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Old 7th March 2005, 09:24 PM   #1
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Default Aleph-X oscillating. What to do?

Hello!

Now I've finished my two Aleph-X, rated at 70W into 4Ohms. But the Amps are oscillating, what can I do against this? Tomorrow I'll try to increase the resistor value in the feedback or some RCs on speaker outputs. Any further suggesstions?

AudioAngel
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Old 7th March 2005, 09:27 PM   #2
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In what manner are they oscillating?
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Old 8th March 2005, 03:59 AM   #3
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I would ask at what frequencie(s) does the oscillation happen
if it happens at all frequencies then most probably you have
a design fault on your feedback loop
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Old 8th March 2005, 07:54 AM   #4
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Hello Audioangel,

did you use the Cīs in the feedback loop and active current source. I also had oscillations when not using 3n3 in the current sources. See this thread for more info:
One Aleph-X working, One to go

hope this helps,

William
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Old 8th March 2005, 08:43 PM   #5
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Hello!

Today, I've done some further measurements. Very interesting. The amp isn't oscillating. I've tried to measure teh frequency of oscillation and noticed that that oscillation frequency is 1kHz, but I used a 1kHz testsignal. So, the oscillation is the testsignal and there is another signal which is modulated by the testsignal. Then I've found out that the other signal is the variation of the rail voltages. But no fault can be found and the main pcb has already been tested with another power supply. After removing the resistors in the pi-filter of the psu, there was nolonger this behavior and the Aleph-X amplifies the testsignal correctly. Very interesting fault.
The Amp is rated @ 6A with 18V per rail, I've measured 1,3A @230V out of wall, I think, this seems to be correct.
Tommorrow, I'll take a look at the pi filter. And I'll try to recall the oscillation.
But thank you for your help!

AudioAngel
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Old 9th March 2005, 08:33 AM   #6
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Are you saying that you are seeing excess ripple from your power supply or are you simply experiencing hum?
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Old 15th March 2005, 12:03 PM   #7
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Ok, now its running, but there is a lot of hum out of loudspeakers and some plopps. I think I've made a mistake in connecting the unbalanced input. How have I to do this? Is it right to connect the RCA jack with the (+)-input via a coupling cap and the shield of the RCA jack with ground and then connect ground via a coupling cap with the (-)-input?

AudioAngel
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Old 15th March 2005, 01:04 PM   #8
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Audioangel,

yes that is right (Thatīs also the way I did it).

William
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Old 23rd March 2005, 11:31 AM   #9
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Ok, now both amps are running or should I say heating?
Very nice amps. All the plops and hum have gone only a little bit hum can be heard if I'm in a distance of maximal 10cm to the loudspeakers.
Thanks to all for the help!
Now I'm going to finish the cases, perhabs in the week after Easter it will be done.

AudioAngel
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Old 24th March 2005, 02:50 AM   #10
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Quote:
Originally posted by AudioAngel
connect the RCA jack with the (+)-input via a coupling cap and the shield of the RCA jack with ground and then connect ground via a coupling cap with the (-)-input?

AudioAngel
Is there any way I can keep both RCA and XLR inputs ? How ?
What are the good values for the coupling cap ? Thanks!
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