Compare of MJL21193/94 with MJL3281A/1302A - diyAudio
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Old 5th March 2002, 08:06 PM   #1
sonnya is offline sonnya  Denmark
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Smile Compare of MJL21193/94 with MJL3281A/1302A

I dont know if this have been discussed before, but i did a sim on MJL21193/MJL21194 against MJL3281A/1302A in a emitterfollower outputstage with perfect +/-40V rails (zero impedance) and a perfect generator to see wich kind of distortion they make.
I wanted to make my design as good as possible without negative feedback before adding it.

I have searched on this site for a previous discussion but did not find any.. So sorry if i make a repost on this subject.

Back to the compare.

The MJL21193/94 generates a dominating odd order distortion.

The MJL3281A/1302A generates a dominating even order distortion.

Idle current is 550mA in each transistor. output is 50Vpp into a load of 8Ohm.

Look for you self in the attached zipfile.
Attached Files
File Type: zip outputsims.zip (63.6 KB, 400 views)
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Old 6th March 2002, 03:32 AM   #2
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Thanks for posting the results. Lacking any technical competence to comment, I would like to ask some questions -

How does the spectrum look with a 4 ohm load?
Are the constant beta charateristics of the 3281/1302 pair the causal effect?
Given the constant beta characteristics, how do the spectrums look with a less than perfect source driving the followers?

The spectrums are quite interesting, especially given Self's writings on 'load invariant' output stages.

Thanks,
Paul
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Old 6th March 2002, 07:13 AM   #3
sonnya is offline sonnya  Denmark
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I have just found out that onsemi has a bad model for the MJL1302A!!! The BF was set for 10000 where it should be around ~120 - ~140.

After i uses they gain from a 2SA1302 model the FFT is identical for both types of BJT...

Sonny
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Old 6th March 2002, 07:50 AM   #4
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There's an old saying in the computer business:

"Garbage in, gospel out!"



A simulation is only as good as its models - and it seems your models weren't very close.

I would be surprised to see a qualitative difference between two sets of BJT devices, and that's what your initial results suggested. I would not be surprised to see a modest quantitative difference.

There's another old saying:

"The purpose of computing is insight, not numbers."

What can you learn from the incorrect model? Specifically, what has to change in a real-world transistor to get the kind of results you want?
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Old 6th March 2002, 08:51 AM   #5
sonnya is offline sonnya  Denmark
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Hi chucko.

For the models .. They are good, but it is done by hand and therefore subjected to human error. Wich is the problem in this case.
It is not some homebrew ones but from onsemi.
The models are made by engineers not software designers.

"I would be surprised to see a qualitative difference between two sets of BJT devices" - If you take another BJT like TIP31 and 32 you will get another result.. So it will give you a indicator how the transistor behaves.

The sims also showed me that the higher the idlecurrent is => lower distortion to some degree and lot of other things.
This you only will see if you have done your PCB and maybee ended up making a new one.

Wich is a lot cheaper than make 10 different PCB's just to discard 9 of them.

You can test your design down the level just before the PCB to see if you have done any miscalculation.
At this point sims are good.
And when do not have a audioanalyzer this is a really good tool. You are not in total darkness.

(Sorry for english and my temper)

Sonny
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Old 6th March 2002, 10:12 AM   #6
Geoff is offline Geoff  United Kingdom
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Sonny

The OnSemi model for the MJL1302A is not necessarily bad. Agreed the bf is set at 10000, but this is modified by other factors during use. If you simulate gain v collector current for the MJL1302A and the 2SA1302 you will find that the gain is not too dissimilar. These figures are simulated at 27degC and rounded to the nearest 5:

Ic (A) ... MJL1302A ... 2SA1302
0.1 .......... 100 ............ 105
0.5 .......... 120 ............ 115
1.0 .......... 115 ............ 115
2.0 .......... 100 ............ 110
5.0 ............ 75 ............. 90

Before using any new model, I always run this check and compare the results against the datasheet graphs for a typical device. Some models are very close to the datasheet figures and others show significant discrepancies and must be used with caution.

