone signals through the relay, but again I wasn't sure if it was safe to dump the voltage to ground or not. I would be happy to get a response to this from someone more knowledgeable than me on this subject.
Of course, it's possible the voltages I measured aren't harmful at all and this whole thing was an exercise in futility, but at least now I can sleep well at night!
