4 or 8 ohm acoustics?


Created on 18.11.2010 20:56.

Updated on 31.01.2020 23:44.

Author: Igor Illarionov.

Question: I’ve heard the opinion that 8-ohm acoustics are better (meaning the same speakers, but in two types of 4 and 8 ohms), it seems like it creates a higher sound pressure and has less dependence of uneven frequency response on loudness. Who has any opinions on this matter?

Answer: Fundamentally there is no difference, but in practice, given that the question probably refers to:

  1. to the same amplifier with the same final output impedance;
  2. to the same power supply of the amplifier with a final load current (short circuit);
  3. to the same connecting cable with the same length;
  4. to acoustic systems with different ohmic resistances (4 and 8 ohms), but structurally close and with presumably the same percentage unevenness of the characteristics of the dependence of the resistance to alternating current on the frequency;

… there is a difference and can vary from insignificant to significant and even critical.

1. According to paragraphs a) and b), the power released by the same amplifier on 8 and 4-ohm acoustic systems with the same input signal and the same position of the volume control can vary from equal if an amplifier paired with a power supply is the ideal current generator, down to half on an 8 ohm load if it is an ideal voltage generator.

In practice, the truth is, as always, in the middle. The basis is Ohm’s law for a closed circuit. The power delivered at an 8-ohm load is one to two times lower.

So, we figured out the power.

2. According to the same two points, a transformerless amplifier that allows a 4-ohm load to be connected works without problems at an 8-ohm one, with a slight decrease in the power output. On the contrary, not always – an amplifier designed for an 8-ohm load may fail when connected to a 4-ohm one. Basis – exceeding the permissible load current at maximum volume levels. This is a way out of the standard current mode with all the ensuing possible consequences.

3. The contribution of the cable to the signal in the case of connecting a 4-ohm load will be approximately twice as high. The cable, as a complex element of the path with distributed parameters, is a carrier of a number of properties that can affect the signal received at the output.

The recommended length, at which, as a rule, the contribution of a classic speaker cable is practically absent, can be 2 meters for a 4-ohm load and 4 – for an 8-ohm load. The properties of the cable may differ depending on the length, material of the wire and insulation, the type of winding, the thickness of the cores (cores), the quality and direction of pulling the conductive part of the structure through the feeder.

In most cases, this should not be given undue importance, since often, in the case of exotic cable designs, the combination of an expensive cable and expensive acoustics works as an attempt to compensate for some properties (and disadvantages) with others. That is, the need to use a specific cable is actually due to the shortcomings of specific speakers, although the combination can be quite euphonious and even interesting.

As a rule, for a well-executed technical design of a classical acoustic system, an inexpensive universal acoustic cable made with an understanding of the laws of physics is sufficient. And what is higher is already more a fit after the fact, based on the flaws in the speaker design.

4. The back EMF generated by the AC as a response to the input signal is applied to the AC terminals, causing the output volt-ampere characteristic to become unstable. To neutralize this effect, an amplifier with a low output impedance is used (the back-EMF signal is shunted at the output terminals). That is, in a closed AC circuit of audio frequency, which is made up of the capacitors of the power supply (pump energy source), the transition resistance of the output transistors (pumping energy control), acoustic systems (energy consumer), according to Ohm’s law for a complete closed circuit, try to minimize voltage drop across the amplifier (first two links) and maximization across the speaker. Thus, a high efficiency of the sound reinforcement and reproduction system is achieved.

Obviously, a system with a higher resistance will be more stable here. But only except for those cases where the modes of the output stage are determined by the load resistance and / or are formed dynamically.

In total, we have:

  • On points 2,3, and partly 4 – the preponderance is in favor of 8-ohm acoustics.
  • On point 1 – in favor of 4 ohms.

So choose the acoustics for yourself, and do not forget to read the instructions.