Ultra-LD 200W Power Amp Mk2 SC Aug 2008

[Franc Zabkar]

I've been trying to understand the theory behind the design of the
Ultra-LD 200W Power Amp Mk2 in the Aug 2008 issue of Silicon Chip. The
amp uses On Semiconductor's NJL3281D/NJL1302D ThermalTrak power
transistors. These have integral diodes.

This is the full circuit:
http://us1.webpublications.com.au/static/images/articles/i1107/110797_10mg.jpg

Photo of amp module:
http://us1.webpublications.com.au/static/images/articles/i1107/110797_12mg.jpg

Bias circuit:
http://us1.webpublications.com.au/static/images/articles/i1107/110797_9mg.jpg

The article:
http://www.siliconchip.com.au/cms/A_110797/article.html

Transistor datasheet:
http://www.onsemi.com/pub/Collateral/NJL3281D-D.PDF

In the absence of an adjustment, how can you guarantee that the
quiescent current settles at a level that is optimum for crossover
distortion and power dissipation purposes? Wouldn't the actual bias
current be highly dependent on the characteristics of Q10 and Q11?
Would a standard Vbe multiplier arrangement where DQ12/DQ13 and
DQ14/DQ15 were wired in parallel (with series resistors ?), and
Q10/Q11 were at ambient temperature, be a better way to do this?
Wouldn't that guarantee optimal thermal tracking, and wouldn't that
allow the amp to be optimally biased?

BTW, the article states that the quiescent voltage across the 0.1 ohm
emitter resistors is 70-100mV. I believe this should be 7-10mV.

- Franc Zabkar

 

[Phil Allison]

"Franc Zabkar"

I've been trying to understand the theory behind the design of the
Ultra-LD 200W Power Amp Mk2 in the Aug 2008 issue of Silicon Chip. The
amp uses On Semiconductor's NJL3281D/NJL1302D ThermalTrak power
transistors. These have integral diodes.

This is the full circuit:
http://us1.webpublications.com.au/static/images/articles/i1107/110797_10mg.jpg
(snip)

In the absence of an adjustment, how can you guarantee that the
quiescent current settles at a level that is optimum for crossover
distortion and power dissipation purposes? Wouldn't the actual bias
current be highly dependent on the characteristics of Q10 and Q11?

** The Vbe of Q10 and Q11 ( at 8mA and 55V ) IS critical to the actual
output device bias current in that circuit.

The SC boys likely tried a few MJE15030/1 devices, all from the same batch
and came to the nonsense conclusion Motorola ( or ON Semi) made them all
identical and would do so into the indefinite future.

The circuit needs a bias trim pot somewhere to allow for production
variations in the MJEs.

Would a standard Vbe multiplier arrangement where DQ12/DQ13 and
DQ14/DQ15 were wired in parallel (with series resistors ?), and
Q10/Q11 were at ambient temperature, be a better way to do this?

** That is gobbledegook.

Vbe multipliers use transistors, not diodes.

Wouldn't that guarantee optimal thermal tracking, and wouldn't that
allow the amp to be optimally biased?

** The whole idea of having integral bias diodes inside the output devices
is so the bias voltage compensation will track the ACTUAL chip
temperature and so the bias current stays as close as possible to constant.
Thing is, the chip temp of an audio output transistor in a class B amplifier
varies suddenly and over a wide range - ie in a couple of seconds it may
go from 25C to 150C.

A Vbe multiplier circuit can only track the temp of the ambient air OR the
heatsink used by the output devices - which will not vary anywhere near as
quickly OR over such a wide range as the output device chips. Means bias
voltage compensation is slow arriving and always less than needed to
maintain the same bias current.

This in turn means that after a sudden increase in output power, the output
devices are over-biased and remain that way until well after the output
power level drops back to a low level again.

In the new SC circuit, there are four Vbe voltages to compensate - the two
MJEs and two of the NJLs (cos output devices in parallel behave as one).
However, the MJEs pass only 1% of the current the NJLs do (since Hfe =100)
and so dissipate only 1% of their power as well. Means the chip temp of the
MJEs is controlled more by the output device heatsink temp than their own
puny dissipation.

As stated previously, the heatsink temp is nothing like the output device
chip temp so the SC bias circuit is wrong - it over-compensates.

Bias current in the output devices will fall significantly during a burst of
power - but the previously set level will return very quickly, a few
seconds after drive is stopped, when all the chips acquire the same temp as
the heatsink.

This gives a false illusion of good bias stability.

