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JLH 10 Watt class A amplifier
JLH 10 Watt class A amplifier
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Old 2nd May 2011, 11:44 PM   #1941
Bigun is offline Bigun  Canada
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JLH 10 Watt class A amplifier
I'm not looking for significantly lower distortion from the JLH 1969 except that which might be gained from the dual rail supply and possibly a bit more current through the input device. There are many ways of reducing the distortion, the easiest being to increase OLG and hence increase nfb factor. I don't consider more nfb a step in the right direction although I know some people do and that's OK with me too. There are some obvious ways to increase OLG through the use of some constant current sources which also reduce the Vbe variation of the devices so you win twice over - but in my version of the JLH I don't want any (active device) CCS's near the signal which means they are out of the question for the input device or the phase splitter device. I know it has been tried (see Geoff's website) but to me, this is no longer the JLH.


At the same time I don't see TIM as an issue in the JLH either. The OLG extends well past the audio region even using the MJ480 device and there's nothing to suggest it would be slew rate limited within the audio spectrum. I'm not in favour of using 'modern devices' that require additional compensation to remain stable. I view compensation as an unfortunate consequence of nfb. Faster output devices would allow more OLG at higher frequencies and hence extend higher nfb factors further up the spectrum. But I'm not looking to push the JLH into such territory, I'll leave that for you fellas.
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Old 2nd May 2011, 11:49 PM   #1942
Bigun is offline Bigun  Canada
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Quote:
Originally Posted by alibear View Post
Hi all, Bigun, the cap multiplier I used was one from the class A website. There was no extra capacitance fitted after it, large value caps were used before it as per the diagram.
I suspect that it's important to have large capacitance after the multiplier. The cap multiplier has a relatively high output impedance and you don't want any of the signal current flowing through the active device or it will add it's exponential characteristic to the signal distortion even if at a very low level. With large caps after the multiplier you have a chance to isolate the amplifier signal current from the multiplier. This is just my theory on the matter but perhaps it means that the cap multiplier is still in the game... as I see a variation on the multiplier being useful in controlling bias current runaway.
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Old 3rd May 2011, 12:57 AM   #1943
AllenB is offline AllenB  Australia
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Quote:
Originally Posted by Bigun View Post
I'm not looking for significantly lower distortion from the JLH 1969
I assume this means that 2nd order is no problem for you, but I also assume you'd like to reduce anything higher in order.

Quote:
I don't consider more nfb a step in the right direction although I know some people do and that's OK with me too.
For what it's worth, I'm OK with nfb as long as it isn't being used as a method of reducing problem distortion.

So I can see your point about the CCS's as a trade off between increased higher order distortion and reduced 2nd order...versus lower overall distortion. I wouldn't go there either in this case.

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Originally Posted by Bigun View Post
I suspect that it's important to have large capacitance after the multiplier.
No argument, but for what it's worth, it doesn't sound bad with all such capacitance removed.
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Old 3rd May 2011, 01:23 AM   #1944
Bigun is offline Bigun  Canada
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Yes indeed, I don't have a problem with 2nd order although too much can be bad for IM on complex music. And yes again, higher orders, especially 5th and above are to be despised from all that I've read. But I already have to live with some consequences of starting out with the 2N3055's for now...

I've worked out, I think, how to glue the bits of heatsink together to give me something useable. Maybe not great or elegant but all in the spirit of DIY ! If I can find time to actually assemble these metal pieces I'll post a photo or two.
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Old 3rd May 2011, 07:12 AM   #1945
mikelm is offline mikelm  England
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Quote:
Originally Posted by AndrewT View Post
wrong conclusion.
A ClassA amplifier, whether a Power amplifier or Pre-amplifier or a single stage single ended amplifier is only constant current when there is Zero output current, i.e. in the quiescent state.
Hi Andrew,

Surely this depends on the design - if the o/p stage has a CCS as a "load resistor" then the current, to all intents & purposes, is constant.

