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Old 22nd March 2020, 11:05 PM   #651
mbrennwa is offline mbrennwa  Switzerland
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Quote:
Originally Posted by Zen Mod View Post
it would be , in case that I know that you have entire chain fully differential, properly balanced and floating

which nowadays is rarely the case, most output stages being electronically balanced, and often not having equal Rout of phases

though , you can try , and leave option which most suits you , sound-wise and hum-free wise
The source has a transformer output, so it is properly balanced and floating. I just wanted to be sure that I am not missing anything. Changing from one setup to the other will not be difficult, so I'll just give it a try and see.
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Old 5th April 2020, 06:03 PM   #652
mbrennwa is offline mbrennwa  Switzerland
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Default Oscillation!

So I just finished building the first channel of the amp as shown in post 644. As far as I can tell, the thing almost works. However, the lower MosFET starts oscillating if I turn up the bias voltage.

What I tried so far:
  • Removed the global feedback loop (for easier debugging)
  • Tried a larger gate stopper (did not help)
  • Controlled the gate from a regulated DC power supply instead of the input/driver circuit (oscillation is the same).

Any ideas what else I could try?
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Old 5th April 2020, 07:13 PM   #653
Zen Mod is offline Zen Mod  Serbia
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try usual 1nF capacitor cure , 2 positions per channel



see attached file
Attached Files
File Type: pdf Aleph30 by Algar_emi.pdf (29.9 KB, 26 views)
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Old 10th April 2020, 11:27 AM   #654
mbrennwa is offline mbrennwa  Switzerland
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Quote:
Originally Posted by Zen Mod View Post
try usual 1nF capacitor cure , 2 positions per channel
Ok, I tried that. Didn't help. I believe it was even a little bit worse.

The only thing that kills the oscillation was to use a different MosFET in the Aleph source. I found an IRFP150 floating around the bench, and tried that (leaving the 2SK214 in the lower position). Oscillations gone!

Are the 2SK214 particularly sensitive to oscillation?

I have to say that I don't understand a thing about those oscillation issues. How do they happen?
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Old 10th April 2020, 11:45 AM   #655
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well, I didn't catch that detail , that you're using some other mosfet in Aleph CCS

2SK214 there ........

now it seems that I didn't catch anything ...... are you making some mini iteration , with T0220 mosfets?

however..... main problem you had is lower Ugs of mosfet used in Aleph CCS(comparing to IRFP), so small bjt is having lesser voltage window, causing oscillations

no special difference if you use ( even if yuo can without problems) Audio-approved part there, most things are dominated with small bjt and even its job isn't critical

but everything needs to be optimized , as is already in original Papa's concept
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Old 10th April 2020, 11:57 AM   #656
mbrennwa is offline mbrennwa  Switzerland
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Quote:
Originally Posted by Zen Mod View Post
well, I didn't catch that detail , that you're using some other mosfet in Aleph CCS

2SK214 there ........

now it seems that I didn't catch anything ...... are you making some mini iteration , with T0220 mosfets?

however..... main problem you had is lower Ugs of mosfet used in Aleph CCS(comparing to IRFP), so small bjt is having lesser voltage window, causing oscillations

no special difference if you use ( even if yuo can without problems) Audio-approved part there, most things are dominated with small bjt and even its job isn't critical

but everything needs to be optimized , as is already in original Papa's concept
See my pre-previous post: I built the amp as in post 644 (here).

Of course I could save the precious 2SK214 and use something cheaper in the Aleph source (and I probably will). But I'd like to understand what's going on with those oscillations. What's the story with that voltage window thing?
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Old 10th April 2020, 12:03 PM   #657
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IRFP needs approx. 4V for gate being above source , and you have put small bjt in that voltage window

you used 2SK there , which is having half of IRFP voltage ( max 2V ) from gate to source , and you still putting small bjt in that window .... so it's sorta voltage starved

same as when you put sack of cement on your shoulder and need to bring it 100 feet

not so hard, even if boring, if you are walking with your back straight

try the same, while walking through 1.3m high corridor.... after some time you'll have your back snapped, or - in best case - you're going to fight for your breath

same with small bjt ....... he's fighting for its breath, while behaving hysteric (with reason)

transistor demands some minimal rail voltage for at least so-so linear operation
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Old 10th April 2020, 01:15 PM   #658
mbrennwa is offline mbrennwa  Switzerland
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Quote:
Originally Posted by Zen Mod View Post
IRFP needs approx. 4V for gate being above source , and you have put small bjt in that voltage window

you used 2SK there , which is having half of IRFP voltage ( max 2V ) from gate to source , and you still putting small bjt in that window .... so it's sorta voltage starved

same as when you put sack of cement on your shoulder and need to bring it 100 feet

not so hard, even if boring, if you are walking with your back straight

try the same, while walking through 1.3m high corridor.... after some time you'll have your back snapped, or - in best case - you're going to fight for your breath

same with small bjt ....... he's fighting for its breath, while behaving hysteric (with reason)

transistor demands some minimal rail voltage for at least so-so linear operation
Ok, I am tall, so I need a bigger tunnel. Makes sense. Which MOSFET would work well? IRF610? IRF530? A TO220 part would be nice so I can re-use my contraption to fix the outputs on the heatsink.
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Old 10th April 2020, 01:35 PM   #659
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any of IRF small ones you have in drawer, use one with greatest xconductance
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Old 11th April 2020, 03:13 PM   #660
mbrennwa is offline mbrennwa  Switzerland
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any of IRF small ones you have in drawer, use one with greatest xconductance
I couldn't find a TO220 IRF, so I ordered a few. Left the IRFP150 in there for the moment.

In the meantime I had some fun with the function generator and the scope. I found that the square-wave response was slightly deformed on the falling edge. I don't know what this is, but I could make it go away by inserting a 15 pF capacitor from gate to drain of the gain MosFET (directly on the pins), as shown in the attachment. Any ideas what's going on with this?

(sorry, I forgot to take some screenshots from the scope)
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