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Vote and Comment: SRPP vs. Aikido vs. Grounded Grid


2008-12-18 2:29 am
Could you perhaps show some examples of each topology in it's best application?

The grounded grid is pretty common in HF gear but has not many applications in audio mainly because of its low input impedance. However there is one where this could be desireable: input stage of MC phono preamp.

I personally don't like SRPP even as a simple line preamplifier. Mu-follower is a lot better.

I think the Aikido is the best of the three thanks to its excellent performance and flexibility.
Tube747 said:
What's your choice and comment for each philosophy?
These are not philosophies but circuit architectures.

Is your question to be interpreted as
"how long is a piece of string?"
"please write a textbook chapter and post it here free of charge".

As has been said already, grounded grid is rarely used in audio - it comes into its own for UHF and microwave front-ends. The audio circuit sometimes wrongly called 'grounded grid' is not grounded grid but cathode coupled - or short-tail pair.
What do you mean by "grounded grid"? Bruce Rozemblit's circuit is not a "grounded grid" in any meaningful use of the term.

As was indicated by DF96, Rozenblit's circuit is semi-misnamed, and is better referred to as a coupled-cathode. This is a compound gain stage comprised of a cathode follower input driving a grounded grid output, the halves coupled via their cathodes. J. Broskie is fond of this configuration, and his analysis (plus his own variations) can be found on his site.
The MM input and line stage are somewhat unusual active load stages, with upper cathode resistors bypassed. This may be an attempt to cancel out the significant second-order distortion which an ECC82 can generate. I suspect a proper active load stage might be better, but maybe the designer has done measurements and finds better results with his unusual design. The MM output stage is SRPP.

The low MM input stage gain (half mu) means that signal/noise is significantly compromised. At mid-band (1kHz) the input stage plus RIAA network will attenuate the signal!

As Ian says, no sign of an Aikido.
Sorry, the mod 2a and 2b are Aikido based on what my friend told me yesterday.

This is frequenct source of puzzlement for newbs, so just to clear things up, this is an Aikido stage (i.e. cathode follower with PSU noise cancelling):

This is a half-mu stage (the label on the image is wrong; it's not an SRPP):

This is an SRPP:
The first one is a White cathode-follower with noise cancellation. Give some credit to Eric White who invented the original circuit.
Strictly speaking, the "White cathode follower" name is now reserved for the push-pull version of the circuit where the lower triode gets an input signal from a resistor in series with the anode of the upper triode. It's similar to this Aikido version, but perhaps unwise to start mixing names. Better to have one name for each circuit, so we all know where we are!
No, I believe Broskie uses the term Aikido for the general principal of PSU noise cancellation by injecting it somewhere else. However, he doesn't really have the monopoly on that as it's been around a long time, so it's rather a personal gimmick of his.

"The name "Aikido," on the other hand, is a fairly recently coined word, which I use to label the technique of eliminating power-supply noise from the output by anticipating its presence and countering it simultaneously " Aikido Push-Pull

It's not a white CF.
"Then there the problem of a name being given too much scope; for example, for many tube lovers any time two tubes are in a totem-pole position, with one triode atop another, such as in the cascode or the White cathode follower or in the Aikido cathode follower, the name "SRPP" is applied." Same page.
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