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Author Topic: Microphonic Stanford Omega Mic  (Read 663 times)

Michael O.

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Microphonic Stanford Omega Mic
« on: December 07, 2016, 11:46:11 pm »

Hi there,

I've got a Stanford Omega nuvistor tube-based microphone that is exhibiting unwanted microphonic behavior. Otherwise, it operates as intended and has surprisingly low self-noise, but it rings out at a high frequency sometimes. It produces this ringing even when decoupled from all floor borne vibration, and it is excitable by sources as quiet as acoustic guitar (so it really sings when placed as a drum kit overhead ;)).

I understand nuvistors are particularly prone to going microphonic, so I'm assuming and hoping the tube is the culprit. So, is there a best replacement nuvistor for this mic? And, assuming the tube tests properly on a hicock, is the only way to test it for microphonic behavior within the circuit/microphone itself? Can one physically dampen a nuvistor? Or maybe the problem lies with the capsule?

Additionally, the power supply is labeled high impedance, and is assumedly unbalanced at the out. Any insights into what ratio transformer to wire in (I think UTC A or even O series will work), and whether or not to simply wire it in series with the current output circuitry?

Thank all of you for this and for your general collective brilliance

david royer

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Re: Microphonic Stanford Omega Mic
« Reply #1 on: December 23, 2016, 08:52:23 pm »

I would suggest using a subminiature glass tube, like a 5703 or a 5840 wired as a triode.
You are probably having problems with microphonics, which Nuvistors are notorious for.
I have heard that the Stanford Omega was originally designed around a triode connected 5840
but that they switched to the 6CW4 Nuvistor when it became available, since it was easier to find and less expensive, since RCA made millions of 6CW4's for the tuners of TV sets and FM radios up into the seventies.

You will need to make a slight change in the power supply to accomodate
the greater heater current if you go to a subminiature glass tube. If in doubt, consult with a technician. The Nuvistor needs 6 volts at 130 mA for its heater and the various subminiature glass tubes that would be possible substitutes would need 6 volts at either 150mA or 200 mA.

Note that the circuit is a cathode follower so it is already fairly low impedance, even without an output transformer.

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Michael O.

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Re: Microphonic Stanford Omega Mic
« Reply #2 on: December 28, 2016, 02:14:53 am »

Mr. Royer, that info is more thorough and informative than I could have hoped, and it will help me immensely; thank you so much! Converting to a superior tube is a great idea, and that makes sense about the impedance and jibes with my experience with matching it to preamps.

Also, thanks for creating some of the only truly classic production microphones to come about in my lifetime!
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