Of course this current is shared between the load & the o/p devise - so in as much as there is a signal it is not constant in the o/p devise - but to the PSU it should be more or less constant.

This would be even more so in a balanced working design where even tiny variations will be equal & opposite and will cancel each other.

mike

Last edited by mikelm; 3rd May 2011 at 07:37 AM. Reason: corrected mistake
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Old 3rd May 2011, 11:37 AM   #1946
alibear is offline alibear  United Kingdom
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Hi all,
BIGUN, I am confused regarding using high value caps after the capacitance multiplier or indeed other types of series regulators. Rod Elliot has designed a cap multiplier for the JLH ( ESP project 15 )and he quite clearly recomends not using a large value cap after the series transistor due obviously to the charging current for that cap that would have to flow through the transistor.
Have I missed something?
Can I have your views ( or anyone else ) about this please, I am very willing to learn.
Alan
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Old 3rd May 2011, 11:45 AM   #1947
lineup is online now lineup  Sweden
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Isnt the 'only' 1000uF capacitor in ESP proj 15
because it does not make sense to have a big capacitor
when already we have an output of multiply capacitance
in the transistor.

The charging of the 1000uF will be a fairly slow process
because the caps at the base of transistor will graduly charge.
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Old 3rd May 2011, 12:34 PM   #1948
Bigun is offline Bigun  Canada
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I can't comment regarding Rod Elliot's thoughts, he's far more experienced than I but I will try to describe my thoughts.

There are two separate tasks to be performed by the supply rail capacitors, a) low pass filter for the ripple from the rectifiers, b) low impedance return path for signal current flowing through the load.

For a) the objective is simply to create a smooth dc supply. The Cap Multipliers main purpose is to simulate a very large capacitance which has the function to reduce ripple and noise on the supply. I believe it's a very bad choice of circuit to create a low impedance path for signal current returning from the load because it's a non-linear element and has a relatively high impedance. I don't really see a problem in having a large capacitor after it - so what if this large capacitor must be charged through the cap multiplier ? the power supply including cap multiplier will supply several amps of current, quite sufficient to charge up some capacitors.

For b) I want high quality capacitors. I'd prefer not to have power supply ripple filtered by the same capacitors as those in the return path for the load current because then I have intermodulation between the signal current and the power supply ripple. I don't know exactly why that would be bad, but it sounds as if it would be. I also believe with suitable design that it's possible to improve channel-channel separation too.


Alibear - there are 'other series regulators' that might well complain about having capacitance after them. I'm not familiar with these issues as I've never used them. These 'other' regulators I refer to are feedback regulators, often an i.c. - they are amplifiers in their own right and their stability can be adversely affected by the load they are driving into. I don't see that as an issue which affects a capacitance multiplier - it has no loop feedback.
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Last edited by Bigun; 3rd May 2011 at 12:38 PM.
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Old 3rd May 2011, 03:22 PM   #1949
AndrewT is offline AndrewT  Scotland
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Quote:
Originally Posted by mikelm View Post
This would be ...so in a balanced working design where even tiny variations will be equal & opposite and will cancel each other
This statement is almost true, if you take out "even tiny".

All the rest of your post is untrue.
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Old 3rd May 2011, 04:30 PM   #1950
Bigun is offline Bigun  Canada
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Quote:
Originally Posted by AndrewT View Post
All the rest of your post is untrue.
That's a bit harsh maybe ? Perhaps there is a misunderstanding of what is being said. I interpreted Mike's comments as related to an example of a Single Ended output stage, with a CCS load. On this basis, I thought his comments were spot on.

I understand that there has been some debate as to whether the JLH is a push-pull or a kind of modulated single ended output. My view on the project I'm building is that this is a push-push output so the case of a simple CCS load doesn't apply.
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Last edited by Bigun; 3rd May 2011 at 04:34 PM.
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