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A simplified universal differential or single ended phono preamp
A simplified universal differential or single ended phono preamp
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Old 7th April 2017, 06:54 PM   #21
scott wurcer is offline scott wurcer  United States
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Quote:
Originally Posted by EUVL View Post
Remember the spice files I sent you a while ago for the version with 300x gain at 1kHz ?
In spice at least, THD is quite decent at 2Vrms out, using 4x 2SK372 at the input.
Maybe you would have another look at see if I have done something really silly......


Patrick
Will do, I'm not fully set up with LTSPICE on my new laptop. Could you resend just to make sure to my gmail account some things got mixed up with my retirement.

EDIT - I found the one with 2SK170's and 200 Ohms degeneration, I will look at it more closely. What I worry about is reducing the degeneration of the folded cascode and at the same time increasing the impedance level of the RIAA network will make parasitic capacitances/resistances have a larger role.

It might all work out that we can have it all, but I would like to have a version that uses the Linear Systems FET's so folks don't have to worry about obsolescence and fakes.
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Old 7th April 2017, 07:57 PM   #22
EUVL is offline EUVL  Europe
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I'll send you all files again by email.

We have enough stock of everything, even if it ever becomes a public project.
I guess you are not hired by LS to come up with application examples for them ?




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Old 7th April 2017, 08:25 PM   #23
scott wurcer is offline scott wurcer  United States
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Originally Posted by EUVL View Post
I guess you are not hired by LS to come up with application examples for them ?
Patrick
To be clear no, not one cent. I've worked with John Hall (RIP) since 1977, but in that case I was paid by my day job.
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Old 7th April 2017, 08:39 PM   #24
EUVL is offline EUVL  Europe
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Was just pulling your leg.

Documents sent by email.


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Old 7th April 2017, 09:08 PM   #25
john curl is offline john curl  United States
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I feel that Scott's new discrete circuit has a lot of merit, and the topic that it includes is worth contributing to, especially on this thread. Scott has the skill and parts to do world class work, and that is what we should discuss.
Here is one example of one of my similar designs, taken as Scott did, from the AD797 topology and its predecessors (Harris), that I made for Parasound, but they decided not to build but a few prototypes.
There are a few differences, please note the use of bipolar parts for the second stage. (Parasound did not want to use any more pch jfets than necessary), servo (IC designed by Scott), and a number of other features. Parasound got the right to first refusal, so I can publish it now. Scott's design has other advantages, and perhaps we might discuss the general design together, with any one else invited to contribute.
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File Type: jpg old phono1.jpg (292.3 KB, 766 views)
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Old 7th April 2017, 09:50 PM   #26
Mark Johnson is offline Mark Johnson  United States
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A simplified universal differential or single ended phono preamp
My headphone amp uses that topology as its first two stages: NJFET diff pair -> PNP folded cascode -> enhanced NPN current mirror. But I've added a double EF output stage to drive 15 ohm loads; John has no 3rd stage so he's got a transconductance amp driving a 2.62K load resistor. I felt it was important to heavily bypass the PNP folded cascode transistors' bases, to the top rail, so I used 100uF + 0.1uF. John's schematic doesn't include these.

And we can wonder aloud whether the 220uV 35V capacitors on the gate nodes of the capacitance multipliers, are "less harmful" than an electrolytic bypass capacitor would be on the supply rail itself. Since the MOSFET is a unity-gain source follower, any electrolytic-caused badness on the gate, gets copied onto the source, n'cest pas?
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Old 7th April 2017, 10:00 PM   #27
john curl is offline john curl  United States
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Mark, the cap has to be 'doing something' in order to act up. It is not drawing any current. I use GOOD electrolytic caps as well.
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Old 7th April 2017, 10:00 PM   #28
EUVL is offline EUVL  Europe
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In simulation at least, I found that a 4-FET current mirror has lower distortion than a 4-BJT transistor, with the same degeneration resistors.
I wonder why FET mirrors are not used more widely ?


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Old 7th April 2017, 10:06 PM   #29
scott wurcer is offline scott wurcer  United States
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Quote:
Originally Posted by EUVL View Post
Was just pulling your leg.
Documents sent by email.
Patrick
I know, thanks if we could squeeze in the extra gain without sacrificing other parameters it would be an improvement.

Thankyou John, if one can make an open-loop phono stage with -87dB THD it deserves a listen. The circuits all come from the past that is not important.
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Old 7th April 2017, 10:14 PM   #30
john curl is offline john curl  United States
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And at what output level did you measure -87dB Thd?
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