My dear Carlos,
do we both speak about the output stage Iq only (1302/3281), or do you mean Iq of the complete 1 channel?
Cheers,
Pavel
do we both speak about the output stage Iq only (1302/3281), or do you mean Iq of the complete 1 channel?
Cheers,
Pavel
PMA said:
Hi ! Thanks for the efforts and beta evaluations !
Sadly, i am quite busy here at work right now... 🙁
Pavel, if you can reproduce this instability, can you please trace it back ?
Means, check the signal at the base of the output-bjt, and drivers.
If the 6.6mhz signal is not present (or much smaller) in vas or driverstage, the outputbjts are oscillating on their own... In this case, the zobel maybe needs improvement, this was the best cure i found when i had that problem.
Which outputdevices are you using ? Looks like MJLs on your photo... 🙂
I can check closer this evening...
Michael
Yes, we had harsh in the first models...and modifications were done.
But do not remember exactly...those 47K or 100 k resistor that have parallell capacitor to ground....the capacitor position size..those things...Mike may remember...we had!...but we have not more...my units do not harsh...and Michael told me that their are OK too!
The entire amplifier...the final result...early first amplifier had some harsh!
Now i remember.... Michael found 10 milivolts of some high frequency....i suggest him to switch off all the electronics he have that can have oscilators and measure with an RF prepared test probe...switching of also the Scope, because the oscilators....but do not remember the result...but i think he found the cause...MICHAEL!!!
regards,
Carlos
But do not remember exactly...those 47K or 100 k resistor that have parallell capacitor to ground....the capacitor position size..those things...Mike may remember...we had!...but we have not more...my units do not harsh...and Michael told me that their are OK too!
The entire amplifier...the final result...early first amplifier had some harsh!
Now i remember.... Michael found 10 milivolts of some high frequency....i suggest him to switch off all the electronics he have that can have oscilators and measure with an RF prepared test probe...switching of also the Scope, because the oscilators....but do not remember the result...but i think he found the cause...MICHAEL!!!
regards,
Carlos
Carlos,
the oscillations in the amp I test are just Iq dependent. Decrease Iq and they disappear. This is an effect of Gm doubling in the output stage for higher Iq (class A to some Iq, then class B).
There is nothing mysterious, no oscillators in the vicinity etc. Just stability problem. The setup must be done the way no oscillations appear. There must never be oscillations in the amp, as they indicate a serious problem. They change with temperature and operating points of transistors. So there must be considerable stability margin to avoid unwanted problems. At at the edge of unstability setpoint they can appear and disappear, amp may sometimes sound harsh and sometimes not.
the oscillations in the amp I test are just Iq dependent. Decrease Iq and they disappear. This is an effect of Gm doubling in the output stage for higher Iq (class A to some Iq, then class B).
There is nothing mysterious, no oscillators in the vicinity etc. Just stability problem. The setup must be done the way no oscillations appear. There must never be oscillations in the amp, as they indicate a serious problem. They change with temperature and operating points of transistors. So there must be considerable stability margin to avoid unwanted problems. At at the edge of unstability setpoint they can appear and disappear, amp may sometimes sound harsh and sometimes not.
Good that nothing wrong was found.... so... Herman will put the beer again inside the
Freezer....i took them out..... someone with problems!!!..... now beer will go in again....hehehe
For Herman....for him, everything is OK!...problems means beer out of freezer, and herman drinking without stop....if no problems..he will wait to have beer latter.
He asked for a Forka..... hehe... Fork with "a".... he is speaking wonderfull prutrugeeze. (aonde vai vem fica garfa?)
that's good!
regards,
Carlos
Freezer....i took them out..... someone with problems!!!..... now beer will go in again....hehehe
For Herman....for him, everything is OK!...problems means beer out of freezer, and herman drinking without stop....if no problems..he will wait to have beer latter.
He asked for a Forka..... hehe... Fork with "a".... he is speaking wonderfull prutrugeeze. (aonde vai vem fica garfa?)
that's good!
regards,
Carlos
Harshness etc.
Now I have decreased iq as Pavel suggested and been listening to the same record which sounded a little harsh at first. The harshness has disappeared completely.
I had 16mV over the 0.22 resistor before the adjustment.
Thanks for the tip.
Jarno
Now I have decreased iq as Pavel suggested and been listening to the same record which sounded a little harsh at first. The harshness has disappeared completely.
I had 16mV over the 0.22 resistor before the adjustment.
Thanks for the tip.
Jarno
Carlos, i have exactly the same amount of HF-rubbish (3mv) visible on scope, independent on the amp beeing switched on or off...
I had often scope attached while listening and never observed such stability problems !
I am quite sure that this instability is not caused by the feedback, it looks more like the outputdevices making a problem.
Pavel, maybe your 4.7ohms in the zobel is wirewound ?
Mike
I had often scope attached while listening and never observed such stability problems !
I am quite sure that this instability is not caused by the feedback, it looks more like the outputdevices making a problem.
Pavel, maybe your 4.7ohms in the zobel is wirewound ?
Mike
Re: Harshness etc.
Hmm, obviously we have a problem ! I don't know why i don't have that symptom...
I tested with many different biasings, the difference was minimal, with bias below 10ma it started to sound worse.
Mike
jarkaa said:
Hmm, obviously we have a problem ! I don't know why i don't have that symptom...
I tested with many different biasings, the difference was minimal, with bias below 10ma it started to sound worse.
Mike
The 6.6 MHz were well more than 50mV in amplitude, not 3 mV. No ghost game. After dicreasing Iq they disappeared, but when amp got cold, they appeared for some time after turn-on again. So I decreased Iq again and now the amp is stable even for 1500pF capacitive load (some speaker cables may have similar capacitance). There were problems with 1500pF load before, but initial problems were even for no-load, only scope probe connected.
