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Old 2nd June 2013, 10:40 PM   #531
PMI is offline PMI  United States
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Quote:
Originally Posted by shaan View Post
Damn, look at that bbox!
Quote:
Originally Posted by wiljj78 View Post
Wow... nice box..
Thanks, I am hoping to make a couple small revisions before I wire it all, and looking forward to getting all the bits off the bench and into the chassis...
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Old 2nd June 2013, 10:55 PM   #532
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Default A few more tests... Finding the high frequency -3db point

Caution: All of the tests were done without a zobel network. At high frequency, a normal-value Zobel resistor might get VERY hot during a high frequency test.

First three pictures are finding the -3db point.

#1 - 100 kHz since wave, input and output in phase, no attenuation.
#2 - 1.1 MHz, -2db
#3 - 1.3 MHz, -3db

In the first pic, the two channels of the o-scope were adjusted to different scales to show the input and output signal superimposed at 100KHz, well before any high frequency rolloff. Second pic, getting close. Third pic, -3db point reached at about 1.3 MHz. Conclusion, there is plenty of bandwidth, well above the audio range.
Attached Images
File Type: jpg VSSA bw test11.jpg (215.2 KB, 1362 views)
File Type: jpg VSSA bw test12.jpg (223.5 KB, 1300 views)
File Type: jpg VSSA bw test13.jpg (221.9 KB, 1248 views)
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Old 2nd June 2013, 11:16 PM   #533
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Default Square Wave, 20KHz to 500kHz

Next three pics show a square wave test, 20, 100 and 500kHz.

#4 - 20 kHz square wave, no attenuation or slew rate limiting seen
#5 - 100 kHz, rise and fall time can be seen, but still clearly a square wave
#6 - 500 kHz, practical limit for a square wave, distortion is obvious

Conclusion, clean square wave well above audio range.
Attached Images
File Type: jpg VSSA sqw test14.jpg (212.2 KB, 1205 views)
File Type: jpg VSSA sqw test15.jpg (234.0 KB, 1169 views)
File Type: jpg VSSA sqw test16.jpg (224.6 KB, 272 views)

Last edited by PMI; 2nd June 2013 at 11:40 PM.
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Old 2nd June 2013, 11:38 PM   #534
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Default Slew Rate

Next picture shows a rise time measurement with +/-30V supply, resulting in a maximum estimated slew rate of around 80V/usec. (A previous measurement under better test conditions and higher supply voltage showed around 100V/usec).

#7 - Slew rate, using rise time from 10% to 90% of Vp-p.

Final pic shows the test setup.

#8 - Tektronix PS280 bench supply @+/-30V, Wavetek 182A signal generator, Tektronix 2221A oscilloscope, Fluke 77 Multimeter. Tested w. resistive load only.
Attached Images
File Type: jpg VSSA sr test17.jpg (242.6 KB, 276 views)
File Type: jpg VSSA sr test18.jpg (280.3 KB, 291 views)
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Old 3rd June 2013, 01:04 AM   #535
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Great and very precise work PMI, thanks for the measurements.
I find a little difference with VSSA but not in a major scale, -3 dB point at 3 MHz (input filter & Zobel connected) and a little higher slew rate, around 150 V/us, but I am sure you'll extensively measure it by yourself later. Can you please also measure Zout and some RC loading, would be interesting too?
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Old 3rd June 2013, 02:04 AM   #536
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Quote:
Originally Posted by Lazy Cat View Post
Can you please also measure Zout and some RC loading, would be interesting too?
Yes, although I cannot adjust the source (the Wavetek) accurately on the highest range (1-4MHz).

(Everything is manual, and just too sensitive, even to bumping the cart. I like the Wavetek, b/c it is all analog, but I never intended to be testing with it at those frequencies. For 1M and up I will need something else if we want accuracy.)

If you suggest a specific test or tests, and I will give it a shot.
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Old 3rd June 2013, 06:14 AM   #537
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Default VSSA T03

2khz-2khz clip - 2khz -20khz-100khz-500khz

8Ω Load no zobel

Mr Jason thanks for all and the board again .

the litle noise solve look at pic 3 the green jumpers.
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File Type: jpg IMGP1926.JPG (400.0 KB, 171 views)

Last edited by nikosokey; 3rd June 2013 at 06:40 AM.
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Old 3rd June 2013, 06:22 AM   #538
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Default VSSA T03

8Ω with 100nf no zobel

1khz-20khz-50khz
Attached Images
File Type: jpg IMGP1929.JPG (324.0 KB, 154 views)
File Type: jpg IMGP1930.JPG (320.9 KB, 141 views)
File Type: jpg IMGP1932.JPG (343.5 KB, 188 views)
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File Type: jpg IMGP1939.JPG (555.9 KB, 260 views)

Last edited by nikosokey; 3rd June 2013 at 06:26 AM.
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Old 3rd June 2013, 09:30 AM   #539
shaan is offline shaan  India
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Hi PMI and Nicos!

Spectacular job guys!(are we really talkin' 'bout a little class-ab amp? )

Now that's what we call 'potential'. Yeah, this amp can chew fast sines like a piece of cake.





Thanks again Andrej.
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Old 3rd June 2013, 10:03 AM   #540
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HI Shaan
I change the Vas transistor and the first power up i get 25mv now the resistor is 910k
not noisy at the start at all , detailed sound.. ... ... ... ... ... ... !!!!!
I'm impressed neighbor Andrej and I thank you for your offer.
Regards Nikos
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