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Old 13th June 2011, 03:37 AM   #1021
AKSA is offline AKSA  Australia
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Hmm, gotta think about this.

First impressions:

#1 Only if the first stage is a source follower will there be no phase reversal
#2 Grounded base needs constant current to deliver good linearity
#3 There is no feedback in this circuit to deliver the necessary low Zout and distortion
#4 Output is single ended, not push pull, therefore Class A with commensurate low power
#5 Jfet selection required to deliver appropriate operating point
#6 Miller cap would be difficult to implement because no phase reversal at the VAS
#7 There will be inevitable phase shift, like any other amplifier, from input to output


Hugh
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Old 13th June 2011, 04:58 AM   #1022
sonnya is offline sonnya  Denmark
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SWF:

Yes that is what i thought too. The oscillation is in the negative voltage swing.

Hugh:

You cannot build an amp that does have 100% identical positive and negative slew-rate. And it is the nature of this circuit layout. I have no problem with it,

Keantoken:

Is it not better to be more pessimistic then optimistic!?
If one build a better quality board layout with low parasitic inductance, don't you think that the circuit should be stable too in this layout as well as in an sluggish layout!?!?!?

I did this to be able to follow up on this thread (Which has a very very high pace at the moment). I do not think that it is a bad circuit and it is easy to stabilize. One way of doing it was to modify the feedback loop as shown.
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Old 13th June 2011, 05:49 AM   #1023
WuYit is offline WuYit  Sweden
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Hugh,
Quote:
#1 Only if the first stage is a source follower will there be no phase reversal
Only the common emitter / source / cathode configuration introduces a 180 degree phase inversion, not to be confused with phase displacement.
Quote:
#2 Grounded base needs constant current to deliver good linearity
All topologies do. Constant DC values are a condition of linear function.
Quote:
#3 There is no feedback in this circuit to deliver the necessary low Zout and distortion
#4 Output is single ended, not push pull, therefore Class A with commensurate low power
The circuit is not adapted, meant for demonstration purposes only.
Quote:
#5 Jfet selection required to deliver appropriate operating point
Itīs always the case with J-FETs.
Quote:
#6 Miller cap would be difficult to implement because no phase reversal at the VAS
There are many ways to accomplish phase compensation.
Quote:
#7 There will be inevitable phase shift, like any other amplifier, from input to output
Certainly so.

Thatīs an overall description of the common base amplifier not a description of that particular circuit.
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Old 13th June 2011, 05:53 AM   #1024
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Sonnya, if SWF prefers to leave the circuit without compensation, then construction will mess with stability no matter what. This is one of the trade-offs with having no compensation... It is up to the builder to find a layout that works, if he is not comfortable with the original one.

- keantoken
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Old 13th June 2011, 06:35 AM   #1025
AKSA is offline AKSA  Australia
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Hi WuYit,

OK, you understand the limitations. Problem is, if you suggest something new, it's best to deliver a schematic that actually improves on the existing circuit, which our SWF is valiantly attempting to do. I think your post is a little discouraging, and I strive to foster SWF's efforts in this thread.

You clearly have all the advantages/disadvantages in your mind, but your circuit was rather simplistic.

Incidentally, I have built a grounded base VAS, and if you use a LTP the phase issue is a non-event, including the compensation. But in your circuit, about the only way you could implement LC would be via shunt, which is a bit compromised. The other option is to use a bootstrap with a lousy, highly inductive cap, to pull back loop gain by the upper pole.

I have always felt that a successful audio amp design is the artful management of compromise, and that the measure of excellence may not necessarily be a number, THD or otherwise. As a group, we diyers really are hung up on the psychology of numerical appraisal.

I should be clear that I'm not trying to poke fun at your circuit, but it was too simple to meaningfully contribute. Mind you, I can't talk, I'm not contributing anything on the forum, although I do speak to Greg off air. He's taking a well earned break, BTW!

Cheers,

Hugh
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Old 13th June 2011, 06:47 AM   #1026
lineup is offline lineup  Sweden
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Quote:
Originally Posted by AKSA View Post
I have always felt that a successful audio amp design is the artful management of compromise,
and that the measure of excellence may not necessarily be a number, THD or otherwise.
As a group, we diyers really are hung up on the psychology of numerical appraisal.
A compromise ..
Not numbers ..


I think you are very right, AKSA.

One thing this thread have teached me
is the sane topology importance.

Secondly a good selection of parts values comes before SPICE numbers.

If I was to build Fetzilla, it is not max lowest THD that should guide me,
but to have no or very moderate compensation capacitance.

Regards
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Old 13th June 2011, 07:04 AM   #1027
sonnya is offline sonnya  Denmark
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Quote:
Originally Posted by keantoken View Post
Sonnya, if SWF prefers to leave the circuit without compensation, then construction will mess with stability no matter what. This is one of the trade-offs with having no compensation... It is up to the builder to find a layout that works, if he is not comfortable with the original one.

- keantoken
You don't get it... do you?

I am not trying to get him change his circuit. I was only giving my idear on how he could remove the oscillation.

As i stated: I am not sending it to the graveyard!
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Old 13th June 2011, 07:39 AM   #1028
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Sonnya, thanks for clarifying. Why not just put a 500R resistor in series with the phase lead cap and not mess with the series feedback resistor?

Your way puts a 100pF+150R network between the input and output of the output stage. I'm not sure if this is doing anything but increasing the load on the VAS.

- keantoken
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Old 13th June 2011, 07:41 AM   #1029
AKSA is offline AKSA  Australia
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Actually, Kean, increasing the (resistive) load on the VAS tends to exploit its asymmetry and create higher levels of H2 and H3. Some people like this; it sounds tubey.

I agree about messing with the series feedback resistor. I've found that even small caps across this can actually reduce stability because of the additional phase shift in the output stage.

Hugh
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Old 13th June 2011, 09:09 AM   #1030
mikelm is offline mikelm  England
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Default progress report

I am building boards using blank vero board, pins & copper wire, one finished the other 1/2 done, these will be great for experimentation but are a labour of love to build. Have to test & match some components now. As you can see I'm not rushing but should have one working cct in a few days.

The number of versions bouncing around here is enough to make a potential builder giddy but initially I'm interested to two versions only and I thought I would see what the experts think about how they compare theoretically. They differ only in VAS. 1) irf9610 2) casoded bd140s

Earlier in the thread a BJT VAS was discarded due to lack of OLG ( about 75dB ) but if a cascode is added the gain increases to almost 100dB which is flat to about 1Khz which with my CLG target of 20 - 24 dB should be plenty of FB.

Many here have stated that they prefer a BJT for VAS and I think the cascode makes it a viable contender.

In simulation I saw advantages in keeping the VAS current high so I'm begining with 20mA. I would be interested in alernative device suggestions.

My new PC scope arrived today which is great but I can tell you that building when all your usual parts, tools & testing gear are on a boat on the Atlantic is quite a challenge !
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