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Author Topic: U87 transistor inquiry  (Read 28472 times)

Offline Jim Williams

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Re: U87 transistor inquiry
« Reply #15 on: October 11, 2010, 10:35:58 am »
Readers should know a scope will not show THD levels below about 3%, the limits of the trace errors. 3% is very high and should be trimmed down to a lower level.

As David says, a distortion meter is best for this. An analyzer like Audio Precision will show THD vs amplitude and will allow you to fine tune the THD. You will see the distortion change with levels so you can optimize it for high or low SPL's.

Then run a THD vs frequency sweep to see the effects at 20 and 20k hz. Small tweaks may allow you to minimize THD at one end or the other.

One can also use that rig to test jfets for noise. Then you can place various loads on the mic and test for loading effects.

Test gear like that eliminates any guessing and the inherent limitations of the ear to detect small distortion effects. It is necessary if you want to "nail it".
Jim Williams
Audio Upgrades

Offline MDM,

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Re: U87 transistor inquiry
« Reply #16 on: October 12, 2010, 10:00:10 am »
I once used a spectrum analyzer on a DAW input, feeding a sine-wave.

As the transistor went in and out of the linear region harmonics would spring-up in the high frequencies.

when the harmonics are at their lowest then the transistor is probably biased reasonably well, I imagine.
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.
John Adams (1735-1826) 2nd President, United States

Offline Klaus Heyne

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Re: U87 transistor inquiry
« Reply #17 on: October 12, 2010, 11:48:14 am »
Indeed, the "trough" between the two types of distortions is what I have been aiming for when I bias these fets.

Unlike some others who chimed in, this old ludite still uses his ears to find that spot, but that requires a very quiet environment, as the spot is easiest to find at very low listening levels of the sine wave.
Klaus Heyne
German Masterworks

Offline Galil

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Re: U87 transistor inquiry
« Reply #18 on: October 13, 2010, 11:11:24 am »
Would it be proper to assume that the less expensive condenser microphones on the market have not had their FETs tuned in an optimal fashion, as described in this thread?

Stephen Galiley

Offline permeke

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Re: U87 transistor inquiry
« Reply #19 on: October 14, 2010, 12:33:55 pm »
Thanks again for the excellent explanation.

Back to the resistors: Is t just a matter of bridging the resistors or should something be rewired at the transformer as well ?

Kind regards-