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Old 9th June 2008, 11:59 AM   #41
Mooly is offline Mooly  United Kingdom
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Hi Space,
The reason I say select on test is that I dislike pre-sets, you never know what they get up to when your not watching
However I would initially use a good quality multiturn pot for R8 and when alls O.K. make the value up from a couple of parallel resistors, R8 and R8A on the PCB. A 470 ohm preset should be fine, initially set to Zero ohms for minimum current.
The only Zener is the 12 volt one for the IC supply, nothing else to add. If you were testing with a much lower supply, say +/- 20 or so then the 10 K feeding it would need dropping-- thats all.
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Old 9th June 2008, 12:18 PM   #42
Mooly is offline Mooly  United Kingdom
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Hello Lumba Ogir,
"Why does this topology create a distortion spectrum so close to the ears liking" That surely is the ultimate question ! The ear ( well certainly mine ) does not seem to like technical perfection.
I suspect going back to a single ended input stage has a lot to do with it, yes the distortion is initially higher, but it is mainly even harmonic and rises at a much lower rate with increasing frequency than the more usual LTP ( which produce more odd harmonics ).
It never ceases to amaze me that the ear can be so discriminating, but it really does seem to have this ability.
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Old 9th June 2008, 12:19 PM   #43
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Hi Mooly

Approx how many watts might it generate into a 3 ohm load?

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Old 9th June 2008, 12:30 PM   #44
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Quote:
Originally posted by Lumba Ogir
Lateral MOSFETs are however extremely fast
Ah, so that is why the rise time of a Lateral MOSFET is so much longer compared to a HEXFET, sure glad i finally understand it.
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Old 9th June 2008, 12:33 PM   #45
Mooly is offline Mooly  United Kingdom
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Hello Otto88,
I can't really say it's designed for sustained output into 3 ohms certainly not with a single output pair. Theres a world of difference between a real speaker with impedance dips to 3 ohms reproducing music and a 3 ohm resistive test loading.
As I say at the start, it's designed for a domestic setting where it will cope just fine with any "normal" speaker loading.
You can always parallel outputs or use "double die" FET's and tweak the driver stage to cope with the additional extra capacitive loading but I can't recommend doing ANY of this without listening tests.
The whole reason for the amps existance is it's sonics, if you start changing things it's almost certainly going to effect this.
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Old 9th June 2008, 12:45 PM   #46
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I think the Zeners space2000 was referring to are the gate protection Zeners.
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Old 9th June 2008, 12:57 PM   #47
Mooly is offline Mooly  United Kingdom
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Good thinking Batman
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Old 9th June 2008, 12:59 PM   #48
Mooly is offline Mooly  United Kingdom
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Space,
Just build as per the circuit, the only ommision is a 10 ohm 2 watt carbon or metal film resistor across the output inductor.
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Old 9th June 2008, 01:34 PM   #49
lineup is offline lineup  Sweden
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Quote:
Originally posted by Mooly

The 270 ohm gate stopper is higher than usually seen (47 or 68 ohm).

R13 the 330 ohms-- these just provide a little "isolation" for the VAS
from the loading and junction capacitance of the drivers Q6/7. Again no magic--- just a sensible value.
Quote:
Originally posted by Samuel Jayaraj

Regarding gate stopper resistors,
Randy Slone uses 470 ohms for the 2SJ162 and 680 ohms for 2SK1058.
I have tried these values with other circuits and I think they sound very good.
I can add to this about MOSFET gate stoppers.
Nelson Pass uses, almost constantly, 220 Ohms for IRF240 / IRF9240.
(I know, that Lateral is a bit diffrent than IRF HEXFET.)
--------------------

But anyway,
some time ago I did some spice mosfet amp design, using IRF240 / IRF9240.
I tried different values for Gate Stoppers.
Then I had a look at Bandwith Curve for the transistor ( AC Analysis ).

I started out low 22 Ohm, 47 Ohm, 100, 150, 220, 330, 470.
Result was almost Frightening
Master nelson certainly has not chosen 220 Ohm, just by chance, randomly.
Because at or around 220 Ohm (150-300) my simulation showed the most nice frequency curve sloop, without overshoot and bad PHASE Angle.

Of course I have seen Nelson Pass use a few other values, than 220 Ohm, in some circuits.
For example 100 Ohm for IRF240/ IRF9240.
So, to a certain degree, it depends on the circuit use, what value is optimal.
-------------------

Another thing.
If the P-MOS is way different regarding capacitance, we should theoretically,
use different resistor value for this. Compared to the N-MOSFET


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Old 9th June 2008, 06:14 PM   #50
Mooly is offline Mooly  United Kingdom
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Hello Lineup,
It's nice to know I'm not too wide of the mark with 270 Ohms
I must confess that I don't use a simulator, I tend to work I suppose empirically, particularly on matters like the gate stoppers.
The nice thing with the laterals ( and HEXFET'S ) is that the gate stopper is a user chooseable value, allowing you to define your own upper limit for the particular application.
JLH once mentioned something as well, that because of the extremely high Ft for a lateral that a Lecher Line type oscillator could be formed around a single device depending on the surrounding values of inductance/capacitance etc. Now they are weird things--- I first came across Lecher lines in UHF tuners, and I always wondered why sticking a pin through a piece of coax TV downlead thus shorting the inner and outer could "improve" the signal seen by the receiver on certain channels.
With anything like this amp I put a lot of faith in squarewave testing, it really is very revealing and well, 270 Ohm seemed just right all things considered.
It would be an interesting exercise to modify the design for HEXFET's including a vgs multiplier and see whether the sonics are unchanged.
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