Aleph-X offset problem

[Andypairo]

I use 8 mosfets, 4 per side.
Each one carries about 1.25 A, which means 275 mV on source resistors, slightly different from the 0.5V you use as "reference" value.... where does it come from?

On the Aleph30 manual this voltage value is of about 250 mV (and the equivalent of AlephX's R34 is 820 ohm), in the Aleph60 this voltage is 370 mV and the resistor is about 1K.. does this mean something?

Cheers

Andrea

[jarek]

but the behaviour of the absolute offset puzzles me: it starts at about a couple of volts, then slowly decreases to about 300 - 400 mV. It stays around there jumping now and then to 500 mV; after that is keeps going negative, stops for a while at about - 300 mV and behaves as when of the positive side.

Decreasing resistance of resistors from output to the ground and resistors from output to sources of differential pair decreases range of change of absolute offset with temperature.

[Netlist]

Originally posted by jarek


Decreasing resistance of resistors from output to the ground and resistors from output to sources of differential pair decreases range of change of absolute offset with temperature.

Indeed, I didn't use the output to ground resistors but only the output to diff-pair sources. I used 2k, which is fairly low, but with good results.

/Hugo

[Andypairo]

I used 2k//4k7 which is about 1k5 and also the 30 ohm from output to gnd.

While testing I also noticed that while the differential output can output a perfect sinewave of more than 13Vrms, the single ended outputs dont'.... did anyone noticed that also?

Cheers

Andrea

[Edwin Dorre]

Originally posted by Andypairo
I use 8 mosfets, 4 per side.
Each one carries about 1.25 A, which means 275 mV on source resistors, slightly different from the 0.5V you use as "reference" value.... where does it come from?

I use 0.5V because this is the value used by Grey Rollins in the original first Aleph-X and by Nelson in the Aleph 2 and Aleph 4 which were both role models for my hyped up Aleph-X which runs at about 7A and on a voltage of -25V and +25V . I never thought of using another reference voltage. Actualy I am running at 0.45V because the lower bias! But with the VR1 and VR3 trimmers I can move it up and down over 0.4V to 0.7V or so...

Edwin

[jarek]

While testing I also noticed that while the differential output can output a perfect sinewave of more than 13Vrms, the single ended outputs dont'.... did anyone noticed that also?

What did you mean? Isn't it a bridged amp?

[Andypairo]

Well, I mean that, running it unbalanced mode, the output waves of the two halves (referenced to GND) are very different in amplitude (but this seems normal).

The funny thing is that, when the input signal is strong enough to make the more "driven" side clip the other half compensates for it, giving a ugly waveform that, summed to the clipped other, makes a almost perfect output waveform.

I wish I had a digicam... a image is better than 1000 words sometimes...

Another thing: I tried to see on my scope the AC current flowing in the lower mosfets but, while I expected to find a sine wave, I only see it at very low levels; raising the input voltage it becomes a more peaky waveform.
The current on the upper side is instead perfectly sinusoidal: does this mean that the current source is doing all the job?

Cheers

Andrea

[jarek]

Look at: http://www.diyaudio.com/forums/showt...636#post196636

[Netlist]

Originally posted by Andypairo
absolute offset becomes more and more negative and stops at about -7V.

Andrea,
After reading through various posts of the original Alephx thread
my guess would be that there is something wrong with your
diff-pair current source.
-7V is simply not normal. Are you sure you have no bad soldering or wrongly connected components?
Also, the 1k5 resistors could indeed be too low.
What are your values of R12/R34?
Greetings
/Hugo

[Andypairo]

Jarek - the link you posted was very useful, I followed some of Nelson's suggestions (R46-R47 are now back to 4.7k) and things got better.

The big change was when I also changed R14 (R31) and R32 (R15) to 1.5 k and now things are going even better.

Hugo - The waveforms I obtain are now quite good, the only thing left to trim is the AC gain of the current source, which seems to have too much gain: if I measure the AC current on the low side I get about 300 mA pk-pk, while on the positive side I have almost 900 mA pk-pk... maybe R12/34 is too high (1.2k, as from Grey's schematic) ?

I'll try changing it and see what happens...

Cheers

Andrea