phono hum


2012-09-04 4:50 pm
I'm sure someone out there has the solution that has evaded me so far:

I built a perfboard version of John Broskie's Aikido phono stage. It worked out really well and sounds beautiful. BUT...I seem to have a problem when I connect a 'Le Pacific' MC Pre-Pre. Then I get the hum from hell. The stage is perfect (dead quiet) using medium output MC with a 5x step up transformer, medium output MC with no transformer, low output MC with 10x transformer. But it hates the head amp connection. The PrePre is powered from a SLA battery and it works just fine into my Copland CTA301 phono section with the low-output MC. So, on the surface it would appear that the builds are fine. I've tried every combination of grounding and internal wire shields that I can think of and I am assuming there is a compatibility issue where the phono expects independent grounds and the head amp output is not there a fix? One piece of additional info, I have a 4PDT rotary switch in the phono to select between 2 turntables; again it works fine until the head amp is connected...the object of the project is to run the LOMC through the head amp to the phono, and the MOMC through the 5x step up to the phono. Best sounds all around..'cept for the hum.

Anyone have a solution for this?