BTW, the article states that the quiescent voltage across the 0.1 ohm
emitter resistors is 70-100mV. I believe this should be 7-10mV.

** Another SC typo ......



...... Phil

 

[Franc Zabkar]

Yes, I'm aware of what the "be" in "Vbe" stands for.

I was thinking of an arrangement like this:

|__________
| |
__|__ |
| | |
R R |
| | |
DQ12 DQ13 |
|____| |
| |
|--|-| |
| | |
R R |
| | |
DQ14 DQ15 |
|____| |
| |
| |
VR |
| |/
|--------| Q
| |\
R2 |
|----------|
|

- Franc Zabkar

 

[Phil Allison]

"Franc Zabkar"
"Phil Allison"

Yes, I'm aware of what the "be" in "Vbe" stands for.

** You snipped my entire post without any comment

- you fucking, arrogant wog pig.



....... Phil

 

[MoiInAust]

"Franc Zabkar"
"Phil Allison"



** You snipped my entire post without any comment

- you fucking, arrogant wog pig.



...... Phil

Oh dear! Here he goes again. After lots of really good informative posts,
out again comes the foul language and rotten expressions. Back to the
killfile...

 

[Phil Allison]

"MoiInAust"


** Drop dead you pathetic wanker.

 

[Phil Allison]

"Terry Given"

has anyone done an amp yet using, say, an MSP430 running a thermal model
based on sampled output power to control the bias setpoint? it would be
really easy to do, and the thermal model itself is almost trivial.

** There are two, long proven ways of solving the problem of class B bias
setting.

1. Use the output BJTs in common emitter mode - ie with the load connected
to the collectors.

2. Use no bias at all in the output devices.


Solution #1 removes the output transistor B-E junction temp from the
equation and the heatsink temp as well, long as the drivers have their own
heatsink.

Solution #2 is achieved by using drive transistors to cover the first 25mW
or so of output power, then the output devices act purely as current
boosters after that. This is the preferred way if multiple parallel devices
are used.

Then, there is the Quad "Current Dumping " method, still covered by patent
AFAIK - a neat variation on solution #2 that is just soooooo elegant.

Digital solutions have neither need nor appeal.



...... Phil

 

[Eeyore]

If its a decent amp, use a decent processor (spend $10) and get 1Ms/s or
so. but I do like the idea of a tiny DSP sitting on the output, directly
measuring each transistors current and Vce.

I've thought of that using analog circuitry and a multiplier chip.

The thing is though, you CAN exceed the DC safe area quite happily by LOADS for
short periods. That makes it difficult to calculate.

Graham

 

[Eeyore]

"Terry Given"


** There are two, long proven ways of solving the problem of class B bias
setting.

1. Use the output BJTs in common emitter mode - ie with the load connected
to the collectors.

2. Use no bias at all in the output devices.

Solution #1 removes the output transistor B-E junction temp from the
equation and the heatsink temp as well, long as the drivers have their own
heatsink.

Solution #2 is achieved by using drive transistors to cover the first 25mW
or so of output power, then the output devices act purely as current
boosters after that. This is the preferred way if multiple parallel devices
are used.

That is the way I like to do it.

QSC designs use both methods albeit with a common heatsink. However this leads
to a very non-linear transfer characteristic at low levels, which a Singapore
distributor brought to my attention once.

Then, there is the Quad "Current Dumping " method, still covered by patent
AFAIK - a neat variation on solution #2 that is just soooooo elegant.

Yet I've never heard one I liked the sound of.

Digital solutions have neither need nor appeal.

For SOA monitoring ? Overkill really. Just use enough output devices and design
sensibly. Seems to work.

I've spent ages thinking through suitable alternative protection schemes but if
you do (as you do) need to drive a highly reactive load (which it may be at
certain frequencies only) it isn't simple. Not by a long way.

Graham

 

[Phil Allison]

"Eyesore"
Phil Allison

"Terry Given"


QSC designs use both methods albeit with a common heatsink.

** All the QSC amps I ever saw had forward biased output transistors and
puny driver transistors.

Which one uses zero bias ?????

However this leads
to a very non-linear transfer characteristic at low levels, which a
Singapore
distributor brought to my attention once.

** As usual - no details provided.

Just more bollocks.

Yet I've never heard one I liked the sound of.

** More bollocks..............................

For SOA monitoring ?

** Not what the context here is - pal.


The Stevenson troll simply choses NOT to read what people actually post.

Cos it gets in the way of his insatiable, psychotic urge to bullshit wildly.




