John Curl's Blowtorch preamplifier part II - Page 1869 - diyAudio
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Old 3rd December 2011, 05:27 AM   #18681
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Hi John,

Quote:
Originally Posted by john curl View Post
Thank you JohnloudB. I think you are correct. For the record, there is a 100 ohm resistor at each output. There is also provision for a current offset using a fet to the negative supply. Looking again at the schematic, I can't find any problem.
I think the problem is that you quite sensibly decided that the lowest load this Phono would ever drive would be 10K//1nF and that only when JA or other 'scope jockeys test it.

So someone tested it at 600 Ohm, for which it was not designed... I deal with such rank tomfoolery by writing 10K minimum load impedance into the Spec's and doing a Steve Jobs on anyone who decides to ignore that spec.

JA also tested the AMR PH-77 with 600 Ohm and got 3% 2nd order THD, I'm not worried (the output is directly from a Tube and not from a Follower), as my Spec says "Minimum Load Impedance: 10KOhm".

Ciao T

PS, something someone send me recently, I would like to share with you John:

Don't try to
win over
the hater's
You are not
the JERK
whisperer
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Old 3rd December 2011, 05:47 AM   #18682
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Quote:
Originally Posted by john curl View Post
Just give me your 'faves' either single or dual, fet or bipolar input. 1'st stage single bipolar input, second stage dual fet input.
It's downright phishing.....
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Old 3rd December 2011, 06:32 AM   #18683
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Not really, I don't need to take anyone's secret find of the best sounding op amp. After all, many here think that all IC's sound the same anyway. If you have a technical secret, please keep it to yourself, but please don't ask anything extra out of me, either.
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Old 3rd December 2011, 06:40 AM   #18684
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Quote:
Originally Posted by SY View Post
Or market positioning. One needs to differentiate when competing against established brands, whether or not the differentiation is relevant.
This is funny reasoning. I just shared the experience that I have had for 10 years with both solutions of output stages, i.e. plain opamp and buffered output. Walt Jung also described buffered link stage outputs in his articles - will you also call it differentiation and plain marketing?? I wonder why John is open to parts selection, capacitor types, input stage devices, power supply solutions, transformer types - would you call it differentiation as well? But he does not admit the possible effect of output buffering vs. simple opamp, and uses very simple reasoning - this is surprising! In case that everything except for NE5532 + uA7815/7915 is a differentiation, then I understand. You SY use tubes. Differentiation?
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Old 3rd December 2011, 06:47 AM   #18685
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Quote:
Originally Posted by ThorstenL View Post

JA also tested the AMR PH-77 with 600 Ohm and got 3% 2nd order THD, I'm not worried
Thorsten, the 600 ohm plot of JC 3 in the Stereophile has shown CROSS-OVER DISTORTION (not 2nd and 3rd order THD). Maybe not everyone seen it immediately in the plot. Just for this reason I asked what is the design of the output stage, and several posts later I read it is the simple opamp. That's why I pointed to a possible problem. I would not expect a simple opamp output in any of John's design, for the stand-alone unit (not integrated inside another product).
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Old 3rd December 2011, 06:54 AM   #18686
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Well, I thought that todays IC's were free of crossover distortion. That is what everybody tells me. However, this design was NEVER made for 600 ohm operation, so testing it in that way will show distortion. Everyone using the JC-3, please refrain from using less the 5K ohms as a load, 10K or more preferred.
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Old 3rd December 2011, 06:59 AM   #18687
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PMA on my test bench I have an augmented HEADPHONE AMP design for Parasound consisting of an op amp and a discrete mosfet output stage. It HAS to drive 600 ohms or less, (30 ohms actually), so I am not relying on the IC itself, for optimum drive. Satisfied? (I've been playing with it for the last 6 mo or more)

Last edited by john curl; 3rd December 2011 at 07:11 AM.
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Old 3rd December 2011, 09:43 AM   #18688
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Quote:
Originally Posted by PMA View Post
TYou SY use tubes. Differentiation?
I don't sell anything so there's no "market" for me. I design for fun.
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Old 3rd December 2011, 11:53 AM   #18689
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Pavel,

Look at figure TPC7 of the AD845 data sheet.
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Old 3rd December 2011, 12:40 PM   #18690
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Quote:
Originally Posted by john curl View Post
Well, I thought that todays IC's were free of crossover distortion.
You're getting old enough to appriciate the advances in low power circuits, that will go into portable patient monitoring, etc. You are the one who said "Universal". Don't use them for audio.

There are plenty of op-amps that will take a 16mA current source at the output, and you have your no crossover at 600 Ohms.

BTW Walt was always convinced something like the AD815 was needed for the best line stage.
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