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Old 9th March 2011, 07:55 PM   #11
artu is offline artu  Chile
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Hi Kenpeter,

I always being afraid about mosfets because of the self oscillating issue, and the use of gate stoppers that indeed works, but for me it is not a elegant solution, it adds distortion and noise, and the 'voltage drive' property holds only for low frequency as it rises the mosfets becomes current hungry, maybe the HF response compared to BJT is an indisputable advantage, but with BJT with 4MHZ Ft or higher I don't see clearly the advantage at audio, maybe the harmonic profile ??

Cheers
Arturo
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Old 9th March 2011, 08:57 PM   #12
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So use bipolar Darlingtons, and ditch the Zeniers.
MOSFET is not a religion with me yet. (or is it??)

You asked alternative topology, all its about...

Bias and square law crossing the same either way.
Managed by the cold Schottky stack, not by which
style of output device does the brute work.
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Old 9th March 2011, 09:08 PM   #13
artu is offline artu  Chile
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Quote:
So use bipolar Darlingtons, and ditch the Zeniers.
MOSFET is not a religion with me yet. (or is it??)

You asked alternative topology, all its about...
Hi Kenpeter,
Oh yes, nothing to criticize about mosfets (despite I managed to blow some ...), I was arguing about my personal taste (truly biased), but you are right, I must open my mind (what I am requesting), nothing to do with religion, when they do the work OK that's enough, maybe I am getting old ...

Cheers
Arturo
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Old 10th March 2011, 11:06 AM   #14
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Thumbs up very good design

Quote:
Originally Posted by artu View Post
...
Hi Arturo!
I found your topology best for my new amp,
I would only consider to use cfp pairs for the input devices...
could you show pcb you designed for your amp?
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regards, Pawel
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Old 10th March 2011, 12:45 PM   #15
artu is offline artu  Chile
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Default No PCB yet

Quote:
padamiecki :
Hi Arturo!
I found your topology best for my new amp,
I would only consider to use cfp pairs for the input devices...
could you show pcb you designed for your amp?
Hi Pawel, you will see I am at the very beginning with this amp. The protoboard shown (which is similar but not exactly the circuit posted) was mainly to check the concept and verify DC stability. Also picked up some music from the buffer (BD139/140) to a headphone (despite the mess and the 'wires' and the ground sounds good!!!).

I've tried CPF inputs but it makes almost impossible to bias the circuit (tested in practice), if you wish there are better signal BJT's than the 2sc945/2sa733 (unmatched pairs), I use them because is what I have on hand. Currently I am working on the PSU. Maybe in a week I will have the PCB design finished, but I must warn you that it will be a two layer PCB, I don't have any facility to do my own PCB's.

Cheers
Arturo
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Old 10th March 2011, 10:16 PM   #16
ontoaba is offline ontoaba  Indonesia
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The thing that I like from CFA is bandwidth. That also the reason I am using CFA for my new VAS. 10MHz and up of bandwidth is easy to reach without compensator.

A733-C945 is Vbe matched, the datasheet shows many differences but their Vbe characteristics is matched. I think its pair is chosen by designer, and not offered by manufacturer, this is rare condition.


Add: Hi, artu. Did you tried the output stage with loudspeaker? I remember similar output stages in other thread. Something like current feedback classA thread.

Last edited by ontoaba; 10th March 2011 at 10:25 PM.
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Old 11th March 2011, 12:49 AM   #17
artu is offline artu  Chile
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Quote:
Ontoaba:

The thing that I like from CFA is bandwidth. That also the reason I am using CFA for my new VAS. 10MHz and up of bandwidth is easy to reach without compensator.

A733-C945 is Vbe matched, the datasheet shows many differences but their Vbe characteristics is matched. I think its pair is chosen by designer, and not offered by manufacturer, this is rare condition.

Add: Hi, artu. Did you tried the output stage with loudspeaker? I remember similar output stages in other thread. Something like current feedback classA thread.
The A733/C945 was the 'best' choice a had, and yes this pair is not quoted as matched by the manufacturer because other characteristics differs widely, only same power and voltage rating, but it is also a good thing that this topology doesn't need strict matching components, even the VAS is unsymmetrical 2sa733/2n5401 upper leg and two 2n5551 in the lower leg (done to correct the shape of clipping).

CFA's are free from the 180 phase issue of VFA differential inputs designs, that's why you can reach higher frequencies safely, and I need only a 3.3pf FB cap. and a 10pf miller cap. in the cascode to shut up a 37MHZ instability, both completely immune to PCB parasitics.

About the output stage, yes it is very suitable to CFA because the way the current flows driving the OPS that makes easy to cancel DC OFFSET for a CFA which are known to be more difficult to achieve low offsets. Yes I know and heard, this output topology (buffer+OPS) with several different loudspeakers years ago and sounded very good. Also simulations reveal 0.066% THD at full power this stage alone, that is easily corrected by the NFB loop at 90db OLG. I've added a thermal compensation scheme that lowers the AB quiescent current inversely to the OPS temperature, as temperature raises the quiescent current lowers maintaining OPS within SOA.

Cheers
Arturo
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Old 11th March 2011, 01:22 AM   #18
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Tube cad has an interesting circuite the xpp. I wonder if this cuircuite
would work using a DN2540 for the first "triode" and a IXTH20n50 for the
second "triode"
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Old 11th March 2011, 01:41 AM   #19
artu is offline artu  Chile
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Hi Ontoaba,
BTW, can you share your CFA circuit?

Cheers
Arturo
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Old 11th March 2011, 04:51 AM   #20
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Quote:
Originally Posted by woody View Post
Tube cad has an interesting circuite the xpp. I wonder if this cuircuite
would work using a DN2540 for the first "triode" and a IXTH20n50 for the
second "triode"
Probably not, MOSFETs lack "Mu" internal negative feedback that style of circuit would require.
Schading MOSFET to act like triode, complicated if you care to maintain high input impedance.
I think you selected a triode dependant circuit that doesn't translate terribly well to sand.
If you had a box of rare SIT's, yeah maybe...
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