Ultra-LD 200W Power Amp Mk2 SC Aug 2008

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:

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Photo of amp module:

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Bias circuit:

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The article:

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Transistor datasheet:

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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

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Reply to
Franc Zabkar
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"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.

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

Reply to
Phil Allison

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

Reply to
Eeyore

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.

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

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

Reply to
Eeyore

MJE15032/33s are PUNY ?

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

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

Graham

Reply to
Eeyore

"Eyesore"

** 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.

** Completely FALSE.

All output BJTs are biased on.

** All you are doing is posing UTTER BULLSHIT.

...... Phil

Reply to
Phil Allison

On Sat, 23 Aug 2008 09:26:01 +1000, Franc Zabkar put finger to keyboard and composed:

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

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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

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Reply to
Franc Zabkar

"Franc Zabkar"

** 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.

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

.... Phil

Reply to
Phil Allison

On Mon, 25 Aug 2008 16:57:06 +1000, Franc Zabkar put finger to keyboard and composed:

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

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Reply to
Franc Zabkar

I guess that depends on your definition of difficult, but its the

*entire* point of using a DSP. A second order thermal model is easy, and should give quite good results. However its not really much more complex to have a higher order thermal model - think chain matrices, for example, or cascaded second-order sections

Cheers Terry

Reply to
Terry Given

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