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Old 12th June 2011, 09:40 AM   #981
wahab is offline wahab  Algeria
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Originally Posted by keantoken View Post
I use the models from Cordell's website. They should be very accurate.

- keantoken
Thanks Kean, and thanks to Bob as well.
Will do some comparisons to see the differences.
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Old 12th June 2011, 09:53 AM   #982
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I don't see why any models should be more accurate than Bob's. Using other available models tends to be a big gamble.

Wahab, your amp is nice, but it's totally different than the one we're discussing.

- keantoken
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Old 12th June 2011, 10:03 AM   #983
wahab is offline wahab  Algeria
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Originally Posted by keantoken View Post
I don't see why any models should be more accurate than Bob's. Using other available models tends to be a big gamble.

Wahab, your amp is nice, but it's totally different than the one we're discussing.

- keantoken
Well, i dont have doubts about Bob s abilities..

As for the amp being totally different , i wouldnt go as far since
this is a very little variation , but for sure that linearity wise, yes,
it s totally different.

Indeed, IF a darlington is used as VAS , one will conclude that the
apparent component count is still the same !!

Unfortunately, there are not darlingtons that have as good
caracteristics as a BC546+2SA1360.

Overall, i think that the use of a JFET at the input and mosfets
at the output, with the Bjts providing very high gain between these
two stages , is the most logical way to go to achieve performances
that are good enough to make the circuit preferable to an IC amp.
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Old 12th June 2011, 10:26 AM   #984
juma is offline juma  Serbia
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Hi swordfishy,
few months ago I completed a topologically similar amp (main difference is that I used the single rail PSU) so it may be of some use to you and the others. I also tried different types of popular FETs, MOSFETs and BJTs. In the end, this is what sounded best to me :
25W cl. A amp with Lateral MOSFETs
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Old 12th June 2011, 10:40 AM   #985
AndrewT is offline AndrewT  Scotland
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Having been reminded of Juma's earlier BJT inverted version, I am considering an inverted version of this one. I have stock of 2sj74, zvn2110, zvn3310, 2sk1058, 2sj162 and various CCSs. Might start it soon.

Any comments on an inverted version?
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Old 12th June 2011, 10:54 AM   #986
GregH2 is offline GregH2  Australia
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Juma,

Very very nice. Great square wave there too.

Your timing with this post is eerie - Hugh and I were just discussing the benefits of a bjt vas and input. I am actually getting tempted to go this way too. I did actually try it a week or two back, and thought the bjt input and vas was a little more sterile sounding. But I did not experiment too much with it and maybe could tweak some warmth into it.

Can I ask, why would one choose to, or choose not to use a inverted or non inverted configuration? It seems to me it would be preferable to use the more linear NPN device in the vas, but the input isn't exactly non important either.

AndrewT,

Can't see a problem with the inverted version. Please do and let us know how it goes.

Keantoken,

Can't see any signs of a 4khz dip, but in the mean time I've temporarily removed the decoupling capacitors. By the way, my oscillations are occurring at many hundreds of kHz, at least.
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Old 12th June 2011, 11:00 AM   #987
mikelm is offline mikelm  England
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inverted version:

I was thinking about this for a while because VAS seems to the the critical element and I have understood ( at least in the past ) that nchannel devices are faster than pchannel ones.

Does this still tend to be the case ?

mike
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Old 12th June 2011, 11:18 AM   #988
juma is offline juma  Serbia
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Quote:
Originally Posted by swordfishy View Post
... Can I ask, why would one choose to, or choose not to use a inverted or non inverted configuration? It seems to me it would be preferable to use the more linear NPN device in the vas, but the input isn't exactly non important either. ....
Sorry but it's not completely clear to me what you refer to when using terms inverted/non-inverted...
Through the amp, the signal is inverted twice (at Q1 and Q2 collectors) so the whole amp is of non-inverted type (output signal is in phase with the input signal).VAS is NPN BJT (Q2=BC550c) and input (Q1=BC560c) is strongly degenerated and without contribution to OLG (its task being to provide high and linear Zin and Low Z NGFB point).

Last edited by juma; 12th June 2011 at 11:20 AM.
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Old 12th June 2011, 11:23 AM   #989
GregH2 is offline GregH2  Australia
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Juma,

Sorry for not being clear.

What I meant was what a the pros/cons of using a npn input and pnp vas versus a pnp input and npn vas?

As I mentioned in my post, and mikelm too, i assume it is preferable to use the more linear npn device in the vas where most of the voltage amplification takes place (as you have done).

This is the opposite to what the current design of fetzilla uses.

Last edited by GregH2; 12th June 2011 at 11:26 AM.
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Old 12th June 2011, 11:30 AM   #990
AndrewT is offline AndrewT  Scotland
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Originally Posted by swordfishy View Post
This is the opposite to what the current design of fetzilla uses.
The downside is that it forces the adoption of 2sj74 for the input device if we retain the jFET as the input device.
I am lucky, I have a stock of all of the required devices.
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