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Old 13th February 2013, 06:48 PM   #2251
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Here are the pics.

The first is 10V rms output
The second around 26V
The third almost 30V and clipping

the upper trace is the output (via a voltage divider) and a INA134
the lower trace is where the McMillans meet at the sources

William
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File Type: jpg IMG_2967.jpg (49.9 KB, 243 views)
File Type: jpg IMG_2968.jpg (52.1 KB, 239 views)
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Old 13th February 2013, 06:53 PM   #2252
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and here is the summed signal (purple) between the two outputs.

Output was 4,00V rms. Here one side is 1,94V, the other 2,06V (summed also 4,00V). Not sure what causes the difference or if there is one in reality.

William
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Old 14th February 2013, 09:00 AM   #2253
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Quote:
Originally Posted by wuffwaff View Post
Hi,

I added both outputs on the scope and got the same signal I see at the JFet sources only 20x bigger. So there seems to be quite a lot of common mode distortion. Not sure where this comes from.......

William
I've not investigated this in detail but isn't quite a lot of common mode distortion to be expected? The X topology trades lower differential distortion for higher common mode on the grounds that the latter will have no influence on the speaker.

Ian
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Old 14th February 2013, 09:28 AM   #2254
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Hi Ian,

yes it does. But I just dont know if this is a "normal" amount for an Aleph-X not driven anywhere near its limits.

William
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Old 14th February 2013, 09:35 AM   #2255
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This is best answered by another Aleph-X builder. My project is based more closely on the XA which doesn't need the Macmillan resistors and hence I have not looked into this aspect of performance. It is an interesting question though.

Ian
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Old 18th February 2013, 05:58 PM   #2256
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Interesting graphs, William. I am not sure what to make of them..

It is curious, though, every now and then the conversation sends me back to make some more measurements and explore something that I thought I understood some time ago. I was making some minor changes to one of my amps and I decided to go back and study the whole Absolute DC Offset vs. time thing again and this time I also measured temperature. The result was rather interesting:

Time......Offset....Temp
0mins.....14v........13c (ambient)
1min......11.7v......didn't measure
2mins.....10.6v......26c
3mins......9.7v.......30c
4mins......9.0v......didn't measure
5mins......8.3v.......33c
10mins....6.1v.......38c
15mins....5.0v.......45c
30mins....3.7v.......46c
45mins....3.0v.......46c
60mins....2.7v.......46c
90mins....2.5v.......46c

Earlier this summer, I had readjusted my DC offset after 2 hrs to read zero volts (it was off by about 2.5v), but the ambient temp was closer to 19-20c, thus final amp temp was closer to 53-54c. Now that the weather is colder, offset is running about 2.5v higher again.

But the part that stands out to me is the DC offset vs Temp readings in the 15min-90min window. Temp has reached equilibrium, but Absolute DC Offset keeps moving. My current configuration runs 21.5v rails, 8.5A bias, has 10K McMillan resistors, and still uses 9610s as the input differential. House mains voltage seems to move between 122-124vAC depending on the time of day.

Any ideas what is causing offset to move after the amp has reached thermal equilibrium?
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Last edited by Eric; 18th February 2013 at 06:20 PM.
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Old 18th February 2013, 06:34 PM   #2257
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simple - it didn't reached temp equilibrium
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Old 18th February 2013, 07:17 PM   #2258
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Eric,

did you measure the mains voltage?


William
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Old 18th February 2013, 08:17 PM   #2259
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Zen: I can see this in the time up to the first hour, but I find it strange beyond the first hour... I'm starting to remember some of my early frustrations with tweaking

William: At the time that I measured, mains were steady at 123v.
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Old 18th February 2013, 09:16 PM   #2260
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OK,

Zen is probably right that it is still changing temperatures inside. Mine take a lot longer than an hour.

Im almost ready with preparations for a dc-servo. I think the amp sounded better with the higher McMillan resistors and I would like to try another solution.

William
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