Another Aleph-X coming up!

[Edwin Dorre]

Originally posted by Netlist
The nicest way to reduce offset is what Peter says in post #11.
Second, I used the resistors from common input sources to outputs and I set the value at 4.7k. Bringing it further down to 2.2k increased stability of the offset.
Coupling caps at the input also helps a lot, not only for the zero offset, but also for offset drift.
I never used 100ohm resistors at the output and it works great: 1 to 2mV after 10 to 15 minutes, and very stable.
Look at http://www.diyaudio.com/forums/showt...696#post173696 and you will see how I mounted the input fets; very handy to change them till they match perfectly.
There's also a post somewhere that says to first adjust V1/V3, then V2. V2 has a great infuence on offset.
These are my findings when building the amp.
What value did you use for R19/29 and for feedback and input R's?
Did you use C2/4?

/Hugo - can't wait to see pics of the finished amps.

Hi,

First of all; I am talking about DC offset between OUT- and OUT+, I am NOT talking about absolute DC offset to GND...

Ok, I use 5pF for C2 and C4. I have the 4k7 from source to output in place. R19 and R29 are 10k (like in the 'final' from hifizen's PCB).

I have found this article from Klaus; DC-offset in spite of perfect matched FETs!?

in which he writes the higher zener output gives him problems. When I measure between my two amps (the prototype 25W Aleph-X with 4 mosfet and my 130W Aleph-X with 16 mosfet) the first has a zener voltage of 9.2V and the latter of 9.4V , can this be a source of my problem?

I have to go to work now, but I will test it tonight by swapping the zener...

Edwin

P.S. All my 1/2W resistors are 0.1% too!

[Edwin Dorre]

Originally posted by grataku
R17 doesn't do anything to the DC, it just control how much current flows through the voltage reference.

That is what I thought; but what is 'the resistor' of the CCS of the input differentials??? The only thing I can come up with is the R24, R26, VR2 thing... But these have a VR2 trimpot as default. Where o where to put another trimpot?

I just have put in the 100 ohm resistors to ground, it does not do a thing for the OUT- OUT+ DC offset


Edwin

[jh6you]

Originally posted by Netlist

I never used 100ohm resistors at the output and ...

Hi Hugo

The 100 ohm resistors are to minimise DC offset drift.
If there is any negative effect, what would it be?

JH

PS. Life in Shanghai is not bad... Many pretty girls (just for looking)

[nar]

Edwin,

The 100 ohms between outputs and ground do nothing to differential DC offset. They only help a little so settle absolute DC offset ( i.e. between outputs and ground ) .

With 100 ohms they help only a little . If you reduce this value down to 30 ohms you will see a more effective control of absolute DC,but as Nelson stated we begin to charge the circuit with loads which affect the circuit , if you go below 30 ohms .

The key for me is to match merely 9610s first,at 10 mA each and have several pairs to swap in and out,and play on the VR trimmpot of the 9610 current source until you achieve lowest offset at iddle.

Implement 4k7 magical , or between 2k2 and 4k7.You will see it gives a different absolute DC offset at turn on , and that you will have to readjust the trimmpot to get a 0V DC between outs and grounds.

I don't think it is a good idea to adjust current in the 2 output sections to control the offset. In my prototype 23 V rails 4,25 A bias I used fixed resistors on both sides to get proper current in each Aleph out , and stays the same R value on each mirror-side.

Hope that helps !

Regards

[grataku]

Edwin,
the zener should be no problem, you could always go to the other extreme and install a 6.9 voltage reference instead of the 9.1 zener. I would stick with the 9 V reference anyways. The trimpot in parallel with the resistor was put in to reduce the adjustment range, and it only affects the absolute dc offset (btw each output and the gnd) not the actual offset at the speaker, which is the one you are interested in. If you can bring the absolute dc down to within 100mv using the ccs pot V2 then it means you are in the correct range and you shouldn't mess with it further. Of course things may change when you play with V1 and 3 and you may need to readjust V2 and that may bring you out of range.
There is no question in my mind that your problem right now is mismatched mosfets and the bigger problem for you right now would be to take them out and re-match them. It is not straightfoward to find the perfect balance.


The only tolerance I am worried about is the 3W 0.56 source resistors (r 1-6, r40-45).

