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Old 25th February 2012, 09:24 PM   #1
popilin is offline popilin  Argentina
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Default 25W Class A Hybrid Amp

Hi All
Valves are optimum as voltage amplifiers and transistors as current amplifiers, so common sense indicates a hybrid power amp.
I accept criticism.
Thanks.
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Last edited by popilin; 2nd June 2012 at 12:55 AM.
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Old 25th February 2012, 09:47 PM   #2
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I have no critics but would like to know why you'd choose some topologies. Why use a currentsource instead of an anode resistor? Why not shift the fase with a tube also? And why do you use 2x2 transistors as outputdrivers? Why not FET's only?

These are no critics, I'd like to learn
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Old 25th February 2012, 09:59 PM   #3
popilin is offline popilin  Argentina
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Hi boyofwax
1) CCS minimize distortion
2) I hate the long tailed pair, and if you think in a push-pull amp there is no reason for a fase shifter.
3) The BJT/MOSFET cascode avoid (I hope) thermal runaway, the other MOSFET driver is for best square wave response.
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Old 25th February 2012, 11:54 PM   #4
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Quote:
Originally Posted by popilin View Post
1) CCS minimize distortion
Not really. The tube is heavily loaded. It doesn't just see the anode resistance where you put the ccs. there is a path through: C3-R10, C3-R14-C8, C3-R14-Q3-.... and the same for the complementary part.
Just putting a ccs anywhere without proper design won't do the job.
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Old 26th February 2012, 03:09 AM   #5
popilin is offline popilin  Argentina
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Hi the_manta
I'm sorry but I knew a CCS is an ideal load for the valve and mimize distortion, if you have a better implementation of the CCS is wellcome.
Because the amp has zero feedback any help with distortion will be grateful.
Best Regards
Johann

Last edited by popilin; 26th February 2012 at 03:29 AM.
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Old 26th February 2012, 09:07 AM   #6
Cassiel is offline Cassiel  Libya
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Quote:
so common sense indicates a hybrid power amp.
LOL. Common sense won't get you far in audio. There are a lot of things that SHOULD work but then, they don't. The empirical world is full of surprises. Not that I have anything against solid theory......just saying.

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I hate the long tailed pair,
You see?
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Old 26th February 2012, 10:09 AM   #7
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When you post the SPICE file I would take some time and test a few ideas.
And I need to where you get TubeLib.inc
I only have Koren_Tubes.inc and dmtriodeep.inc
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Old 26th February 2012, 11:27 AM   #8
godfrey is offline godfrey  South Africa
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With Q3 and Q4 current biased like that, I wouldn't expect DC offset to be very stable. The cascodes will reduce self-heating of Q3 and Q4, but can't help with ambient temperature changes etc.

The circuit below may be useful (or at least of interest). It's intended to replace the middle part of your circuit (Q1, Q3, Q4, M1 and M2). M3 and M4 are the same as in your circuit.

How it works is interesting: The DC current I1 through R4 and R5 is always exactly equal to the DC current I2 through R6 and R7 (because there's nowhere else for the current to go), so the output is always biased exactly half-way between the supply rail voltages.

Now what happens if the current gain of Q1 is double the current gain of Q2? The base currents have to be the same, so the current flowing through Q1 and Q4 will be half of the current flowing through Q2 and Q3.

Input impedance is very high, so you should get good linearity from the CCS loaded triode.
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Last edited by godfrey; 26th February 2012 at 11:32 AM.
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Old 26th February 2012, 12:31 PM   #9
DF96 is offline DF96  England
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Quote:
Originally Posted by popilin
I'm sorry but I knew a CCS is an ideal load for the valve and mimize distortion, if you have a better implementation of the CCS is wellcome.
A CCS minimises distortion provided you don't add a load. You have added a load.
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Old 26th February 2012, 01:56 PM   #10
popilin is offline popilin  Argentina
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Hi the_manta
TubeLib is the Koren lib with other name. Thanks for your interest, if you can, help me please.

Hi godfrey
Thanks for your help, Q1 is in the same heatsink with output transistors, Q3-M1, Q4-M2, M3-M4 has their own heatsink.
I tried gyrators, but the frecuency responce was a disaster, I think to put a CCS at the input of SS stage for bias. (I'm afraid DF96 critics for this)
I tested (a month ago) the topology that you post with poor results, but I will try again, your circuit is a bit different.
I'm afraid of C1-C2, is possible some feedback?

Hi DF96
Thanks for your critics, you are a complete scientist and I respect and appreciate your advice, can you help me with a better idea?
Thanks all
Johann

Last edited by popilin; 26th February 2012 at 02:06 PM.
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