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DFroehlich 25th April 2006 07:14 AM

Problem with aleph 5
I 'm in the building process of an aleph 5 with boards that have the old brian gt layout. The output section is hardwired. Now the problem: one board and output section is working ok, but the other board with the same output section does strange things.
First R11 (221 Ohm) smoked, so I changed the zener Z5(and of course R11). Now R14 smoked and I changed Q3 (and of course R14). Now theres no smoke and the voltage measurements on R11 and R14 are ok. The positive bank of output fets is ok too ( there are 0,6 V on each 1 Ohm resistor and the heatsink gets warm. On the negative bank there are 0,6 V on each fet too, but the heatsinks stays damn cool. i don't think that the negative fet's are blown, because there was no smoke, but to be sure i have to change to my other board this evening.
Is there anybody who have a clue, what's the problem ?
maybe I have to change Q4 and Q5 ore Q1 and Q2 ?
Thanks in advance,

BobEllis 25th April 2006 12:24 PM

What is the output DC voltage? I'll bet it is close to the negative rail. Measure the Gate-Source voltages on the gain devices (negative side), I'll bet that you find it is high. This means the current in the input section is high (quite a bit more than 4V).

Check the zener is the proper voltage and that you get around 4V at the differential pair sources.

I don't have the A5 schematic, but is R11 connected between the positive rail and the source of the input CCS? What is the voltage across it? Your smoked R11 may have been indicative of a failed IRF9610. Are the gates of the input pair at approximately at ground potential?

DFroehlich 27th April 2006 06:52 AM

Hallo Bob,
thanks for your reply. It is like you said:
The DC Voltage is at 32,9 V (nearly the Supply-Voltage). The gain to source voltage of the gain devices is 8,8 V.
The differential pair sources are 3,5 V above ground.
R 11 is in fact between positive rail and source of the input ccs and the voltage above it is 5,4 V.
The gates of the input pair are: Q2 33,4 V above ground and Q1 0 V above ground. Q2 is the fet who's drain is directly connected to the negative rail.
Does that mean there is something wrong with Q3, that I just have changed or is there another problem?
Thanks for help

BobEllis 27th April 2006 10:31 AM

The voltage across R11 is a little high - perhaps the zener voltage is just outside its tolerance, but at least the CCS is working, since you have 4V on the sources. Once you get the main problem solved you may have to go back and change the zener to get the proper current in the differential and therefore good DC offset

Q2 is blown - it will not survive that much gate-source voltage. That explains why you have 8.8V on the gates of the output devices. You'll have to replace both Q1 and Q2.

Before replacing the input devices, find out why you have positive rail voltage on that gate. Are you sure that you didn't have your leads swapped and it is really near the negative rail? The latter seems more likely, and would be the result of the feedback network. Check for solder bridges.

DFroehlich 4th May 2006 09:37 AM

Thanks Bob.
The problem is gone.
Now I have to make some hardwork for the chasis and a better matching for Q1 and Q2 because in one channel I have unmatched devices and because of that an dc-offset of 0,25V.
I let you know when my aleph is playing music, but until that there may be a few weeks, because of the little time I have.

BobEllis 4th May 2006 10:55 AM

Glad I could help.

DFroehlich 1st November 2006 08:03 AM

It is unbelivable, but I' m still not ready with my aleph 5.
But the proplem is that I'm short on time.
Now I have one channel of the aleph running. there is a little humming in the speaker, but that is not the problem.
The second channel has a big problem:
All measurments are ok except for Q 5. There is only 0,6 V across emitter and basis instead of 5V and there is no Voltage across the 1 Ohm resitors at the output stages. The voltage on Q4 is ok. Both MPSA18 are ok, because I changed their position on the board.
Has enyone of the experts an idea, what the problem may be ?
Excuse for bad english, but I worked on the alephs last night until 5 o clock and now I'm searching for some sleep at my work :o
Thank's for any ideas.

DFroehlich 2nd November 2006 06:53 AM

No problem anymore, my alephs are singing and the sound is realy excelent.
I made a wrong connection of the sense points and this took me a night and I didn't see that. Yesterday afternoon it took me two minutes and I fixed that. Sometimes it is better to sleep then to solder a whole night .
Now I'm very pleased with my work and I wan't to thank Nelson Pass for his genourosity.
The next project will be a good preamplifier for the alephs and I'm not sure if this will be an active or a passive one.

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