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Old 11th March 2006, 04:36 PM   #61
daly2k is offline daly2k  United States
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Default XA 100

GL-I think you are on to something related to the dc offset vs AC voltage variation in the house mains. My house mains during the winter are around 122-125 volts with a 60hz +/- 3hz frequency.
During the summer this can drop to 115 volts easily and I am not sure what the frequency does. I have a monster ac power supply filter which measures these ac variations. Now I am wondering if I should build a humongus regulator for the power supply similar to what NP recommends. I can't think of anyother way to cure this problem with DC offset (if it indeed contributes).
I still will carefully match all the parts. Thanks for keeping these ideas flowing. dave
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Old 12th March 2006, 10:12 AM   #62
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Hi,

it does contribute since abs. dc offset is set with the neg. rail as a reference. When this changes, abs. dc changes.
In my amp ca. 3V of mains (232-229) change, changes offset by 1.5V

William
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Old 13th March 2006, 04:47 PM   #63
gl is offline gl  United States
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Hi Dave,

Thank you for the comments. This started out as a "hey look what I built" type of thread and has evolved into a pretty constructive sharing of experiments and ideas.


Hi William,

I redid the AC gain measurements and found that increasing the bias from 500ma to 540ma did not change the AC gain of the Aleph CCS at all. I did the test at 100Hz and 75Hz.

I am next going to use my variac to "adjust" the AC mains voltage to the amp and see what it does to the abs. DC offset. I am not sure I completely understand why the two are related. The voltage across the 392 ohm resistor is set by the 10ma from the CCS. The 10ma from the CCS is essentially set by the zener. Adjusting the mains voltage slowly should not affect these values. However, it will change the voltage across the diff pair.

In the Salt Lake Shootout thread I noticed that another member tried ZVP3310 parts as the input to an Aleph with very favourable sonic results. I recall that Cheff also had this idea. I am now thinking about building new driver boards with ZVP3310's cascoded with ZTX550's as Cheff suggested. I have all the parts. I am going to buil new boards because I am using the amps regularly and I don't want to perform major surgery on the existing boards.

Enough has changed in the "as built" AX100 and enough interest has been shown by other members that I have decided to post an updated schematic and circuit description.

Cheers,
Graeme
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Old 13th March 2006, 04:53 PM   #64
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Quote:
Originally posted by gl
Enough has changed in the "as built" AX100 and enough interest has been shown by other members
Maybe WraffWraff can post his servo circuit too ?
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Old 13th March 2006, 04:58 PM   #65
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Grrrrrrrrrrrrrrrrr........
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Old 13th March 2006, 05:09 PM   #66
gl is offline gl  United States
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Hi Jacco,

The purpose in creating the AX100 was to take Grey Rollins original circuit and 1) scale it up to 100W, 2) move the design closer to the factory XA amps by incorporating all the crumbs dropped by NP over the past 4 years.

Ideas like cascodes, new CCS designs, servos and JFETs create new designs that I think should be subject of new threads. That is why I used the term "as built".

Cheers,
Graeme
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Old 13th March 2006, 05:16 PM   #67
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Graeme,

as far as I understand the current through the diff pair is not affected by the power supply voltage / mains voltage.
Also the voltage between the gate of the output transistors and the negative rail stays the same as the current through the drain resistors (392R) stays the same.
What is affected is the voltage between the gate of the output transistors and ground. This changes the same amount as the neg rail voltage.
If Vdg stays the same (wich it probably will not) the absolute dc will change the same amount as the negative rail.

mmmm, maybe said another way:

you set the absolute offset by setting Vgs on the output fets and thus changing the drain-gate voltage. If this reaches rail voltage minus Vgs then your absolute offset is zero.

Jacco,

it is a standard servo with an OP77GP, 2x 1uF, 2x 330k and an output resistor. It is connected to ground and to the gate of the current source fet. I would feed it with +18V and -6V with a few zeners connected to the supply rails. The output resistor must be determined by trying but could be between 2 and 4k.

William
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Old 13th March 2006, 06:32 PM   #68
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Hi William,

Yes that makes more sense. Thank you.

It seems like a shame to damage the sound of the amplifier adding DC offset control components just because the mains voltage is fluctuating. Oh well.

Graeme
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Old 13th March 2006, 06:36 PM   #69
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Well,

the question is more like:

will putting in a servo in this way hurt the sound?

-it is not really in the signal path as it feeds the current source
- caps and reistores can be set for a very slow following of the output dc voltage
-you can probably leave out the McMillan resistors
-you can raise the value of the output resistors to ground

I think it should be tried before saying it´ll hurt the sound.

William
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Old 15th March 2006, 07:10 PM   #70
gl is offline gl  United States
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Here is an updated schematic for the AX100.

Although the schematic shown on page 1 of this thread is still valid this updated version accurately reflects the current "as-built" amps. The changes shown include tweeks done over the past few weeks some of which are discussed in this thread.
These are:

1) The value for the MacMillan resistors R46 and R47 are changed from 4.75K to 10K. I may try 22K again but not for while.

2) The protection diode scheme for the inputs has changed. I did this after examining the close-up photos of the XA200 PCB that NP published on pages 58 and 59 of the big Aleph-X thread. This scheme was previously used by NP on the Aleph 0. I don't believe it affects the sound but it does delete two parts and two ground connections.

3) R11 and R33 are now shown as 100K which gives 540ma of bias per output transistor on the Aleph CCS. I am going to stick with this value for now. I have not tried 600ma or higher yet. The AC current gain of the Aleph CCS maesures the same for either 500ma or 540ma of bias.

4) I have now decided that the PS inductors are not saturating.

5) The notes have been cleaned up.

I am not going to publish another schematic for this particular amplifier because I don't foresee any significant changes. Besides I am having too much fun listening to these so I am moving on to other projects. The ideas coming out of the on-going discussions here will go into a new project rather than being retrofit into this one. The only thing I might try in these amps at this point is a set of "Hansen Snubbers" as described recently over in the Chip Amps forum.

Cheers to all,
Graeme L.
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