Is the Zen really a push-pull? - diyAudio
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Old 17th August 2004, 02:12 AM   #1
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Default Is the Zen really a push-pull?

I have been working with the Aleph Current Source (reference from DIY Project: The Penultimate Zen’s Current Source).

Low-valued resistors (R13, R14) at the output sense current and inject the signal into the current source Q2, thereby modulating the current source with the music. Therefore, the current source stops being “passive” in the circuit, and takes on an “active” role.

I’m tempted to call this arrangement a form of push-pull: during 180 degrees of a sine wave, the bottom transistor Q1 goes high-impedance, thereby routing current into the load. The top transistor Q2 goes low impedance, thereby sourcing more current for the load (pushing).

In the opposite 180 degrees, the bottom transistor goes low-impedance, drawing current from the load (pulling), and the current source goes high-impedance, and limits current.

Now, it’s not a formal push-pull, but it’s close enough for me. The amount that the current source acts as an active device is programmable by changing some resistor values (R15).

Notice the signal appears on the power supply rails as the “current source” modulates power supply current, as it pushes and pulls. It surprises me that the arrangement works well with the capacitance multiplier in the Zen v4! Can someone explain this to me?
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Old 17th August 2004, 03:49 PM   #2
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I once referred to the Aleph output as "a current source driving a current source." Given that the gain device in the Aleph output presents its Drain to the output, that would make it a high impedance, transconductance amplifier. Then you have the current source, which is operating "backwards" with its Source presented to the load, hence a low impedance. Now, normally a current source would be operating at a single, defined current with the voltage being the parameter that varies. But here the CCS runs steady at idle and varies in use, deriving its signal from the output of the gain stage (itself a current source), hence my calling it a current source driving a current source.
So what is it?
Quasi-complementary?
Current source?
Single-ended?
Push-pull?
All of the above. And none.
The problem is one of semantics, shoals upon which many threads here at DIY have come to grief.
There are a certain number of commonly accepted phrases that everyone can agree on, but the variations and elaborations and permutations on those themes can drive you crazy. An elaborate taxonomy could be invented that might allow for easy classification of circuits, but I'm afraid that I don't know enough Latin to come up with the families and species and whatnot that would make such a system work properly.
Perhaps we could start by calling the Aleph output stage Quasi Constantus Currentus Outputus Alephus
Lacking the nomenclature, just grok the concept and all will be well.

Grey
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Old 17th August 2004, 05:39 PM   #3
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I concur. Push-Pull isn't the right language, but it got the job done.

However, the question still stands: How does the capacitance multiplier work with this circuit? The Zen v4 uses this modulated current source and a capacitance multiplier at the same time (generally, a capacitance multiplier likes constant current, and doesn’t work well with varying current).

This also violates the presumption of the original Zen, that multiple channels can share a common power supply without experiencing crosstalk though the power supply rails.

Using the modulated current source, the power supply is also modulated, and the signal appears on the power supply rails. Multiple channels of the Zen v4 shouldn’t share power supply rails – oops!
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Old 17th August 2004, 09:07 PM   #4
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I don't think that the presumption of the Zen is that each
channel must have its own power supply, and many of them
have shared shupplies.
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