Input Impedance of NVA Passives?

[Yomanze]

Hi Doc,

Am looking for a passive preamp to complete a second system, but ideally want something with 10K input impedance. Could you please advise on the NVA range of passives and their input impedance?

Thanks,

Neil

[Dr Bunsen Honeydew]

They don't have one they are passive All impedances are variable between zero (as though the pre-amp doesn't exist so the source is looking at the input impedance of the power amp) at full out on the volume control. To a max of what ever the resistance of the pot is at closed or zero on the volume control. That can be 10k or 47k which are the two options I give, but to be honest it means absolutely ferk all on all usable solid state amps (some like Naim do not like passive pre-amps) and has a very marginal effect on SOME valve amps.

I am afraid this is another forum folk law and brainwash put around by ignorant people.

Active pre-amps have input and output impedance because they have active input and output buffer stages that fixes the input and output impedance across the volume range.

So little understanding it is sad

EDIT - not a personal dig, just that it gets repeated so many times everyone believes it.

[jammy395]

So little understanding it is sad

It is indeed Sad.....

[Dr Bunsen Honeydew]

A major part of the problem Jammy is that people don't seem to understand the difference between impedance and resistance. It causes much confusion in the understanding of specs, especially when manufacturers refer to DC resistance as impedance. It just confuses people and turns specs into crapology which then infects the forums. An active pre amp has input and output impedance because it is an active circuit. A passive pre-amp is just a variable resistance in the way of the signal.

[Dr Bunsen Honeydew]

Per usual none of these subjects are simplistic - see if you understand this.

Electrical impedance is the measure of the opposition that a circuit presents to a current when a voltage is applied.

In quantitative terms, it is the complex ratio of the voltage to the current in an alternating current (AC) circuit. Impedance extends the concept of resistance to AC circuits, and possesses both magnitude and phase, unlike resistance, which has only magnitude. When a circuit is driven with direct current (DC), there is no distinction between impedance and resistance; the latter can be thought of as impedance with zero phase angle.

It is necessary to introduce the concept of impedance in AC circuits because there are two additional impeding mechanisms to be taken into account besides the normal resistance of DC circuits: the induction of voltages in conductors self-induced by the magnetic fields of currents (inductance), and the electrostatic storage of charge induced by voltages between conductors (capacitance). The impedance caused by these two effects is collectively referred to as reactance and forms the imaginary part of complex impedance whereas resistance forms the real part.

The symbol for impedance is usually Z and it may be represented by writing its magnitude and phase in the form |Z|∠θ. However, complex number representation is often more powerful for circuit analysis purposes. The term impedance was coined by Oliver Heaviside in July 1886. Arthur Kennelly was the first to represent impedance with complex numbers in 1893.

Impedance is defined as the frequency domain ratio of the voltage to the current. In other words, it is the voltage–current ratio for a single complex exponential at a particular frequency ω. In general, impedance will be a complex number, with the same units as resistance, for which the SI unit is the ohm (Ω). For a sinusoidal current or voltage input, the polar form of the complex impedance relates the amplitude and phase of the voltage and current. In particular,

The magnitude of the complex impedance is the ratio of the voltage amplitude to the current amplitude.
The phase of the complex impedance is the phase shift by which the current lags the voltage.

The reciprocal of impedance is admittance (i.e., admittance is the current-to-voltage ratio, and it conventionally carries units of siemens, formerly called mhos).

[Dr Bunsen Honeydew]

Or this if you wish to understand impedance matching.

In electronics, impedance matching is the practice of designing the input impedance of an electrical load (or the output impedance of its corresponding signal source) to maximize the power transfer or minimize reflections from the load.

In the case of a complex source impedance ZS and load impedance ZL, maximum power transfer is obtained when

Z_{{\mathrm S}}=Z_{{\mathrm L}}^{*}\,

where * indicates the complex conjugate. Minimum reflection is obtained when

Z_{{\mathrm S}}=Z_{{\mathrm L}}\,

The concept of impedance matching was originally developed for electrical engineering, but can be applied to any other field where a form of energy (not necessarily electrical) is transferred between a source and a load. An alternative to impedance matching is impedance bridging, where the load impedance is chosen to be much larger than the source impedance and maximizing voltage transfer (rather than power) is the goal.

Impedance is the opposition by a system to the flow of energy from a source. For constant signals, this impedance can also be constant. For varying signals, it usually changes with frequency. The energy involved can be electrical, mechanical, magnetic or thermal. The concept of electrical impedance is perhaps the most commonly known. Electrical impedance, like electrical resistance, is measured in ohms. In general, impedance has a complex value; this means that loads generally have a resistance component (symbol: R) which forms the real part of Z and a reactance component (symbol: X) which forms the imaginary part of Z.

In simple cases (such as low-frequency or direct-current power transmission) the reactance may be negligible or zero; the impedance can be considered a pure resistance, expressed as a real number.

[Dr Bunsen Honeydew]

ERGO - a passive pre-amp has no capacitance or inductance or active gain, so cannot be reactive, so contributes no phase shift, so behaves like a pure resistive load to all frequencies (up to extreme HF and RF, after that there is reactance). So can be said to have a purely resistive nature. So as long as its maximum resistance does not exceed the input resistance on the target (input of power amp) it makes absolutely ferk all difference.

[Yomanze]

Thanks Richard an interesting read.

[_D_S_J_R_]

Here's me jumping in where perhaps I shouldn't, but in practise for domestic audio, the rule I believe is to have the output impedance of a source component as low as possible and the input impedance of the other end as high as possible. Some ss designs (and I think many/most valve designs) do this and don't seem to offer much of an issue with different sources and interconnects, but there are some ss amps which offer very low input impedance of 10k or less. It was my feeling that many sources may not work so well into such a load.

Apologies in advance if the above is garbage. Keen to get it straight in my mind

[Andrew]

I've never understood any of it. Even the explanations above seem to make my head hurt. Interesting subject though, because pre/power pairings are probably one the most unpredictable things in hifi. I'm sure the knowledge will be invaluable to anyone who can understand it. As for me, I guess it will still be trial and error :D