Aleph-X oscillating. What to do?

AudioAngel · 7th March 2005, 09:24 PM

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

Nelson Pass · 7th March 2005, 09:27 PM

In what manner are they oscillating?

mastertech · 8th March 2005, 03:59 AM

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

wuffwaff · 8th March 2005, 07:54 AM

Hello Audioangel,

did you use the C´s 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

AudioAngel · 8th March 2005, 08:43 PM

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

kilowattski · 9th March 2005, 08:33 AM

Are you saying that you are seeing excess ripple from your power supply or are you simply experiencing hum?

AudioAngel · 15th March 2005, 12:03 PM

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

wuffwaff · 15th March 2005, 01:04 PM

Audioangel,

yes that is right (That´s also the way I did it).

William

AudioAngel · 23rd March 2005, 11:31 AM

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

jrsun · 24th March 2005, 02:50 AM

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!

7th March 2005, 09:24 PM	#1
AudioAngel diyAudio Member Join Date: Dec 2003 Location: Verden	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

8th March 2005, 07:54 AM	#4
wuffwaff diyAudio Member Join Date: Aug 2001 Location: Ingolstadt Germany	Hello Audioangel, did you use the C´s 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 __________________ een ooievaar is geen konijn want zijn oren zijn te klein!

9th March 2005, 08:33 AM	#6
kilowattski diyAudio Member Join Date: Aug 2003 Location: Long Island, New York	Are you saying that you are seeing excess ripple from your power supply or are you simply experiencing hum? __________________ ----------------------------------------------- Kilowattski

15th March 2005, 01:04 PM	#8
wuffwaff diyAudio Member Join Date: Aug 2001 Location: Ingolstadt Germany	Audioangel, yes that is right (That´s also the way I did it). William __________________ een ooievaar is geen konijn want zijn oren zijn te klein!

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7th March 2005, 09:27 PM	#2
Nelson Pass The one and only Join Date: Mar 2001	In what manner are they oscillating?

8th March 2005, 03:59 AM	#3
mastertech Banned Join Date: Mar 2005 Location: Australia	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

8th March 2005, 08:43 PM	#5
AudioAngel diyAudio Member Join Date: Dec 2003 Location: Verden	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

15th March 2005, 12:03 PM	#7
AudioAngel diyAudio Member Join Date: Dec 2003 Location: Verden	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

23rd March 2005, 11:31 AM	#9
AudioAngel diyAudio Member Join Date: Dec 2003 Location: Verden	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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