Low frequency content problem with Royer-MXL mod.

Hi all,

I finished the standard MXL2001 Royer tube mod, and now I've got a very nice mic with a slight problem. 

If I use the mic as a drum overhead, when I hit the snare drum, there's significant extremely low frequency content in the signal (<20Hz).  Otherwise the mic sounds great with voice, guitar, etc. 

I saw one other post on a forum where someone else had the same problem, but an answer was never found (or never provided if one was found).  I'm absolutely positive that the gain structure of the entire signal path is ok.  This is not clipping.  It seems as if it only happens on fast transients like a snare drum hit. 

Using the high-pass filter on my mic preamp fixes the issue, but there's something wrong with the mic and I'd like to fix it.

Any idea what might cause something like this?

Thank you very much for your help!
-MattM

That sounds like mechanical transmission through the floor/mic stand/shock mount, rather than an electrical problem. Can you try a different shock mount? Or get a heavy paving slab and put that under the mic stand - see if things change any.

Thank you, I really appreciate the reply! 

I'm using a very high quality shockmount, and I've never had this issue with any other mics.  Also, the vacuum tube is physically isolated from the body of the mic (it's "floating" inside the mic body via the tube pins).

Do you think a microphonic tube might exhibit these symptoms?  Perhaps I'll try another tube...  I suppose I'll tap on the tube tonight and see if it's microphonic.  If the tube was microphonic, perhaps just the air pressure might trigger this LF oscillation?  I really don't feel that the transmission route would be through the stand, because the stand (extra large K&M) and the drums are sitting on thick carpet, and I'm using the high-quality elastic shockmount (Neumann-style).

Let's assume that it were an electrical issue... any idea of what might electrically cause these symptoms?

Thanks again!
-MattM