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Old 14th January 2010, 07:19 PM   #1
Tyimo is offline Tyimo  Hungary
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Default Differential push-pull design question

Hi!
Could somebody explain what is a differential (balanced) push-pull design?
What is the difference between differential (balanced) push-pull and "normal" Push-Pull?

Tyimo

Last edited by Tyimo; 14th January 2010 at 07:24 PM.
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Old 14th January 2010, 08:03 PM   #2
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There really is no difference between the two. It is a question of terminology within the English language. A "normal" push-pull stage is balanced by it's very nature. A "differential' stage (with few exceptions) is also a balanced one. So in a maner of speaking, one is saying the same thing twice by saying "differential push-pull". A push-pull circuit is assumed to be, and intrinsically is, a differential circuit, and therefore balanced.

For audio amplifier purposes a push-pull stage is one that amplifies the sum of it's two inputs since they are combined at the output. A differential amplifier can also be used to amplify (or null) the difference between the inputs.
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Last edited by HollowState; 14th January 2010 at 08:13 PM.
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Old 14th January 2010, 08:15 PM   #3
tomchr is offline tomchr  United States
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A push-pull output stage is differential by design.

But a complete amplifier can be single-ended (the signal is referenced to ground) up to the phase splitter where it becomes differential ("the signal" then is the difference signal between two nodes). I would consider this a "normal push-pull design".

One can also implement the entire signal path using differential circuits. Morgan Jones did this in the Crystal Pallace -- except he's using a single ended input to the differential input stage and driving the inverting input of the differential input stage with the global negative feedback signal. In case of this "differential push-pull design", a differential input stage is used and the output of it is used to drive the output stage directly (typically via a pair of cathode followers). No phase splitter is needed as the signal is already differential. Had Mr. Jones done what he states he intended to do and driven the input stage with a differential signal (as found on XLR connections), the design would have been fully differential input-to-output.

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Old 14th January 2010, 09:38 PM   #4
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Quote:
Originally Posted by Tyimo View Post
Hi!
Could somebody explain what is a differential (balanced) push-pull design?
Balanced push-pull is just what it sounds like: a two phase system where both phases have the same absolute amplitude, but which are 180 degrees out of phase. This design nulls out even order harmonic distortions, but not odd ordered harmonic distortion.

Quote:
What is the difference between differential (balanced) push-pull and "normal" Push-Pull?
Using a differential amp as a phase splitter gives better AC phase-to-phase balance, and, more importantly, balanced harmonic distortion between phases. The line-up of the various paraphase splitters do not do this, as one phase will typically show greater harmonic distortion than the other phase. Sometimes, this might be desirable (i.e. guitar amps) but is not for audio reproduction.

In order to get the most from your LTP (differential amps) splitters, it's best to use an active (CCS) load on the tail.

The only other non-diff amp splitter that offers phase-to-phase balance is the cathodyne, if it doesn't see an overdriven load that will throw it out of balance. That, and the reduced output voltage swing, is why the Williamson design moved the cathodyne back in the signal chain to a lower signal level.
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Last edited by Miles Prower; 14th January 2010 at 09:41 PM.
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Old 15th January 2010, 07:05 PM   #5
Tyimo is offline Tyimo  Hungary
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Thanks for everybody!
Quote:
A "normal" push-pull stage is balanced by it's very nature. A "differential' stage (with few exceptions) is also a balanced one. So in a maner of speaking, one is saying the same thing twice by saying "differential push-pull".
So it is only a linguistics question.

In practice:
- a "normal" push-pull is for example the Bevois Valley or Dynaco ST35 etc. (with a grounded cathode voltage stage and a cathodyne phase splitter and PP output stage)
- a differential PP is with a differential input stage and a PP uot put stage?

and than what topology has the Karna amplifier designed by Lynn Olson?
or Allen Wright's DPA-300B ?

BTW. What are the differences between Balanced and Push-Pull amps??
Sorry for these question, but I got confused by this link:PCL86 UL Push Pull Full Differential Amplifier

Tyimo
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