POWER SUPPLY FEEDBACKS

How they act and how to counter them effectively

Negative feedback is a technique commonly used in most systems, not just audio. In general terms, it consists of taking a part of the output signal and returning it to the input of the device in phase opposition.

The use of negative feedback in audio can have several objectives, such as providing greater stability, reducing distortion, or increasing bandwidth.

In addition to this type of feedback, which is exclusively electrical, there are other forms of feedback, for example acoustic feedback, with the typical Larsen effect that we have all experienced (the classic ‘whistling microphone’).

There is, however, a lesser known and much underestimated form of feedback, probably because it is not intuitive and difficult to measure.

This is the FEEDBACK of Power Supply.

What is it and how does it work?

We must first consider that the impedance of the supply line is low but never zero, and increases proportionally with frequency due to the reactive components. This causes voltage variations proportional to the current absorbed by our system.

The current is not constant, but varies over time in relation to the musical signal. The immediate consequence is that the voltage on the multi-socket that typically supplies our system also varies according to the message being played.

Looking now at our system from this point of observation, we see that the electronics differ from one another: those that operate at signal level (phono preamplifier, streamer, CD player, D/A converter, preamplifier) have very low and usually constant absorptions, while the power amps have absorptions that are several orders of magnitude higher, and always correlated to the musical signal.

These ‘correlated’ absorptions are ‘seen’ by the power supply, which as we know has a non-zero resistance and an impedance that increases with frequency, and generate a voltage variation that we find intact on all the primary stages of the signal electronics, from there on the secondary and in the active stages, and finally on the signal.

In fact, we have real feedback: the output signal of the power amp is carried over, indirectly, into the other components of our system.

If it is true that the negative feedback in electronics is desired, foreseen and controlled for the purpose of their optimal functioning, in the case of power feedback the effects affect the entire system, and are neither desired, nor foreseeable, nor controllable a priori.

This type of feedback is obviously complex in nature, and in fact unpredictable in its effects, which depend on the sum of all the interference produced by the individual components and how this interference interacts with the active circuitry within each.

In fact, the same type of interference, although less pronounced, occurs between the power supplies of the signal electronics: each power supply produces a certain amount of electrical noise on its primary.

What are the consequences of this phenomenon?

Power supply feedback is to all intents and purposes a source of pollution, spreading across the audible spectrum and beyond.

The filtering systems present in the power supplies of electronics are designed to eliminate ripple, the alternating component of the mains frequency (50/60Hz), but are far less effective at higher frequencies and on common-mode noise.

The consequence is that much of this pollution is not absorbed by the mains, enters the active circuits and interferes with the musical signal, causing harmonic distortion, dynamic compression, loss of detail through masking, listening fatigue and lack of naturalness.

The solution to the problem: passive parallel filters

The most effective solution is to eliminate this pollution at the exact point from which all electronics draw their energy. This point is also the one where the elimination of noise from the external mains is most effective, and that is at the multi-socket, which is the centre-stack of the power supply.

Our parallel-type filters are designed to greatly reduce, even eliminate, the negative effects of power supply feedback, avoiding the side effects typical of series-type filters.

Our Black Hole filter is a plug-in type and is designed to be easy to insert into the audio system: it fits directly into one of the available sockets.

An even more effective solution is the Dynamics Extender and the new Materik series.

Apotema Audio filters are designed to handle both self-induced and external network disturbances correctly, acting on the specific characteristics of intensity, frequency and duration.

Although different in amplitude and depth of intervention, all filters share the same principle: the use of a network of diversified cells, each dedicated to a precise and unique purpose. The optimal combination of cells, in terms of number and type, is the key to achieving the high performance found in Apotema Audio products.