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Old 28th September 2018, 03:48 AM   #41
Loudthud is offline Loudthud  United States
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See third paragraph on page 5 of the article:

http://www.firstwatt.com/pdf/art_h2.pdf

Last edited by Loudthud; 28th September 2018 at 03:51 AM.
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Old 28th September 2018, 05:55 AM   #42
Loudthud is offline Loudthud  United States
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Nelson also talks about it in this thread:

Sound of various Pass amps

Quote:
Originally Posted by Nelson Pass View Post
SIT-1: Broke the mold in the ability to play with the load line of a no-feedback
Common Source amplifier that still has any damping factor. A very unique
harmonic signature that can be dialed in to taste - Lots or little of 2nd
harmonic of either phase. Literally the sound of one transistor clapping -
without the input buffer (which no one seems to use) there is only 1
transistor in the amp. Has tremendous clarity and depth, depending on
setting, spectral consistency with warmth and detail. It allows you to focus
on a single instrument in an ensemble, almost to the exclusion of all else if
you want. This is the amp that convinced me that absolute phase is not
such a subtle factor.

SIT-2: A SIT-1 with a constant current source load instead of a stack of
power resistors. More efficient but similar to the SIT-1, it is set at a single
load line setting, so is not quite as flexible or fun.
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Old 28th September 2018, 06:21 AM   #43
sarathssca is offline sarathssca  India
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can it be used as preamp with very high input impedance and again of around 10db by changing the registers :
R1 = around 150K or 160K
R3 = 39K
Then i think the input impedance will be around 150k.. is my understanding ok?
Can we increase the input impedance to around 1Mohms?
Also does it needs a output buffer to lower output impedance when used as a preamp with gain.

Last edited by sarathssca; 28th September 2018 at 06:39 AM.
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Old 28th September 2018, 01:35 PM   #44
WBS is offline WBS  United States
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Quote:
Originally Posted by Nelson Pass View Post

...SIT-2: A SIT-1 with a constant current source load instead of a stack of
power resistors. More efficient but similar to the SIT-1, it is set at a single
load line setting, so is not quite as flexible or fun...

Well, my SIT-2 is the most fun I've ever had with an amplifier!
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"Why do we try to fool the ear? Because it makes people happy and it makes it easier to listen to stuff."
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Old 28th September 2018, 01:52 PM   #45
Mark Johnson is offline Mark Johnson  United States
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I posted this schematic in another thread because I was clueless & did not realize this thread exists.

It came from a review of a phonostage in Audio Magazine, February 1979. Sold by a company that's still in business today.

Each of the two Long Tailed Pairs is intentionally unbalanced, by adding an emitter degeneration resistor (330 ohms) to only one of the two legs of the Long Tailed Pair. This creates 2nd order harmonic distortion and apparently that was a design goal. Somebody knew something, long long ago.

I like to analyze the circuit this way: Imagine that the 330 ohm resistor is really a 3-terminal device, a potentiometer. Its wiper connects to the 39K bias resistor, and its outer arms connect to the two emitters. With the wiper at the midpoint (165 ohms left, wiper, 165 ohms right), the circuit is balanced. However the designer has intentionally chosen to turn the pot all the way clockwise, and now it's (330 ohms left, wiper, 0 ohms right). The circuit is no longer balanced.

If you built the circuit and used an actual potentiometer, you could seamlessly dial from lots of positive phase H2, to nearly-zero, to lots of negative phase H2, just by twirling the dial. And, get this, you could have done so 40 years ago! I bet it was designed in 1978 or perhaps 1977, took a few months to build production units, shipped one to an Audio Magazine reviewer, who took a few months to do the review and write his article, turned in his copy, which took a few months to get typesetted and appear in print, in Feb 1979.

By the way, do a google image search for the schematic of the B&K ST140 power amp (mid 1980s), an early lateral-MOSFET amp widely praised for its smooth sound. What do you see? Cowabunga!!

_
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Last edited by Mark Johnson; 28th September 2018 at 02:00 PM.
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Old 28th September 2018, 02:30 PM   #46
kasey197 is offline kasey197
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Although seemingly counterintuitive, I believe/recall that the ‘unbalanced’ resistors actually aids symmetry in the complete circuit ... I’ll give this a twirl on a Sim to make sure my memory isn’t playing tricks on me ...

Last edited by kasey197; 28th September 2018 at 02:49 PM. Reason: Edit to correct balance vs summary
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Old 28th September 2018, 03:21 PM   #47
Mark Johnson is offline Mark Johnson  United States
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Quote:
Originally Posted by kasey197 View Post
Although seemingly counterintuitive, I believe/recall that the ‘unbalanced’ resistors actually aids symmetry in the complete circuit ... I’ll give this a twirl on a Sim to make sure my memory isn’t playing tricks on me ...
Does your memory tell you that having two 330 ohm resistors in place, as shown, is in some way better than having zero 330 ohm resistors, and also better than having one 330 ohm resistor? That would surprise me.
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Old 28th September 2018, 04:48 PM   #48
Rick Miller is offline Rick Miller  United States
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Earlier this year Bob Katz wrote an article about a device he built called "Bobs Blender". He could introduce 2nd harmonic into a signal path with it. He used tubes and in fig 5 of the link below it shows only second harmonic, no third or higher order products. The article on the Pass H2 shows some third and fourth harmonic.
Is this because JFETs are square law devices while tubes are Childs law devices?


Katz's Corner Episode 25: Adventures in Distortion | InnerFidelity
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Old 28th September 2018, 05:28 PM   #49
Nelson Pass is offline Nelson Pass  United States
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I would say the tube is more nearly square law.

That said, if you bias the J112 for "positive phase 2nd" you get a cleaner
2nd harmonic, but more limited maximum output.

Clearly this circuit can be improved, but I popped this out for free fun
giveaway at BAF. Not as an example of the best possible.
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Old 28th September 2018, 05:39 PM   #50
Rick Miller is offline Rick Miller  United States
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Quote:
Originally Posted by Nelson Pass View Post
I would say the tube is more nearly square law.

That said, if you bias the J112 for "positive phase 2nd" you get a cleaner
2nd harmonic, but more limited maximum output.

Clearly this circuit can be improved, but I popped this out for free fun
giveaway at BAF. Not as an example of the best possible.
Thanks for the reply Nelson, and thank you for giving all of us this circuit.
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