Aleph X - on 25V/4A 8mosfets

[yoke]

Hi...
I have put together my Aleph-X couple months ago.
I have use PCB from first group buy and the part list is the sime as in Aleph-X revision 1.0.

But I have use 8 mosfet per chanel instead of 4 and I have rise voltage from 15 to 20-25V.

Now I have problem with adjusting ac current gain to 50%.

I have wrote about this problem, couple months ago in ax builders tread, but since there is number of posts and I really would like to make this work, I have open new tread.

Now, since I could not adjust AC current gain to 50%, the conclusion was that my AX is oscillating. I have no oscilloscope...
I would set everything as it should be but after some time when I measure again I would get some other figures...


Now after some time finally a gat some free thyme to try playing with it again...

I have plug everything and amplifier is playing music, but the problem is sime...when I play 1kHz sound and measure ac voltage on R5 and on R2,3 I get different readings from time to time ( after 1 hour,2,3 )


As I remember the voltage should be the sime on R5 and R2,3 since I have use 0.5 Ohm for them...

I have notice that when I move V+ wire near ext wire for R7 the 1kHz sound changes, distort...So I believe it's evidence of oscillation...

And my source resistors get really hot ( R5,6,40,41) , about 65*C !!!

So my question is what could be wrong ?

Could the problem be that I have rise voltage and the circuit is left the sime ?

Should I change some resistors in circuit to adopt this for 20-25V ?

Does any one have schematic for 25V/4A and 8 mosfets per channel ?

Shoul placing some capacitor in place of C9,7 help...what value ?

Thanks

[mpmarino]

I havent built mine yet..but this seems to be a common schematic people go by..
http://web.vip.hr/pcb-design.vip/alephx.html

[wuffwaff]

Hi,

you could look here for some answers. I used a 22V (at the Fets) supply and changed some resistor values and added caps.

One Aleph-X working, One to go

There´s a list with used values somewhere at the end.

Hope this helps,

William

[yoke]

Thanks...
I will take a look and try to change some values to see what will hapend...
will post my results...

Thanks
Regards
Josip

Wuffwaff... how hot your's source resisitors get ?

[wuffwaff]

Hi,

don´t know...... The dissipated power is 1,33^2 x 0,33 = 0,6 watt. They´re rated at 3 watts so I didn´t check the temperature.

William

[yoke]

whell I have use 0.5 Ohm 5W ( 1^2x0.5=0.5W) ... so no nead for panic, even if they are 65*C ???

[wuffwaff]

Yoke,

are you shure the amp isn´t oscillating?

William

[yoke]

I think my amp is oscillating, but don't know for sure. As I wrote before I don't have oscilloscope.

I was wondering what could be the reason for oscillations ?

Could reason be, that I have use circuit for 15V and 4 mosfet per channel ? ( but I place 20-25V and 8 mosfet per channel )

... or that should not be the problem, since I can easily adjust 0.5V on source resistors and DC offset ~ 0V which stay stabile even after 2 hour.

or I should find some oscilloscope, test amp and then come back for some advice ?

[wuffwaff]

Yoke,

what you could do is place some caps. 10pF for the feedback loop and 4n7 for the active current source. This will probably be enough and if it is working you can optimise these with an oscilloscope.

William

[yoke]

Thanks William, I will try...