Crack as preamp

[Doc B]

Take the output from the jack.

 

[mightygrey]

Tried my newly-finished Crack as a pre-amp upchain from on old 20wpc NAD 7020e, sounded awesome.

Tried using it as a pre-amp on my Sherbourn PA 2-250 (250wpc) power amp, and immediately had an incredibly loud humming noise come out of my speakers, even with the volume pot at zero. I was running a Schiit Mani pre-amp in front of the Crack from my record player.

Any thoughts as to what might be happening here?

Cheers,

Matty

 

[Paul Birkeland]

Tried using it as a pre-amp on my Sherbourn PA 2-250 (250wpc) power amp, and immediately had an incredibly loud humming noise come out of my speakers, even with the volume pot at zero.
Any thoughts as to what might be happening here?

This sounds a lot like a ground loop hum.  I find it odd that the Sherbourn only has a 2 prong power cord, is the NAD a 3 prong cord? (I can't find an image with the AC plug included)

Noises like this can often be arrested by connecting a piece of wire between the metal plate of the Crack and the metal chassis of your amp.

-PB

 

[mightygrey]

Thanks Paul - I'll check this out after work this afternoon!

 

[AllanMarcus]

so I'm a little confused. the female jack on the crack had 6 pins. See the picture:

lug 1 is obviously common
lug 2 and 3 are right and left into the amp
I assume lugs 4-6 are mirrors of 1-3, but only when there is no plug inserted, but all these lugs are connected to gound, and to the hots via the resisters. I guess I would love to hear an explanation of what's going on here (pop protection?). I would also like to know if I want to use these lugs to as taps to feed RCA jacks for preamp use, what do I need to do and what will I be giving up.

or should I just come off 1,2, and 3 for the RCA preamp jacks?

Thanks.

 

[diynewbie]

Yes, connections from 1 to 4, 2 to 5 and 3 to 6 exist when no plug is present and the connections are broken when a plug is inserted.

While you could make changes to the wiring, but why not just buy or make a TRS to RCA connector instead?

While I could give a guess as to “what is going on here”, I think it would be best to leave it to the knowledgeable.

I will just say this is a modification of the original wiring (no 4-5-6 connection) and it eliminates a voltage swing of up to ~ 9 volts then back to 0 measured from tip or ring to ground when the Crack is first powered on with no headphones attached.

 

[AllanMarcus]

I added pre-outs to the crack. Sounds pretty nice. The wiring simply comes off the headphone tap (1, 2, and 3 in my pic above), so something needs to be plugged into the headphone jack for the pre out to work. I use a simple 1/4" to 1/8" adapter to serve this purpose.

I prefer the jacks as I think it's cleaner than a 1/4" to RCA cable.

 

[2wo]

I like that solution, sort of like an arming pin ...John

 

[Paul Birkeland]

You'll have to cut the connections between pins 4, 5, and 6 on the photo you posted, or just use a 1/4" TRS to RCA adapter in the headphone jack.

-PB

 

[AllanMarcus]

Thanks PB. Can I ask, what am I giving up by cutting that connection?

 

[Paul Birkeland]

That connection eliminates the small voltage spike while the Crack warms up if no headphones are plugged in.

-PB

 

[AllanMarcus]

interesting. I assume that voltage spike is not good, therefore I guess I shouldn't remove that little wire. right? What's the downside?

 

[Paul Birkeland]

To be honest, the voltage spike isn't a problem.  When we put out the first iteration of the Crack, we encountered endless threads about the voltage spike, so when we revised the kit I altered the design to make it a non-issue.

As a preamp, the coupling caps in the Crack are kinda big, but if you turn your amp on last, you're not going to have any problems.

-PB

 

[AllanMarcus]

Awesome. Thanks.