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Old 16th August 2014, 05:53 PM   #2471
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Default Taming the beast

OK Soldering iron out again and more tweaking.

I have 10K trimmers over R23 and R25 - I can trim out most of the relative DC offset (20mv remaining) but still have issues with differences in the amplitude (and shape) of the waveforms on the outputs, also the two output waveforms don't cross over at the middle - I expected that they would (and should). I can adjust these factors a little by adjusting the 10K's over the top of R23/R25 but what you see on the scope traces is the best I have achieved so far.

If I invert channel 2 then the output looks like a clean sine wave, without the inversion it looks like a lower amplitude "M" - I can't recall when that came about - might have been when I increased the value of the McMillan resistors - I might have to go to back to a lower value (currently 20K).

The absolute DC offset is proving difficult to tame - after warming up it seems to sit around a sweet spot for some time and then wanders off without any good reason, I am therefore reluctant to attach a speaker to the outputs (currently just 6.6 ohms of power resistor). I would be grateful for any further advice.
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Old 16th August 2014, 05:55 PM   #2472
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William - I did get a 10 ohm trimmer but have not soldered it in yet, might try that a little later but think my current problems are probably bigger than just having that potentiometer present.
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Old 16th August 2014, 05:57 PM   #2473
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X output need to be measured balanced , without gnd involved
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Old 17th August 2014, 06:44 AM   #2474
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Hi Zen Mod, not sure that I can simply connect my scope probes across the outputs without some kind of ground isolation.

Any thoughts about needing to invert channel 2 on the scope in order to see a good waveform?
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Old 17th August 2014, 08:04 AM   #2475
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either se to bal converter , or lifting scope from safety gnd

mine cro is fed via isolation mains xformer (not connected to safety gnd)
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Old 18th August 2014, 05:31 AM   #2476
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and don´t worry about absolute DC offset, your speakers won´t see it!

As I see you are measuring near clipping level? Try something near 1 or 2V to start with.

And have a look at the current limiters on the negative halves, it could be that they are already working (Q4, Q9 and resistors 10,13,35,39. Maybe you have to change those to change the limit.
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Last edited by wuffwaff; 18th August 2014 at 05:35 AM.
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Old 20th August 2014, 12:25 AM   #2477
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Hello
I would like to ask some opinion about the Aleph X to be replaced by the UGS power amp.
I don't know if someone already done it.
Do it worth the investment, the extra job etc.
I do have the Aleph X and I love it , also I planed to mode the front end to J Fet.
Now I really don't know do I stick with the Aleph X just mode the front end and enjoy it or it worth the investment to change it to the UGS ad some extra power mosfet and bias the hell out of it.
About the UGS I read someone compared with the Aleph J and the J performed way better.
Any advise welcome.
Greetings
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Old 25th August 2014, 08:44 AM   #2478
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Hi William,

"and don´t worry about absolute DC offset, your speakers won´t see it!" - If it were a few millivolts I wouldn't worry - problem was that it was creeping up to 14v or so. I appreciate that the speaker won't see it but it doesn't leave much headroom for the circuit to work as an amplifier. I've since swapped a few things around - McMillans back down to 10k and changed the way I trim out the relative DC offset on the diff pair. I now have just a few millivolts relative offset and absolute offset drifts +/- 100mv (but is usually less and now doesn't drift skyward). Things are looking much more like they should. I plan to run it for a while and confirm that it continues to behave and then start putting the speaker protection together (wanted to start that a while ago). I'm still not sure about that output inversion issue on one channel that I mentioned previously, to my mind inverting the scope channel should not be needed ....
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Old 27th August 2014, 11:50 AM   #2479
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Does your scope have a Vert Diff position? A-B? Ch1-Ch2? Connecting scope probe Gnd to your Gnd will be fine. Your measurement will be between two Channels of the scope. 1 Channel will be inverted in typical operation.
I always have the Gnd lifted on my scope AC input but that is not something I'd recommend.
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Last edited by flg; 27th August 2014 at 11:54 AM.
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Old 27th August 2014, 09:06 PM   #2480
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Hi flg,

I have dual channels and channels 1 & 2 are both grounded to the central grounding point with one scope probe on each output. I expect one output to be swinging negative whilst the other swings positive, the sum ( ie channels 1 & 2 combined) should be a nice healthy sine wave but instead I see a wobbly m wave as the output. If I invert one of the channels then the resulting waveform is the large bridged sine wave output that I expected in the first place. I just don't understand why I need to invert one scope channel in order to see it how it should be.
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