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Author Topic: Self Bias vs. Fixed Bias (in the context of microphone design)  (Read 16937 times)

maarvold

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Re: Self Bias vs. Fixed Bias (in the context of microphone design)
« Reply #15 on: September 22, 2010, 01:21:38 pm »

Klaus Heyne wrote on Wed, 22 September 2010 09:43

...That's another reason why modifications of U47 mics to run with EF14 tubes are questionable: they often include self-biasing, due to feeding a separate heater voltage to the mic. The tube's sound is already a bit anaemic, even with fixed biasing.



According to Merriam-Webster's On Line Dictionary:

anemic (which is how I would spell it, although anaemic is also acceptable):

a : lacking force, vitality, or spirit <an anemic rendition of the song> <anemic efforts at enforcement>
b : lacking interest or savor : insipid <anemic wines>

I would never think of the word "anemic" when describing my 2 U47-inspired mics, both of which utilize EF14's.  1-2 times/week, for the past 6 weeks, I have spent several hours listening to a big, baritone voice cutting Sinatra-like vocals at Capitol on a mic that I would describe as a classic U47-type sound: maybe more so than any 47 I've come across in LA, probably close to a dozen at this point (although the actual Capitol mic is a K47-based U48).  In the midst of these sessions, I did a date with another big, baritone voice using my mic.  Was it exactly like the Capitol U48?  No.  Was it very close in overall character?  Yes.  Did it have a similarly satisfying and powerful lower midrange?  No question.  Was it lacking in "vitality" compared to the Capitol U48.  Nope.  I felt like there might be a slightly better 'see through' quality in the 1 kHz-3kHz area on the U48.  But my mic has a Thiersch PVC M7 instead of a K47 and this might be part of it.  Or maybe the coupling capacitor... or the different mic pre.  If anything I would say that my mic has a touch more character than the U48 and, if I had my 'druthers', I would sacrifice a bit of the character for a bit more of that 'see through' quality.  But lacking in force or vitality or interest are factors that don't equate in my experience.  

EDIT: According to Merriam-Webster's On Line Dictionary:

vitality:
1
a : the peculiarity distinguishing the living from the nonliving

2
b : lively and animated character

1a could maybe be equated with the see-through aspect I described that is enhanced in the U48 (compared to my mic)

2b is definitely not lacking in my mic

For the record, my engineer friend that has been doing the Capitol/U48 dates came to my date and, unsolicited, said he liked my mic a lot.  This is not something I expected to hear him say because he doesn't profess to liking much of anything generally.  I realize that this might violate the "third party" rule; please delete if you think it does.  
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Michael Aarvold
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Klaus Heyne

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Re: Self Bias vs. Fixed Bias (in the context of microphone design)
« Reply #16 on: September 22, 2010, 06:38:20 pm »

Thanks for sharing. I would not venture an argument to contradict your impressions. I am happy for you!

I take my sound impressions and "anaemic" description from U47 specimens where I personally witnessed the "before" (VF14) and "after" (EF14) and the "before" (fixed bias VF14) and "after" (cathode bias of the same mic).

There are strong, characterful mics out there of any stripes and you obviously ended up with one (though I wish you could hear your mic with a genuine M7!)

Best,
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Klaus Heyne
German Masterworks
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Marik

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Re: Self Bias vs. Fixed Bias (in the context of microphone design)
« Reply #17 on: September 23, 2010, 12:03:20 am »

Klaus Heyne wrote on Tue, 21 September 2010 18:59

I hear the phase aberration of bypass capacitors that were not chosen carefully through hearing experimentation...




Klaus,

If I remember correctly, a few years ago I already asked this question, but did not get an answer, so if I may, can I ask again--how do you hear it? How do you know that what you hear is the phase problem or any kind of other anomalies and what makes you feel to attribute the anomalies you hear specifically to phase ones?

Best, M
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Mark Fouxman
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MDM,

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Re: Self Bias vs. Fixed Bias (in the context of microphone design)
« Reply #18 on: September 23, 2010, 07:40:36 am »

I prefer a grounded cathode and fixed bias myself.

self-biasing is more of a plug-n-play thing, fixed bias has to be adjusted for each tube, ideally.

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Klaus Heyne

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Re: Self Bias vs. Fixed Bias (in the context of microphone design)
« Reply #19 on: September 23, 2010, 02:38:21 pm »

Quote:

Klaus,

If I remember correctly, a few years ago I already asked this question, but did not get an answer, so if I may, can I ask again--how do you hear it? How do you know that what you hear is the phase problem or any kind of other anomalies and what makes you feel to attribute the anomalies you hear specifically to phase ones?

If I limit the variables in a comparison test to one, the changes I hear are more than likely attributable to the varied component.

In this case, I examine, let's say, the contribution to phase smear of the stock bypass capacitor in a self-biased mic set up.
When I double the cap's capacitance, or when I change out its material while retaining the same value, and if I now hear immediately an improvement (or deterioration) in sound, I can confidently assume, it's the change I effected that is responsible for the new sound, i.e. lower of higher phase aberration.

If I now repeat the same test dozens of times, and I always get the same trend-line, more or less, I can even go further in my assumptions and eliminate other possible (but hidden) variables, like the age and condition of the original capacitor, as a factor in my judgement.
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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 #20 on: September 25, 2010, 03:18:29 pm »

Quote:

As to "Testing a mic on high impact sources like drums can reveal the difference between self & fixed bias mic amps", one has to be extremely cautious to make sure that the "all other parameters being equal" rule is enforced.
As an example, comparing an M49 (w/ fixed-bias AC701) and an M49c (self-bias AC701) is not a proper comparison, since the 49c runs at 60% more quiescent current (0.73mA vs. 0.45), lending it a 4dB advantage.
Since gain null of two channels can be a useful guide for determining differences in sound characteristics of similar devices (practically rendering 4dB difference irrelevant), can I re-state your statement as a conclusion that "any perceived difference in sound quality between an M49b fixed bias (HPF removed) and an M49c is due to the difference in plate current"?

