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Old 13th August 2013, 01:20 PM   #11
mcd99uk is offline mcd99uk  United Kingdom
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Beginner question again.

How do you extract this info from LTspice?

I have tried dropping the feedback shunt resistor to various values between 0.1 and 0.0001. Is this the correct way?
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Old 13th August 2013, 01:23 PM   #12
Mooly is offline Mooly  United Kingdom
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Quote:
Originally Posted by Bonsai View Post
Ok that's a good idea.

I've opened a thread up.

Mods, you may want to move the CFA stuff to that thread.


Quote:
Originally Posted by mcd99uk View Post
All we need now are the posts moved from D Self's thread to here.
There are some 500+ posts from when the CFA debate started, and not all are relevant to the subject.

If someone wants to list all the post numbers as they stand now and put that in an easily readable form (for example a .txt file saying 501, 503, 506, 507 etc etc) then it might be possible to move them over.
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Old 13th August 2013, 01:26 PM   #13
Mooly is offline Mooly  United Kingdom
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Quote:
Originally Posted by mcd99uk View Post
Beginner question again.

How do you extract this info from LTspice?

I have tried dropping the feedback shunt resistor to various values between 0.1 and 0.0001. Is this the correct way?
Yes, you have to run the amplifier "with no signal feedback". If the amp has offset problems these might be compounded and need a workaround (AC coupling or inclusion of a static DC voltage source to counter the error).
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Old 13th August 2013, 01:30 PM   #14
mcd99uk is offline mcd99uk  United Kingdom
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Quote:
Originally Posted by Mooly View Post
Yes, you have to run the amplifier "with no signal feedback". If the amp has offset problems these might be compounded and need a workaround (AC coupling or inclusion of a static DC voltage source to counter the error).
Thank you. I was getting some strange results. With 0.1R I was getting around 80dB then with 0.0001 I was getting a strange looking plot with a peak of about 20dB. Never considered the offset. Shall try again tonight.

Edit: This is on a CFA so not totally off topic

Last edited by mcd99uk; 13th August 2013 at 01:35 PM.
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Old 13th August 2013, 01:49 PM   #15
Bigun is offline Bigun  Canada
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my first 2c:

1) It seems to me that both CFA and VFA can get the job done and the differences between them are only to be heard when comparing two different amplifiers (since they start with different topologies). Hence, more often than not, differences between them are difficult to isolate since there are many factors known and suspected to affect the sound of the result. Compensation is a key difference, as already mentioned above. There are many topological options that use CFA and many that use VFA which allow the designer to provide for certain performance parameters (e.g. PSRR, slew rate, harmonic profile, gain-bandwidth etc.) So I bring into doubt the usefulness of the question, which is better between CFA and VFA.

2) Without casting opinions about anybody in particular, I would imagine that a good results is achieved when a designer uses the tools he/she knows best, whether CFA or VFA, or indeed FETs vs BJTs, or SS vs Tubes - amplifier artists and engineers might do best when using certain approaches because this is where their talent and experience is.
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Last edited by Bigun; 13th August 2013 at 01:52 PM.
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Old 13th August 2013, 02:00 PM   #16
wahab is offline wahab  Algeria
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Quote:
Originally Posted by Mooly View Post
Carried over

Mine has and more and is 100% stable.
I think Mikek is talking of global NFB and in this matter
he s right , you cant go over 30-35dB GNFB at 20Khz
without stability issues, of course the total feedback
including local loops can be much higher at said frequency.
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Old 13th August 2013, 02:05 PM   #17
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Originally Posted by wahab View Post
I think Mikek is talking of global NFB and in this matter
he s right , you cant go over 30-35dB GNFB at 20Khz
without stability issues, of course the total feedback
including local loops can be much higher at said frequency.
30-35dB GNFB @ 20K for the CFA or VFA?
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Old 13th August 2013, 02:06 PM   #18
wahab is offline wahab  Algeria
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Quote:
Originally Posted by mcd99uk View Post
I have tried dropping the feedback shunt resistor to various values between 0.1 and 0.0001. Is this the correct way?
If you drop this resistor you ll change the caracteristic
of the circuit , either it s a CFB and this will increase
the OLG or it s a VFA and this will change the feedback
loop phase/gain response.
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Old 13th August 2013, 02:08 PM   #19
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Bigun, it seems i'm the only one, here, to had made a CFA version of a VFA amplifier in real world.
The problem is i have no way to share the sound in a forum.
I proposed that an expert in simulation (I'm not) take the L.C. VSSA schematic (with its permission, and because it is simple) and make a VFA of it, removing the F.B. resistances and adding an other input pair in LTP topology instead.
Trying to get the best results of this VFA version with appropriated currents.
Then publish its files and results for both versions.
It would be a good way to COMPARE. To see the differences in distortion, phase, slew rate, bandwidth, feedback levels etc...
A very useful collaborative work, full of interest. A reference.

I wonder why, no one from the VFA addicts, master of simulations witch were attacking-me about my models etc does not met the challenge.
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Old 13th August 2013, 02:14 PM   #20
wahab is offline wahab  Algeria
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Quote:
Originally Posted by MagicBox View Post
30-35dB GNFB @ 20K for the CFA or VFA?
Whatever the topology , the higher the total gain the more
you ll eventualy have to "dump" high frequency gain either
using local linearizing loops or more brutaly with a shunt
at said frequencies.

Last edited by wahab; 13th August 2013 at 02:26 PM.
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