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Old 8th April 2005, 09:17 PM   #631
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Protos,

the zener wonīt isolate from power supply noise. Itīs only there to provide a biasing voltage for Q6. Thatīs why some people wanted a better (less noisy) reference voltage for the CCS. The other posibility would be to use a precision voltage reference like the LM329 and adjust R24/26 VR2.

William
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Old 8th April 2005, 10:01 PM   #632
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I took some time to re-measure the DC values on my AlephX and the behavior of it over a period of time.
As a reminder, I don’t use the output resistors to ground as recommended mainly because I never felt the need to do so.
When cold, I measure 3.5V absolute DC which goes down very quickly to below 0.5V and stays there with some +/- 100mV of wobbling. This is with the 2k resistors from common diff pair sources to outputs. The differential DC stays around +/-0mV-35mV, depending on how loud I play.
I then changed the 2k’s in 4.7k because I remember Nelson saying:
Quote:
The lower the resistance, the more effective the control
over absolute DC, but then you start depending more
on the common-mode output cancellation to get better
input common mode rejection. I feel it is probably better
to go to higher resistance and use output resistance to
ground to additionally stabilize the absolute DC.
Whatever that may mean but I do understand not to go too low.

Difference with 4.7k is a greater tendency to drift and a smaller range to adjust with V2.
One particular thing I noticed now and in the past is that all adjustments affect each other.
V2 takes care of absolute DC as well as differential DC be it of a smaller magnitude.
Adjusting V1/V3 also has influence on DC in some way. Up or lower the bias with say 0.5A and a new adjustment has to be made to the offset. I tend to believe there’s a sweet spot somewhere.
The amp is now rather stable, knowing it’s a full open design. Literally every breath I take has effect on DC values.
Because the amp is now fully warmed up I’ll measure again tomorrow at first startup and report.

Schematic in link, notice also that the ccs is the one with the TL431. Feedback resistors have also been changed to 150k.
http://www.diyaudio.com/forums/showt...538#post222538

/Hugo
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Old 9th April 2005, 03:04 AM   #633
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One thing I found out doing simulations with the Aleph-X is at different DC biases the absolute DC offset responds to different values of the Rs from output to diff-pair source. At 3A bias on each side of the balanced output, 2.5k resistors yield about 20mV of offset at the +/- output. At 4A bias 3.3k works best at about 3mV offset at each output. These were with or without resistors to ground at each output. I suggest using 5k trimpots to adjust the DC offsets. Easier than substituting in and out different R values.

Of course, simulations does not take into account heat variations and other factors but it's worth giving it a try.
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Old 10th April 2005, 02:52 PM   #634
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Quote:
Originally posted by Nelson Pass
Within a lot code, matching Vgs is perfectly adequate.

The bottom half are common-Source connection. The
top half would probably be considered an unusual
application of Common Drain.


Thanks for your help Nelson. You will recall that all the mosfets I purchase have the same date code. I completed the mosfet matching with fairly good results and was able to get many pairs to match within 0.002 volts at current values of 10mA, 100mA and 200mA. I don't know if anyone else has noticed, but each mosfet has a part number stamped on it which I discovered after measuring and matching them. Well, it turns out that the ones which have consecutive part numbers (ie; 5J and 5k) match almost perfectly in most, but not all, cases. You still have to go through the matching process to be sure.

Regarding the DC offset problem that most are experiencing, it appears this is thermal related. If one adjusts the DC offset to be minimum at operating temperature, at start-up , it is way off. Just a thought, has anyone ever tried using a thermister in parallel or in series with R46 or R47. For a negative gradient thermister, the resistance decreases with increasing temperature at a well defined relation for each type of thermister. If a thermister were selected that could match the DC offset R adjustment required, this "thought" should work. In short, it would effectivly serve as an automatic trim pot controlled by temperature. Again, just a thought.

If this doesn't work, I will resort to a microcontroller based DC correction tool which would select various resistors in parallel with R46 and R47. The key here is to sample the DC offset an make adjustments at a time when there is a lull in the music ie: no input signal. This condition occurs frequently even when playing music. This will work but it is a lot more complicated that the thermister option.
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Old 10th April 2005, 02:56 PM   #635
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Just to clarify, Substrate is my son, I am using his computer. I am lcleven.

Larry
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Old 10th April 2005, 06:15 PM   #636
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Hi,

it would be nice if everybody would say if they talk about the relative or absolute DC-offset.
The first is fairly constant over the temperature range, the second changes quite a lot during warm up and should be set near zero after an hour or so.

William
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Old 10th April 2005, 06:59 PM   #637
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Quote:
Originally posted by Netlist
Because the amp is now fully warmed up I’ll measure again tomorrow at first startup and report.
I had a pretty hard time to re-adjust both DC settings and finally ended with adding the output R's to ground. 43ohm is what I had at hand. I can tell for sure that it is possible to get both values well within specs being <1V differential DC and <100mV absolute DC.
In fact both values are now close to zero after some hours playing. When cold, things are worse but only take half an hour to meet the specs.
Hope this helps.

/Hugo
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Old 10th April 2005, 07:10 PM   #638
lcleven is offline lcleven  Canada
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Wuffwaff,

I have to apologize for not being up to speed yet on the Aleph-X. My son bought the PCBs last year but decided not to build it because he heard of the "DC offset" problem inherent to the design. I said what the heck and am about half way to finishing the amp.

I have found nothing in the wiki or the elsewhere describing "relative DC offset". What is it, what does it affect and what is done to get rid of it, if indeed it is a problem. The Wiki only discusses the Absolute DC offset adjustment and I have not come accross it in the posts. I have not read all the posts as yet and doubt that I will ever have the time to.
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Old 10th April 2005, 07:18 PM   #639
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Relative Dc offset is the same as differential DC offset.

/Hugo
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Old 11th April 2005, 03:03 PM   #640
protos is offline protos  Greece
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I played around with the zener and ccs.The part that is most temperature sensitive and affects abs dc offset is - believe it or not- the zener.Then the ccs mosfet and last the diff pair.Even just putting a coldish screwdriver on the zener would affect offset values quite quickly by hundreds of mv.The ccs is also quite sensitive.
In my highly biased ccs the amp begins at 10v offset drops after 5-10 min to around 4v and then very slowly until it reaches final temp stability after two hours to quite a few hundred mv - I gave up trying to get it lower.
What I see happening is that there are two variables one is the temp stability /working temp of the ccs which is reached quite quickly in the first 5-10 min and then the temp inside the case which affects all other parts but which takes very long to stabilise because of the large external heatsinks.
Because of my higher bias I believe my problems are multiplied by a factor of two or three.There is not much you can do with the ccs mosfet however I was thinking of changing the zener for another biasing arrangement that is less temp sensitive.I still donīt understand why the zener affects the mosfet so much.Normally the resistors biasing the mosfet at the source should be the most sensitive part.At 9v bias the mosfet should be fully switched on so a few mv here and there at the gate shouldnīt affect it .
In any case I donīt know if itīs worth the trouble in the end.The amps sound really fine and a a few hundred mv abs offset is not going to affect anything but a very small percentage of rail swing in terms of operation.
You just have to switch it on for a few minutes before you put it in gear.However if I am around the house I just leave them on more or less permanently.
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