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    Performance Evaluation of VTA ST-70


    Posts : 8
    Join date : 2018-08-28
    Location : Philadelphia, PA

    Performance Evaluation of VTA ST-70

    Post by bennytehcat on Sat Sep 15, 2018 9:58 pm

    Howdy folks!  Who's ready for today's lab assignment?  You got pants?  Closed-toe shoes?  Lab notebooks?  Fantastic...let's get started!

    Objective: Evaluate the performance of my new VTA ST-70 and kill a Caturday.

    (1) VTA ST-70 (See here for my interpretation)
    (1) Fluke 115 Multimeter
    (1) Atten ATF20B Digital Function Generator
    (1) Agilent MSO-X 2014A Digital Oscilloscope
    (2) Oscilloscope Probes: 10:1, 150 Mhz, 10M Ohm, 15 pF
    (2) 3' BNC Cables
    (1) BNC Tee
    (1) Female BNC to Male RCA
    (1) Female RCA Mono to Male RCA Stereo
    (2) UX-Cell 8 Ohm 100 Watt Dummy Load Resistors
    (4) Strong Beers
    (1-2) Cats

    Experimental Procedure:
    To start, a beer was opened.  After a short period the 8 ohm reistors were installed on the corresponding output terminals of the amp.  The output of the function generator was connected to a BNC tee which split the signal into two paths.  The first path was converted to the RCA connection and sent to the amplifiers input.  The second path was sent to ch1 of the scope to record the original input signal.  Two scope probes were attached, one to either output channel of the amplifier which then fed into ch2 and ch3 of the scope.  All equipment is powered up and 30 minutes are allowed to ellapse.  Time for a second beer.

    The first step in the diagnostic is checking the bias across all tubes, using the same naming scheme as the VTA assembly guide.  

    V2: 423 mV
    V3: 421 mV
    V6: 423 mV
    V7: 425 mV

    Data was then recorded for the following inputs at 200 mV:

    A) Sine at [10,20,100,1k,5k,10k,15k,20k,30k] Hz
    B) Square at [500,1k,10k] Hz
    C) Step at [500,1k,10k] Hz

    The oscilloscope recorded the following:

    Ch1: Original Signal
    Ch2: Left Amp Output
    Ch3: Right Amp Output
    F1: Ch2-Ch3

    As a last step, the bias was checked once again to ensure nothing changed:

    V2: 416 mV (Diff: -7 mV)
    V3: 416 mV (Diff: -5 mV)
    V6: 417 mV (Diff: -6 mV)
    V7: 418 mV (Diff: -7 mV)

    All signal traces were read into MATLAB for some simple math and plotting.  This is probably a good time to open the third beer.

    I was curious about investigating 3 main things:

    1) Gain
    2) Signal Reproduction
    2) Difference between Left and Right channel (ch2-ch3 on the scope)

    Gain was measured as V(ch3)/V(ch1).  In the sine signal test, the average gain across the frequency range was 36.6 +/- 0.2 (V/V).  Both channels were in sync in terms of power, but that slipped when the 20 kHz test ran. The square and step test also looked pretty good at 500 and 1k Hz, but they also slipped around 10 kHz.

    If anyone has any suggestions for additional testing, please let me know.

    As for beer #4... Well, now that I'm done that one will be for the next listening session.  We're going with Nicola Conte's Rituals on the TD160.

    Note, all the plots below have the original signal scaled by a factor of 10 to make it visible with the amplified signal.

      Current date/time is Mon Sep 24, 2018 4:32 am