Buzzing noise after 270k resistors installed to reduce gain

[Merlocpm]

I have been loving my crack ever since I built it a couple of weeks ago.  With no music on, I can turn the volume pot all the way and not hear any humming for buzzing. 

The only thing is the maximum that I  could turn the volume pot was about the 8 o clock position before things got too loud.  I decided to install a couple of 270k resistors on the rca jacks as suggested.  I'm now able to go to go to the 12-1 o clock position.  However, I am getting some faint humming/buzzing now when I turn it up with no music playing.  Is this considered normal?

 

[Paul Birkeland]

Is this noise present when your source is connected?  Can you describe the 270K resistors you used?

 

[Merlocpm]

Yes the noise is present with the source connected.  I don't hear the humming/buzzing until I turn the volume pot at least to the 11:00-12:00 position.  These are ceramic resistors that I'm using.

I did not have any humming/buzzing prior to installing these resistors even with the volume pot turned up fully with no music playing

 

[Paul Joppa]

It seems likely that the increased impedance is making the input more sensitive to electric fields.

You can make an L-pad at lower impedance this way: Change the 270K to 75K (actually, anything 50K to 100K is close enough). Add a 33K resistor in parallel with the level control (end to end, i.e. not touching the center contact). This is an experiment to determine whether it's the impedance effect, or something else.

 

[Merlocpm]

Hi Paul,

Just to clarify...I understand about changing out the 270k resistor to a 75k ohm resistor.  I'm not sure where to connect the 33k resistor though?

 

[Paul Birkeland]

I'm not sure where to connect the 33k resistor though?

You can install them across the outside pair of lugs on each level of the volume pot, or you can solder them between the ground tab on each RCA jack and the junction of your 270K resistor to the wire that feeds the volume pot. 

-----signal hot ----->
                              |
                              \  270K
                              /
                              |--------------------to volume pot input-------------->
                              / 33K
                              \
                              |
__signal ground__ |--------------------to volume pot ground----------->

 

[Merlocpm]

I just left the 270k resistor in place and added the 33k resistors as suggested.  The humming/buzzing level decreased.  I have to turn the volume pot to the  the 1:00 to 2:00 position now to start hearing it.  Resistors are carbon film 1/4w.

 

[Doc B]

The clock position is not particularly relevant except that the pot may track better at higher level settings. What matters is how loud the music is when played with the knob set at the onset of the noise. If the music blasts your head off at the level setting where you hear noise, then noise probably won't be audible at a normal listening level. On the other hand if the noise is there at a level setting at typical listening level (regardless of what that clock setting may be), there is definitely room for improvement.

Without knowing the source's nominal output level and the headphone sensitivity it's difficult to make a more certain suggestion about where to address the problem. But if you haven't got the noise beaten down to a satisfactory level it might be that padding the output of the source rather than the input of the Crack would allow less interference into the system and allow the Crack volume control to be turned higher with less noise.

 

[Merlocpm]

Thanks for all the helpful advice.  I decided to take out the resistors and am back to the no humming/buzzing with the volume pot at max. volume.  I am going to be doing the speedball update and then look at attenuating from the source output