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Old 13th January 2005, 09:27 PM   #1
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Smile One Aleph-X working, One to go

Hi,

today I finally finished (almost) one of my Aleph-X monos. After some minor wiring problems (half the amp was dead) I got it up and running.
So far it seems to be allright. Relative offset is around 10mV, absolute offset could be set to zero.
Bias was around 4A with 27k for R11 and R33 (VR1 and VR3 set to zero)
One thing that isnīt like it should is the frequency responce and the square wave.
Frequency responce is not falling like it should but rises maybe 1-1.5dB at 100kHz then is flat again at 200kHz before it falls like expected.
Square wave at 10kHz as quite some overshoot but looks very wel dampened with only two halve waves until itīs flat.

So I suppose I should place a cap parallel to the feedback resistor or is there another place more suited?

Until now I didnīt connect R12 and R34 to keep things simple.


William (very happy )
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Old 13th January 2005, 11:57 PM   #2
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If you've got a rise in the frequency response, then try a few pF across the feedback caps. A small value like 2 to 5pF should do the trick. Certainly no more than 10pF. Look for a -3dB point around 100-150kHz (although I'm not shy about going higher).

Grey
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Old 14th January 2005, 09:04 PM   #3
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Hi Grey,

I tried 10pF as I canīt find any 5pF caps. This makes the square wave perfect at 1Khz, very slightly rounded at 10kHz and not so perfect anymore at 100kHz. I didnīt measure frequency responce yet.
I will activate the active current sources next and go for 50% to start with.

The amp is behaving very well although my voltage is a bit higher than expected (22V instead of 20) so heatsink temp is also a bit higher at 8A of bias. I now get 53°C at 22°C ambient for around 350 watts dissipation.
Rel offset starts at 12mV and goes down to 6mV after warm up. Absolute offset is now 0V when warm but Iīll have to look at the startup value at the next start from cold.

The diodes are not as hot as expected but the chokes canīt be touched 60°? I will measure them as soon as everything is working.

The transformer is very, very quiet (I canīt hear it at all!). Never had one of these before.......

William
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Old 14th January 2005, 10:56 PM   #4
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ok,

some more information.
Iīve connected the 10pF caps (C2, C4). This looks good now although maybe a bit too rounded at 10kHz (Iīll try to post some pics tomorrow).
Then I put some pots at the R12 and R34 position (started with 680 Ohms), turned on the amp and gor a nice signal on the scope although I didnīt put one into the amp Oscillation!
So I tried some 1nF caps for C9 and C10. This got rid of the oscillation but the square wave had a bit overshoot again and was somewhat "sloped". I changed the caps to 4n7 after trying 1n5 and 2n2. This took care of the overshoot but not of the sloping........
Next I measured the power into 8, 4 and 2 ohms:

100watts, 178 watts, 256 watts

This was a bit too much! Especially the 2 Ohm sinus had some nice distortion at the 0V position long before clipping.

Looking at the AXE-1 sheet this should be more than 75% ac-current-gain.
So I set the pots to 1k. The result was the same so I went to 2k5 wich was a bit too much (56 watts/4 Ohms).
After trying a few more settings I came to 1k3 wich gave 100 watts/ 8ohms, 160 watts/ 4 ohms and 80 watts/2Ohms. This is around 55% ac-current-gain and I will leave it there for the moment.

After this I looked again at the 10kHz square wave. The "sloping" was gone and it actually looked very nice I think it was caused by the way too high ac-current-gain setting.

Frequency response is 140kHz -3dB, 217kHz -6dB. At 20 kHz it is down by 0,2dB.

to be continued.........

William
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Old 16th January 2005, 08:44 AM   #5
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Hi,

here are some pics. First is the square wave response at 1kHz / 3V into 8 ohms. Looks good to me
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File Type: jpg diy 3v-8ohm-1khz.jpg (33.5 KB, 3583 views)
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Old 16th January 2005, 08:46 AM   #6
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the same at 10kHz. Itīs somewhat rounded so maybe Iīll buy some other caps but Iīm not shure if this is so important.
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File Type: jpg diy 3v-8ohm-10khz.jpg (66.6 KB, 3456 views)
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Old 16th January 2005, 08:53 AM   #7
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and here is the amp section. You can see the two heatsinks and the back. Heatsinks are Fischer SK57 (300x200x85mm, ca. 0.2°K/W). The fets on the minus side are matched to <0,01V (All have 4,58V Vgs at 1.1A current) The positive fets are matched in two pairs of three. I tried to get the same value when adding Vgs for the left and right side and took the same value for the sence fet (where the bias is set) So plus left was 4,56 (sence) 4,57 and 4,61,
right plus was 4,56 (sence), 4,59 and 4,60.

Source resistors are 3x1Ohm/1% per fet.
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File Type: jpg diy-fets.jpg (77.0 KB, 4443 views)
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Old 16th January 2005, 08:56 AM   #8
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and here a view from above. Here you can see the output resistors (12x1Ohm 1%) connected directly to the output terminals. Feedback is also taken from here (so I could use the same cheap silverplated copper wire for the output connection )
The resistors from output to ground (4x390Ohm) are on the board.
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File Type: jpg diy van boven.jpg (89.5 KB, 4289 views)
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Old 16th January 2005, 09:10 AM   #9
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input caps are 10uF BG N (no bypass yet), resistors are mostly 1% CONRAD metalfilm with some 0,1% resistors for feedback and diff pair. The input fets are matched to 0,001V (3,739 and 3,740).

Upon startup rel. DC offset is now 18mV, absolute offset is smaller 0,3V and somehow stays there.

Iīve reduced bias to 7A to make it possible to touch the heatsinks for a few more seconds. 58° somehow makes me feel uneasy but maybe Iīll convince myself that everything is allright some day.......

Iīve listened to it (mono , Dynaudio audience 42) The music is sounding nice but I canīt say a lot more at this moment.

The last picture shows both amps. The second one still needs the fets mounted, board populated etc. This will take me at least 6 weeks as Iīm of to South Africa for the next three weeks and after that in Sweden for two.

I would like to thank Nelson, Grey, Chad and all the other who made this project possible!

William
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Old 16th January 2005, 11:03 AM   #10
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Hi William,

did you try C6(Aleph2) the so called Cdom?

Uli

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