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Old 16th July 2012, 06:42 PM   #11
Calvin is offline Calvin  Germany
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Hi,

You canīt simply replace the PNP Q8 with an NPN transistor. Q doesnīt have any base-voltage. So it doesnīt open and doesnīt conduct current.
Either You use a PNP BD140 for Q8 or You redesign the output stage for complementary Transistors. In this case You need parts like diodes or other parts networks that generate a sufficiently high voltage (the Vbe of the transistors) that the transistors can open up.
Hereīs the sim-file and the associated models.
bipolar discrete OP.asc
bjt_oli.txt

jauu
Calvin

Last edited by Calvin; 16th July 2012 at 06:47 PM.
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Old 16th July 2012, 06:54 PM   #12
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I think it's great that he wants to do a discrete line stage. Wonderful! There's much to learn about these things. So many people just throw chips at the problem that's it's refreshing to see someone really dig into the details.
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Old 16th July 2012, 07:20 PM   #13
Mooly is offline Mooly  United Kingdom
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Quote:
Originally Posted by dirkwright View Post
I think it's great that he wants to do a discrete line stage. Wonderful! There's much to learn about these things. So many people just throw chips at the problem that's it's refreshing to see someone really dig into the details.
Couldn't agree more. You always have to try things for yourself.
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Old 17th July 2012, 12:29 AM   #14
owdeo is offline owdeo  Australia
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If you happen to have a copy of Self's Small Signal Audio Design there's a discrete class A opamp on page 91 (fig 3.33). It uses a simple emitter follower with current source load instead of push pull. If you replace the input LTP resistors with a current mirror it simulates, measures and sounds fantastic. Only 2nd and 3rd harmonic measureable and at vanishingly small levels.

Mooly you seem to be onto something re single ended vs push pull output - I tried a push pull version of the same thing and it wasn't nearly as nice to listen to. I also tried increasing the VAS gain with both cascoding and darlington arrangements per Self's power amps, but this made it sound worse and reduced stability even though it pushed the THD down further.
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Old 17th July 2012, 06:26 AM   #15
Mooly is offline Mooly  United Kingdom
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Oliphant...

Have a play with these LTspice files too. Just quickly put together based on your design.
Attached Images
File Type: png 80Khz 600 ohm load.PNG (113.7 KB, 251 views)
File Type: png 80Khz Squarewave 600ohm load.PNG (111.7 KB, 239 views)
Attached Files
File Type: zip Test Files.zip (3.1 KB, 21 views)
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Old 17th July 2012, 06:26 AM   #16
Calvin is offline Calvin  Germany
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Hi,

I havenīt simmed the two stages in my post for optimal compensation.
But the second stage with the current mirrors showed clear oscillations with 20pF dominant pole compensation.
So I changed the compensation to the RC-network between the input stages collectors. Unfortunately compensation measurements, where, how and why to place is hardly to find in literature. In D.Selfs power amp book for example only pole splitting (dominant pole) compensation is discussed. In the interest of THD under highlevel conditions a compensation close to the input may be preferrable.
One can find this in a lot of Elektor amplifiers, that feature a RC-network between the input stages collectors. But again, without any explanation at all.
Anyway, the RC-network in the second circuit seems to keep the stage free from oscillations and also raisens bandwidth slightly.
If someone wants to use this circuit, go ahead, but bear in mind, that this is just a quick sim. The optimal values and design will probabely differ and need proof.

jauu
Calvin
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Old 17th July 2012, 07:05 AM   #17
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First off, I'd like to say a big thank you to everyone that's helped so far. This is my first decent attempt at a discrete amplifier and the help that I've recieved so far is fantastic. Once I've finished playing with everything and I have a circuit that I'm happy with I'll design a circuit board for it and share the files via this thread


Quote:
Originally Posted by Mooly View Post

What kind of load is this preamp going to drive ? You show a 47K load which can be driven via a single ended output rather than a push-pull and I'm sure it would sound better.

But don't let anything put you off experimenting

How about equalising the input currents to the LTP ?
Do you really want the feedback return DC coupled ?
The amp I currently have has an a 10k input impedance but this will be changed to 47k once this preamp is finished.

Please excuse my ignorance but how would you propose I equalise the current going into the LTP? Are you talking about the use of a Wilson current mirror or similar?

Feedback is now AC coupled following the circuit modeled by Calvin. I probably will end up using it with SE output but I would still like to experiment with a push-pull output stage.


Quote:
Originally Posted by dirkwright View Post
Do you really need 4dB of gain?

I'd recommend replacing the output stage with a diamond buffer. You connect it directly to your VAS.
Yup I do need 4dB of gain. My current power amp is a OPA549 with a voltage gain of 8 and the phono preamp I use doesn't have as large a gain as a CD player. After this project is over however I want to use the lessons learned to design and build a low distortion 50W class AB or B amp and that will likely have a gain of 21 - then I can try the diamond buffer.

