External speaker

[ValvoStef]

Afternoon all. I have an understanding problem with impedances and loads.
For example, lets assume a radio has an power pentode with 10k impedance, the transformer connected to it has primary 10k and secondary 4 ohms, the speaker has 4 ohms. If I connect an external 4 ohm speaker which is then in parallel to the internal speaker, the output load of the transformer would then be 2 ohms. Does this mean the primary impedance of the transformer will drop as well and therefore overload the valve? The reason for this question is, for many years I use a valve radio downstairs, on a daily basis with an external speaker connected to it and it does not seen to affect the performance of the radio. Am I wrong with my assumption?

[paulsherwin]

The primary impedence will fall, but no harm is likely to be done. Anode loading is a lot less critical than is often assumed.

Damage is much more likely if the radio is used without any speaker at all connected, as the increased impedence will cause very high voltages across the transformer primary. Even then you usually get away with it though.

[ValvoStef]

Thanks Paul.
However, would a lower impedance not also mean a higher anode current?

[paulsherwin]

The current is mostly determined by the bias in class A operation. The output volume will be somewhat reduced but this may not be noticeable.

https://en.wikipedia.org/wiki/Amplifier#Class_A

[ValvoStef]

thanks for the info, Paul.

[Ed_Dinning]

Hi Stef, when we loosely talk of valve impedances etc it can be confusing.

The valve is operating under a set of static, almost DC conditions and draws a certain anode current. It also operates under (usually ) small signal AC conditions that are "on top, or applicable at the same time as the static conditions.
A total short of the op trans secondary will short out the AC part of the signal, but leave the static (DC) part relatively unchanged. Changing speaker impedances over a 4 to 1 ratio thus has negligible effect on valve volts and amps (it may alter signal gain and distortion levels.
If the speaker goes open circuit there is then the unloaded inductance of the output transformer in the anode circuit and a rapidly changing signal. This generates high voltages much as a coil does in a car, with possibly bad results for the valve and the transformer insulation.

This can be a bit difficult to understand as resistance and impedance are both measured in ohms and have similar numerical values for a speaker, so leading to confusion.
Used in this application the op trans is an Impedance Trandformer, not an impedance
per see.

Hope this helps to clarify things


Ed

[TonyDuell]

This reminds me of another little 'puzzle'...

Suppose you have an output valve operating in class A, transformer coupled to a speaker. And assume the valve introduces no distortion.

With no input signal, there is a certain anode current, which is constant. Therefore no power is transferred to the speaker, there is no output.

Now apply a symmetrical sine wave input. There is now a symmetrical, changing anode current centred on the steady value above. So the average anode current is unchanged. The power taken from the PSU is unchanged. But there is now an output signal in the speaker, so some power ends up there. Where does it come from?

[julie_m]

The power going into the speaker would have been dissipated as heat in the valve under no-signal conditions. Class A is inefficient like that. Also, just because the mean value over time of an alternating current is zero, certainly does not mean that no energy is transferred.

[TonyDuell]

Exactly. The valve actually runs hotter with no input signal (which most people do not expect)

[Herald1360]

In an ideal class A 5W amplifier, the zero signal dissipation in the output device will be 10W. At full output, 5W in the device and 5W output. If overdriven, you'll get more output and less dissipation but the output will be heavily distorted.

[ms660]

Originally Posted by TonyDuell

The power taken from the PSU is unchanged. But there is now an output signal in the speaker, so some power ends up there. Where does it come from?

The power in the speaker comes from the power supplied by the power supply, average and instantaneous can sometimes be confusing.

Lawrence.