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-   -   Output stage CFP to EF on LJM amp? (https://www.diyaudio.com/forums/solid-state/360246-output-stage-cfp-ef-ljm-amp.html)

Rallyfinnen 16th September 2020 06:19 AM

Output stage CFP to EF on LJM amp?
 
2 Attachment(s)
I have a couple of LJM L12-2 that I would like to play around with, and try EF output stage instead of CFP.
I don't have the spice model for the 12-2, so I used one I found for the MX50SE, which is very similar, but only has one output pair.

Basic simulation seems ok, but before going ahead and soldering, I thought I would check with the experts :)

There are two things that I have been thinking about:

-compensation/stability, should probably be re-tuned?
-Current through drivers seems to give little chance in distortion FFT. What would be general advice/considerations for choosing the resistor between drivers?

OldDIY 16th September 2020 07:18 AM

Output stage CFP to EF...
CFP has the order of the transfer function +1. This will require a change in correction to ensure stability.
Output transistors with a higher cutoff frequency are needed.

Rallyfinnen 16th September 2020 08:28 AM

Since I don't know how to simulate stability of the circuit (pole/zero/OLG etc), and don't have the correct model for the 12-2, all I can do is check stability on scope. I'm thinking no load, 8ohm & 4ohm, and with a small cap on the output.


I think the outputs used should be very fast (SA1186 / SC2837), so finding even faster ones is not 'in the scope' :) Only use the ones I have.
These boards have been left unused since I did not like the sound much(even if they measured fine), so I just thought I would play around with them and see if I can make them more pleasing to my ears.. Very basic level here..

And I also plan to mount the 'bias compensation transistor' thermally coupled to the outputs, and not the drivers as it is now.

BesPav 16th September 2020 12:47 PM

Quote:

Originally Posted by Rallyfinnen (https://www.diyaudio.com/forums/solid-state/360246-output-stage-cfp-ef-ljm-amp-post6343047.html#post6343047)
Since I don't know how to simulate stability of the circuit (pole/zero/OLG etc)

Share your *.spice file, i will add necessary components and settings.

Rallyfinnen 16th September 2020 01:54 PM

2 Attachment(s)
Thank you!

Here is the model for the MX50SE. Similar, but not the same..

I hooked it together now (using leads to the output transistors), but it's oscillating like crazy..

BesPav 17th September 2020 05:19 AM

3 Attachment(s)
Ok, let's check.
Change extension of *.zip file to *.plt (because *.plt can't be attached) and put *.plt file in same folder with *.asc.
Try model and compare with my printscreen *.png.

Rallyfinnen 17th September 2020 05:59 AM

Thank you! I ran the sim, and it seems to work (same result as you posted)

Forgive me for being a novice, but if I'm reading this correctly, phase is around 90deg where OL gain is 0? This should make a stable amp right?

BesPav 17th September 2020 07:27 AM

Quote:

Originally Posted by Rallyfinnen (https://www.diyaudio.com/forums/solid-state/360246-output-stage-cfp-ef-ljm-amp-post6344074.html#post6344074)
Thank you! I ran the sim, and it seems to work (same result as you posted)

Pretty fine! Save this sheet with _OLG appendix.

Quote:

Originally Posted by Rallyfinnen (https://www.diyaudio.com/forums/solid-state/360246-output-stage-cfp-ef-ljm-amp-post6344074.html#post6344074)
Forgive me for being a novice, but if I'm reading this correctly, phase is around 90deg where OL gain is 0? This should make a stable amp right?

Clearly!
Comment .step directive with ; in front, put voltage signal source at the now grounded input and use .tran instead of .ac.
Save modified sheet with _TRAN appendix.

Rallyfinnen 20th September 2020 07:11 AM

I have some trouble with the bias adjustment, have to check that.. Current gets too high when I increase voltage, can't adjust it low enough even if trimpot is set to 0..

Rallyfinnen 23rd September 2020 07:34 AM

2 Attachment(s)
I got one modified board up and running. Only running one output pair at the moment and low output power (which I think is more significant). I tried 120 and 75 ohms as emitter resistor for the drivers, and there was a big difference in what Iq was needed to get good distortion. I did not see such a difference in the simulation. Now the Iq is around 50mA. Seems stable too. I get ringing with a cap across the output and square wave in, but it's damped. No LR on output.
Looks promising I think.

EDIT: broke a lead to the load resistor while replacing the emitter resistor to the drivers, so now I need to up the Iq again to get low distortion.


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