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Old 31st December 2012, 10:56 PM   #11
TheGimp is online now TheGimp  United States
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All of my tests are at 2.8Vrms (8.24Vp-p actually) as that is the largest signal I can feed into my sound card without clipping. I try to keep the test signal between -.25dB and -.5dB if possible.

I've seen several tubes with 0.025% thd under these conditions.

I plan on reconfiguring the socket and testing some EF86s as well to see how they compare.
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Old 31st December 2012, 10:56 PM   #12
euro21 is offline euro21  Hungary
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Happy New Year folks!

My amp: D3a, green LED bias, 10mA cascode CCS as anode load, capacitor coupling to FET source follower (5mA), direct coupling to 300B, 5K:8 OPT.

At A2 mode 8W output (A1 limit about 6W power) CCS out is 95V RMS, 0.2443% THD, source follower output (300B grid) also 95V, 0.2430% THD.
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Old 1st January 2013, 11:10 AM   #13
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Quote:
The title to this thread is because I found that for every current source I tried, and for every FET I tried, Source Follower induced distortion was inversely related to current.
Hey,
One important parameter is missing: Load of the SF! It would be natural that distortion lessens with more current if load is lowZ. Have you calculated needed drive current??
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Old 1st January 2013, 01:57 PM   #14
TheGimp is online now TheGimp  United States
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The load is an M-Audio 192 sound card, with a specified input impedance of 10K.

4V peak into 10K is 0.4mA peak. Rule of five says over 2mA should be good. Maybe just not good enough?

Last edited by TheGimp; 1st January 2013 at 02:20 PM.
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Old 1st January 2013, 03:17 PM   #15
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The rule of 5 was probably written when -30 db was considered good. -70 couldn't be measured then. A follower can source more current than it can sink, and the sink current is controlled by the source resistance. The lower the resistance, the lower the asymmetry. This is why the second harmonic is affected most by current. Yes, the load impedance and capacitance matters because of the asymmetric output impedance of the follower.
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Old 1st January 2013, 03:44 PM   #16
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With MOS devices, the "Threshold Region" of low drain current is the worst-performing region.

Diodes (the new Zetex owner) don't publish the curves any more, so here is a grab from the old data book, illustrating the threshold region of the ZVN0545.

At low current the Id vs Vgs characteristic is highly curved, compared to higher-currents.

In effect, for a source-follower, this means that the gm is changing as the load current increases (eg due to capacitance charging).

The output drive impedance for the FET is 1/gm, so the distortion arising from the load is also changing as the current increases.

For a follower, the small-signal output voltage is related to gm and the load RL by:

Vout/Vin = RL/(RL + (1/gm))

so the nonlinear component of gm is sure to cause distortion.


gm for the ZVN0545A is about 100ms at 100mA - and a great deal lower at 3mA.

The bipolar transistor performs much better at low current, but the base current brings problems of its own.

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Old 1st January 2013, 06:06 PM   #17
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Rod hit the nail squarely. Turn MOSFETs on hard, for good performance. That 3 mA. ID is IMO/IME a good value for use with the ZVN0545A.
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Old 2nd January 2013, 02:13 PM   #18
mogliaa is offline mogliaa  United Kingdom
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What will be the best setup to test Pentode distortion? Clearly the approach with a CCS used in a triode is not practical as the gain of the stage will be too high and difficult to measure in practice. Options would be a resistor load but probably better a choke or gyrator?

How have you done this before?
Thanks
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Old 2nd January 2013, 03:45 PM   #19
TheGimp is online now TheGimp  United States
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I plan on using a gyrator as the anode load when I test in pentode mode. I already have one built in to the test-set.

I built a buffer using a LF353. FFT 1 is the low distortion oscillator throught the buffer. FFT 2 is with the SF but no buffer, and FFT 3 is with the SF and buffer.

The buffer has reduced the 3rd and 4th harmonics, but second is worse.

I tried two different current sources and a gyrator and the results are the same.

I have also tried different tubes and get the same results.

I've tried the tube driving the buffer without the SF and the results are the same.

I'm at a loss for the high second order harmonic with the SF and buffer.

Out of curiosity, I tried a resistor which resulted in the same operating point and it looked really bad compared to the CS and Gyrator!
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File Type: jpg SF-w-Buffer.JPG (163.0 KB, 197 views)
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Old 2nd January 2013, 03:54 PM   #20
mogliaa is offline mogliaa  United Kingdom
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Steven,
Are these triode tests? What is the output level? 10Vrms?
Regarding the buffer, I think I read somewhere that the op amp could be forced in class A by adding a couple of CCSs and improve the distortion. Not an expert in op amps, so probably someone can shed some light.

The RL (or RA) will impact the distortion. If you are loading the valve with a CCS or Gyrator, then the input impedance of your soundcard/test box is the one driving distortion up. If using a resistor load, then this resistor is in parallel with the soundcard input impedance (as in case of CCS/gyrator) but given that the value is typically below 100k this is what drives distortion even higher.

Coming back to pentode test setup, the gain of the pentode is typically gm*Ra. If we use a gyrator then the impedance at the testing frequency (e.g. 1kHz) has to be not that high (e.g. 20k for a gm of 5mA/v ) otherwise the gain will be too high and clip easily?
Ale
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