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Old 19th December 2003, 11:04 AM   #1
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Default JC-3 class-a power amp

I was looking at geoff's class-a site and noticed this amp. It uses jfet upfront, and is fully complementory.

I would like to know:

1) how does it work? it has a fully complementory vbe generator, . and it also has a very interesting feedback scheme. I would appreciate any insight into how it works.
2) I am interested in what has to be done to replace the jfets;
3) and I would also be interested to use some irfs as output devices, .

Any insight is apprciated.
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Old 19th December 2003, 05:18 PM   #2
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Where is the site? Without a schematic, it is difficult to say anything.
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Old 19th December 2003, 05:20 PM   #3
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john, here it is:

http://www.tcaas.btinternet.co.uk/jc-3.gif

your comments would be greatly appreciated.
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Old 19th December 2003, 05:33 PM   #4
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That design is correct in principle, but the design is over 25 years old, and the parts are obsolete. The basic design can be modified with FET's just about everywhere. I recommend Fairchild Mosfets, as they will be better than IR in this application. Jfets can be 2sk389 and j109.
This design is somewhat limited to inverting operation, but can be more useful with a servo added to control the offset.
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Old 19th December 2003, 05:37 PM   #5
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thanks, John.

can you speak in a little bit more detail as to how the feedback works? thanks in advance.
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Old 19th December 2003, 06:08 PM   #6
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This is a dual feedback arrangement. Especially in the early days, it seemed to be useful to reduce the amount of global negative feedback, because of potential TIM. This technique lowers the over global negative feedback to the ratio of the internal feedback resistors and the overall feedback resistor. This should be about 14-20dB. The reason for this is that the drive impedance to the output stage is lowered and this removes the dependence of very linear beta in the output stage. Another way of looking at this is that the pre-driver stage (6178-6180) generates a very high drive impedance. The darlington output stage reflects back from the speaker impedance another fairly high impedance, which is essentially 2*B(1)*B(2)*R(L). It is the relative levels of these two impedances that implies whether the output stage is voltage or current controlled. Lowering the drive impedance to the output stage makes the output stage into a voltage follower, rather than a beta multiplier (which is more nonlinear).
With fets in the circuit, this is unnecessary, as the gate is never current controlled.
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Old 19th December 2003, 07:15 PM   #7
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i need to digest it more but it is appreciated, john.
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Old 20th December 2003, 07:37 AM   #8
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Upon re-reading my previous comments, I regret that I did not express things properly.
This is the situation: A transistor output stage can be thought to be controlled in two ways. It can either be a BETA MULTIPLIER or a VOLTAGE FOLLOWER. What matters is the source impedance that the output stage is driven with. If it is driven with a HIGH impedance, then the output stage will behave as a Beta Multiplier and will therefore be VERY dependent on BETA.
If, on the other hand the drive to the output stage is LOW impedance, then the transistor output stage will behave as a Voltage Follower to the drive voltage. Beta nonlinearity will then be less important.
The extra feedback loop reduces the drive impedance to the transistor output stage, so it should become more linear, and also be less sensitive to load variation on overall linearity.
I left out one important word in my previous statement:
It should have said "removes the dependence of HAVING a very linear beta in the output stage"
I hope you can understand what the extra feedback does better now.
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Old 20th December 2003, 05:24 PM   #9
jcx is online now jcx  United States
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http://www.ne.jp/asahi/evo/amp/J200K1529/report.htm

circuits 6,7 and 13,14 seem to show some relation to the JC-3 topology and could be interpreted as illustrating Mr Curl's point about output stage impedance loading of the VAS stage - of course i'm "interpreting" the schematics, not the Japanese commentary
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Old 20th December 2003, 05:48 PM   #10
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Mr.John,
Could you tell me why do you not use Non-inverted mode in JC-3?
The reason is easy to make amp stable?

I think that adding a resister (10K,f.e.) between the pre-driver stage and GND could obtain the same effect which lower the impedance of the pre-driver stage output,and you didn't do that.could you tell me why? Maybe the impedance of the pre-driver stage is still higher and the open loop gain of amp is lower ?

Sorry for my poor English,
thanks for your replies.

X.G.
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