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Old 19th April 2002, 04:03 PM   #91
Geoff is offline Geoff  United Kingdom
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Ergo, Thijs

I would not omit R9. I think the comment regarding distortion/R9 value that Thijs is referring to is the one Rod made in relation to one of his prototypes for the DoZ. This was not the JLH circuit but was a Darlington output transistor with a resistive collector load. Rod found that with no base resistor for the Darlington, the distortion was predominantly 2nd harmonic but was three times higher than when a base resistor was fitted. With the base resistor, the distortion was mainly 3rd Harmonic.

Of more importance, without a base resistor the positive going slew rate was degraded due to the lack of a base discharge path which resulted in severe degradation of the leading edge of a 10kHz square wave.

A similar situation will exist for Tr1 in the JLH circuit so I suggest that R9 remains, though I don't think its value is too critical.

I'm afraid I don't follow Ergo's friend's line of reasoning but I will give it some more consideration over the weekend. My initial thought is that the voltage across R9 (and hense the current through it) is determined by the Vbe of Tr1. This will vary with the collector current of Tr1 and with its c-e voltage. Indulging in a little speculation, pehaps what is required here is a transistor with a linear variation of Vbe with collector current (since it is not possible to obtain a device with no variation). Comparing the Vbe/Ic characteristic for an MLJ3281a with that of the MJL21194 over the operating range of current (say 0.1A to 2.5A) will show what I mean.

Ergo, I agree with your reservations about Tr5. Simulation shows that distortion with the Tr5 circuit is about twice that of an ideal current source or even the 1969 bootstrap arrangement. If you want to avoid Tr5, the ccs shown in the outline schematic in my previous post can be a standard two-transistor or LED/transistor circuit. An alternative is to revert to the 1969 bootstrap arrangement. As explained by Hugh Dean in another thread, the stability of the amp should be better with the bootstrap since the high frequency performance of the bootstrap capacitor will cause the gain of Tr3 to be reduced at higher frequencies thus improving the phase margin. There is some empirical evidence of this in that a 1969 version of the JLH worked using 2SC3281 output transistors whereas a 1996 version using the MJL3281a oscillated.

Again I will quote JLH from another of his articles:

"Relatively few design decisions now remained. Of these, the most significant was whether to give the second, class-A amplifier stage a bootstrapped collector load or to use a constant current source. The merits of these two arrangements are fairly well balanced. For example, the bootstrapped driver will give a slightly larger output power for the same HT voltage, and is simpler, while the constant current source arrangement will give a slightly lower second harmonic distortion figure which allows the same overall THD figure to be obtained with a lower open-loop gain - which makes the preceding circuitry simpler"

The input stage referred to in this quote was basically the same as that in the Class-A design. My simulations of the Class-A indicate that there is very little difference in THD between the ccs and the bootstrap except at very low frequencies, where the bootstrap capacitor begins to loose its effectiveness and the distortion rises a little.

Geoff
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Old 19th April 2002, 04:18 PM   #92
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Thanks for all the replies .. I must say I find this thread very educative for me.

Thanks alot!!


Thijs
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Old 19th April 2002, 04:25 PM   #93
Geoff is offline Geoff  United Kingdom
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Thijs

And I'm beginning to wish you had never started this thread :-)

Only joking. I welcome debate about this design since the more I have to think about it the more I learn.

Geoff
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Old 19th April 2002, 04:38 PM   #94
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Default JLH

Thanks Geoff for your very detailed analysis. I too feel the use of a constant current source for the positive leg of the output is the way to go. This the arangement that I prefer for my Aleph 3. It was not in jest that I said that the JLH topology shares some resemblance to that of the Aleph series. Perhaps a hybrid of Fet and BJT transistors would be interresting. A 2SJ74 p channel Jfet with CCCS might even allow direct coulpling for the amp. Also an IRF 510 for the driver of the output BJT would be interesting......

H.H.
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Old 19th April 2002, 05:05 PM   #95
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Shame on you dirty Harry..

I have posted such designs !

the total MOSFET version of the JLH 10Watter shows even lower distortion than the BJT version.. the hybrids I simulated were less satisfactiry..

All simulation .. that's a shame...

thijs
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Old 19th April 2002, 05:37 PM   #96
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Exclamation Make my day!

Contrairy to popular opinion, I have not seen every amplifier schematic on the web. A man has to know his limitations.... Scott Nixon has seen every amp schematic, and I've seen the rest.

H.H.
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Old 19th April 2002, 05:38 PM   #97
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here it is... full MOSFET version..

simulates great... but haven't heart a single note out of it!
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Old 19th April 2002, 06:04 PM   #98
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Default And you won't....

Gate resistors or the thing will oscillate. IRFP 240 on outputs, IRF510s won't last long for a power amplifiers and don't have enough transconductance. The source resistors for output Mosfets should be at least 0.5 ohms for bias stability. Some kind of current limiter or fuse is a real good idea. Everybody simulates..... doesn't anyone actually build something?

H.H.
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Old 19th April 2002, 06:16 PM   #99
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yeh yeh yeh... I know... I was just playing around.. I did make such remarks on my first post... but heee.. you askes for it ..

so don't be smart with me boy..

The funny thing is that it simulated so good... very linear.. and good bandwidth (also because of high Iq of the driver stage) I still think it might be something nice to try someday... transductance is good enough of high lineairity.. You may be right about the 0.1R being to low but the negative temperature coefficient helps in this case...The Iq is 1.2Amps is this case and so current is limited allready (???? or is it not?)

And heeeeee .. This thread started with my latest finished project!

You only build passive preamps.. my guinea pigs can do that


Thijs
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Old 19th April 2002, 06:19 PM   #100
jam is offline jam  United States
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Dirty Harry,

Looks like fighting words!

Jam
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