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Output stage CFP to EF on LJM amp?
Output stage CFP to EF on LJM amp?
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Old 16th September 2020, 06:19 AM   #1
Rallyfinnen is offline Rallyfinnen  Sweden
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Default Output stage CFP to EF on LJM amp?

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?
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File Type: jpg image.jpg (50.3 KB, 136 views)
File Type: jpg mx50ef.jpg (163.5 KB, 137 views)
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Old 16th September 2020, 07:18 AM   #2
OldDIY is online now OldDIY
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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.
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Old 16th September 2020, 08:28 AM   #3
Rallyfinnen is offline Rallyfinnen  Sweden
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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.

Last edited by Rallyfinnen; 16th September 2020 at 08:33 AM.
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Old 16th September 2020, 12:47 PM   #4
BesPav is offline BesPav  Russian Federation
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Quote:
Originally Posted by Rallyfinnen View Post
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.
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Old 16th September 2020, 01:54 PM   #5
Rallyfinnen is offline Rallyfinnen  Sweden
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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..
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File Type: asc MX-50EF.asc (10.0 KB, 3 views)
File Type: txt Cordell-Models.txt (19.6 KB, 2 views)
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Old 17th September 2020, 05:19 AM   #6
BesPav is offline BesPav  Russian Federation
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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.
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File Type: png MX-50EF.PNG (85.7 KB, 63 views)
Attached Files
File Type: asc MX-50EF.asc (10.4 KB, 2 views)
File Type: zip MX-50EF.zip (604 Bytes, 3 views)
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Old 17th September 2020, 05:59 AM   #7
Rallyfinnen is offline Rallyfinnen  Sweden
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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?
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Old 17th September 2020, 07:27 AM   #8
BesPav is offline BesPav  Russian Federation
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Quote:
Originally Posted by Rallyfinnen View Post
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 View Post
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.
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