Quote:
Originally Posted by dirkwright View Post
I think it's great that he wants to do a discrete line stage. Wonderful! There's much to learn about these things. So many people just throw chips at the problem that's it's refreshing to see someone really dig into the details.
Thank you very much! I have used opamps in the past for everything that I've built and while that has taught me a lot, I have always had a guilty feeling as if I was cheating in some way. Learning about and building a discrete amplifier has been an absolute revelation for me; being able to control the individual elements and the amplifier characteristics is fantastic. I still have a long, long way to go and a lot more to learn, but I feel confident knowing that we have such a helpful community here at diyaudio.

Quote:
Originally Posted by owdeo View Post
If you happen to have a copy of Self's Small Signal Audio Design...
I purchased this yesterday and will be reading it thoroughly.

Quote:
Originally Posted by Calvin View Post
Hi,

You canīt simply replace the PNP Q8 with an NPN transistor. Q doesnīt have any base-voltage. So it doesnīt open and doesnīt conduct current.
Either You use a PNP BD140 for Q8 or You redesign the output stage for complementary Transistors. In this case You need parts like diodes or other parts networks that generate a sufficiently high voltage (the Vbe of the transistors) that the transistors can open up.
Hereīs the sim-file and the associated models.
Attachment 291954
Attachment 291956

jauu
Calvin
I've added a pair of diodes as recommended (see circuit diagram) and I have a 1.45V drop accross them. But now I don't have any output on the bread-boarded circuit. I'm currently taking it apart (slowly and methodically) to make sure I haven't killed any transistors. I'll report back later when I've finished this (it may take some time as I'm also changing the layout)
Attached Images
File Type: jpg Push pull mk3.jpg (140.8 KB, 251 views)
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Old 17th July 2012, 07:10 AM   #18
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Quote:
Originally Posted by Mooly View Post
Oliphant...

Have a play with these LTspice files too. Just quickly put together based on your design.
Thank you!


Quote:
Originally Posted by Calvin View Post
Hi,

I havenīt simmed the two stages in my post for optimal compensation.
But the second stage with the current mirrors showed clear oscillations with 20pF dominant pole compensation.
So I changed the compensation to the RC-network between the input stages collectors. Unfortunately compensation measurements, where, how and why to place is hardly to find in literature. In D.Selfs power amp book for example only pole splitting (dominant pole) compensation is discussed. In the interest of THD under highlevel conditions a compensation close to the input may be preferrable.
One can find this in a lot of Elektor amplifiers, that feature a RC-network between the input stages collectors. But again, without any explanation at all.
Anyway, the RC-network in the second circuit seems to keep the stage free from oscillations and also raisens bandwidth slightly.
If someone wants to use this circuit, go ahead, but bear in mind, that this is just a quick sim. The optimal values and design will probabely differ and need proof.

jauu
Calvin
I'm rebuilding the circuit on the bread board at the moment so I think i'll give this a try. Thanks again Calvin. Also, I'm new to LTSpice, how do add the components that are contained in the .TXT file? I can't sim the circuits until I've done this. Also, does anybody have a pentiometer symbol? I've found the model but I don't have a symbol.
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Old 17th July 2012, 07:31 AM   #19
Mooly is offline Mooly  United Kingdom
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When an amp is DC coupled it is usually important to keep the base currents of the two input transistors equal. Not doing so results in a large DC offset at the output.

If the amplifier sees for example a 47K resistor to ground at the input, then the feedback resistor should also be 47K (assuming the feedback return is AC coupled). AC coupling the feedback return is normally advised to prevent the amplifier from amplifying its own "errors". The AC coupled feedback return ensures the amp has a gain of 1 at DC.

So in your picture in post #17 (which I see has AC coupling on the feedback) that means that R24+R15 should equal R10 which of course they don't.
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Old 17th July 2012, 07:44 AM   #20
Mooly is offline Mooly  United Kingdom
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Quote:
Originally Posted by Oliphant View Post
Thank you!




I'm rebuilding the circuit on the bread board at the moment so I think i'll give this a try. Thanks again Calvin. Also, I'm new to LTSpice, how do add the components that are contained in the .TXT file? I can't sim the circuits until I've done this. Also, does anybody have a pentiometer symbol? I've found the model but I don't have a symbol.
Adding models took me ages to sus out too

Put the text file into the same folder as the LTpsice circuit you are working on.

Here are 4 pictures showing how. The "cordell models" is my text file.

You must add a spice directive of "include cordell models.txt (exactly what the .txt file is called).

To use a part from the text file right click the part type number (2SK1056 here) and type the new type number you want to use exactly as it appears in the text file into the box. So I am changing the 2SK1056 to a 2N5401.
Attached Images
File Type: jpg Add 1.jpg (83.5 KB, 233 views)
File Type: png Add 2.PNG (40.6 KB, 210 views)
File Type: png Add 3.PNG (111.3 KB, 72 views)
File Type: png Add 4.PNG (119.3 KB, 76 views)
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Last edited by Mooly; 17th July 2012 at 07:49 AM.
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