Mini Aleph, some output problem

[Giobax]

Hello. I apologize for my bad english ..... I built a mini aleph using this scheme. Two identical board, same problem


This is the input and output curve at 10Khz. 17,5V each rail, 850mA bias, under 5mV DC offset. 3Vpp input, 27Vpp output.(input on the bottom side)


Same values ​​but 1Khz input signal:


10Khz, 2Vpp input, 20Vpp output:


1Khz, 2Vpp in, 20vpp out:


1Khz square, 1Vpp in, 10Vpp out...it seems good.


The problem manifests itself around 5Khz and I can find it just in the output side of the differential pair of irf9610, matched under 3mv difference.
Does anyone have any idea?
Thanks.
Giovanni

[Loudthud]

I wonder about the two 1nF capacitors. I don't remember seeing caps in those positions in other Aleph schematics.

[rtirion]

You are driving your amp to hard at only 0,8A bias.
The Aleph current source is out of steam and shuts off.
I also suspect that you are not driving a 8 Ohm resistive load.
This makes matters worse.
Is the AC Current gain exactly at 50%?
Given the bias and 20dB (10x) overall gain 2Vpp input is your amp's max with the given reactive load (speaker) at 1Khz.
At 2Vpp / 10KHz input the impedance of the reactive load is lower, which means more current that's not available.

A solution would be large heatsinks and more bias.

[hotrodmaniac]

There is a resistor missing from bc560 base to collector. Some Alephs also have a pot
in series with this resistor. This is for adjusting the current source to a lower value of
current. The lower output mosfet is probably going into current limit. The resistors from
the source of the mosfet set the maximum current of the respective output mosfet. Check
these values as they are different with different Aleph amps.

[HIPCHECK]

Originally Posted by rtirion

You are driving your amp to hard at only 0,8A bias.
The Aleph current source is out of steam and shuts off.
I also suspect that you are not driving a 8 Ohm resistive load.
This makes matters worse.
Is the AC Current gain exactly at 50%?
Given the bias and 20dB (10x) overall gain 2Vpp input is your amp's max with the given reactive load (speaker) at 1Khz.
At 2Vpp / 10KHz input the impedance of the reactive load is lower, which means more current that's not available.

A solution would be large heatsinks and more bias.

WOW, just noticed your join date, kind of like being a diyaudio fossil, that is way back.

[rtirion]

>Fossil...
Has a nice ring to it :-)
I still like designing/building/fixing stuff.

[Giobax]

Originally Posted by rtirion

You are driving your amp to hard at only 0,8A bias.
The Aleph current source is out of steam and shuts off.
I also suspect that you are not driving a 8 Ohm resistive load.
This makes matters worse.
Is the AC Current gain exactly at 50%?
Given the bias and 20dB (10x) overall gain 2Vpp input is your amp's max with the given reactive load (speaker) at 1Khz.
At 2Vpp / 10KHz input the impedance of the reactive load is lower, which means more current that's not available.

A solution would be large heatsinks and more bias.

Bingo! At 1A bias whith 2V 10Khz input problem almost disappears. No problem whit heatsink, after 30 minutes at 1A (with shorted input) mosfets stay at 43°C, so I think there is still room for further increasing the bias 100mA more. I'm driving a 8 Ohm resistive dummy load.
Thanks to all.

Giovanni