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Old 4th September 2004, 12:43 AM   #1
Pelle is offline Pelle  Sweden
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Default Yet another N-Channel Amplifier

Hi!

I would like some comments of my implementation on a N-channel amplifier. I hope no rules have been broken by borrowing pieces from different amplifiers to assemble this one.
The output is very much similiar to the one used by Nelson Pass in his MOSFET Citation 12 and the input stage is inspired by some of Anthony Holton's schematics. I've also added a DC-Servo to remove the need of a big electrolytic cap in the feedback.
The BD136 on the schematic will be replaced by MJE350, but I did not have the spice implementation of it. A zobel network is also present at the output but not shown in the schematic. My goal was an amplifier that used only parts that I already had = cheap.
I'm exhibiting some crossconduction at high frequencies but much less than in both the Citation 12 and Anthony's N-Channel amp, so I guess this is something that might need to be tweaked once the amplifiers is build?
Feel free to make any comments as i would appreciate that very much!

-Pelle

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Old 4th September 2004, 01:01 AM   #2
johnnyx is offline johnnyx  United Kingdom
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I would like to know how you can post a nice big and clear diagram, while I have to post smaller ones sideways.
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Old 4th September 2004, 02:11 AM   #3
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I can't seem to post anything at all.

But your cross conduction is probably in Q13 or the
value of its Collector resistor.
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Old 4th September 2004, 03:32 AM   #4
Pelle is offline Pelle  Sweden
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Thanks! I'll try to change the value of the collector resistor and/or Q13. I'll probably end up having to change them once the amplifier is done and tested.

-Pelle

The pictures if btw a gif-image that I've put on my server. I did not manage to attach any pictures directly when I posted so I used the 'Insert hyperlink' instead.
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Old 4th September 2004, 04:29 AM   #5
Eva is offline Eva  Spain
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To prevent cross-conduction I would try replacing R36 by a lower value resistor or maybe by a higher value resistor and a 10mA current source like the one that charges the gate of M18 in paralell. This would ensure that M19 turns off fast enough

To get big schematic pictures in small manageable files they should be captured as 16 color GIFs or PNG files, never as JPEG

To place images directly in the forum they should be uploaded to your own web-server and referred from the post as images, not urls or attachments

Images placed as attachments are not directly visible from the threads and increase information traffic demands on the web-server of the forum
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Old 4th September 2004, 11:42 AM   #6
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Pelle: Nice and straightforward design! However the big problem is (like the Citation 12 clone) that there is really high rail loss. At each rail this design starts clipping at some 10 Volts under the rail.
In practical life this means that to build a 100W amp you will have to use not 44V rail voltage but 54V, to satisfy the amplifier's rail loss. It wastes power like a 180 Watt amplifier, so you power supply and heat sinks need to be size: 180 Watt. Still the amp put out only 100 Watt.
Not a very efficient design.

R36 why not make it 50 Ohms, and then place Q13 on the heat sink as well. R17 could be 10 Ohms. Then it will work without cross conduction.

All the best from

Lars
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Old 4th September 2004, 12:00 PM   #7
MikeB is offline MikeB  Germany
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Hi pelle !

As the bd136 has a rating of 45volts, it's very likely that it will blow
if you turn up the volume with this amp. (your supplyvoltage is 70volts)
Maybe you should use mje350 instead ? (q20/q13)

Mike
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Old 4th September 2004, 12:40 PM   #8
Pelle is offline Pelle  Sweden
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Thanks for your replies! It's great that there are no bigger faults to the design. All the things you have commented on are easy to adjust even when the PCB is made.

Lars Clausen: Why is this special to this design? M18 is driven as a source follower and would have the same minimum voltagedrop as many other mosfet-amps? According to simulation this design clips at 30V positive and 29V negative, so that's more like 5V at each rail. Will there be any difference when I build the amp?

MikeB: As I said in the begining "The BD136 on the schematic will be replaced by MJE350, but I did not have the spice implementation of it."

-Pelle
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Old 4th September 2004, 12:48 PM   #9
MikeB is offline MikeB  Germany
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Quote:
Originally posted by Pelle

MikeB: As I said in the begining "The BD136 on the schematic will be replaced by MJE350, but I did not have the spice implementation of it."

-Pelle
Sorry, next time i will read...

Edit: you can download spicemodel here:

http://www.onsemi.com/site/products/...MJE350,00.html
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Old 9th September 2004, 03:15 PM   #10
pro is offline pro  Italy
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Hi Pelle.
I tried to simulate your schematic, but I can't controll the very high gain of the amp. How do you controll it? Changing the value of R24 or R10 it doesn't go under 80 dB.
Ciao.
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