amplifier oscillations

If an amplifier is "unstable" it will oscillate. The characteristics is a very high frequency at the output of the amplifier, in the MHz often. This frequency is dependent on the speed of the feedback loop. Oscillation is very undesirable. Even though it is inaudible, the transistors are switching at that speed and waste a ton of energy and will get very hot. The transfer function of the amplifier must not have "poles" on the right half plane.

BeanZ
 

lowfi

Member
2002-03-22 10:15 pm
i just got an aleph 5 up and measuring okay, without R21 loop. the fellow that was helping me, suggested that because the amp worked only when the R21 loop was omitted, that i may have oscillations. are there any other possibilities? all the key measurements are within spec. from the service manual and there is only between 27 and 51 milivolts dc at the outputs.

any thoughts?
 

lowfi

Member
2002-03-22 10:15 pm
i'll try to be more specific, but not sure how much i can add.

when i have R21 hooked up, VR14 is too low to turn on the input pair, or whatever it's supposed to turn on. when i disable it, boom, well i guess that's not the correct word. how bout voila, outputs turn on and everything measures fine.

hope that helps.
thanks,
scott
 

lowfi

Member
2002-03-22 10:15 pm
H.H.
whoah!!! i tried reading that article, but it's gonna take a few or a hundred passes for it to sink in. i've never seen this material before...way over my head.
i guess i need a scope to really find out if my amp is unstable in open loop, which would lead to instability in closed loop, eh?
what are the implications of me not having that current source loop in my amp? and why on God's green earth would my amp work without it in?
:( :confused:
 
Harry's idea of humor is to hand a drowning man an anchor. We don't need a lecture on stability, we need an oscilloscope. I'm assuming that you don't have one, or you would have already tried that.
You said at first that all the voltages read properly according to the schematic, but then you said that the voltage across R14 is low...
With R21 in place, what voltages are you reading across the back end of the amp, meaning R40-42, R64-66, and across Q5. And just for fun, is there any DC offset when that's hooked up?

Grey
 
You replied while I was typing...
The amp will work either with or without R21. The traditional way to run a single-ended amp is <i>not</i> to have R22-25, R21, C10, and R19. Those parts are the ones that make it an "Aleph," remove them, and you've got what amounts to a Zen amp with a front end. Granted, the rest of the amp might could stand a bit of retuning if you disable that part, but it will work. Sometime later--after you get the amp running properly--read Nelson's patent for the Aleph current source (it's actually pretty readable). The objective is to get more power out of a single-ended design with less heat dissipation...i.e. make it more efficient. Compare the original Zen schematic (not the recent version) with the Aleph output stage, and you'll see what I'm talking about.
But, like I said, it will work, just not as well.

Grey
 
Harry's idea of humor is to hand a drowning man an anchor

And your idea of humor is to post schematics of amps that have no high frequency compensation. Tell us what you plan to tell this guy to do about it when he finds out with a scope that his amp is oscillating, and who is doing the greater diservice to beginers. You seem to imply that it is fine to leave caps out of Nelson Pass's origional designs without the basis of knowing what constitutes an amplifier with adequate high frequency stablity. There are plenty of tested and working designs by Nelson Pass for beginers to copy (with all the neccesary caps hopefully). Those who want to design amps are well advised to know a few fundamentals. Phase margin is not that tough to understand or measure. Spice models can give good indication before building the amp.

H.H.
 
Harry,
If he's built an Aleph 5, then I'm assuming that all the parts--including the caps--are there. Perhaps they aren't, but he hasn't mentioned any modifications yet. Perhaps something isn't hooked up properly. Perhaps there's a solder bridge. Perhaps there's a cold solder joint. Perhaps any number of things. But unless and until he gets to an oscilloscope and finds an oscillation, I'm going to assume that an Aleph 5 is stable. If you feel otherwise, feel free to argue the point with Nelson Pass, it's his design, not mine, caps and all.
If you intend to help, then help. If all you intend to do is be a back seat driver, then please do so in another car. Your idea of 'help'...isn't.

Grey
 
feel free to argue the point with Nelson Pass

I never agued that NELSON PASS'S designs are not stable. The question is, if that is that what he built..... Since it means so much to you, I will let you help him out. We are interested to see to see your troubleshooting skills in action. I look forward to your expert advice.