The 4.7R Zobel resistor is metaloxide, not wirewound. Sorry, I have daily worked in the development labs for more than 25 years.
The 4.7R Zobel resistor is metaloxide, not wirewound. Sorry, I have daily worked in the development labs for more than 25 years.
Easy out. Try a small film cap across C-B on each output. Not ceramic. Start at 47 pF, that might be enough. Another solution may be to bypass the supplies right at the collectors to the dirty ground. Ceramics may work here okay due to constant voltage across the cap.
-Chris
-Chris
Chris,
this I had to do for my push-pull follower with same output devices years ago. I will probably try it.
Mike,
I am using genuine Toshiba's 2SC5200/2SA1943.
this I had to do for my push-pull follower with same output devices years ago. I will probably try it.
Mike,
I am using genuine Toshiba's 2SC5200/2SA1943.
PMA said:
Sorry, i really had not the intention to insult you ! It just could have happended accidently and i wanted to be absolutely sure...
I just have heard/read too many people saying that wirewounds are never a problem ! 🙄
PMA said:
Maybe the Toshibas are even more sensitive to this problem ?
I could try to buy some, but will take a few days...
Mike
Hi Mike,
I wouldn't apologize if I were you. You're just checking the basics. These are the problems that can stump a tech for weeks. You are showing attention to detail. Seen that problem too.
I once serviced a cheap line of amps with WW emitter resistors. They all oscillated. Non-inductive resistors completely solved the problem. These amps made it past manufacturing approval and into the market. I'm sure the original designer had spec'd non-inductive (cost accounting engineers are great!).
PMA,
I don't think anyone doubts your abilities. I think everyone has respect for all the others in this thread. I too, will tend to ask really basic questions that have an obvious answer. Just to cover basic things that could be overlooked.
-Chris
I wouldn't apologize if I were you. You're just checking the basics. These are the problems that can stump a tech for weeks. You are showing attention to detail. Seen that problem too.
I once serviced a cheap line of amps with WW emitter resistors. They all oscillated. Non-inductive resistors completely solved the problem. These amps made it past manufacturing approval and into the market. I'm sure the original designer had spec'd non-inductive (cost accounting engineers are great!).
PMA,
I don't think anyone doubts your abilities. I think everyone has respect for all the others in this thread. I too, will tend to ask really basic questions that have an obvious answer. Just to cover basic things that could be overlooked.
-Chris
Simulation shows that stability of this amp is a very very delicate thing. There is a lot of OLG gain, high F-3db OLG point, excessive phase shifts and fast devices. So this topology probably would be component-dependent. I would rather like to see all the resistors placed horizontal, as vertical position increases wire length and lead inductance as well. But it seems that reducing Iq solves stability problems.
I think there is nothing wrong to point this at. Not every DIYer is equipped with costly measuring instruments and this experience might be useful for him.
Anatech - I am not touchy at all, I only sometimes bite when bitten, take it easy.
It is a nice co-operation we are having here, I am very glad.
I think there is nothing wrong to point this at. Not every DIYer is equipped with costly measuring instruments and this experience might be useful for him.
Anatech - I am not touchy at all, I only sometimes bite when bitten, take it easy.
It is a nice co-operation we are having here, I am very glad.
Hi PMA,
No problem, I am a little touchy right now. Just under the weather.
Anyway, I am concerned that under load we may see some spurious oscillations. Hard to see unless you are looking for them. I'm sure this amp is very component dependent. (not really a bad thing) Layout is always tricky with a wide bandwidth amp.
A little messing around with it as others build it may help for worst case compensation. I am having difficulty with drawing the traces by hand. A little shaky today too. You guys are doing a great job!
-Chris
No problem, I am a little touchy right now. Just under the weather.
Anyway, I am concerned that under load we may see some spurious oscillations. Hard to see unless you are looking for them. I'm sure this amp is very component dependent. (not really a bad thing) Layout is always tricky with a wide bandwidth amp.
A little messing around with it as others build it may help for worst case compensation. I am having difficulty with drawing the traces by hand. A little shaky today too. You guys are doing a great job!
-Chris
Clipping is decent and smooth, no phase inversion. This is very important.
Michael - excellent work.
I will take a break for some time now, and later will describe further results on my web page.
Michael - excellent work.
I will take a break for some time now, and later will describe further results on my web page.
Ahahaha...very good!.... Herman is sleeping with the dog...too much beer.
Iinvited him to go to School of Samba...were have Samba music and drums...all those things, more those indians alike the one i am showing ya.
I asked Herman.... the dog made...grrrrrrrr!
My dog!...this is not fair!....i invited him...he said:
Ya...gut!...and head fell down and sleep again...hic!..hic!
We will visit Indians....yeah!...nature..... only love for the art, of course.
I am very happy that Pavel did not dislike.... too much knowledge will always find something strange, as always have, and he is not finding nothing hard!....Party day for ME!
hehe.
Carlos
Iinvited him to go to School of Samba...were have Samba music and drums...all those things, more those indians alike the one i am showing ya.
I asked Herman.... the dog made...grrrrrrrr!
My dog!...this is not fair!....i invited him...he said:
Ya...gut!...and head fell down and sleep again...hic!..hic!
We will visit Indians....yeah!...nature..... only love for the art, of course.
I am very happy that Pavel did not dislike.... too much knowledge will always find something strange, as always have, and he is not finding nothing hard!....Party day for ME!
hehe.
Carlos
