Altec M11 mic system impedance/termination (21B with "coke bottle" 150 base)

[emrr]

Interesting!

Those are the values listed under "input impedance" in factory documentation. I suppose this is a classic case of "knowing just enough to be dangerous," because I'm clearly missing something.

At any rate, it sounds good now!

Cheers

Read a whole lot more here.  That is 'intended source impedance' in most cases.  Read the BA-31 manual....actually you should wire it for 150 instead of 600.....it's still bridging.  The API is 1500....still bridgingish enough.  REDD47 document floating around says 2K loading, 200 source Z.

Thing about it another way again....the  BA-31 would  be close enough to tame a resonance if those were true impedances, and it doesn't right? 

Don't take 250 literally.  The 330 you picked is a good choice, but maybe it could also be higher.  470 might do it, far closer to 250 than 1500 is.  You always want same or higher, never lower. 

 

[soapfoot]

Thanks!

The short-term important thing is that the microphone sounds excellent with all of our preamps now.

 

[thecr4ne]

Would a 4665 work as a substitute? (also backwards as in the Altec 525A "lipstick" psu)

I've got a pair of coke bottle's on the way, sans power supplies. I DO have some 525As, so I'm looking into:
A- whether a 525A could be used either as is or modified for a Coke Bottle
B- if not, can I at least use the 4665 as a substitute for the TBB-103 in a DIY built PSU for the coke bottles?

 

[soapfoot]

Remember that the output transformer for these mics live in the power supply. That’s possible because the tube is configured as a cathode follower

 

[dale116dot7]

The 4665 or 15095 will work fine with the Coke Bottle. The UTC O-2, O-8, and O-11 also work here. Some are wired backwards depending on the impedances. I’ve always built separate supplies for Coke Bottle mics, they are a bit different with the bootstrapped suppressor grid.