"what are the implications of me not having that current source loop in my amp? and why on God's green earth would my amp work without it in?" Grey,you want to explain this in layman's terms?



H.H.
 
Explain a current source?
In layman's terms?
Why, sure, glad to.
Let's come at this from the backside. Let's talk about voltage regulators. So, what is a voltage regulator? A voltage regulator is a circuit that takes a raw, incoming DC voltage with all its warts and imperfections and smooths it out into a constant, predictable voltage. No bumps, no wiggles, just pure, clean, steady DC voltage. If you look at it on an oscilloscope, there's nothing there; it's just a straight line, nice and boring, just the way we want it to be.
Let's say that we've got a load that draws current in spurts, like the output stage of a class B amp. If there's no regulator in the circuit, just a transformer and rectifier feeding some caps, the voltage on the caps will drop when the circuit demands a large amount of current (unless the power supply is very, very large indeed). This isn't good. It changes the behavior of the amp. So we stick a regulator in.
The regulator works by chopping off all the voltage above a certain preset level (yes, this is wasteful, but necessary). If you set a regulator for 20V, then feed it 30V, it will lop off the 10V that it doesn't need, allowing 20V to come on through. If the circuit demands more current, the regulator delivers more current, but at that same preset 20V. This is the crucial point for our present purposes--a voltage regulator holds the voltage steady, but allows the current to vary according to the needs of the moment.
Okay, let's turn back around and look at current sources. The thing that makes a current source mysterious is that it has multiple names: current source, current sink, and...wait for it...current regulator. Fasten onto that last name and things will become clearer. Whereas a voltage regulator maintains a steady voltage and allows the current to vary, a current regulator (aka current source) maintains a steady <i>current</i> and allows the voltage to vary.
Think of a circuit as a variable resistor. From the current source's point of view, there's somebody out there twisting the knob back and forth, back and forth, with no rhyme nor reason. The load changes unpredictably. That's okay. Current sources are patient little fellows. They're also obedient. If you tell your current source to deliver 10mA, it will do everything in its power to deliver 10mA no matter what the load does. If the load goes to an effective 10 ohms, the current source will set itself to .1V (10 ohms * .010A =.1V, simple Ohm's Law stuff). If the load suddenly goes to 1k, then the current source will set itself for 10V. At all times and in every way possible the current source will do its dead level best to deliver 10mA, simply because that's what you told it to do.
Okay, so where's that leave us in terms of an Aleph output stage?
The bottom transistor is the actual amplifier. It gets its signal from the front end differental (the two IRF9610s back-to-back to the left of the schematic) and amplifies it, in this case adding both voltage and current. The current source is the MOSFET and NPN (plus a few resistors and caps) up above. Normally, and there are numerous commercial and DIY examples of this, you would expect to see a conventional current source up there. The resistor on the upper MOSFET's Source sets the current and the MOSFET delivers it. The NPN is there to oversee the operation and kinda nudge the MOSFET back into line if it does a less than perfect job; this being the real world, the MOSFET benefits from a little help in accomplishing its task. (In fact, you can build an even simpler current source without the NPN, but performance would suffer a bit.)
If we were to leave the circuit here, this is what would happen: signal would enter the lower MOSFET and cause an output signal to appear at the Drain. The current source, seeing that the voltage is varying, and seein' as how varying voltage is second nature to a current source, goes along for the ride. As the voltage varies, the speaker (whose other side is connected to ground--nominal 0V) will see current travel back and forth, depending on whether the signal happens to be swinging above 0V or below. Music will come out of the speaker.
Back to the Aleph part of the deal, which is the crux of the matter. What Nelson did was add a few parts to the current source, so that it's even more attuned to what the lower MOSFET is doing.
As the signal leaves the circuit, the very last thing it does is pass through an array of resistors (R22-25 in the Aleph 5). The fact that these resistors are in series with the load means that they serve to sense the current going out the back door. This signal travels back up into the innards of the current source via R21 and tells the current source that it's okay to vary a little bit, as long as it does so in accordance with what the lower MOSFET is doing.
Seems a little weird to be telling a current source <i>not</i> to be delivering constant current, but as long as it does so in synchrony with the lower MOSFET, it's cool. If you want to see a voltage analog to this, take a look at Part 3 of the current Zen series over at www.passdiy.com, wherein Nelson puts together a voltage regulator that...well...it varies the voltage. Again, in synchrony with the output of the amp.
If you want more of the grisly details, check out the patent. It's not all that difficult to follow.
Incidentally, Nelson likes to point out that although the Aleph current source varies on an instantaneous basis, when taken as an average over time, the current is indeed constant.
So what happens when you remove R21 is that you end up with a "normal" current source, i.e. one that doesn't dance along with its partner. With R21, you give the current source a little bit of wiggle room to dance along with the music in the current domain.
Incidentally, the IRF9610 sitting by its lonesome up above the differential is also a current source. It's one of the conventional kind that doesn't dance, current-wise. The Zener diode and the resistor to ground set a reference voltage. The resistor above its source is what sets the current, when referenced against that 9V. In principle, you could use a similar arrangement for the output current source, using the same 9V Zener and 10k resistor at the Gate of the upper MOSFET, but substituting a 10 ohm resistor below the source (better make it a 5 or 10W resistor), but you'd have to give up the "Aleph" part of the circuit to do it. You're better off with the NPN.
Howzzat?

