All About the Reflex Enclosure - Part 2 - P.G.A.H. Voigt - This is part 2 of a series by the legendary Voigt. http://www.itishifi.com
Friday, March 11, 2011
Optimal Source Impedance....
For years now I have tried to explain (preach? scream?) that tube amplifiers do not have "slow" or "flabby" bass. This common misconception originated when solid state amplifiers came onto the market and the speakers boxes began to be tuned to the lower output impedance of the new amplifiers. If a box is optimally tuned for a solid state amplifier's one ohm source impedance, then it will be severely under-damped (woolly, slow, flabby, etc) with a zero feedback tube amp of a few ohms output impedance.
But what happens in the upper regions? Do compression drivers need to be driven from an optimum source impedance or is lower always better? After all, lower output impedance means a higher damping factor. We have all been told that damping factor must be as high as possible. Well, I am saying that just is not true... please prove me wrong! I was measuring anyway, so I took these plots:
Impedance plot of a JBL 2435 on LC600 horn:
I know that the mighty Bell Labs actually specified a driving impedance for their Fletcher system. It basically was the root mean square of the peak and valley of the impedance plot. They published that this driving impedance was required to meet their frequency response specification. We do not quite meet that formula here, but look at the difference in the region from 1kc to 2kc when the driver is driven directly, through a four ohm resistor, and through a twenty ohm resistor.... and this region from 1kc to 2kc is precisely where the speaker impedance peaks, dips, peaks... driving impedance will change frequency response.. that we already knew...
Frequency response plot of JBL 2435 on LC600 horn - ignore everything above 10kHz, new computer and mic calibration has not been corrected.. sorry...
This is a tight zoom on an impulse response of the JBL2435 driven directly from a solid state amp, through a four ohm resistor, and through a 20 ohm resistor... I did not move the speaker for any of these tests... just clipped in a resistor on the amplifier end...
oh... I know... I should run a square wave through there... I wonder if my measurement gear is up to it... stay tuned!
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