Ultimately, the same current should flow on both sides ie the voltage drop should be equal on all the source resistors and both absolute and actual DC offset should be as close as possible to 0. Whether you want V1 and V3 in place or not it's up to you.

[Edwin Dorre]

Originally posted by grataku

If you can bring the absolute dc down to within 100mv using the ccs pot V2 then it means you are in the correct range and you shouldn't mess with it further. Of course things may change when you play with V1 and 3 and you may need to readjust V2 and that may bring you out of range.

There is no question in my mind that your problem right now is mismatched mosfets and the bigger problem for you right now would be to take them out and re-match them. It is not straightfoward to find the perfect balance.


The only tolerance I am worried about is the 3W 0.56 source resistors (r 1-6, r40-45).

Ultimately, the same current should flow on both sides ie the voltage drop should be equal on all the source resistors and both absolute and actual DC offset should be as close as possible to 0. Whether you want V1 and V3 in place or not it's up to you. [/B]

Thanks Grataku for the time you take to help me out, much appreciated;

1. I can change the absolute witin 100mV without any trouble so the V2 range seems ok. So I will not touch it!

2. I have set VR1 and VR3 to about .47V over the .056 ohm resistors. Which gives me a total bias per side of 3.35A . This is about what I am aiming for (between 3.3A and 3.5A) ... With this bias I can change the absolute within 100mV with V2 .

3. My 'old Aleph-X' had about 0.07V difference in differentials and it work ok... still a mystery...

4. I have adjusted the ACS gain to a perfect 50%

5. My 0.56 are 2% intertechnik metalfilm 5W resistors 300ppm.

Some questions; (again!)

1. I can take out the 9610's from my test Aleph-X and put these in, just to see if these are better 'matched'. Is this wise to do?

2. When I rematch; should I do it on 5mA 10mA and 20mA ??? What is the best way?

3. Should I buy new resistors and match them? But how?


Thanks,

Edwin

[grataku]

The 9610 are certainly a big part of the problem, but the biggest problem maybe the output mosfets, both the ccs and gain, since I seem to recall that substitution to a better pair caused you worst offset.
I would suggest to try to re-match them at two different current levels by changing the resistor in the matching circuit. To match the 0.56 you should be able to insert them in the mosfet matching circuit by keeping the mosfet the same and changing resistor. In any case, the best thing is to use a matching current that is similar to the current at which the mosfet will operate in the actual amp circuit.

[Edwin Dorre]

Originally posted by grataku
The 9610 are certainly a big part of the problem, but the biggest problem maybe the output mosfets, both the ccs and gain, since I seem to recall that substitution to a better pair caused you worst offset.
I would suggest to try to re-match them at two different current levels by changing the resistor in the matching circuit. To match the 0.56 you should be able to insert them in the mosfet matching circuit by keeping the mosfet the same and changing resistor. In any case, the best thing is to use a matching current that is similar to the current at which the mosfet will operate in the actual amp circuit.

When I changed the 9610 for a new pair, the DC offset changed from 0.221 to 0.132 ... So it halved! However the voltage difference between the 390 ohm resistors did not change much!

Edwin

[Peter Daniel]

Originally posted by Edwin Dorre


When I rematch; should I do it on 5mA 10mA and 20mA ??? What is the best way?

Should I buy new resistors and match them? But how?

To calculate the current when matching input differential divide expected voltage drop on 392ohm resistor by 392 (ohms law).

Measure the resistors to see how close they are, they shouldn't be too far off.

[Edwin Dorre]

Originally posted by Peter Daniel



To calculate the current when matching input differential divide expected voltage drop on 392ohm resistor by 392 (ohms law).

Measure the resistors to see how close they are, they shouldn't be too far off.

Thanks,

I know this; the 9610 mosfet's give over 390 ohm (european E-serie) after 15 minutes of running 4.52V and 4.50V which means 11.58mA and 11.53mA .. So they look matched close enough so it seems... still I keep an DC offset of 118mV ...

I just found out the values of my ACS source 0.56 resistors after 20 minutes of running (hot heatsink), they all start higher but they decrease each about the same amount; otherwise said; the voltage difference between the mosfet's stay the same but overall they decrease when they heatup:

leftside:

4.63
4.59
4.62
4.48

= avg 4.58V

and

rightside:

4.55
4.59
4.64
4.51

= avg 4.57V

I will swap the 4.48V and 4.41V with new resistors to see what happens!

Edwin