R-AP.SCI wrote on Tue, 21 September 2010 00:00 |
A great example of this type of biasing is the U67, after one considers other aspects of its design (the feedback/forward & e.q.) to get the most out of the tube its plate voltage is run higher than the "average" 120v. I am not sure of any other mic that uses an EF86 at around 200v. |
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. |
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... |
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? |
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. |
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 |
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. |
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. |
dbock wrote on Tue, 28 September 2010 17:04 | ||
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. |
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.) |
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. |
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. |