2. amps-preamps
  3. 191157-18279

DC Offset Issue in Don Sachs Preamp

I have a nice Don Sachs preamp.  It's making an incredibly loud popping sound on power off.  Obviously, it's always good to power preamps first then amps and reverse the order for power down. But, I'm still concerned about potential damage to my loudspeakers. 

Would this be do to a leaky coupling cap at the output stage?  Or not enough uF/capacitance to take out all the DC? 

I could email Don but I bought the amp used and I got the sense he may have retired. 

 

Any insight is very welcome. Thank you.

128x128jbhiller
jbhiller's avatar
jbhiller OP
1,403 posts
 

Amp is used (3-4 years old) but new to me.  

Today, I swapped out the CV181Zs for classic 6sn7s, and things may be better. Would that make sense? Doesn't the CV181Z have higher heater current value than 6SN7s?  

invalid
1,186 posts
 

The only cv181 tubes that have a higher heater current are the nos tubes, the newer production cv181 tubes are identical to the 6sn7.

lynn_olson's avatar
lynn_olson
328 posts
 

Don is still with us, contact him by email. My guess is the load resistor on the output of the cap has gone open, but I wouldn’t expect both channels to do that at the same time. Maybe something has gone wrong with the regulator. It would help to know what power amp you are using and how efficient your speakers are. (Khorns? Magnepans?)

The only way to be sure is look at the output with an oscilloscope and see how big that pulse is. You can get handheld Chinese models for about $200.

jbhiller's avatar
jbhiller OP
1,403 posts
 

Thank you! 
 

I’m running 100dB sensitive horns with larger woofers.  I’m currently playing the Sachs into a Marantz Ruby set to Power Amp mode.  This hybrid setup sounds sublime. 

gs5556
1,234 posts
 

Definitely call Don. But before you do, disconnect all inputs into the preamp and cycle it on and off to rule out the signal source as the problem. Without a schematic it is difficult to guess what the problem is. 

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