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Old 28th January 2009, 11:59 PM   #1
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Default The Rush Cascode: Possible Wiki page

Hi all.

If you don't know about the Rush cascode I suggest you use Google and your other preferred resources to get some background information. Here are some threads and pages where it has been discussed:

http://www.4qdtec.com/pwramp.html

http://www.4qdtec.com/preamp.html

Input stage idea

First of all, I need to add that I don't have much experience in electronic design. My comprehension and understanding at this point has come entirely from resources such as DIYAudio, Doug Self's site and many other free internet resources. Even though comprehension has come relatively easy for me, everything I know has still come from theoretical sources and not true time-tempered experience.

The Rush Cascode is a manifestation of the differential long-tailed pair. Same concept, different ways to implement it. It was first used by englishman Christopher Rush of THX fame, who is still designing today (info provided by AKSA).

In some posts here on DIYAudio, the Rush cascode has been dismissed quickly after theoretical and mental comparisons to the conventional long-tailed pair (henceforth referred to as the LTP) as an input scheme. I think more thought and work should be put into its understanding.

The Rush cascode can be difficult to get working because of its speed and gain, not to mention DC offset and temperature compensation if one intends to make a DC-coupled amplifier. However, there are things we can do to get around this, which I intend to expand upon.

The rush cascode can be difficult to make stable for the following reasons:

1: High gain.

2: High speed.


The common solutions to this are:

1: A degeneration resistor between the emitters to decrease gain.


Firstly, it seems there is a misconception that like the conventional LTP, the Rush Cascode benefits from using high-gain transistors. In my simulations I have found this to be inaccurate.

I have the following observations to present:

1: The distortion of the Rush Cascode is more dependent on the degeneration resistor than it is the transistors used. A low degeneration resistor will in my experience get lower distortion.

2: Stability decreases with higher-gain transistors.

I have the following suggestions when implementing the Rush Cascode:

1: Use low-gain devices, like the 2N5771/5769, with as small a degeneration resistor as possible.

2: Increase open-loop gain in other parts of the circuit instead of the cascode itself. This will result in better stability and distortion than the other way around.

3: Use both legs, rather than one, to increase OLG and VAS speed.

I believe that the Rush Cascode should have some 'net coverage, which is why I am doing this. More people should know about it.

So I want to make a Wiki page and/or PDF that talks about it along with some example circuits. I also have designed and amp on the sim that has a nifty thermal and DC offset compensation scheme, although it requires some calculations.

I want to see more circuits with it, and I want to see how others get past its shortcomings.

Attached are some ways I have used it in the simulator.

- keantoken
Attached Images
File Type: gif rushcascode.gif (10.8 KB, 638 views)
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Old 30th November 2016, 05:30 PM   #2
maiko is offline maiko  Finland
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Post Rush Cascode

Great initiative collecting info on the "Rush Cascode" Keantoken!
I took some time trying to find more information about this intriguing circuit and will just add it here for any ones information, first of this sweet child seems to have a few names, some controversial or used in other type of transistor circuitry, which are:

* Rush Cascode
* Complimentary LTP (Long-Tailed Pair) (However, this name applies as well to ordinary LTP's, eg. two LTP's, where one is N-channel LTP and the other a matching P-channel LTP, used together)
forr, a user here on DiyA thinks it's called:
* Series Differential Pair (renders very meager search engine result, but a suitable name indeed, or how about Vertical Diff Pair, VDP)
* Emitter Coupled Pair (this is incorrect as it is another name for LTP, see -> https://wiki.analog.com/university/c...ext/chapter-12 )
forrs' comment: Emitter Follower Vas stage, is this new?
And adds "I think it was used in an Akaļ amplifier around 1980."
I tried to search anything related to Rush Cascode + Akai but didn't find anything.

An odd search engine collecting "key-phrases" gave the following result
"Cascode" related terms, short phrases and links
"AKSA John, My understanding is that it is a Rush Cascode, discovered by Christopher Rush, I believe an Englishman, in 1964."

