Kent Crack

[palmfish]

My Crack is now 95% complete and operational! I have to replace the power switch (as explained in my other thread) and I still need to stain and satin poly my base.

But I've got my headphones plugged in and music is playing! Before I post the pictures though, I have a few issues/questions I'm hoping for some feedback on...

#1 When I did the voltage check per the instruction manual, all of my numbers are 25% lower across the board than those published in the manual. Is this OK?

#2 There is a background "hum" that, while masked by music playing, is easily audible without music. I hear it with the volume knob at "zero" and it is the same loudness with all of my headphones - low impedance (Koss KSC75) and high impedance (Senn HD 800).

#3 I notice some channel imbalance (right side is louder) and I can only use about 1/4" of the volume knob travel before it gets too loud. Are these issues normal?

 

[palmfish]

And my pictures...

 

[Paul Birkeland]

#1 When I did the voltage check per the instruction manual, all of my numbers are 25% lower across the board than those published in the manual. Is this OK?

#2 There is a background "hum" that, while masked by music playing, is easily audible without music. I hear it with the volume knob at "zero" and it is the same loudness with all of my headphones - low impedance (Koss KSC75) and high impedance (Senn HD 800).

#3 I notice some channel imbalance (right side is louder) and I can only use about 1/4" of the volume knob travel before it gets too loud. Are these issues normal?

1 - What is your AC line voltage?  If you have ~110V AC, you may find lower voltages. 
2 - This is not normal, I noticed you painted your chassis, if you've insulated the metal to chassis interfaces on the terminal strips and the grounding lug by the IEC socket, then you will end up with a noisy Crack.  I would loosen these connections, scrape the paint off, then reconnect them.
3.  There is some channel imbalance with the stock potentiometer at lower volumes (see the FAQ sticky).  You can lower the signal level coming into the Crack to counteract this. 

-PB

 

[palmfish]

#1 When I did the voltage check per the instruction manual, all of my numbers are 25% lower across the board than those published in the manual. Is this OK?

#2 There is a background "hum" that, while masked by music playing, is easily audible without music. I hear it with the volume knob at "zero" and it is the same loudness with all of my headphones - low impedance (Koss KSC75) and high impedance (Senn HD 800).

#3 I notice some channel imbalance (right side is louder) and I can only use about 1/4" of the volume knob travel before it gets too loud. Are these issues normal?

1 - What is your AC line voltage?  If you have ~110V AC, you may find lower voltages. 
2 - This is not normal, I noticed you painted your chassis, if you've insulated the metal to chassis interfaces on the terminal strips and the grounding lug by the IEC socket, then you will end up with a noisy Crack.  I would loosen these connections, scrape the paint off, then reconnect them.
3.  There is some channel imbalance with the stock potentiometer at lower volumes (see the FAQ sticky).  You can lower the signal level coming into the Crack to counteract this. 

-PB

1. Yes, I do have 110 VAC. Glad to hear it's nothing to worry about.

2. I did paint the bottom of the plate but I thoroughly scraped the paint off around every hole that the terminal strips are bolted to. What about the RCA jacks? I read somewhere that I shouldn't use the plastic washers on the underside? I did use them...

3. I hate to do this (reducing dynamic range in theory) but if I must, I will.

 

[Paul Birkeland]

1. Yes, I do have 110 VAC. Glad to hear it's nothing to worry about.

2. I did paint the bottom of the plate but I thoroughly scraped the paint off around every hole that the terminal strips are bolted to. What about the RCA jacks? I read somewhere that I shouldn't use the plastic washers on the underside? I did use them...

3. I hate to do this (reducing dynamic range in theory) but if I must, I will.

I'd love to know who/where it is written not to use the bottom plastic washers.  Not using them will result in ground loop hum.

Dynamic range is not reduced by this method, this is another common misconception, that would be implying that a potentiometer acted as a compressor, which it does not. 

Can you check the resistance between terminal 3 and #8 ground screw at the rear of the chassis?

 

[palmfish]

Almost forgot. Here's the final picture with new power switch and volume knob...

