PassLabs BalancedZen free download

File name PassLabs-BalancedZen preamp.pdf

Balanced Zen Line Stage Copyright 1997 Nelson Pass Introduction The popularity of the Zen projects points out the interest in very simple linear circuits. They are intended to fuel that interest. The Zen, Bride of Zen, and Son of Zen have been explorations in how much objective and subjective performance can be achieved with a single gain stage. This extreme simplicity has an aesthetic appeal, which speaks to the purist in audiophiles, and the presumption that simple circuits sound better. At least one "objectivist" has complained (objected?) that the Zen projects do not measure up compared with more sophisticated and complex amplifiers. This is mostly true, but beside the point. The literature and store shelves are full of multi-stage amplifier circuits using generous amounts of negative feedback. These are single stage amplifiers. The Design Getting good performance from a single gain stage is a fine technical challenge, and I would say a proper beginning for those who would go on to design and build more complex circuits. Simple circuits have particular value as DIY projects. They are more understandable, they are more likely to be attempted, and they are more likely to work. So is this the anticipated Bride of the Son of Zen? I suppose it is. It has an identical topology and is perfectly suited for driving the Son of Zen, but there is much utility to this circuit. It also serves as a nice balancedin to unbalanced-out or unbalanced-in to balanced-out converter. Like the Bride of Zen and the Son of Zen projects, this circuit performs linear amplification without negative feedback. A lot has been written pro and con about the use of negative feedback, and I don't propose to bring the debate into this article. It just so happens that we will be getting quite good performance without it. Balanced Operation Let's review why balanced operation is desirable. Audio circuits operate in an environment of electrical noise; crosstalk from other channels, ground loops, magnetic pickup from transformers, power supply ripple and other noises. In a balanced circuit, two opposite phases of the signal are present on two otherwise identical input lines. The input of a balanced circuit has a plus and minus polarity, and the output of the circuit also has a plus and minus. The balanced amplifying circuit will amplify the difference between the two inputs and will display a larger difference signal at the output. What the circuit doesn't do is as important as what it does; it does not amplify any portion of the signal which is the same at both inputs. Ideally it completely rejects the common input signal, and the quality of this rejection is referred to as the Common Mode Rejection Ratio (CMRR), which tells how much of the common input signal gets through. Pass D.I.Y Project: Balanced Zen Line Stage This balanced line stage is the classic "differential pair" topology with two identical gain devices connected to develop voltage gain from a differential voltage input