Friday, November 25, 2016

Carver TFM amplifiers

I have my last chance to measure the Carver TFM-224 amplifier that I have been borrowing.

Just how does it sound like a tube amplifier?  (In my sighted testing, it sounded more like the Krell than a Parasound HCA-1500A.  In DBX testing, I couldn't reliably tell the difference.  Anyway, I believe Bob Carver was doing some trick here that I might be able to reliably tell in DBT eventually.)

Well, for one thing it appears to have rather low damping factor.  I'm seeing close to 9 across the audio spectrum.  Tube amplifiers often have a damping factor of about 8, and sometimes less.

Meanwhile, I'm not seeing significant variation in output vs frequency (and, actually, my I didn't re-measure the input, it's a long story but the impedance of the input matters a lot, and I chose the "high z" output mode of my Keithley 2015 analyzer, since the amplifier input is fairly high z.  So, you can't necessarily even believe the measured rolloff  (around 0.1dB at 20kHz, about 0.3dB at 20Hz, relative to 1kHz) since those could be generator errors, to some degree, but the generator errors would not likely be making the response look better.

BTW, Damping Factor is surprisingly easy to calculate, measure output voltage under load and then unloaded, and then: Df = Vl / (Vu - Vl).

8 ohms: 2.6858v
unl.: 2.9806v
df: 9.1

8 ohms: 2.7307v
unl.: 3.0343v
df: 9.0

8ohms: 2.6608v
unl.: 2.9530v
df: 9.1

The damping factor is remarkably low for a transistor amplifier, and it is remarkably constant across the audio spectrum for any amplifier.  Usually amplifiers have lower damping factor at 20kHz, not slightly more.  But the low and constant damping factor is not the real limits of the amplifying circuitry, it is the dialed in characteristic, "fake" damping factor if you will, so it can be as constant as anything so long as it doesn't go higher than the "actual" damping factor the amplifying circuitry is capable of.

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