I assume due to the nature of operation of the Rush Cascode, the type of distortion is leaning towards even harmonics,
perhaps someone could fill in more here on the Rush Cascode characteristics.
Also, a Rush Cascode could probably be made up of a hybrid mix of different types of gain devices, for instance a N-ch JFET input, and a PNP BJT for the feed back.

-------------------
ps. Attaching some of the schematics from the links' Keantoken gave in case those web pages would ever disappear.
pic.1 4QD-TEC: Low distortion Audio amplifier
4QD-TEC: Electronics Circuits Reference Archive
Low distortion (0.02%) audio amplifier, circuit is dated January 1977
pic. 2 4QD-TEC: Audio preamplifier circuits
Simple to use it as a low impedance mic amp or as a high input impedance preamp for mic or, with a suitable RIAA equalization, for phono pickup or similar.
pic. 3 https://mrevil.asvachin.eu/amp/topologies/rush/
A simple Rush Cascode input stage
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Attached Images
File Type: gif pwramp.gif (24.9 KB, 115 views)
File Type: gif pre3.gif (14.2 KB, 113 views)
File Type: png rush_schematic.png (5.6 KB, 115 views)

Last edited by maiko; 30th November 2016 at 05:44 PM.
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Old 30th November 2016, 06:08 PM   #3
maiko is offline maiko  Finland
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Default Hybrid Rush Cascode

A quick schematic of the hybrid proposal, will it work?
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File Type: jpg Hybrid Rush Cascode N-ch JFET and PNP BJT.JPG (24.2 KB, 99 views)
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Old 30th November 2016, 06:22 PM   #4
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Ha, that was almost 8 years ago. Of course I never got around to the wiki page. Since then DIYaudio has learned quite a lot about this circuit. A dual complimentary version has been used in CFA amps.

At some point someone suggested we call it the NTP, No Tail Pair. Most people here could figure out what you meant if you called it a Rush Cascode, although that isn't really accurate.

I suspect the main reason it hasn't caught on in the current generation of super-amps inspired by OStripper and the CFA group, is because it's asymmetric behavior isn't conducive to super-clean square waves. A dual symmetric version has been used to get around this problem.

It can also be more stressful to use, since it can have latching problems and overload issues can be tricky to solve.
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Old 30th November 2016, 06:24 PM   #5
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Quote:
Originally Posted by maiko View Post
A quick schematic of the hybrid proposal, will it work?
I think I've seen this in app notes and phono preamps. It will certainly work, if used correctly.
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Old 30th November 2016, 06:35 PM   #6
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Quote:
Originally Posted by maiko View Post
A quick schematic of the hybrid proposal, will it work?
Yes, it will work...and current setting resistor between source and emitter may not be needed.
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Old 30th November 2016, 10:48 PM   #7
PRR is offline PRR  United States
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I found this in a 1967 book, unattributed. Sadly I can't remember which book, don't have a scan. I did a write-up of a mike-amp working this way, but web-rot has lost the images. I recently dreamed-up a fully "push-pull" complementary form with diff-output, sim-verified, but I don't think the world wants it.

Yes, even my apparently "push-pull" form really works push-push. All even-order nonlinearities up to clipping.

Many ways to skin cats.(*) You can always stitch-up skins into a coat. As far as I see, this way leads to lop-sided coats. You can tug and pull. Or you can skin the cat symmetrically, get a symmetric coat shell, and "lop side" it with snips and trims. (*I do NOT endorse cat skinning!! It's just an old american saying.)
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Last edited by PRR; 30th November 2016 at 10:53 PM.
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Old 30th November 2016, 11:52 PM   #8
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Patent US 3578911 uses the topology. The application is odd by audiophile conventions. The input and the output are the same. This connects directly across an existing (and long and installed) audio line. It makes a negative capacitor which sucks-out the capacitance in a line. Further, telephone line-center power supplies are much higher than signal voltages. The topology "stacks" two 24V amplifiers on one 48V supply.
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