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Old 25th October 2009, 11:56 AM   #1
xcel is offline xcel  Netherlands
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Question Aleph-X: strange signals ....

Fellow DIYers,

I’ve run into some problems during the construction of my second Aleph-X, and I was wondering if you could provide some assistance in solving them. This is a high powered version at about 8.3 A bias, with 16 IRFP 240’s. I’ve added the schematic of both the amp and the power supply

The amp itself is functioning properly, as far as I can tell:

•The gain is 20dB
•The bias comes up nicely (about 1.02 A per MOSFET), total of about 8.3 A bias.
•Differential offset is 25 mV, and absolute offset about 300 mV after an hour.
•Across R23/R25 4.33V
•Across the 0.39 ohm resistors in the bias sections 400-410 mV
•Across the 0.39 ohm resistors in the output section 390-410 mV
•Across C3,C5 4.35 V.

The supply lines are heavily filtered by a CRCRC filter, resulting in a ripple of about 15 mVpk on both lines. But here’s the problem: I see a nasty distortion in the residual ripple (picture 1). This ripple appears at Q6 (current source) and is “changed” into some sort of square wave as shown in picture 2. It is has a peak voltage of about 2.5 mV and the square wave appears at 10ms intervals (so it’s a 100 Hz residual). This ripple is present at the voltage feeding the diff amp Q5/Q7 (2) and at the gates of MOSFET’s in the bias section (Q21/20/12/1/10/14/16/17).

It finally ends up at the outputs (slightly bigger, about 5 mV peak) and although the amp is dead quiet, it bothers me…On my scope it’s clearly visible that there is a connection between the distortion of the mains residual and the square wave: the square wave and the distortion of the residual appear at exactly the same moment.

I’ve used this CRCRC configuration in my other (medium power) Aleph-X monoblock and although the bias was about half (4 A) it functioned flawlessly. In my High Power Aleph-X the residual ripple has the same amplitude but it seems like something is “cutting” into the ripple.

Actions taken so far:
Replaced the capacitors: no effect, problem remained
Bypassing the RCRC filters: same kind of distortion on both outputs.
Replaced the mains filter: no effect, problem remained
Replaced the NTC: no effect, problem remained (I have my doubts about the NTC however, they should have a resistance of 10 ohm at 20°C but they measure 4.4. ohms…). Maybe they’re too small ?

Questions:
•Do you have any idea what could be causing this ?
•Is it important or does the square wave gets cancelled at the load (sorry if this is a dumb question)?

Looking forward to your comments . Many thanks in advance !

Xcel
Attached Images
File Type: jpg Picture 1a.jpg (173.4 KB, 183 views)
File Type: jpg Picture 2a.jpg (193.2 KB, 171 views)
File Type: jpg IMG_4499.jpg (214.3 KB, 175 views)
Attached Files
File Type: pdf Power Supply Design High Power .pdf (39.3 KB, 60 views)
File Type: pdf Aleph-X High Power Construction.pdf (54.4 KB, 70 views)
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Old 25th October 2009, 01:59 PM   #2
radule is offline radule  Serbia
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Quote:
Originally Posted by xcel View Post
I see a nasty distortion in the residual ripple (picture 1).

Smell like rectifier recovery.
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Old 25th October 2009, 02:52 PM   #3
xcel is offline xcel  Netherlands
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Radule,

That's what I thought so I replaced the rectifier bridge with a new on./ Nothing changed, sadly.

Maybe I should look into the specs of this thing since it was originally bought for another project. On the other hand, these things are pretty rugged....

Anyway, thanks for the reply !
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Old 26th October 2009, 09:23 PM   #4
gl is offline gl  United States
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Hi Xcel,

I can't help with the ripple problem except to suggest you check for possible ground loops.

But I would like to suggest the following changes:

1) R19 and R25 to 10K since that's the value Passlabs used on the original factory XA amps.

2) R46 and R47 to 10K. On my amplifiers this made a huge difference in opening up the sound.

3) The output to ground resistors to 100 ohms @ 25W. Same reason as 2) above.

These components and their values have been discussed a great deal in other threads.

Cheers,
Graeme
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Old 26th October 2009, 10:46 PM   #5
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I answered your email today:

"•Across C3,C5 4.35 V."

is your big clue.

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Old 26th October 2009, 11:11 PM   #6
xcel is offline xcel  Netherlands
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Default Much appreciated advice !

Nelson,

Thank you very much for pointing out my big clue: as soon as my Cabernet Sauvignon wears off I'll get straight on it ! That will most likely be tomorrow morning, since we're allready burning the midnight oil overhere in Europe.


Graeme,
Big thanks to you also for the advice. I've followed your work closely from your posts and you strike me as someone who knows what he's doing. I allready had plans in this particular direction, and I hope we can exchange views and experiences somewhere in the near future.
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Old 27th October 2009, 11:01 AM   #7
xcel is offline xcel  Netherlands
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Default Oops

I have to admit I made a mistake. When I wrote “across C3,C5 4.35 V” I should have written “across Q3,Q8 4.35 V”. My bad and I do apologize.

I checked the voltages in that area:
Across R31: 8.49 V
Across R32: 8.11 V
Across R33: 11.87 V
Across C5: 12.48 V
Across Q8: 4.40 V
Supply Voltage : 21.7 V

This looks pretty ok to, the other side of the amp showed very similar results. The parts checked out ok, although I am using a slightly smaller cap (100 uF instead of the reported 220uF).

So, I’m back at square one...

Help is still very much appreciated
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Old 27th October 2009, 12:39 PM   #8
radule is offline radule  Serbia
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Is it second picture with shorth circuit on input signal terminals (PCB)?

My opinion is that signal on the output coming from PS pulsed current, and not from rail voltage ripple (that is job for PSRR). 8A Dc mean 30-40A pulsed current.

Anyway, I think that you dont have any real problem with amp
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Old 28th October 2009, 09:39 AM   #9
jazz is offline jazz  Netherlands
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Hi,

I'm rather interested in what the clue leads to. Would it be ok for xcel to post your mail mr Pass?

regards,
Joris
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Old 28th October 2009, 10:12 AM   #10
xcel is offline xcel  Netherlands
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Default Additional info

Well jazz,

Keeping in true Nelson-style the mail I recieved from him was as brief as his post.


He advised me to check out the components surrounding the caps. But since I made a slip of the pen, his advice was based on wrong information.

I wrote Nelson an email yesterday with the correct data (look above) and a number of things that I had encountered when comparing this amp with my working medium power Aleph-X:

One thing that boggled me was that in my other amp the power supply ripple gets eliminated after Q6 (current source). In this amp the “mutilation” of the ripple somehow survives and gets turned into some sort of square wave . I’ve already replaced the MOSFET but with no effect. The diff amp amplifies this problem and through R46/47 this signal ends up at the gates as well. Is it unlikely to think that through C1/ R88/R12 and C6//R89/R34 ends up at the base of Q3/Q8 ?

The same things happens at R14/R31: after this component the ripple is gone and the square wave appears… Maybe the power supply rejection ratio of this particular amp is off ?

I have a strong feeling that this is the root of all evil but to be honest I’ve no clue on how to fix it. Once again, help is appreciated.

Thanks,

Xcel
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