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Old 28th April 2007, 02:46 AM   #21
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Hi, PB2,

If you modify this CCT, and if you have the final drawing, what will it be like? Could you attach it here?
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Old 28th April 2007, 03:21 AM   #22
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Oh, I wish I had the time to really work on it but I've currently got the old
Tiger amps in the que to do. Not a priority, just for the enjoyment.

The first and second gain stages in that design look reasonable, I'd add
bootstraping, and the top half of the output stage emitter follower looks
reasonable, a bit strange but more or less reasonable.
The lower half is, what? A sliding current source? I'd have to simulate it
to get more insight.

Pete B.
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Old 28th April 2007, 11:30 PM   #23
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HI all
Surely some mistake in the diagram?
The second stage diff pair is a class A to drive the output stage in class A, but it seems to me that the left hand decoupling capacitor (between 2.7k - 390 tap) should be bootstrapped while the right hand should go to ground. Don't think this is a sliding bias- just a push pull class A?

And it would probably be a lot better with newer transistors...
all those roll-off capacitors probably mean it has a high susceptibility to overloading the input stage at high frequencies....

cheers
John
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Old 29th April 2007, 02:35 AM   #24
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Quote:
The second stage diff pair is a class A to drive the output stage in class A, but it seems to me that the left hand decoupling capacitor (between 2.7k - 390 tap) should be bootstrapped while the right hand should go to ground.
Possible, because the upper output NPN's base is dynamicly changing (follower) while the lower output NPN's base is relatively steady.
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Old 2nd May 2007, 03:48 PM   #25
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Hi Lumanauw

...in which case the 4.7 ohm filter resistor and second 2200 uF capacitor in the psu are not needed.

Here's what I suggest: the original circuit probably is intended to run with "fixed" collector loads, because the left hand side needs a higher voltage to make sure there is enough drive for the upper output stage.

The bootstrapped right hand side makes sure that the RHS sees the same load as the LHS. But this arrangement forces the diff pair to swing through almost all of its operating conditions. I estimate that the diff amp transistors run at 7 mA and have to swing between 1 and 13 mA to get full output.


Changing the bootstrap sides over has some advantages: the additional voltage on the LHS is not needed, so the PSU filter is not needed, and higher output power can be achieved with the same psu. In addition the current swing in the diff pair is reduced to be only approximately that of the base current needed by the drivers, i.e. about 1..2 mA, improving linearity and lowering distortion.

Only trouble is that, as with JLH69, any increase in base drive on one side is achieved by a reduction in base drive from the other, so if you use transistors with a gain which falls with increasing current (2N3055 for example) the quiescent current has to be set a little higher than half the peak output current

Cheers
John
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Old 3rd May 2007, 03:03 AM   #26
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Hi, John,

Thanks for the advice. I also think changing the topology like you suggest is better, changing bootstrapp from left to right.

I'm thinking about using slow transistors throughout the amp for small transistors (MJE340-350 for all small transistors, 3-4mhz) and 30mhz for output (2SC2922). Will this give problem?
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Old 4th May 2007, 03:18 PM   #27
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Quote:
Originally posted by john_ellis
HI all
Surely some mistake in the diagram?
The second stage diff pair is a class A to drive the output stage in class A, but it seems to me that the left hand decoupling capacitor (between 2.7k - 390 tap) should be bootstrapped while the right hand should go to ground. Don't think this is a sliding bias- just a push pull class A?


cheers
John

I am not sure if it was intended to have it as a sliding class A . I personally would basically design it as you have proposed, but the mullard design seems to do it different on purpose. Have a look to the voltage rating of that 220uF bootstrap cap, 6.3V. This is fitting to the bias settings of all stages. If intended to be put to GND the voltage rating should be min 30V, better 50V. For me it is looking like Mullard intended to run the lower half mostly as a constant current source... but then I am wondering how we should get output currents above 1A without clipping... What's the opinion of PSPICE? As In understood there are already some simulations around...
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Old 6th May 2007, 03:10 PM   #28
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Hi chocoHolic

Yes I agree that the circuit probably was deliberately designed the way it was published, but I simulated the distortion as was and with the bootstraps swapped. CUriously, the distortion as was at about 15V output was 0.35% but with the swap, was <0.1% at higher output. If you swap the bootstrap side, then it will be necessary to choose the right voltage of capacitors.

Hi Lumanauw

I would not recommend MJE340's in the input stages - my usual input stage transistors are BC307 (PNP) or BC237 (NPN). Once these were made with the TO-18 triangle pin-out but as manufacturers have sought to lower costs the pin-out is now in line like many TO-92's. BC547, BC557 are similar. For the diff-pair VAS with the swapped bootstrap you could use BC546 rated at 60V or BD139's, as you could reduce the current a little to say 5..6 mA in each side.

Calculations of the optimum resistors needs some thought. I'll publish my conclusions later.

Cheers
John
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Old 6th May 2007, 04:35 PM   #29
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..thanks for the detailed answer! I have to a little bit more about this circuit...



P.S.
But "class D Gen2" is binding all my DIY capacity in the moment....
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Old 11th May 2007, 03:02 PM   #30
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Hi Lumanauw, chocoholic

Here's my proposal for a mod version of this amplifier.

Some comments: I have used BD139s for the drivers and diff-vas.
I prefer these to MJE340's because they have higher fT and gain. The MJE340's seem to me to be like old video transistors - fine at high voltage, but for linear apps tend to have ft limitations below 10V because of the epi. BD139's also have a low voltage ft-reduction but this tends to be below 5V rather than 10V.

The driver stages keeps the capacitor bootstraps but the load resistors are split. To improve LF response, the capacitors all need to be bigger than in the original. But with capacitors directly connected to the collectors of the drivers, a short circuit could cause discharge - especially if bigger values are used- which might kill the o/p transistors. With the split load resistor, short circuit output current is limited, admittedly dependent on the gain of the output transistors, to about 5 or 6A.

The output capacitor is increased to 4.7 mF - this is better for LF, but the decoupling bias capacitor needs to be 22 mF to keep pace. But with modern low-R, low-Z capacitors a few parallelled 4.7 mF might be fine.

Frequency compensation uses the phase-lead approach John Linsley Hood and Bailey adopted (and the method I prefer). The input filter is split into separate frequency-stability concerns and actual HF filter. THe first RC pair (2.2k, 330 pF) are to prevent input stage overloading at high frequencies, the second is to provide stability (the 330 pF is too high). By all means if you think the input filter ruins audio performance you can take this off, and most audio material won't cause TIM in any case.

I noticed during simulations that there is a sliding bias effect. Unfortunately this is in entirely the wrong direction. The quiescent current DROPS in both output transistors, at the start of a signal, and this means that to stay in class A the quiescent current has to be quite big. Hence the reason for the 1.5 ohm and approx. 2A quiescent current. But this amp gives 25W into 8 ohms at 0.02% distortion which is only 2nd harmonic, no evidence of crossover distortion (as there should not be).

Think this is better than the original but I hvaen't tried it for real yet.

cheers
John
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