Geoff
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Old 6th March 2002, 10:58 AM   #7
sonnya is offline sonnya  Denmark
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maybee we are not using the same model or i am using a bad simulator (switchercad 3 from LTC).

But i got a peak gain of ~240 with a vce of 40Vdc.

But thanks for the feedback Geoff.

Sonny
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Old 6th March 2002, 12:42 PM   #8
sonnya is offline sonnya  Denmark
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I am a big idiot!!
The first sim i have done is OKAY. The MJL3281A/1302A pair do generate lower odd order harmonics.

And Geoff your are right about that the simulation model is OKAY.
I have not taken the low va into acount.

The gain of 240 was calculated at a vce of 40V. Lowering it to 10V shows a gain of ~130.

The MODEL IS OKAY!!!! for the MJL1302A.

Regarding the MJL21193(PNP) high Ic (When driving low impedance loads) it shows off a rise in vbe drop.

It is not my day .. Maybee i should get some sleep now!

Sonny
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Old 6th March 2002, 05:04 PM   #9
sonnya is offline sonnya  Denmark
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Quote:
Originally posted by pmkap
Thanks for posting the results. Lacking any technical competence to comment, I would like to ask some questions -

How does the spectrum look with a 4 ohm load?
Are the constant beta charateristics of the 3281/1302 pair the causal effect?
Given the constant beta characteristics, how do the spectrums look with a less than perfect source driving the followers?

The spectrums are quite interesting, especially given Self's writings on 'load invariant' output stages.

Thanks,
Paul
It looks like it is "constant beta" characteristics of the 3281/1302 pair. The current gain peaks at ~640mA and is nearly flat over an area of 1Amp. (1 - 2% gain change) where the MJL2119X has a gain who falls all the time. It has no flat "spot".
I have observed the same thing with smallsignal BJT in a emitterfollower.
If you can place the idlecurrent at the peakpoint and the transistor is flat over a wide area then the harmonics falls off in a nice slope at a relative "heavy load".

When you increase the load to 4 ohm or lower. Then you will see a rise in odd order harmonics.
At this point it matches D.self's comment about loading ,currentgain curve and multiple pairs of outputtransistors.

All this is when perfect source driving the followers.
All this would also apply to driverstage because of the heavy change in base current from the outputpair.

The driverstage also have to have a high early voltage. If it is low it will lower the input impedance on the driverstage.

I have not seen the article on 'load invariant'? But i would like to.

I can be wrong about all this. It would not be the first time today.

Sonny
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Old 6th March 2002, 11:20 PM   #10
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Sonny,
Thanks for the response. The reson for my query is that at this very moment a pair of AKSA 100watt amps are wending their way from Australia to me in the Nasty Apple. Hugh's amps use that very same 3281/1302 pair (or possibly their equivalent 5200/1943 pair). As to why I'm seeking technical validation of my subjective view, it is simply a reflection of my typical 'audiophile' insecurity. I wouldn't buy the AKSAs until I heard them, and I was very impressed, to say the least.

With regards to Self's article on Load Invariant Amplifiers, its not available at his site -
http://www.dself.demon.co.uk/ampins.htm
like his article on Sources of Amplifier Distortion, but was published orignally in EW Jan 97. My reference was to the 1st part of 'Load Invariant Power Amplifiers' in the current, March 2002 "Audio Xpress" magazine. I don't know if they're the same article. If you can't find access to the article, email me with your address (my email server is currently down), and I'll send you a copy, or if you want to wait, I could email you the full article when the second part comes out.

To torture you, I'll quote the last part of the first part -

""increased driver nonlinearity is caused by beta-droop, which is caused by increased ouput device collector current" There are three points of attack here. First, increase the linearity of the driver system; this can't be done if it's a single transistor, as is usually the case. Secondly, the output device types can be selected for the least beta droop. This is very device dependant... Third, the per-device collector current can be reduced by using paralell output devices..."

Just as you said.

Thanks,
Paul
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