Grey
 

lowfi

Member
2002-03-22 10:15 pm
holy crap!
i leave for a barbeque and look what happened! sorry about stirring up a debate, but that last post from Grey was fantastic! i read it a couple times over and think i follow it. so i guess for now, i'll work on getting the hum out of the amp (another thread/prob i'm trying to work on).
am i to understand that
a) the amp should work fine w/out the r21 loop, but that it was put there for a reason.
b) it may mean that something is unstable with my amp, but without a scope, i really don't have the tools to diagnose it further.
and because it is presumably a stable design, that this safeguard loop will not adversel affect the operation of the amp, but not help if the amp ever needs it.

i really appreciate the replies folks, and i WILL read those referenced articles/patent

cheers,
scott
 

lowfi

Member
2002-03-22 10:15 pm
well,
how 'bout this. i hook up a tweet and run some muzac through it. that should tell me if everthing's ok, no? i'd cross is over so it only "hears" the highs, and see if anything yucky happens. i'm having a devil of a time tracking down a ground loop right now. is it possible that the two problems (current regulator and grounding prob) are somehow related?

thanks,
scott
 
Okay, if you've got a tweeter getting hot, you've probably got an oscillation. It would still be nice to get to an oscilloscope, as it will keep you from roasting that tweeter. Waiting for a tweeter to get hot is about like doing brain surgery with a hammer; it's not exactly the best tool for the job.
Assuming that you're using the full Aleph 5 with no modifications, I'd start checking to make sure that all the parts are installed correctly.
Yes, I know, you've probably already done this ten times. Maybe you could get a friend to go over it for you. They don't necessarily have to know electronics, just treat it like jigsaw puzzle and look for physical match between what the layout is and what its supposed to be. (For instance, if you're using MPSA18s for the NPNs, the leads should be EBC from left to right with the flat front side facing you.)
You might want to reflow solder joints. Sometimes solder <i>looks</i> like it did a good job of connecting, but it doesn't. There could be no connection or a poor connection, and either one can make things act poorly.
Something I forgot to ask earlier--are both channels doing the same thing?
Another question--what are you using for a PC board?

Grey
 
i'm having a devil of a time tracking down a ground loop right now. is it possible that the two problems (current regulator and grounding prob) are somehow related?
Could be -- I've had marginally stable amplifiers before that sounded as if they had ground-loop hum. You might try adding a temporary local bypass capacitor (an electrolytic of 100 uF or more, observing correct polarity) from each power supply rail to ground as close as possible to the output stage.

If this makes the problem go away, remove the bypasss caps and try to cure the problem by shortening or redressing the output stage wiring (here I assume your output devices are mounted remotely from the main amplifier PCB). If the local bypass doesn't do anything, remove it and keep looking per Grey's suggestions above.
 

lowfi

Member
2002-03-22 10:15 pm
grey,

- yes, it's in both channels

- boards are Brian Bell's (brianGT) pcbs he had made.

i'll reheat the boards and re-examine population orientation tomorrow, with a somewhat fresher set of eyes.

i think i'll have to try fixing this sans scope.

thanks for the help so far,
scott