Designing amp with 2SJ201/2SK1530

[Rikard Nilsson]

Hello guys!

I´m new to this forum, but really glad I found it!

I have been thinking of designing an amp including Mr Cordells error correction output stage, and a symmetric twin differential input/driver. My question is, would the Toshiba 2SJ201/2SK1530 be suitable for the Hawkfords error correction approach? And alsp important, are they lateral? I would like to benefit from the self protecting characteristics at high currents. It would be wonderful not having to use non transparent protection circuitry. I know they have slightly positive temperature coefficient, but that´s no problem. I plan to use only the standard zener across gate/source as protection, and maybe a pair of schottky diodes at the output.

[lineup]

About Lateral or Vertical for 2SK1530 / 2SJ201
we have discussed this a number of time = search forum

http://www.diyaudio.com/forums/search.php
>>> 2SK1530 lateral

In my opinion they are more lateral if we look at main characteristics.
Anyway, they are very good powerful mosfet.
Quite easy to drive (not too much current needed).

About temp coefficient.
I would use some vbe compensation voltage (not pure resistive output bias control).

About Hawkford / Cordell Error correction approach with these
it is better Robert Cordell will answer you on this
Besides I do not use such fancy corrections .. only normal feedback correction.

[AndrewT]

and in my opinion they are vertical FETs but designed for audio use rather than switching use.

[traderbam]

Originally posted by lineup
About Hawkford / Cordell Error correction approach with these
it is better Robert Cordell will answer you on this
Besides I do not use such fancy corrections .. only normal feedback correction.

Don't be misled, "Hawksford EC" is normal feedback. Cordell's circuit is just a bizarre way of implementing NFB.

The difficulty you will have, Rikard, is with the low Vgs thresholds of those FETS, 0.8V to 2.8V. The Cordell circuit requires 2V or more. So to Use those FETs you would have to have to pick two with above average Vgs th, and preferably very close to one another. If that is not practical, you should use a simpler implementation of local NFB.

[Rikard Nilsson]

Ok, so the lower drive voltage will make the HEC ineffective? If so, a simple resistor voltage divider to the gates would make the drive voltage seen by the HEC circuit just as it was originally designed.

Well, I´m gonna have a try at least.

PS. I agree that Cordells circuit is a kind of local NFB. But nonetheless it seems very effective. I was also thinking of developing something that never lets the current drawn by the FETs down to zero. Technically this would still be class A operation, but not that ineffective. maybe somebody has already done this? (Not simply raising the idle that is)

[AndrewT]

Originally posted by Rikard Nilsson
I was also thinking of developing something that never lets the current drawn by the FETs down to zero. Technically this would still be class A operation, but not that ineffective. maybe somebody has already done this?

Leach and a few others claim that preventing the inoperative device in the Push-Pull Topology from actually turning off is ClassA. They are wrong.
Both devices must be actively controlling the output to be classed as being in ClassA.

[Rikard Nilsson]

But if the "inoperative" device is not completely turned off it will still control the output to some degree, right?

As far as I know, if we never completely turn of any of the output FETs, it is technically known as class A operation. Still, I consider this "politics", the important thing is to get rid of any crossover distorsion.

[lineup]

If I can use Class A, for the power I need to my speakers
than I use Class A push-pull.

If I have good sensitivity speakers ( 90-96 dB SPL )
and normal sized home living room for listening,
I have no problems using my Class A amplifier output stages
No lack of power, whatever, for Hi-Fi audio performance

If Class A is impractical = too much idle power (say for me is >=60 Watt Idle per channel)
than I go for 'normal Class AB'
... as you say, to remove as much crossover effects as possible

[AndrewT]

Originally posted by Rikard Nilsson
But if the "inoperative" device is not completely turned off it will still control the output to some degree, right?

As far as I know, if we never completely turn of any of the output FETs, it is technically known as class A operation. Still, I consider this "politics", the important thing is to get rid of any crossover distorsion.

no,
if the nearly turned off, but not quite, device is simply passing constant current then it is not controlling the output current/voltage. The push pull is operating as Push or Pull depending on which device is idling.
That is no better than ClassAB and may be worse if the bias has been set to other than optimum ClassAB (Vre between 15mVre and 25mVre).

[traderbam]

Originally posted by Rikard Nilsson
Ok, so the lower drive voltage will make the HEC ineffective? If so, a simple resistor voltage divider to the gates would make the drive voltage seen by the HEC circuit just as it was originally designed.

Yes, you can address the gate voltage by reducing the transconductance by a factor of 2 or 3.