 

[palmfish]

I'd love to know who/where it is written not to use the bottom plastic washers.  Not using them will result in ground loop hum.

Dynamic range is not reduced by this method, this is another common misconception, that would be implying that a potentiometer acted as a compressor, which it does not. 

Can you check the resistance between terminal 3 and #8 ground screw at the rear of the chassis?

Zero resistance between ground screw and terminal 3

 

[Paul Birkeland]

I noticed in your build photo that the white washers on the bottom sides of the RCA jacks aren't present, is that correct?

 

[palmfish]

No, they are there. I read that blurb (wish I could remember where) about not using them after I had already installed them. I left them there because it didnt make sense to me to remove them.

Could my hum have anything to do with moving that wire to 14U (to reduce noise) as instructed in the revised manual?

 

[Paul Birkeland]

It may be worth moving that wire to be sure.

Otherwise, I'd leave the Crack on overnight to let the tubes cook a bit, sometimes that can help.


-PB

 

[palmfish]

Thanks Paul, I will definitely try that. I also have anoer set of tubes on the way so maybe that will help. I notice that I get a "tuning fork" type sound when I gently tap any part of the amp, if its related...

And I want to thank you for all your advice. I never would have attempted this build if I hadn't heard the Crack at the last BHQ hosted head-fi meet.

 

[Paul Birkeland]

And I want to thank you for all your advice. I never would have attempted this build if I hadn't heard the Crack at the last BHQ hosted head-fi meet.

Well, if you're ever in the neighborhood, feel free to bring your amp by and we can look it over!  Sadly, I was out of town for the Headfi meet  :-\

 

[palmfish]

Thanks. If I can't figure it out on my own, I will do just that - and maybe pick up a Speedball for good measure!

;D

 

[palmfish]

I got back from my trip and my new tubes were waiting for me! The good news is that the microphonics I was hearing with the original tubes is now gone with the new ones. The bad news is that my 60 Hz "hum" is still present. I retraced the ground from point to point and checked resistance with a multimeter and I can't find any evidence of a bad connection/cold solder (I also checked resistance between the main ground screw and the headphone jack ground lead and it is zero). I even removed the main ground screw and scraped the metal underneath the washer to make sure it was making contact.

I think I'd like to take you up on your offer and come by to see if you can help me out...

Is there any particular day/time that's good?

 

[Paul Birkeland]

Next Monday or Tuesday are great, beyond that I'll be on vacation.

-PB

 

[palmfish]

OK! How about Monday afternoon? Say 2:00?

 

[caffeinator]

This may sound like a dumb idea, but have you tried other power outlets at your place, or maybe taken it to a friend's house to see if the problem is still present?  I had a hum problem I couldn't figure at my old place, which mysteriously disappeared when I found a better outlet elsewhere in the house...of course, this was an old, old house, that should have been rededicated as "The Museum of Residential Electrical History" with exhibits ranging from knob & tube to silver romex to catgut and whale blubber, but that's another story...anyway..just a thought.

 

[palmfish]

Hmmm, that's an idea I hadn't considered. Thanks! I haven't taken it to another home, but I have tried it all over my house. I'll bring it to work with me on Monday (before I head to BHQ) and try it there.

 

[Paul Birkeland]

OK! How about Monday afternoon? Say 2:00?

Sure, I'll see you then!

-PB

(Unless the outlet or location is the issue)

 

[palmfish]

Thanks Paul, I've got it on my schedule!

I actually might have made some progress tonight. I was listening to music tonight and when I turned off my source, I noticed the hum was gone. I unplugged my headphones and plugged them back in - and no hum.

So I repeated it like a dozen times and while the hum came back for most, I got no hum a couple more times. Finally, it hummed every time and I gave up.

I brought my Crack to my workbench and resoldered all the TRS connections, but it didn't help. I once again get "hum" every time I plug in headphones.

I don't think it was a fluke though - maybe the jack itself is bad or maybe I need to resolder the wire further upstream from the jack. I'm done tonight but I'll tinker some more tomorrow.