2. amps-preamps
  3. 192478-110266

Preamp issue

I am having a problem with my preamp would like some feedback/guidance from the members of this forum.

For the past two years my system has been a WIIM Pro to PSAudio Stellar Gain DAC to Backert Labs Rhumba 1.0. preamp to Classe CT2300 amp to Harbeth SHL5 speakers, 

System worked fine and I liked the sound. But, then you get the itch. . .

I was interested in trying to an r2r DAC so I purchased a used (one year old) Denafrips Pontius II 12th DAC. Today I swapped out the PSAudio DAC, fired up the system and listened for about 5-10 minutes, then there was a POP in the speakers and the Rhumba preamp went into its mute mode. I shut everything off and restarted the components now have low frequency(100Hz?), low volume, vibrating sound coming out of both speakers. I have swapped out pieces from a second system an definitely isolated the problem to the Rhumba preamp. 

I am going to have the pream repaired. But since the Rhumba preamp failed shortly after swapping in the Denafrips DAC-

Could the Denafrips DAC have caused the preamp to fail? How? Was it just coincidence? Should I have the Denafrips tested as well or just return it and find a different r2r unit? Your input will be appreciated. Thank you in advance.

Happy Holidays

128x1282tuby
pdspiegel
22 posts
 

If you have a voltmeter here’s a test you can do:

Check the Denafrips for DC on the output.  If you are using the single ended RCA then measure the voltage from the center to the outer shell.  For balanced XLR measure pin 2 to 1 and 3 to 1.  Check both channels.  All should be near zero.  If you find any significant voltage then the Denafrips could be the problem.

Most tube preamps have no DC blocking capacitor and connect the input directly to the triode grid that is grounded through a resistor.  A significant voltage messes with the tube bias (idle current).  On their website Backert Labs describes using an automatic bias circuit that might be affected by an input DC voltage offset (deviance from zero).

I’m just speculating, of course.

2tuby's avatar
2tuby OP
35 posts
 

Pdspieel,

Thanks for the reply. Should this test be done with the DAC seeing an input signal or no input signal?  I just tested the voltage with no signal and both RCA’s have zero voltage. 

elliottbnewcombjr
5,518 posts
 

I can fix physical, but not electrical things, so I would get rid of the DAC the best way possible, i.e. back to the source if you can. no reason to take a chance IME.

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