Help needed: Firing up the Aleph-X100

[hifiZen]

R11 and R33 are supposed to be 47.5K - it's the correct value, straight from Grey's original schematic. I have built the prototype channel over a year ago with these values and it has always worked properly. Note that R11/V1's only function is to supply a wee bit of extra bias current to the base of Q3. The more current is provided at Q3's base, the lower the current through Q1 will be. However, 4.7K seems a fair bit too low to me. Grey used high values specifically so that this little resistor network wouldn't load and interfere too much with the voltage dividing duties of R8 / R12.

Note that MPSA18 has a DC h(fe) of 500 - 1500, while BC550 has 110 - 800 (fairchild datasheets), so it is possible that you got a part with low h(fe) and this contributed to the problem. I suspect there is still another reason, though. Have you actually measured all the other component values carefully? I'm thinking especially of R14 and R15 here, but it wouldn't hurt to check R7 and R8 as well... it's possible some of them have been damaged and are no longer their proper value. You may also want to re-solder the joints in this part of the circuit, in case you've got a cold joint somewhere.

One other noteworthy point: Q4 and Q9 will normally kick in to protect Q2 and Q11 from damage, if you've got the voltage dividers set up properly for the desired current limit (R10 & R13, with R6 being the current sensing element). If this happens, and the current source is not happy, it may start dumping current through the load, and subsequently activate the current protection on the other side as well. When this happens, the errant current source will not be able to deliver the current it wants to, and thus your gate voltage may 'run away' to some high value... like, say, 14V... Keep in mind, the max Vgs for these power mosfets is around 20V, and once you've exceeded the SOA or punched a hole through the delicate oxide, all sorts of bad things will happen to your circuit.

[hifiZen]

Just want to make a minor correction to my previous comment about R11 and R33... 4.7K should be fine in this position. For some reason, I wasn't keeping the 100K trimpot in mind at the time I posted that (doh!). See also Netlist's comments here .

[Blitz]

Yup. I changed back to 47K and this does not give me enough headroom for adjustment.

OK, Here are the results for the moment:

Successes:
- 0,02V absolute offset
- less than 0,01V offset between + and -
- with a simple choke-input-supply 30mV ripple in the rail which results in
- 0,18mV ripple at the speaker, I guess it is already very quiet

Still challenges:
- On one side I have two 044n in the gain stage. Now, one gives me 0,33V, the other 0,5 V over the 0,33 resistor. Now, when I exchange both with each other, the problem does not move with the mosfet, the values stay ! Ok, move all resistors and cabeling around as well: The problem stays at the same location. OK, dis-mounted the mosfet with the lower value: Now we have normal values of 0,5V...Ok, changed the insulator to mica...stays..back to silpad...stays, anyhow no connection can be measured to the heatsink...but when mounting it back to the heatsinks, it behaves again strange...

...to be precise: It starts fo a short time with the right value and than the voltage drops from 0,45V to 0,33V over 60 sec.

Ok, may be ocillation, so put in a 47pf mica capacitor between gate and collector...same behaviour...

By the way: The differences between voltages over the 392R-resistors are still there and don't move as well when swapping the mosfets against each other. So, THere's still something in the circuit which does not work as intended...any suggestions on how to trace it back are highly welcome.

[Blitz]

By the way: Changed as well from 4k7 to 2k2-resistors for reduction of DC.

Hifizen: What are good values for the anti-oscillation-caps ?

A BC550C has a hfe of 500-800, so should be OK.

[rtirion]

I recently had a similar experience in a aleph3.
I used a BC550 bipo and IRFP150 FET.
Ugly oscillation in the Aleph current source.
Fixed it using a 1nF from collector to base on the BC550.

You could study the Aleph service manual to compare.

Regards

[Blitz]

Thanks, will try that. What types do you use ? Mica ? Ceramic ?

Just for the records (in Hifizen's schematic):
- Exchanged R14/R31
- Exchanged R15/R32
- Exchanged R8/R37
- Exchanged Q3/Q8

No effect.

In the meantime I made the following observations:

1. On the one side, the current sources work nicely at the same current, but as described above the voltage across the 0,33R of the gain 044ns the situation becomes worse: Now I have 0,50V at one mosfet and 0,21V across the other, so one mosfet has to work harder and harder

2. On the other side, the gain side is nice, but I have similar phenomens on the current side: there I have one mosfet working with 0,6V very hard while the other one with 0,3V is a lazy guy.

Differences across the 392 stayed as before.

One question: I measure the supply voltages at the mosfets and they are all the same. Nevertheless, I made them p2p with strained Teflon cable, specified for 6A. May this be a source of problems as well ? Or are the 044n in combination with the BC550C something bad ?

[Blitz]

I just found this Aleph-X-schematic:

http://www.myclie.info/images/ALEPHXSCH.pdf

There the Anti-Osci-Cap is not from collector to base but from collector to emitter. What is adviseable ?

Best Regards

[rtirion]

I have used 1nF MKT. From C to B

I also tried using a capacitor for C to E. I had to use 47nF to tame
the oscillation. This big a cap will have an influence on the
operation of the Aleph current source. So I switched to the smal
valued 1nF on C to B at the bipo.

Try sticking to a single schematic. HifiZen's for instance. Your link
points to a pdf with a modified version. No values in the
schematic. I don't even know if there is an amp build based on
this modified version. I would not use it, if I were you.

Regards

[hifiZen]

Blitz:

The stabilizing caps should go between base and collector. This forms the local feedback loop around the transistor which lowers it's gain at high frequencies, thus taming the oscillations. Also, the Miller effect essentially multiplies the cap's value by the gain of the transistor, so a much smaller value can be used than would otherwise be necessary for a given pole frequency. I'd use a good ceramic type in this position.

1nF might work OK, but I'd try for to find the smallest value that works properly. If you're in the sub-100 pF range, you can use a 'gimmick' to find what value works... just take apart some cat5 cable or telephone wire and use one of the twisted pairs as a really long capacitor... usually they'll specify the capacitance per length in the datasheet, or you can make your own gimmick from solid-core hookup wire. 1pF per inch is approximate, if you don't have a cap meter. Keep trimming bits off the end of the gimmick until the oscillations reappear, then you know where the stability limit is. That'll give you the capacitance needed for a teeny-tiny phase margin. Multiply the value a few times, and test with a real cap of that size. If you have a square wave generator and oscilloscope, you'll be able to see overshoot and ringing, which is more useful than just watching for full-blown oscillations.

Sort out the oscillations first, then deal with the DC problems. My bet is, if your speaker output has a low differential DC offset (and it does), then the diff pair is doing it's job just fine, and the difference in voltage you see across the 392 ohm resistors is what the diff pair is doing to compensate for differences in the output stages on each side of the amp. I'd be interested to know what happens when you remove the lower BC550s (Q4/Q9)... be careful when you do this, as these transistors protect your lower output devices from too much current! Watch the voltage across R6 as you power up and if it gets too high, then shut it off before anything blows up.

If your amp is built exactly as in Netlist's previously posted schematic, then the output transistors on one side should share current very nicely. If not, then you have some matching / heatsinking problems (one transistor is getting hotter than the others), or possibly a source resistor which is damaged, and changing value as it heats up. Have you also checked the 221-ohm gate resistors to make sure they're still OK?

[Netlist]

After some searching I found the post that led to the explanation of the famous compensation caps in my first WIKI write-up.
There has been a whole lot of discussion in the AlephX thread long ago about this issue.
Finally, the answer came from Nelson himself and is here:
http://www.diyaudio.com/forums/showt...3550#post33550

/Hugo