...... Phil

 

[Eeyore]

"Eyesore"
Phil Allison

** All the QSC amps I ever saw had forward biased output transistors and
puny driver transistors.

MJE15032/33s are PUNY ?

Which one uses zero bias ?????

All the RMXs for sure. The main output device bias is set just below turn-on,
typically around 500mV. Fairly certain the MXs too.

** As usual - no details provided.

It was YEARS ago man FFS. I'm just sharing info.

Graham

 

[Phil Allison]

"Eyesore"

Phil Allison

MJE15032/33s are PUNY ?

** Not used in any of the USA or MX series.

And the drivers do not drive current into the load in any case.

So you are TOTALLY WRONG as bloody usual.

All the RMXs for sure.

** Completely FALSE.

All output BJTs are biased on.

It was YEARS ago man FFS. I'm just sharing info.

** All you are doing is posing UTTER BULLSHIT.



....... Phil

 

[Franc Zabkar]

I've been trying to understand the theory behind the design of the
Ultra-LD 200W Power Amp Mk2 in the Aug 2008 issue of Silicon Chip. The
amp uses On Semiconductor's NJL3281D/NJL1302D ThermalTrak power
transistors. These have integral diodes.

This is the full circuit:
http://us1.webpublications.com.au/static/images/articles/i1107/110797_10mg.jpg

In the absence of an adjustment, how can you guarantee that the
quiescent current settles at a level that is optimum for crossover
distortion and power dissipation purposes?

Leo Simpson has told me that the amp design was based closely on this
On Semi application note:

http://www.onsemi.com/pub/Collateral/AND8196-D.PDF

He also says that the September issue will explain how to adjust the
bias to account for variation in the diodes (not Q10/Q11). No trimpot
is used.

- Franc Zabkar

 

[Phil Allison]

"Franc Zabkar"

Leo Simpson has told me that the amp design was based closely on this
On Semi application note:

http://www.onsemi.com/pub/Collateral/AND8196-D.PDF

** ROTFLMAO !!

That fraudulent document was debunked by me on AAPLS back in February of
2005 - when Arny Krueger raised it.

For one thing, the MPSA06 and MPSA56 transistors (Q7 & Q8 ) have to cop
nearly double their rated maximum Vce - so the amp will immediately blow
up !!

For another, the text indicates that the bias current in the second version
is HUGE - around 1 amp per device !!

(The bias voltage between the bases of Q7 and Q8 was raised by 200mV -
meaning the idle drop across each 0.1 ohm ballast resistor is raised by
around 100mV too )

So the second version is operating in damn near full class A.

BTW:

The multi-colour THD curves are HILARIOUS !!

4 out of 6 are done with NO LOAD on the amplifier !!!!!

How informative !!!!!!!!!!!!!!!!!

Mark Busier sounds like a fake name - and his article is fake too.

He also says that the September issue will explain how to adjust the
bias to account for variation in the diodes (not Q10/Q11). No trimpot
is used.

** I can hardly wait ......



..... Phil

 

[Franc Zabkar]

Leo Simpson has told me that the amp design was based closely on this
On Semi application note:

http://www.onsemi.com/pub/Collateral/AND8196-D.PDF

Something doesn't seem quite right about the bias circuit in Fig 1.

The text states that the "small signal transistor (TO-92) [is] mounted
on the heat sink between the (TO-220) power output drivers". Since the
MJL3281A and MJL1302A devices come in a TO-264 package, I suspect this
is a typo. In any case I don't think it makes sense to place Q5
(MPSA06) near Q9 and Q10 (MJE15032/MJE15033) which are the only TO-220
devices.

As for the biasing circuit, it seems to me that the Vbe multiplier
consisting of Q5 is set for 6 diode drops. Therefore I would think
that for proper thermal tracking, the 6 "diodes" in the multiplier
would need to be matched by 6 PN junctions on the heatsink. AFAICT we
only have 4, namely Q9 and Q14/15/16 and Q10 and Q11/12/13, unless Q7
(MPSA06) and Q8 (MPSA56) are also mounted on the same heatsink.
However, the article is unclear as to the location of the latter. In
fact it is not even clear as to the location of Q9 and Q10. To cloud
matters further, the introduction to the article states that "one of
the major design concerns in the output section of a typical class AB
audio amplifier is output bias and stability over the operating
temperature range. In the past, this was typically accomplished
through the use of a single bias transistor mounted on the amplifier’s
heatsink in close proximity to the output devices."

- Franc Zabkar