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The AMNESIS amp: a good amplifier, like a gentleman, has no memory.
The AMNESIS amp: a good amplifier, like a gentleman, has no memory.
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Old 15th April 2019, 01:59 PM   #581
Hans Polak is online now Hans Polak  Netherlands
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Location: Blaricum
Quote:
Originally Posted by JOSI1 View Post
@Hans:
The attached .asc file is very close to my real amp.
Differences and modifications which did not solve the problem
are marked in red in the pdf-File.
Remarks:
Power Supply is +-32V
I do not use a coupling cap at the input since my audio source has a low DC offset <1mV
Changing the BC5xx transistors to your suggested 2Nxxxx types
does not solve the problem.

@Max
I use 330R gate stoppers for the JFETs and 150R gate stoppers for the MOSFETs.
What NTC thermistor (value, brand) did you use in series with power.
I found something. Will come back in a few hours.
At least the VAS is playing a big role in this.


Hans
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Old 15th April 2019, 03:46 PM   #582
Hans Polak is online now Hans Polak  Netherlands
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Quote:
Originally Posted by Hans Polak View Post
I found something. Will come back in a few hours.
At least the VAS is playing a big role in this.


Hans
Without looking at the PDF, I only tested Amnesis JFET.asc
To get the amp unconditionally stable, I had to do a few things:

1) I had to change R37 and R46 from 1K to 270Ohm. At the same time I could change R22 and R25 to 2k.

2) I inserted from the junction of R17/R36 a 27V zener with a 1N4148 in series to Gnd. And exactly the same zener and diode from Emitter Q6 to ground.
This way, the output stage can never be overloaded and/or start oscillating.
V(out) is now limited to +/- 25V pk-pk or 40 watt into 8 Ohm.
If you want a somewhat softer limiting, you can put resistors in serie.
You can now severely overload the input without any drama. The amp is really bomb proof.

3) I never understood the function of D1. It's better to use a bipolar for C7.

4) In the Circuit Diagram, C7 is 220, but this probably should be 220uF.


Hope this helps.

When you want me to go in the details of the PDF, let me know, but a BD139/BD140 pair for the predriver seems not to be a good idea.


Hans
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Old 15th April 2019, 06:45 PM   #583
JOSI1 is offline JOSI1  Germany
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Location: Bad Hersfeld
Hello Hans,

thank you for your support.

Quote:
I had to change R37 and R46 from 1K to 270Ohm. At the same time I could change R22 and R25 to 2k.
I didn't populate the bootstrap caps C22/C27 so the sum R22/R43 and R25/R46 is in both cases around 2K.

Quote:
I inserted from the junction of R17/R36 a 27V zener with a 1N4148 in series to Gnd. And exactly the same zener and diode from Emitter Q6 to ground.
This way, the output stage can never be overloaded and/or start oscillating.
I checked that the limiting works but it didn't solve the problem.


It seems that it is not a problem of overloading the output stage
but a local problem of cascoding the output transistors. If replacing
the MOSFETs by a wire the problem does not occur.
I guess it is more a problem of bandwidth of the output stage (MOSFETs).
Decreasing the bandwidth/rise time of the input stage helps.

I already mentioned that the problem occurs only at a certain volume level. If increasing or decreasing the volume by 2dB the amp does not oscillate.

There is no need to look at the pdf-file.

Josi
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Old 15th April 2019, 07:44 PM   #584
maxlorenz is offline maxlorenz  Chile
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Quote:
Originally Posted by JOSI1 View Post

@Max
I use 330R gate stoppers for the JFETs and 150R gate stoppers for the MOSFETs.
What NTC thermistor (value, brand) did you use in series with power.
Any CL-60 (10R/5A) make will do: Amphenol, Vishay...etc. They are supposed to be self-healing. As per Mr. Pass' recommendation. In series with the "live" or, if you happen to have twin 110V primaries, in series, joining both.

Great work as usual. I will look at it when fully awaken from siesta.

I asume those elements with lines accross them are not connected (?).

*Do you have 5pF cap//FB Resistor?

*Do you have the 2K resistor (R51) that connects Mosfets' sources together?

*Output R (10 Ohm) is not there?
Try also Zobel after output R//L filter, for example at speakers' binding posts. If the problem only presents itself at power peaks, it coud be the case of injection of HF into ground from Zobel and passing it to sensitive (input) lines...though your competent design should make that possibility improbable...

Know that the output (driver-output) section works as a complete sub-circuit (or at least that is how I see it) so the bootstrapped output only makes visible a "local" (or global) resonance problem. If you don't succeed, maybe it is time to try those crazy R-C bypasses...

Good luck.
M.

PS: another clip.

YouTube

First slow movement, the fast movement starts at +/-4min 15s.

Last edited by maxlorenz; 15th April 2019 at 07:59 PM.
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Old 15th April 2019, 08:13 PM   #585
Hans Polak is online now Hans Polak  Netherlands
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Join Date: Mar 2005
Location: Blaricum
Quote:
Originally Posted by JOSI1 View Post
Hello Hans,

thank you for your support.

I didn't populate the bootstrap caps C22/C27 so the sum R22/R43 and R25/R46 is in both cases around 2K.

I checked that the limiting works but it didn't solve the problem.

It seems that it is not a problem of overloading the output stage
but a local problem of cascoding the output transistors. If replacing
the MOSFETs by a wire the problem does not occur.
I guess it is more a problem of bandwidth of the output stage (MOSFETs).
Decreasing the bandwidth/rise time of the input stage helps.

I already mentioned that the problem occurs only at a certain volume level. If increasing or decreasing the volume by 2dB the amp does not oscillate.

There is no need to look at the pdf-file.

Josi
Why didn't you use the bootstrap caps c22/c27, they are vital, just as the 270Ohm / 2K mods. The caps don't have to be so large, 10uF is already enough.
Without you are in trouble.


Hans
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Old 15th April 2019, 10:26 PM   #586
JOSI1 is offline JOSI1  Germany
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Location: Bad Hersfeld
Hello Hans,

I will try this mod in the afternoon and keep you informed about the result.
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Old 16th April 2019, 11:48 AM   #587
maxlorenz is offline maxlorenz  Chile
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Quote:
Originally Posted by Hans Polak View Post
Why didn't you use the bootstrap caps c22/c27, they are vital, just as the 270Ohm / 2K mods. The caps don't have to be so large, 10uF is already enough.
Without you are in trouble.

Hans
Indeed. All bootstrap caps (C16-C18; C22-C27) are needed for a proper QUAD bootstrap. Then tell me about dynamics.

I did try 10uF film caps (Hi Q Vishay) as bootstraps but found no significant sound difference, and they were more expensive and bigger...one can always bypass the electros with small film caps underside, if so one feels.

I think R output is also needed.
I would use only 65mA bias current at first for the tests.

What happens with the signal on the scope just prior to the oscillation? Do you see oscillation at the wave peaks?
If one peak shows tendency to oscillation (widening of the trace) then I would try the RC bypass (for example 20nF-50R if I remember correctly) from the corresponding bootstrapped driver's base to ground.

With the YT uploads, please neglect the HF distortion as it is mic induced. Focus on openness and dynamics.

Good luck.
M.
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