First Build Completed. What's the cause of hair trigger issue?

[Lorider]

Hi all,

I've finally completed my very first Crack.  The build was fun and Crack is working, however I do have hair trigger issue that some people have expressed.  I am aware that this can be fixed by adding resistors to the volume pot as stated in the Crack FAQ.  I do like to confirm a few things and more interested in the theory why this may be happening.

Equipment: Modi 3 DAC, HD6XX, HD800S
Tubes shipped in the box: GE 6AS7GA, RCA 12AU7

I think the voltage check are within range but posting here to double check.  The AC voltage from my outlet measures 123VAC.

T1 79.9V
T2 177V (slightly higher than 170V)
T3 0V
T4 177.2.V (slightly higher than 170V)
T5 77.6V
T6 0V
T7 107.6V
T8 0V
T9 106.5V
T10 0V

1. First observation during resistance check is that the right RCA center pin measures 100k ohms and left RCA center pin measures ~80k ohms.  I am guessing this is due to measurement accuracy of my cheap DMM where it's unable to measure the third decimal point when setting to 20M ohms.  But regardless, this discrepancy is probably not going to cause audible channel imbalance.
2. I do have hair trigger issue where sound is inaudible from 6-6:30am on the dial, then the right channel is the first to drive sound and left channel comes in around 7am on the dial.  I've tried both HD6XX and HD800S similarly rated impedance headphones and by 8am on the dial the volume becomes too loud for comfortable listening.  So effective range of the volume pot is from 7am to 8am. 
3.  I noticed that the big tube is the alternative 6AS7GA as opposed to 6080.  Does this affect the B+ voltage?  Does 177V affect the gain?
4. What's the theory of the cause of the hair trigger?  Is this related to DAC output voltage?  I think the Modi 3 DAC has 2Vrms output that is not adjustable.  Aren't recent DAC all outputting around 2V and that most people would have hair trigger issue from the Crack potentiometer?

I can't wait to add the speedball once I figure out the hair trigger issue.   

[Paul Birkeland]

1. First observation during resistance check is that the right RCA center pin measures 100k ohms and left RCA center pin measures ~80k ohms.  I am guessing this is due to measurement accuracy of my cheap DMM where it's unable to measure the third decimal point when setting to 20M ohms.  But regardless, this discrepancy is probably not going to cause audible channel imbalance.

This is pretty normal for carbon potentiometers and nothing to be concerned about.

2. I do have hair trigger issue where sound is inaudible from 6-6:30am on the dial, then the right channel is the first to drive sound and left channel comes in around 7am on the dial.  I've tried both HD6XX and HD800S similarly rated impedance headphones and by 8am on the dial the volume becomes too loud for comfortable listening.  So effective range of the volume pot is from 7am to 8am. 

You'll definitely want to add the padding resistors to the volume pot.

3.  I noticed that the big tube is the alternative 6AS7GA as opposed to 6080.  Does this affect the B+ voltage?  Does 177V affect the gain?

No, the two tube types are about identical.

4. What's the theory of the cause of the hair trigger?  Is this related to DAC output voltage?

Yes, it's a function of having a potent source and relatively sensitive headphones. 

[Lorider]

Thank you Paul for the clarifications. I’ll report back after padding the resistors.

[Lorider]

I’m reporting back after doing the resistors padding. I didn’t have 75k on hand so I put two 33k in series to both of the rca channels to the left lug and the usual 33k across left and right lugs on the pot. Everything seems to be working and sounds great. The hair trigger issue is less pronounced as I can turn the dial in the range of 7-10am though my comfortable listening volume is around 8-9am.

I suppose this is expected behavior or do I need to put 75k ohms in series to the input lugs?

[Paul Birkeland]

The 75K resistors will increase the effect of the padding and put the input impedance back at 100K, but what you have now is also a valid approach.