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Old 8th May 2013, 10:50 AM   #331
Tyimo is offline Tyimo  Hungary
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Quote:
It depends how high is VAS bias current.
If VAS bias is set to 15 mA you would need to set the trimmer to 234 Ohm to get 3,5 Vgg, so at that point output bias would be 600 mA. Yes 1 k trimmer would do OK
Thanks LC!!!
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Old 8th May 2013, 04:16 PM   #332
naf is offline naf  Indonesia
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Poorman's SMPS + VSSA + SSA
No hum, no noise just music
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Old 8th May 2013, 05:05 PM   #333
PMI is offline PMI  United States
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Nice job Naf!
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Old 9th May 2013, 03:15 AM   #334
naf is offline naf  Indonesia
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PMI, thank you.
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Old 9th May 2013, 06:50 AM   #335
PMI is offline PMI  United States
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Default Test Results

So far I am getting good test results w. the first board I have assembled.

Clean sine wave, well behaved when clipping, nice square wave to about 100KHz with no visible ringing. Most of the overshoot visible in some of the tests comes from a corresponding trailing edge overshoot in the source signal. This is especially obvious in the last pic.

Since the basic design was already tested by Lazy Cat and by Shaan, my goal is to run a few simple tests to make sure the first board is behaving well, and that I have not created any problems in layout, or in component selection.

The board as tested has an input filter, 47pF compensation at the drivers, and no Zobel installed. Power supply rails are set to a nominal +/-35V (36V measured), with the help of a variac which feeds the transformer. Total bias current settled at around 145 mA with the two diodes installed, and with no additional parts (no 470R). DC offset was set and checked at 0V before the test, but does drift a few mV with rising temps during the test. Gain was set to 10, basically for my convenience and test equipment.

Signal source: Wavetek 182A
Oscilloscope: Tektronics 2221A 100MHz Analog/Digital Storage Scope
Meter: Fluke 77

Power Supplies: Tektronix PS280 bench supply, and Transformer/rectifier/filter per Shaan's definition of what might be used with a through-hole version of VSSA.

First pic is a shot of the test setup.

#2 and #3 show a 20KHz sine wave, just below and just above clipping. My goal here was to check that the output matches the input up to maximum voltage, and then clips cleanly.

#4 and 5 are a 20KHz and 50KHz square wave, shown in non-storage mode.


#6, 7 and 8 are square waves at 10, 20 and 50KHz in storage mode. 50KHz rise and fall times are starting to deviate from the input signal.

#9 is a rough measurement of the rise time achievable with the component values as installed, which can be used to estimate slew rate and the bandwidth limit of the amplifier.
Attached Images
File Type: jpg Rev1B_BW_test1.jpg (267.2 KB, 413 views)
File Type: jpg Rev1B_sin_max.jpg (126.8 KB, 369 views)
File Type: jpg Rev1B_sin_clip.jpg (131.7 KB, 335 views)
File Type: jpg Rev1B_1V20KN.jpg (174.8 KB, 316 views)
File Type: jpg Rev1B_1V50KN.jpg (163.2 KB, 76 views)
File Type: jpg Rev1B_1V10K.jpg (155.7 KB, 69 views)
File Type: jpg Rev1B_1V20K.jpg (172.0 KB, 66 views)
File Type: jpg Rev1B_1V50K.jpg (173.9 KB, 73 views)
File Type: jpg Rev1B_Slew_rat1.jpg (867.9 KB, 97 views)
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Old 9th May 2013, 08:08 AM   #336
naf is offline naf  Indonesia
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Awesome PMI!
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Old 9th May 2013, 09:46 AM   #337
shaan is offline shaan  India
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Heyh! Nice job. Thanks for uploading the test results.

Most importantly, Are you satisfied with the overall sonics?
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Old 9th May 2013, 12:27 PM   #338
shaan is offline shaan  India
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Hmmm, 90V/uS if I'm not wrong.

PMI, is this slew rate enough for a sharp hit on the snare drum? What do the speakers say?
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Old 9th May 2013, 01:38 PM   #339
shaan is offline shaan  India
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Quote:
Originally Posted by shaan View Post
90V/uS if I'm not wrong.
A second look at the dT makes me think that may be I was wrong indeed. The slew rate is at least 5 times that!
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Old 9th May 2013, 01:55 PM   #340
PMI is offline PMI  United States
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Quote:
Originally Posted by shaan View Post
Hmmm, 90V/uS if I'm not wrong.
Well, I posted the best pic I had, not necessarily the pic of the best test result... This is an old-style storage scope, so capturing the waveform you want to measure, and then ALSO getting a nice picture of it, is sometimes a bit too much for my patience... In some tests, maximum, calculated from middle 80% of the slope, is well over 100V/usec. The two cursors you see (the x and the boxed x) are used to calculate the figures, the delta V and delta T on the screen.

But, from previous experience, I think the usable slew rate (for audio) is around 30-50% of the slew rate measured in this way.

In any case, there will not be a bandwidth limit with this design...

Quote:
Originally Posted by shaan View Post
PMI, is this slew rate enough for a sharp hit on the snare drum? What do the speakers say?
I think so, I played some jazz with good percussion, and it sounded quite nice. I am happy with the sound.

I only have one channel connected at a time so far, but everything I have tried has tested (and sounded) very very good. Exceptionally clean and crisp highs. I will have some time to test in the next 2-3 days, but until now, only an hour here and there.

In normal listening, the heatsink temperature only rises about 10-15% above room temperature. However, when I intentionally drive the output into clipping or close, the temperature starts to rise gradually, and does NOT level off. So, for anyone else who wants to test like this - please use some caution, and perhaps a small fan.
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