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Old 27th November 2009, 12:56 PM   #1
jazz is offline jazz  Netherlands
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Default Aleph-x trouble

Hi,

After about a month out of service i plugged in my aleph-x's this week and to my surprise they started oscillating. Only thing i changed is to replace the xlr input connectors to conform to standard. nothing else. As the units where working before and have done so for more then six months, what have i blown up?

The amps heat up nicely and all psu voltages are on target. No absolute and relative dc offset at the outputs. No strange signals in the psu. The oscillation is only there with the inputs not connected. If i ground the inputs the oscillation is gone. What is strange is that the input fet's gates measure -4V with the input not connected.

I do have the compensation caps in the output ccs installed. that should not be the cause.

Where should i look? the input fets?

regards,
Joris
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Last edited by jazz; 27th November 2009 at 12:59 PM.
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Old 28th November 2009, 08:13 PM   #2
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What happens if you go back to the connectors you had before?

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Old 29th November 2009, 09:52 AM   #3
jazz is offline jazz  Netherlands
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Hello mr Pass,

Thanks for your reply. Unfortunately changing back to the male sockets did not help. I also removed the input wiring completely, did not help. Next i took out the protection zener diodes on the input which did not help either.

Next I'll try a new set of input fets.

Regards,
Joris
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Old 29th November 2009, 10:00 AM   #4
Netlist is offline Netlist  Belgium
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There is a post somewhere in the long AlephX thread (I believe from Nelson) telling how to add some capacitance around the Aleph current source transistor.
I copied that into the old WIKI and I'll be back when I found the article.
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Old 29th November 2009, 10:10 AM   #5
jazz is offline jazz  Netherlands
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Hi Netlist,

Thanks for the suggestion. I do have those capacitors in place since i had oscillation in the output ccs at first (wuffwaff and zen mod gave me the suggestion in post 1710 and onwards of the aleph x builder's thread but I'm sure it was given before and probably by mr Pass). This does not seem to be the same since it is actually hearable with a test loudspeaker and the oscillation in the output ccs was not.

regards,
Joris
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Old 29th November 2009, 12:15 PM   #6
Netlist is offline Netlist  Belgium
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And what do you hear? Can you post a scope picture?

/Hugo
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Old 30th November 2009, 06:48 PM   #7
jazz is offline jazz  Netherlands
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Hi Hugo,

I just tried to do some measurements with the inputs open and now i measure signal on the psu lines as well. The signal on the output keeps moving around and the noise is a medium pitch tearing noise / distorded sine wave (pic of the ouput below). The whole thing is moving around like a bunch of monkeys. If I even touch the leads from the bridge rectifiers to the psu caps the signal is changing and voltages are not ok with the inputs not connected.

However i just realised I simply just don't have the time this month to really pay attention to this. The job is taking to much time and the holidays and festivities demand their time as well. I'm gonna leave it for now and come back to it sometime in january. Thanks for trying to help though.

Regards,
Joris
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Old 30th November 2009, 07:06 PM   #8
Netlist is offline Netlist  Belgium
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Hmm. looks like motorboating. Short the inputs to ground and see what you get but it seems like somethings broken or became loose.

/Hugo
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Old 26th January 2010, 01:41 PM   #9
jazz is offline jazz  Netherlands
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Hi,

Ok i've got it solved. apparently the input fets have become damnaged. I checked the preamp used and discovered it gives out a short dc signal on power up of about 130V (how this is possible with dc blocking caps is still a mystery to me. Maybe LF oscillation or to high a frequency of oscillation for me to measure). Apparently the protection zeners on the input where not enough to prevent damnage to the fets. Replacing the fets solved the problem (zeners are still fine but i replaced as well just to be safe). As an added bonus matching is much tighter now with less then 40mv of differential dc offset.

regards,
Joris
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Old 19th October 2011, 09:51 AM   #10
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at weekend I had get a diy amp for remooving unwanted oscillation and implementing a Sallen-Key topology (for use as a tweeter amp). The schematic was according that one from
ALEPH-X - Pass Labs Clone Class-A amplifier PCB & schematic
Before I perform the actually work, I must remove similar effects as mentioned by post #1 above.
The reason here was not exactly to determine in retrospect, but at whole follow lacks/failures I have found:

1) Transition resistance between the chassis GND and the GND of PCB (spring and tooth ring at the chassis connection solve this problem)
2) Gate resistors and unknown brand of IRF610 (a non inductive version so as genuine International Rectifier IRF610 devices remove all unwanted oscillation effects, independend of the position of the variable resistor for volume control).

Now the actually questions:

1) Which way I must go to determine the exactly value of the serial input gate blocker resistors (ideal value dependend even from the manufacturer of the IRF610)?
2) How I can check the headroom size between the limit values of amplifier mode and oscillator mode (actually I must perform a frequency response up to 10 MHz at least to investigate the roll off behaviour - but are there an other methody?) ?

Both I want exactly to know for implementing an active crossover (12db/oct) according Sallen and Key without additional integrated or discrete OP AMP devices.

An off-toppic question to the member "jazz" from Leiden:
Are there an URL, where I can read the Dutch television dates of the recorded concerts from
http://www.curacaonorthseajazz.com/en/
and
http://www.northseajazz.com/en/program/2011 ??
Thank you very much for an advice - my email address is follow:
kirschner-hifi@tiefbasswiedergabe.de

Last edited by tiefbassuebertr; 19th October 2011 at 10:04 AM.
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