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Old 24th September 2009, 05:49 PM   #1521
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This is what I got in simulator PMA used! It`s my version of complementary jfet/bjt preamp circuit with NO global negative feedback. The circuit needs buffer because the output is (10K) resistor loaded like in a JC2 preamp, so results of simulation are without output buffer.

That would be 0.00016% second harmonic for a 3V peak output for a single ende in/out and a much less for a balanced outout!
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File Type: gif se-in---se-out.gif (12.5 KB, 362 views)
File Type: gif se-in---bal-out.gif (14.2 KB, 356 views)

Last edited by bogdan_borko; 24th September 2009 at 05:59 PM.
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Old 24th September 2009, 05:50 PM   #1522
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Well, it looks like everybody 'fell off the truck'. Could it be that schematics are intimidating? Or just too simple to bother with?
However, this is the question: How do you make a differential pair (of any active device) that is both lowest distortion and lowest noise (especially over a broad range of input impedance)?
Second question, how can you extend the S curve (always present in differential pairs) so that you are not hard limited by 2 Id ?
Three, who invented this first? and how many decades ago?
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Old 24th September 2009, 06:09 PM   #1523
PMA is offline PMA  Europe
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John, I would like to ask a 'practical' question. Is there a big difference between my simulation results, for some 2.8Vpeak output amplitude, and your measurement on a real circuit? Also, is there a big difference between simulation and measurement in spectral content for SE and balanced input drive?

http://www.diyaudio.com/forums/lounge/146693-john-curls-blowtorch-preamplifier-part-ii-post1931538.html
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Old 24th September 2009, 06:24 PM   #1524
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I don't think so, Pavel. I got .02% harmonic distortion at 2.5V out, with 3rd dominant and 2n'd about 20db below third, at 4KHz. This, of course, is beyond clipping level of my JC-1 or similar power amps or about 800+ watts output/channel into my loudspeakers with the JC-1, so it should be a high enough level for consideration.

Last edited by john curl; 24th September 2009 at 06:28 PM.
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Old 24th September 2009, 06:55 PM   #1525
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Quote:
Originally Posted by john curl View Post
Well, it looks like everybody 'fell off the truck'. Could it be that schematics are intimidating? Or just too simple to bother with?
However, this is the question: How do you make a differential pair (of any active device) that is both lowest distortion and lowest noise (especially over a broad range of input impedance)?
Second question, how can you extend the S curve (always present in differential pairs) so that you are not hard limited by 2 Id ?
Three, who invented this first? and how many decades ago?
I personally jumped out of the truck because I expected something exceptional, but found, like in a story, "a new patented way to screw the bulb", when the main device stands on the stool, puts the bulb into a socket on a ceiling, 4 devices helping it take each leg of the stool and start going in circles screwing the bulb; 4 (or more) additional devices go around in circles just to save the main screwing device from dizziness.
I personally would prefer just one device screwing the bulb by hands.
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Old 24th September 2009, 07:03 PM   #1526
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Quote:
Originally Posted by john curl View Post
Well, it looks like everybody 'fell off the truck'. Could it be that schematics are intimidating? Or just too simple to bother with?
However, this is the question: How do you make a differential pair (of any active device) that is both lowest distortion and lowest noise (especially over a broad range of input impedance)?
Second question, how can you extend the S curve (always present in differential pairs) so that you are not hard limited by 2 Id ?
Three, who invented this first? and how many decades ago?
Hello John, I only know the third question

One question I'm wondering since some time....

Which of the shown variants of input differential would you prefer (I think you used both?)

Thanks, Tino
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Old 24th September 2009, 08:59 PM   #1527
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A is easier to implement because of gain scaling. I suspect that they are about the same in linearity. The reason for this is that it is best to operate at the highest level of operating current, and A allows this, yet allows gain reduction.
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Old 24th September 2009, 09:23 PM   #1528
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To be fair, a differential pair of vacuum tubes, such as the 7788, with variable screen regulators and properly designed by a real engineer, can do amazing things in terms of distortion. If you what to know more about this configuration, please contact Bascom King, of Santa Barbara, CA USA, as he helped design this configuration. It is not as quiet as fets, but the dynamic range is at least, the same as the best 4 jfet configuration.
Even a single triode tube, operating open loop, can be very good, as shown by the Audible Illusions line stage, if you want to not use differential input.
The real advantage of the 4 quadrant configuration over other approaches is the self biasing aspect, so that the S curve can be extended, and linearized, especially at extreme current swings. The differential pairs are in parallel, so the noise floor can be 3dB lower.
The biggest weakness is that the p channel fets have more capacitance for the same amount of transconductance i.e. Gm, and this can get in the way of extremely high Z input operation.

Last edited by john curl; 24th September 2009 at 09:25 PM.
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Old 24th September 2009, 09:30 PM   #1529
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Hi John,
Quote:
However, this is the question: How do you make a differential pair (of any active device) that is both lowest distortion and lowest noise (especially over a broad range of input impedance)?
Fewer active devices in the signal path running at higher currents. Reduce supply voltages (so things run cooler and a selected optimal point) and fix D-S (or C-E) voltages to prevent modulation effects from the supply voltages varying.
My guess for the first question. If you look at RF devices, they are almost always far more simple than audio parts if you look at # of active devices in the signal path.

Quote:
Second question, how can you extend the S curve (always present in differential pairs) so that you are not hard limited by 2 Id ?
That's a good question, by I'd ask why there is a need to remove this limit? Just work within it, and if your bias currents are high, it's very unlikely you would even approach 1 ID. After all, limiting the signal current to a smaller fraction of the operating range buys you a more linear transfer curve anyway. Once you begin to approach the complete operating range of current swing, you are creating your own problems to solve. I think that may force you to a more complicated way of maintaining linearity, and that seems to be opposite to the direction you normally would design in John.

Quote:
Three, who invented this first? and how many decades ago?
No idea. I would suspect that there are examples (in a Bell lab somewhere of) almost every way to deal with an audio signal. Remember that before feedback amplifiers were designed, everything they did was open loop and tubes.

This is all solidly before I was born in the late 50s.

-Chris

Edit: Hey John, we were posting at the same time.
What would be interesting are some papers on this subject, rather than a look at how one man performed this design. Again, Bell Labs may have something here. Did Bascom King ever work for Bell? I'm not familiar with him or his work that I know of.
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Old 24th September 2009, 09:41 PM   #1530
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John,
Indeed, jfets are quieter, however, aren't tubes more linear?
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