Crack channel imbalance

[gregorylipovsky]

Hey fellas,
I finished my crack with Speedball last month. I've been racking up lots of listening hours. Love it love it. After the Speedball upgrade I noticed the sound stage is a tad biased to the right channel. I used the amp for about a day before upgrading to SB. So I'm not sure if the problem has been there all along or just after the upgrade. I saw one solution was to add some 75k and 33k resistors. Did that this week. After double checking I installed them in the correct areas, I still hear the imbalance. Its not as bad but I can still tell everything is off center. It gets especially annoying when I hear the bass more prominently on the right side. I've tried different cables, different sources and listened to my headphones without the amp. No imbalance when I listen directly to my Ifi Micro IDSD or LG V10. Also if I begin playing music when the crack is off, than turn it on while wearing my headphones, the first side to get fired up is the right channel than the left fires up. Guessing this is normal. The imbalance is especially noticeable when I barely rotate the volume pot. I can still hear the imbalance when I crank the volume all the way. At about 60% volume I begin to hear a hum. (Thinking thats just interference from tv/xbox? or volume pot?)
The following are my Crack measurements:
1.*
2.*
3. .3 ohm
4.*
5.*
6. .4 ohm
7. Meter says overload.
8. .3 ohm
9. Meter says overload.
10. .4 ohm
12. .4 ohm
13. *
14. .4 ohm
20. .4 ohm
22. .4 ohm
Measurements before volume pot resistor 75k/33k mod.
Right center pin channel 96k ohm. Left center pin channel 92.6k ohm.
After 75k/33k mod: right 99.3k left 98.9.
Voltages:
1. 101.8
2. 172.8
3. 0
4. 172.4
5. 96
6. 0
7. 120
8. 0
9. 116
10. 0
Speedball before 75k/33k resistor mod:
OA. 101.5
OB. 96.5
G. 0
B+ 173
Do I need a new Volume pot or new tube? I checked the soldering and redid some iffy joints for a couple hours today but I'm at a loss. Any thoughts would be appreciated.
Gregory L

 

[Paul Birkeland]

Channel imbalance in the Crack tends to fall into 3 categories:

1.  The source/headphone combination is overly sensitive, so the level control can only be turned up ever so slightly for a very high listening level.  Since low-level tracking is a difficulty with these parts, the resistor mod can be used to adjust where the optimal listening position is to reduce the imbalance.  It is still possible that your setup is overly sensitive, can you use at least 90 degrees of volume pot rotation?

2.  Sometimes the individual triodes inside the 12AU7 and 6080 can be out of balance.  It's rarely the 6080 that causes this kind of problem, and borrowing or trying a different set of tubes can let you know if this is your issue.

3.  You have a cold solder joint that is obstructing the signal somewhere in the amplifier. 

One step you can also take is to play a 60 Hz tone into both channels of your amplifier, then measure the AC voltage between ground and these points with the level control half way up, then repeat with the level control all the way up:

1.  At the center pin of each RCA jack.
2.  At the middle lug on each level of the volume pot.
3.  At terminals 6 and 10 in the amplifier.

This will give a decent breakdown of where the imbalance is originating.

-PB