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Author Topic: Self Bias vs. Fixed Bias (in the context of microphone design)  (Read 16683 times)

Geoff Emerick de Fake

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Re: Self Bias vs. Fixed Bias (in the context of microphone design)
« Reply #30 on: September 28, 2010, 06:24:53 pm »

dbock wrote on Tue, 28 September 2010 17:04

Quote:

You cannot re-state my statement the way you've done it, because I never said that the difference is due only to the change in quiescent current, although it is certainly the paramount parameter.

I accept this rejection. Please explain why then you chose dissimilar plate current devices as your example to illustrate the difference between fixed and self bias.
It's not my choice! Neumann, in their infinite wisdom, chose to alter the operating point between the M49 and the M49C (and a different AC701).
To my knowledge, nobody has tested side-by-side a fixed bias and a self-bias M49, all other parameters being equal.  
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Klaus Heyne

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Re: Self Bias vs. Fixed Bias (in the context of microphone design)
« Reply #31 on: September 29, 2010, 01:43:52 am »

Yes, I have, and the audible results are similar to the other self-and fixed bias tube mic differences where I have tested for just that one variable (see my  earlier comments.)
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Klaus Heyne
German Masterworks
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Geoff Emerick de Fake

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Re: Self Bias vs. Fixed Bias (in the context of microphone design)
« Reply #32 on: September 29, 2010, 01:41:23 pm »

Klaus Heyne wrote on Wed, 29 September 2010 00:43

Yes, I have, and the audible results are similar to the other self-and fixed bias tube mic differences where I have tested for just that one variable (see my  earlier comments.)

You mean you had modified them for same tube, same current?
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Klaus Heyne

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Re: Self Bias vs. Fixed Bias (in the context of microphone design)
« Reply #33 on: September 29, 2010, 03:01:56 pm »

Yes,
I have, because, aside of the different biasing, I never understood why different levels of current would be run with that tube in the same mic.

I still don't understand that part, but I understand enough of the difference in sound between fixed and self that I modify according to the best sonic outcome, rather than a 'one size fits all' approach.
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Klaus Heyne
German Masterworks
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David Bock

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Re: Self Bias vs. Fixed Bias (in the context of microphone design)
« Reply #34 on: September 29, 2010, 03:53:06 pm »

I found this in the "ground rules" section:
Quote:

* Please use your real name and professional affiliation, if different from your screen name. I think it makes for a better, more honest forum and audio community in the long run. Anonymous posts remove responsibility from the poster. I'd rather talk to people who have the courage to take a risk and trust that their courage will not be violated by the forum's participants.
and wondered, who is this Geoff Emerick de Fake who has, thankfully, added much valuable information to this thread. No sarcasm intended!

Klaus Heyne

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Re: Self Bias vs. Fixed Bias (in the context of microphone design)
« Reply #35 on: September 29, 2010, 04:41:25 pm »

David,
I agree with you in principle. But I give people a little leeway, as long as they at least use their real names in the forum's "profile" section.

In this case, Mr. de Fake's real name is Jean Luc Moncel.

I agree, it would be nice to get a bit more background about him.
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Klaus Heyne
German Masterworks
www.GermanMasterworks.com

MDM,

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Re: Self Bias vs. Fixed Bias (in the context of microphone design)
« Reply #36 on: September 29, 2010, 08:11:49 pm »

Asymmetric waveforms, especially if clipping will also change the bias voltage in a self-biasing amp.. I think.

I don't like self bias on most circuits, and try to avoid it, although sometimes it's the only solution.
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J.J. Blair

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Re: Self Bias vs. Fixed Bias (in the context of microphone design)
« Reply #37 on: October 01, 2010, 11:43:05 pm »

Best thread in a long time.  I appreciate the wealth of knowledge being shared.  Much thanks to all involved.
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R-AP.SCI

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Re: Self Bias vs. Fixed Bias (in the context of microphone design)
« Reply #38 on: October 04, 2010, 12:29:01 am »

All interesting points gentlemen (and thanks for stopping by Mr.Blair).

    I will be experimenting with different cap configurations, makes and values in my circuit and if I can accurately measure the results I will be posting them here. In the end I will simply have to choose what I think sounds best.

    Thank you all again for your insights, thoughts and opinions.
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volki

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Re: Self Bias vs. Fixed Bias (in the context of microphone design)
« Reply #39 on: October 06, 2010, 11:28:20 am »

J.J. Blair wrote on Sat, 02 October 2010 05:43

Best thread in a long time.  I appreciate the wealth of knowledge being shared.  Much thanks to all involved.

That's exactly what I was just going to write...

Thanks everyone!  Thumbs Up  Thumbs Up  Thumbs Up

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Volker Meitz

djosephson

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Re: Self Bias vs. Fixed Bias (in the context of microphone design)
« Reply #40 on: October 10, 2010, 01:29:08 am »

Sorry to join this discussion late.  Indeed we have no tube mics in production but I have studied them extensively, built more than a few and thought several times about doing a tube model. Also, this question is certainly not unique to tube mics. Our standard FET first stage is precisely controlled fixed bias at the gate with the drain driven by a current source, so you can say that I'm a fan of "fixed bias."

But as Oliver mentioned, there aren't any standards for what we call things in microphone electronics, so describing the differences we're talking about as "fixed" versus "self" bias is a bit of a misnomer. Things are different than with voltage amplifier stages; the relation between the first active stage and the capsule is quite complex as the impedance the capsule sees changes according to the current drawn by the grid or gate of the first stage. This is in addition to anything that would be caused e. g. by a cathode bypass capacitor.

My design approach to date has been to get rid of as much "sound" as possible from the electronics. (Thank you Paul Klipsch, good sound is the absence of bad sound.) If I can prove to myself that the signal voltage developed across the polarize resistor is unaffected by the first amplifier stage, I'm happy (and that's hard, when it drives you to operate the input impedance in the gigohms). But, that is by no means the only possible approach. Circuits whose characteristics shift with signal level (I call this signal-history distortion) make a certain sound and this might be entirely desirable in some instances. Many historically valued mics exhibit this characteristic. I try to focus on what aspect of the transfer characteristic is judged to be advantageous (who says you can't build a magic limiter/compressor into the mic?) rather than trying to associate it with a given circuit topology. I would recommend looking at what the circuit is doing, rather than trying to hang it on presence or absence of some resistance and impedance in series with the cathode. In this case I think the real question is whether and how the grid voltage changes with signal level.
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David Josephson -- Josephson Engineering Inc -- www.josephson.com

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