MDM,

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Re: Self Bias vs. Fixed Bias (in the context of microphone design)
« Reply #21 on: September 26, 2010, 10:23:19 am »

let's not forget that bypass caps are far from being ideal, especially in the long-run.

an electrolytic cap has a definite 'sound' to it, due to ESR and other nifty non-linear behaviour.

a more 'ideal' bypass cap is a mix of a very large value film cap which has been 'broken-in' well, say 2-4 uF, and an electrolytic.

the reasoning behind this is that the film cap will work for most of the audio band and the electrolytic will work on bass frequencies, where the film cap can't because it would be too big to fit in a mic.

just placing a small cap in parallel creates some weirdness on the top-end because there is a 'cross-over' point in the audible range.

I suspect that small caps in parallel with electrolytics may also create some problems due to the electrolytic's inductance at high frequency, but that's just a suspicion on my part.
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I must study politics and war that my sons may have liberty to study mathematics and philosophy .. in order to give their children a right to study painting, poetry and music.
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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 #22 on: September 27, 2010, 06:04:35 pm »

Indeed, the gain difference in itself is meaningless, since the signals can be normalised. But the harmonic content is different between a tube running at 0.45mA and one running at 0.73.
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.

Another factor is the variation of quiescent current caused by the non-linearity of the tube. because of the hyperbolic nature of the plate current to grid voltage relationship, the higher the signal, the higher the plate current, and as a consequence, the higher the cathode potential, which in turn tries to reduce current (that is why it's called automatic bias in european litterature).

A tube highly solicited by a durable signal will see its operating point drifting to a lower-current, lower gain region, with a different harmonic balance. This becomes measurable for signals whose p-to-p amplitude is a significant fraction of the bias voltage.
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Klaus Heyne

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Re: Self Bias vs. Fixed Bias (in the context of microphone design)
« Reply #23 on: September 27, 2010, 07:28:20 pm »

I think your post's language is too esoteric for this forum. Please edit it for clarity, using language that can be understood by those not steeped in the matter as deeply as you are.

Thanks.
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Klaus Heyne
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mwurfl

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Re: Self Bias vs. Fixed Bias (in the context of microphone design)
« Reply #24 on: September 27, 2010, 09:57:14 pm »

Yes, I thought I was doing just fine until I got to "A tube highly solicited by a durable signal..."

Mark W
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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 #25 on: September 28, 2010, 07:25:06 am »

mwurfl wrote on Mon, 27 September 2010 20:57

Yes, I thought I was doing just fine until I got to "A tube highly solicited by a durable signal..."

Mark W
OK. I'm glad I haven't lost you at the beginning of my explanation.
So I'll rephrase.
When a self-biased tube stage is submitted to an input signal of relatively large amplitude and a duration of at least several dozens milliseconds, it produces a change in the bias condition of the tube, decreasing the quiescent current, which in turn decreases the gain slightly and moves the operating point to an area where the spectral balance of THD components is different. This phenomenon occurs with a slight delay vs. the incoming signal. This time delay is directly related to the value of the cathode cap vs. cathode resistor.
It is some kind of compression.
In microphone head amps, this would occur only with heavily sustained signals, like guitar/bass amp or Leslie micing (generally drums don't have enough sustain, double bass is not loud enough).
Pushed at the extreme, this phenomenon is the explanation of the particular nature of the overdriven sound of cathode-biased guitar amps examplified by the Vox AC30.
Hope it's clearer now.
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Oliver Archut

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Re: Self Bias vs. Fixed Bias (in the context of microphone design)
« Reply #26 on: September 28, 2010, 10:12:57 am »

I think it is easier just to say that the cathode cap will delay (shift) and reduce (limit) a signal above a certain point (input level). The bigger the cap the more pronounced this effect.

Best regards,
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Oliver Archut
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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 #27 on: September 28, 2010, 01:49:04 pm »

It is certainly easier, but not entirely true.
Cathode bias produces some kind of limiting, yes. The larger the cap, the longer the time it takes to limit and the longer it takes for the mic to return to its normal characteristics after being excited.
In fact an infinite cap would not limit at all (well, only after an infinite time).
To make things clearer, I'm talking of dozens of milliseconds.
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Oliver Archut

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Re: Self Bias vs. Fixed Bias (in the context of microphone design)
« Reply #28 on: September 28, 2010, 03:05:32 pm »

In fact an infinite cap would not limit at all

That might be true in theory, but using something close to infinity a 1F cap will make it actual worse, the time delay is definitely audible and the limiting sounds like breathing.

The big issue is that the theoretic side still has its limitation, it is needed but can not explain everything.

Best regards,
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Oliver Archut
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David Bock

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Re: Self Bias vs. Fixed Bias (in the context of microphone design)
« Reply #29 on: September 28, 2010, 06:04:56 pm »

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.
Quote:

A tube highly solicited by a durable signal will see its operating point drifting to a lower-current, lower gain region, with a different harmonic balance. This becomes measurable for signals whose p-to-p amplitude is a significant fraction of the bias voltage.

Right. So it's not really a phase issue with self bias. Myth busted.
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