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Old 5th December 2013, 01:53 AM   #1
danny92 is offline danny92  Portugal
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Question How to design CFA amps?

Hi everyone,

I know that there is already a very good thread covering this subject opened by Bonsai, but with this thread I want to learn more about CFA design, I don't know almost anything about CFAs, I don't know any good books covering audio CFA design, I wanted to start with something very simple, because I'm really a newbie at CFA design. I've attached 2 very simple CFA amps based on the Analog Devices and Texas Instruments papers on the subject, that have THD and DC offset problems.

Best regards,
Daniel
Attached Files
File Type: asc CFA1.asc (17.1 KB, 51 views)
File Type: asc CFA2.asc (17.3 KB, 23 views)
File Type: txt Cordell Models.txt (19.2 KB, 12 views)
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Old 5th December 2013, 02:03 AM   #2
RNMarsh is offline RNMarsh  United States
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Hi danny92..... hope you learn what you want to and get something built as well. cant help with the sim though.

-Richard marsh
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Old 5th December 2013, 02:16 AM   #3
danny92 is offline danny92  Portugal
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Hi Richard Marsh,
thank you for your concern,

I'm trying to decide what I should build for one year which is sad.
That's my first solid state project and I wanted to do an amplifier with good distortion characteristics, I've learned that CFAs could have better results because of their larger bandwidth but I don't have formation on that area, I've bought some lateral MOSFETs from Semelab and some 2N5551 and 2N5401 transistors, and I want to build an amplifier with them, the academic degree I'm taking takes me lots of time, I only I have time to look at the amplifiers design sometimes at night or at the weekends. The only projects I've made are with audio ICs.
When I've an idea of what frequency compensation scheme I should use, I will start the PCB design.

Best regards,
Daniel
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Old 5th December 2013, 02:18 PM   #4
Bigun is online now Bigun  Canada
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Hi Daniel,

Why not take a look at the VSSA amplifier. There are a couple of threads on this forum about it. It's a simple CFA amplifier that uses lateral Mosfets and many folk around who have built it to help you. There may even be a pcb design ready for you to use.

Main thing is to build something.

Be careful about picking up on 'trends of the month'. The CFA bandwaggon has been in full swing around this forum. A few years ago the 'flavour of the month' was bootstrap amplifiers. There are many ways to build an amplifier, CFA is just one way of looking at things. Keep a broad mind.
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Old 5th December 2013, 02:33 PM   #5
mcd99uk is offline mcd99uk  United Kingdom
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Build something on strip board. Leave plenty of space and experiment. You have to leave the sims at some point. That way you can try something in the sim and then implement in reality. It's a great way to learn.

I figure if it is stable on strip board it will work with a properly laid out PCB. Saves money on failure as well. I speak from experience on this.
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Old 5th December 2013, 03:38 PM   #6
Vostro is offline Vostro  South Africa
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My approach has also been long,
I decided on a blameless design. I built four amplifiers as prototypes.
First was with bootstrap vas and zener 'current source' in ltp and single resistor as load in ltp which was a rheostat to adjust offset by varying ltp tail current.
Also experimented with changing miller cap to find out how it influenced stability.
Was very stable on resistive load with 22pF miller cap, ended using 47pF for comfort.
This simple amp was surprisingly nice, its now been used as a guitar amplifier for someone I know, for what must be about 2 years.

Then built an amp with 'proper' current source in ltp and vas. still with miller comp.
After that I built an amp with cascoding the enhanced Vas (three transistor), and tried to cascode the Vas current source.

Then (still in progress) is a amp with cascoded ltp and enhanced vas with experimental TMC compensation, with options for miller compensation.
This is my final blameless and will find a permanent place in my sitting room.
Each time I did my best to produce my own pcb.
It's a great hobby.

But I also ordered a through hole VSSA, can't wait to build it to compare.
I also have a Apex SR200 to compare it to, which is also a different topology.

I hope you finally decide on a amplifier, the process is rewarding.

Regards
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Old 5th December 2013, 04:26 PM   #7
mcd99uk is offline mcd99uk  United Kingdom
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Quote:
Originally Posted by Vostro View Post
I hope you finally decide on a amplifier, the process is rewarding.
Yes but sometimes frustrating and very hard work. But for all the pain when you sit there listen to your own creation you think "why pay someone to do this for you". Then it all becomes worthwhile.
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Old 5th December 2013, 05:55 PM   #8
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Danny,

I agree with the other suggestions here in that you just need to warm up your soldering iron and build something. I have built the PeeCeeBee flavour of the VSSA and love it. If you are curious about a CFA design that is pretty easy to build and sounds very nice I'd suggest giving that a look. Also, don't get too hung up on how things simulate. Use it as a proof of concept but don't let it keep you from building something.

Sometimes you can learn much from starting with a working design and working with that.
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Old 5th December 2013, 06:22 PM   #9
mcd99uk is offline mcd99uk  United Kingdom
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Default If costs are worrying you...

Another discovery, for me at least, you don't need to use expensive parts to get good sound. Think I've spent about 30 on my latest amp but several hundred on the first. Guess which one sounds best...
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Old 6th December 2013, 07:32 AM   #10
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Quote:
Originally Posted by mcd99uk View Post
Guess which one sounds best...
30 one
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First One VSSA
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