John Curl's Blowtorch preamplifier - Page 1001 - diyAudio
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Old 25th October 2008, 12:05 AM   #10001
syn08 is offline syn08  Canada
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Quote:
Originally posted by john curl
Syn08, I don't think that you understand what PMA and I find valuable about the AD825. You have your preferences, but they do not appear optimum for audio line amp design, or this question would have been settled, decades ago. Please, don't second-guess us, we know what op amps do
John,

Please spend a few moments and clarify what you find so valuable and special about the AD825. So far, I haven't heard anything but pseudo arguments.

Quote:
...but they do not appear optimum for audio line amp design, or this question would have been settled, decades ago.
Unless you can explain consistently what you mean above, I'll take this as just another cheek-in-tongue dismissal.

If I missed something in your ("we") posts, I would appreciate a heads-up.
 
Old 25th October 2008, 01:23 AM   #10002
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Quote:
Originally posted by AndrewT
I do not disagree with your premiss that BJTs can be bettered by FETs at the input. Many designers state and show the improvements to be had.

It is the mixed AC & DC coupling that I don't like.

If the pre-amp has a DC blocking cap at it's output, then the power amp must have a DC blocking cap on the NFB stage to preserve the DC balance at the inputs that we refer to and thus ensure minimal output offset.

Misusing a DC servo to allow bad topology to be incorporated is bad design in my book, even when input stage FETs are used to ameliorate the design error. and that's why a DC coupled amp and pre-amp output, need a DC servo and DC detect with output isolation.

Hi Andrew,

In some ways we seem to agree a lot, but in others I'm not so sure. I hope I am not mis-reading you, but it sounds like you are saying that a power amplifier with an AC coupling capacitor at the input and with DC-coupled NFB with servo for offset control is "misusing a DC servo to allow a bad topology to be incorporated is bad design".

In other words, you seem to be saying that if you have AC coupling on the input side, you must have a DC blocking capacitor in the return leg of the NFB side.

If this is what you are saying, I guess I must disagree. In particular, if one is using JFETs, why is the use of AC coupling on the input side with DC-coupled negative feedback on the other side (with servo for offset control) a "bad topology"?

Thanks,
Bob
 
Old 25th October 2008, 02:10 AM   #10003
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I just love those 'pseudo arguments' they are usually 'ahead of their time'.
 
Old 25th October 2008, 02:40 AM   #10004
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I think if one wanted to waste the time you could search the web and find a group of self appointed experts on either side of the fence on any particular op-amp. The amusing thing to me is the extremes of opinion over such minute differences.
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Old 25th October 2008, 02:42 AM   #10005
syn08 is offline syn08  Canada
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Quote:
Originally posted by john curl
I just love those 'pseudo arguments' they are usually 'ahead of their time'.
All clear. AD825 sounds good. AD825 also improves the sound of ahead of their time recrystallized cryogenic directional solid silver cables with unbleached cotton insulation and is best in combination with ahead of their time Bybee devices.
 
Old 25th October 2008, 03:00 AM   #10006
m2003br is offline m2003br  Brazil
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Default dc amp

An powerful (and beautiful) Brazilian made DC amplifier, Dual FET input, 80V/uS, circa 1979.
worth some commentary?

Best regards,
Marcos
Attached Images
File Type: gif gradiente a1.gif (37.1 KB, 1439 views)
 
Old 25th October 2008, 03:30 AM   #10007
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The inductor in the output definately limits its possibilities. I think you need to go back a few hundred pages.
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Old 25th October 2008, 04:16 AM   #10008
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Very standard mid-fi design. Hi output inductance (3uH) is not good. Standard circuit, fets in the input are OK.
 
Old 25th October 2008, 04:17 AM   #10009
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Syn08, attacking my opinions and beliefs will not make a better audio product. You should cease and desist.
 
Old 25th October 2008, 04:44 AM   #10010
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Dear Scott,

What's your opinion about AD825, AD8065 and AD8067? They are ALL J-FET input Ops with SIMILIAR but DIFFERENT sounds. I did like J-FET input OPs but the appearance of LM4562 did change my mind!

Cheers,
chengtaw
 

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